20000846 Ver _Complete File_20060101

DRAFT MEMORANDUM OF UNDERSTANDING DRAFT
between the
U.S. ARMY CORPS OF ENGINEERS,
STATE OF NORTH CAROLINA
DEPARTMENT OF TRANSPORTATION,
and the
UNITED STATES FOREST SERVICE
.for the
DISPOSITION AND MANAGEMENT OF THE CROATAN WETLAND
. MITIGATION BANK
CRAVEN COUNTY, NORTH CAROLINA
Agreement No. 00-CO-110811 XX-XXX
The purpose of this Memorandum is to establish and record agreed-upon policies and
procedures between the U.S. Army Corps of Engineers (hereinafter the "Corps of
Engineers"), the State of North Carolina, Department of Transportation (hereinafter the
"Department of Transportation"), and the U.S. Department of Agriculture, U.S. Forest
Service (hereinafter the "Forest Service") to govern the implementation, monitoring and
management of the Croatan Wetland Mitigation Bank (CWMB) located in Craven
County, North Carolina upon final disposition of the land to the United States of
America, Forest Service.
The Department of Transportation, in fulfilling its public service mission of roadway and
transportation construction, has developed the CWMB, in cooperation with the Corps of
Engineers, to provide in-kind compensatory mitigation for unavoidable wetland impacts
on Department of Transportation projects for which no on-site, in-kind mitigation is
available. This 4,035 acre tract was identified as suitable for development as a wetland
mitigation bank and will be developed in accordance with the Mitigation Plan
(hereinafter the "Plan"), attached to and made part of this agreement.
This agreement is intended to implement and facilitate achievement of the plan,
establish the responsibility of each party and provide for the long-term management
objectives of the tract and will be subject to the following overall policies:
1. The Corps of Engineers, Department of Transportation and the Forest Service will
cooperatively plan the development, use and management of the CWMB as they relate
to wetlands resources. Such cooperative planning will begin with the approval of the
final Miti ation Plan and con inue throu h ion monitorin and and
management staaes. This planning will be pointed toward inclusion and management
of the tract as part of the Croatan National Forest Land and Resource Management
Plan.
2. Long-term management of the CWMB will include land uses and practices that are
compatible with the mitigation objectives of wetland restoration, enhancement and
preservation incorporating restoration of natural vegetation community structure. This
includes, but is not limited to, the protection of the East Prong Brice Creek watershed;
restoring hydrologic function and sustaining aquatic systems; restoration, enhancement,
and preservation of the natural wetlands communities (including hardwood/cypress
wetlands); providing RCW habitat linkage; enhancing black bear habitat; providing un-
fragmented hardwood wetlands for interior Neo-tropical migratory bird habitat and
restoring hardwoods on suitable sites.
3. The Forest Service will take the necessary steps to designate the CWMB property
under Management Area 7 and an appropriate management prescription as defined in
the Croatan National Forest Land and Resource Management Plan (hereinafter the
"Forest Plan") as developed in accordance with the National Forest Management Act of
1976 (P.L. 94-579, 90 Stat. 2743).
4. Management of land and development of resources will be phased in according to
the following schedule:
a. Upon consent from the Forest Service to grant the Department of Transportation
a Public Road Easement for the construction, operation and maintenance of the
U.S. 70 By-pass (TIP No. R-1015), the Department of Transportation will initiate
the process to transfer a fee simple title to the CWMB property to the United
States of America. Such title shall be free from all encumbrances with exception
to a deed reservation that allows the Department of Transportation to implement
and monitor the restoration of the wetlands as agreed to in the Plan.
b. The transfer of title shall occur in two parts: the first being the prescribed
mitigation tracts for the environmental and social impacts to National Forest
System lands as described in the Environmental Assessment for US 70,
Havelock Bypass (TIP No. R-1015); and the second being a land donation to the
United States of America.
c. The Department of Transportation will transfer title in accordance with the
regulations contained 36 CFR 254, Forest Service Manual 5400 and the Uniform
Appraisal Standards For Federal Land Acquisitions. All expenses to transfer
lands to include National Forest Exchange tracts will be born by the Department
of Transportation.
d. Upon final transfer of title, the Department of Transportation shall have full
authority under the deed reservation [as noted in section 4(a)] to implement the
restoration of the wetlands. During this period, the Forest Service will not initiate
management activities prescribed under the Forest Plan, notwithstanding those
activities necessary for the protection of the site for administrative, fire and
access needs. The implementation phase of the project is expected to be
completed by the fall of 2002.
e. During the five-year monitoring phase of the project, beginning in the spring of
2003, the Forest Service will work with the Department of Transportation and the
Corps of Engineers to move the CWMB toward the reference conditions
contained in the Forest Plan. Any proposed management activities shall not
interfere with the implementation and monitoring of the CWMB and shall have full
approval by the Department of Transportation and Corps of Engineers before
implementation.
f. During the implementation and monitoring phases of the project, the Department
of Transportation will hold annual meetings with the Forest Service regarding the
status of the work being conducted. This will ensure a proper transition and
provide the necessary information for long-term management and maintenance
of the CWMB.
g. Upon final certification of the CWMB by the Corps of Engineers in the fall of
2008, the CWMB will be fully managed in accordance with the Forest Plan.
5. The Department of Transportation will be responsible for the implementation and
monitoring of the CWMB in accordance with the plan. All restoration and monitoring
costs are the responsibility of the Department of Transportation.
6. During the restoration and monitoring period, the Department of Transportation will
take all reasonable precautions to prevent and suppress forest fires on and prevent any
unnecessary damage to lands and resources associated with the project
implementation and to this end will collaborate with the Forest Service in formulation of
fire prevention and control plans and programs, location of access roads and relocation
of transportation facilities, land clearing standards, and other matters essential to the
protection of resources and conservation of wetlands
7. Changes within the scope of this instrument shall be made by the issuance of a
bilaterally executed modification.
8. Any information furnished to the Forest Service under this instrument is subject to
the Freedom of Information Act (5 U.S.C. 552).
9. Either party(s), in writing, may terminate the instrument in whole, or in part, at any
time before the date of expiration.
10. This instrument in no way restricts the Forest Service or the Cooperator(s) from
participating in similar activities with other public or private agencies, organizations, and
individuals.
11. This instrument is executed as of the date of last signature and, unless sooner
terminated, is effective through December 31, 2008 at which time it will expire unless
renewed.
12. The principal contacts for this instrument are:
¦ U. S. Forest Service:
Lauren Hillman
District Ranger
Croatan Ranger District
141 East Fisher Avenue
New Bern, North Carolina, 28560
252-638-5628
¦ North Carolina
Department of Transportation:
¦ U.S. Army Corps of Engineers:
Name
Title
Address
City, State Zip Code
Telephone
Name
Title
Address
City, State Zip Code
Telephone
13. This instrument is neither a fiscal nor a funds obligation document. Any endeavor
or transfer of anything of value involving reimbursement or contribution of funds
between the parties to this instrument will be handled in accordance with applicable
laws, regulations, and procedures including those for Government procurement and
printing. Such endeavors will be outlined in separate agreements that shall be made in
writing by representatives of the parties and shall be independently authorized by
appropriate statutory authority. This instrument does not provide such authority.
Specifically, this instrument does not establish authority for noncompetitive award to the
cooperator of any contract or other agreement. Any contract or agreement for training
or other services must fully comply with all applicable requirements for competition.

IN WITNESS WHEREOF, the parties hereto have executed this Memorandum of
Understanding as of the last written date below:
JOHN F. RAMEY
Forest Supervisor
National Forests In North Carolina
NAME
Title
State of North Carolina
Department of Transportation
NAME
Date
Date
Date
Title
United States Army
Corps of Engineers
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CONCEPTUAL DESIGN REPORT,
CROATAN WETLAND
MITIGATION BANK
CRAVEN COUNTY, NORTH CAROLINA
Prepared for:
Mr. Kevin Markham
Environmental Services, Inc.
1100 Wake Forest Road, Suite 200
Raleigh, NC 27604
March 31, 2000
Project 98014-2
Revised
July 17, 2000
August 25, 2000
EDDY ENGINEERING, P.C.
4428 Louisburg Road, Suite 209
Raleigh, North Carolina 27616
[phone]
Fax [phone]
\1t %J1011811"',
0\A CARq
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1G ,ito e No. 17604
).J ,, IS

EXECUTIVE SUMMARY
This report presents surface water hydrologic and hydraulic analyses and conceptual designs for the
Croatan Wetland Mitigation Bank (CWMB), an approximately 4,100 acre tract owned by the North
Carolina Department of Transportation (NCDOT). The site is located in southern Craven County,
North Carolina, adjacent to Croatan National Forest. Restoration and enhancement of wetland
function will provide mitigation credits for NCDOT projects in the lower Neuse Basin.
Environmental Services, Inc. (ESI) is assessing current site conditions, including geohydrologic,
soil, and biologic function, and is preparing recommendations for site modifications to restore and
enhance wetland function. Eddy Engineering, P.C., in combination with ESI, is performing
analyses of surface water hydrology and preparing conceptual design plans for implementation.
' Data used in the analysis came from public record documents and data collected on-site by ESI,
NCDOT, as well as Eddy Engineering, P.C. Public record data included topographic maps, soils
maps, stream flow records, and rainfall and evaporation data. On-site data included stream gauge
' and velocity data, rainfall data, and topographic data. On-site data collection efforts captured
rainfall events from the middle of 1998 to the beginning of 2000, including several hurricanes.
' Hydrologic and hydraulic analyses were conducted to model surface water runoff potential and
estimate flows for watersheds and flow networks on and around the entire site. Several
independent methods were employed and reasonable agreement was obtained between the
' methods.
In order to better understand and predict the existing and future hydrologic conditions of the site, a
water balance was developed utilizing the theory of mass conservation, evaluating losses consisting
of runoff, evaporation, and evapotranspiration, as well as system input through rainfall.
' Findings and conclusions of this study include:
1. Determination of the area contributing runoff to the project site. Long Lake does
' appear to drain predominantly through the site.
2. Development of a means to predict flood flows within and around the site.
3. Selection of locations and types of site modifications to the existing road and ditch
system needed to enhance or restore wetland hydrology. These will be developed into
construction plans as part of the Final Design Phase.
T4-) Determination that increased peak flood flows are likely after site modifications,
because of the increase in soil moisture and the large percentage of the drainage area
that will be wetlands.
The proposed site modifications include elimination of virtually all prominent site ditches,
including those paralleling the existing road network, improved crossings at select locations where
remaining roads cross natural drainage features, and removal of multiple site roads and culverts.
Conceptual designs based on these analyses are proposed to support restoration and enhancement
of wetland function.

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TABLE OF CONTENTS
TITLE PAGE
EXECUTIVE SUMMARY
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF APPENDICES
1.0 INTRODUCTION ....................................................... - 1 -
1.1 Purpose .......................................................... -1-
1.2 Background ...................................................... -1-
1.3 Authorization ..................................................... - 1 -
1.4 Scope of Services .................................................. - 2-
1.5 Project Personnel .................................................. - 3 -
2.0 PROJECT AND SITE DATA ........................ . . . . .. - 5
2.1 Project Location ................................................... - 5 -
2.2 Project Description ................................................. - 5 -
2.3 Site Visits and Visual Observations .................................... - 5-
2.3.1 Channels and Streams ..................................... - 6-
2.3.2 Long Lake .............................................. .6-
2.3.3 Additional Site Data Collection ............... - 7-
2.4 Survey Data ...................................................... - 7-
2.5 On-Site Rainfall and Surface Water Gauge Data .......................... - 7-
2.6 Off-Site Rainfall Data .............. - 8-
2.7 Evaporation ...................................................... .8-
2.8 Evapotranspiration ................................................. - 8-
2.9 Analysis Period ................................................... - 8-
2.10 Focal Locations of Analysis ....................................... - 9-
.3.0 SUMMARY OF PRELIMINARY PHASE I ANALYSES ....................... - 10-
3.1 Synthetic Hydrologic Models ....................................... - 10-
3.1.1 Estimation of Runoff Potential ............................. - 10-
3.1.2 Rainfall Data ........................................... - 11 -
3.1.3 Stage-Discharge Curve Development ........................ - 11 -
3.1.4 Model Calibration ....................................... - 11 -
3.2 Site Specific Unit Hydrograph Development ........................... - 12-
3.2.1 Base Flow Separation ..................................... - 12-
3.2.2 Estimation of Direct Runoff ................................ - 12-
3.2.3 Unit Hydrograph Deconvolution ............................ - 12-
3.3 Flood Frequency Analysis .......................................... - 13 -
3.4 Preliminary Conclusions and Recommendations ........................ - 13 -
4.0 WATER BALANCE ANALYSES ......................................... - 15 -

4.1 Numerical Integration of Runoff Data ................................. - 15-
4.1.1 Measured Discharge ...................................... - 15 -
' 4.1.2 Hydraulic Model Estimates ................................ - 16-
4.1.3 Composite Stage-Discharge Relation ........................ - 16-
4.2 Watershed Delineation .............................................
4.3 Rainfall Versus Runoff Comparison for Six Specific Storms = 16-
17-
4.4 Comprehensive Site Model Development .............................. - 18-
4.4.1 Rainfall Volumes ::::::::::::::::::::::::::::::::::::::::
4.4.2 Runoff = 18-
19-
4.4.3 Infiltration ............................................. - 19-
4.4.4 Evaporation ............................................
4.4.5 Evapotranspiration - 20-
- 20-
4.4.6 Cumulative Water Volume Comparison ...................... - 21 -
' 5.0 FLOOD HYDROLOGY ................................................. - 22-
5.1 Observed Flood Recurrence Intervals ................................. - 22-
5.2 Flood Flow Prediction Using Synthetic Hydrology ....................... - 25 -
5.2.1 The Unit Hydrograph Method .............................. - 26-
5.2.2 Development of a Site Specific Unit Hydrograph ............... - 26-
5.2.3 Pattern Unit Hydrographs ................................. - 26-
5.2.3.1 Base Flow Separation ..................................... - 27-
5.2.3.2 Rainfall-Runoff Estimation ................................ - 28-
5.2.3.3 Model Calibration ....................................... - 29-
5.2.3.4 Model Validation ........................................ - 30 -

' 5.2.4 Application of the Unit Hydrograph Method ...................
5.2.5 Design Storm Rainfall Data ................................ - 30 -
- 31 -
5.3 Flood Flow Prediction using USGS Regression Equations ................. - 32 -
' 5.4 Flood Flow Prediction using Regional Flood Frequency Analysis ...........
5.5 Comparison of Predicted Floods with Observed Floods .......... - 33 -
- 33 -
' 6.0 PROPOSED SITE TREATMENTS ........................................
6.1 Existing Conditions - 35 -
- 35 -
6.1.1 Existing Ditch and Road Network ........................... - 35 -
' 6.1.2 Relic Channels and Natural Drainage Features .................
6.2 Proposed Site Treatments - 36 -
- 36-
6.2.1 Site Ditch Removal and Modifications ....................... - 36-
6.2. 1.1 Ditch Plugs ............ - 37-
6.2.1.2 Surface Water Diversions .
................................ - 38-
6.2.1.3 Scarification of Consolidated Soils .......................... - . 18-
6.2.1.4 Removal of Existing Conveyance Structures ................... - 38-
6.2.2 Site Road Modifications .................................. - 39 -
6.2.2.1 Improving Road Surface Course ............................
6.2.2.2 Surface Water Conveyance Measures ........................ - 39-
- 39-
6.2.2.3 Subsurface Aggregate Drains ............................... - 41-
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6.3 Locating Site Treatments ........................................... - 41 -
7.0 WATER SURFACE PROFILE MODELS ................................... - 42-
7.1 Hydraulic Models ................................................. - 42-
7.1.1 Existing Conditions Model ................................ - 42-
7.1.2 Proposed Conditions Model ................................ - 42-
7.2 Comparison of Water Surface Elevations .............................. - 43 -
7.2.1 Development of Discharge Estimates ........................ - 43 -
7.2.2 Estimated Water Surface Elevations ......................... - 46-
7.2.3 Travel Time ............................................ - 46-
7.3 Discussion of Results .............................................. - 47-
8.0 CONCLUSIONS AND RECOMMENDATIONS ............................. - 48-
8.1 Watershed Characteristics .......................................... - 48-
8.2 Flood Flow Prediction ............................................. - 48-
8.3 Recommended Site Modifications .................................... . 48-
8.4 Effect of Site Modifications ......................................... - 49-
8.5 Future Data Collection ............................................. - 50-
9.0 LIMITATIONS ........................................................ .51-
FIGURES
APPENDICES
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LIST OF FIGURES
1. VICINITY MAP
2. SITE SCHEMATIC
3. DOMINANT CHANNEL REACHES & NATURAL DRAINAGE FEATURES
4. VOLUME COMPARISON, RAINFALL VS. RUNOFF WITH LONG LAKE
5. VOLUME COMPARISON, RAINFALL VS. RUNOFF WITHOUT LONG LAKE
6. MONTHLY WATER BALANCE, INFLOW VS. OUTFLOW
7. COMPARISON OF SYNTHESIZED HYDROGRAPH VS. OBSERVED HYDROGRAPH
FOR STORM 3
8. COMPARISON OF SYNTHESIZED HYDROGRAPH VS. OBSERVED HYDROGRAPH
FOR STORMS 4 & 5
9. TYPICAL EXISTING DITCH CROSS-SECTION
10. TYPICAL EXISTING ROAD PROFILE
11. PROPOSED SITE LAYOUT - ROAD REMOVALS
12. PROPOSED SITE LAYOUT - PERMANENT ROAD CROSSING LOCATIONS
13. PROPOSED SITE LAYOUT - POINT & REACH DITCH PLUG AND SURFACE
WATER DIVERSIONS
14. ROAD REMOVAL AT NATURAL DRAINAGE FEATURE, PROFILE VIEW
15. ROAD REMOVAL AT NATURAL DRAINAGE FEATURE, SECTION VIEW
16. TYPICAL DITCH FILLING AND ROAD REMOVAL
17. LOWER ROAD ELEVATION AND STABILIZE ROAD SURFACE AT NATURAL
DRAINAGE FEATURE, PROFILE VIEW
18. LOWER ROAD ELEVATION AND STABILIZE ROAD SURFACE AT NATURAL
DRAINAGE FEATURE, SECTION VIEW
19. TYPICAL DITCH FILLING AND ROAD STABILIZATION
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20. PROPOSED POINT DITCH PLUGS & SURFACE WATER DIVERSIONS
' 21. PROPOSED REACH DITCH PLUGS & SURFACE WATER DIVERSIONS
22. PROPOSED TREATMENT FLOW CHART
23. EVALUATION OF WATER SURFACE ELEVATIONS AT KEY LOCATIONS
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LIST OF APPENDICES
A. PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS - PHASE I
B. MONTHLY SITE DATA
C. STAGE-DISCHARGE RELATIONSHIP DEVELOPMENT
D. NUMERICAL INTEGRATION OF RUNOFF
E. COMPREHENSIVE SITE WATER BALANCE
F. STORM EVENT ANALYSIS
G. DESIGN STORM DEVELOPMENT
H. SITE STRUCTURES PRELIMINARY DESIGN
1. HEC-RAS ANALYSIS
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1.0 INTRODUCTION
This report presents surface water hydrologic and hydraulic analyses and conceptual designs for
the Croatan Wetland Mitigation Bank (CWMB). The CWMB is proposed for an approximately
4,100 acre tract owned by the North Carolina Department of Transportation (NCDOT) where
restoration and enhancement of wetland function will provide mitigation credits. This section of
our report presents the purpose of these services, project background, authorization, scope of
services, and project personnel.
1.1 Purpose
Hydrologic and hydraulic analyses were conducted to model surface water runoff potential and
estimate flows for watersheds and flow networks on and around the proiec-ct si e`Tfie purpose
was to analyze and evaluate "tlhe surfaceTydro ogic ari V raulic conditions, both present and
future, within and around the CWMB. Conceptual designs based on these analyses are proposed
to support restoration and enhancement of wetland function.
1.2 Background
The proposed CWMB is to provide wetland mitigation credits for NCDOT projects in the lower
Neuse Basin. Environmental Services, Inc., (ESI) is assessing current site conditions including
geohydrologic, soil, and biologic function and is preparing recommendations for site
modifications to restore and enhance wetland function. Eddy Engineering, P.C., in combination
with ESI, is performing analyses of surface water hydrology and preparing conceptual design
plans for implementation. We (Eddy Engineering, P.C.) performed a preliminary study on the
Phase I portion of this site, approximately 1,500 acres, in December 1998. Our preliminary study
is entitled "Preliminary Hydrologic and Hydraulic Analysis, Croatan Wetland Mitigation Bank
Phase I," and is included in its entirety in Appendix A of this report. Section 3 of our current
report summarizes the findings of our preliminary study.
On March 31, 2000, we submitted our final report on the project; however, based on review
comments and recommended changes provided by NCDOT, minor changes were required. The
changes made were primarily grammatical and format related, no changes were made in the
project analyses previously reported. This revised report incorporates those changes and
supercedes all previous versions of the Conceptual Design Report for the Croatan Wetland
Mitigation Bank as prepared by EDDY ENGINEERING, P.C.
1.3 Authorization
These services were authorized by acceptance of Eddy Engineering, P.C., Proposal 98014-
CDR02, dated May 19, 1999, by Mr. Richard G. Harmon of ESI. ESI has been retained by the
NCDOT as consultant for restoration of wetlands for the CWMB project.
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1.4 Scope of Services
Eddy Engineering, P.C., provided the following services:
'
1. We visited the site for site characterization purposes and took specific
measurements of stream flow and other site features.
2. We visited the site to observe characteristics of the site and surrounding area,
verify surface and channel flow conditions, and observe water conveyance
structures.
3. Drainage catchments were initially delineated using topographic data from USGS
maps. This delineation was confirmed by visual observation during site visits.
'
4. Aerial topographic survey data was used in watershed delineation where feasible.
We developed a conceptual connectivity diagram of the ditch/stream/lake network
based on site data. This network was confirmed by visual observation during site
visits. This information was later used in both hydrologic and hydraulic modeling -" -«
'
5. of site conditions. ?
Soils, cover, and land use data were used to assign Soil Conservation Serv'Ee_ t?
(SCS) runoff Curve Numbers (CN) to the various watershed elements.
6. We measured velocities at select channel locations after rainfall events for use-in
developing stage-discharge relationships. We then used this stage-discharge
' relationships at key site locations to allow for the conversion of water surface
elevations to estimate channel discharges during the collection period.
7. We collected site data, reduced data to a useable format, and selected portions to
be used in model calibration and verification.
8. Using rainfall data from each of the three site rain gauges, we used an: areal'
averaging`technique to determine probable average rainfall over specific
' watershed elements.
9. To develop a hydrologic model of the contributing watersheds, we evaluated

' several options for modeling. Both "existing and future developed conditions were
analyzed. In this process we developed a model for estimating Base Flow
Recession during specific storm events.
10. We developed a hydraulic model of the existing and future ditch/stream channel
network. Models were later calibrated to more closely match site conditions
based on site observations.
11. Site features were selected and evaluated in an attempt to attain site restoration
goals. These included increases in hydro periods for soils in select locations
rather than over the entire site. We located select water control structures in and
around existing channels to meet wetland hydrology goals.
12. To attain site hydrology goals based on our discussions with ESI, we located
roadway cuts and culverts under roadways to restore surface water flow
' conditions.

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13. After proposed conceptual designs for restoring wetland hydrology were
identified, we evaluated their effects on surrounding properties and other areas of
the site.
14. Evaluation of site flood levels before and after recommended hydraulic
modifications was important to reduce the possibility of undesirable effects on
surrounding properties and on the site itself. We evaluated potential for flooding
of off-site properties due to the proposed site modifications.
15. Upon completion of our analysis and development of conceptual designs, we
prepared conceptual site plans and a report documenting our findings and
recommendations.
16. We attended various project coordination meetings with ESI and NCDOT during
the course of the project.
17. We prepared this report of our findings, evaluations, and recommendations. This
report is suitable for review by interested agencies and presents the basis for our
proposed conceptual designs.
1.5 Project Personnel
Analyses and report preparation were performed by John L. Eddy, P.E., Project Manager,
Patrick K. Smith, P.E., Project Engineer, and Christopher G. Ply, E.I.T., Staff Engineer.
John L. Eddy, P.E. (NC License No. 17604) has a Master of Science in Geotechnical and Water
Resources Engineering from North Carolina State University and is licensed to practice
engineering in North Carolina, Virginia, and Florida. Mr. Eddy is a civil engineer experienced in
geotechnical engineering, hydrologic engineering, hydraulic engineering, and dam engineering.
He has performed engineering investigations, analyses, design, or evaluations on building,
roadway, dam, pipeline, airport, harbor, stream, wetland, and landfill projects. He has over 12
years of experience on projects representing a wide range of locations including coastal plain,
piedmont, and mountain physiographic areas in several states.
Patrick K. Smith, P.E. (NC License No. 25525) has a Master of Engineering in Civil Engineering
with a concentration in Environmental and Hydrologic Engineering from Clarkson University
and is licensed to practice engineering in North Carolina. Mr. Smith is a civil engineer
experienced in hydrologic engineering, hydraulic engineering, and dam engineering. He has
performed engineering analyses and design on streams, wetlands, open and closed stormwater
systems, dams, and sediment and erosion control projects. He has over 13 years of design and
construction management experience on projects representing a wide range of domestic and
' overseas locations.
Christopher G. Ply, E.I.T. has a Master of Science in Structural Engineering from North Carolina
State University. Mr. Ply is a civil engineer experienced in foundation engineering, structural
concrete design, and hydrologic engineering. He has performed engineering analyses and design

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on wetlands, dams, and concrete structures to include foundations, retaining walls, headwalls,
and dam spillways.

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2.0 PROJECT AND SITE DATA
This section presents our understanding of the project, as planned, and site data that was
available to us at the time of this report. Numerous data sources were used during our analyses,
including site topographic data, on-site rainfall and surface water gauge data, various regional
weather data, as well as various data collected from a series of site visits.
2.1 Project Location
The CWMB project site is located in southern Craven County, NC, adjacent to the Croatan
National Forest. The project location is shown in Figure 1. The site appears on the USGS 7.5-
minute series quadrangle maps, "Catfish Lake," "Hadnot Creek," "Havelock," and
"Masontown," portions of which were used in the preparation of this report. The project site is
within an area bounded roughly by SR 1100 (Catfish Lake Road) in the northwest, US 70 in the
east, and Great Lake and Little Lake in the south. The project site drains generally into East
Prong Brice Creek to the North. Approximate project site boundaries, as well as key site features
and locations referred to herein, are presented on Figure 2.
2.2 Project Description
In an effort to provide wetland mitigation credits for highway-related impacts in Lower Neuse
River Basin, the NCDOT is considering the development of a wetlands mitigation bank. The
parcel will serve to mitigate wetlands related to the Havelock Bypass, as well as future NCDOT
projects. The tract being considered for the CWMB consists of approximately 4100 acres, which
has been divided into three parcels or "phases." Phases I, II and III consist of areas of 1500, 1600
and 1000 acres respectively. Our analyses, conceptual designs, and this report address all three
phases.
Mitigation is possible because the site was extensively ditched and drained. Hydric soils are
present over large portions of the site, although wetland function has been lost or inhibited in
many areas because of the drainage.
t 2.3 Site Visits and Visual Observations
' We visited the project site numerous times over the period of January 1998 to the present to
observe characteristics of the site and surrounding area, verify surface, lake, and channel
conditions, and to observe both natural and man-made water control structures. For ease of
' explanation and orientation, existing channel sections discussed in this report have been
identified as Channels (CH) 1 through 22. Locations of these channels are depicted in Figure 2.
Select points of interest (POI) have also been identified in the same figure.

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2.3.1 Channels and Streams
During our visits to the site we observed that various channels within the project site
frequently show limited evidence of flow. Portions of some channels appeared stagnant,
and as such, no discernible direction of flow can be observed. Additionally, the
observation of relic stream channels, combined with limited changes in relief and localized
depressions, indicate large potential storage capacity of select portions of the site.

' Strong flow or physical evidence of previous flow has typically been observed along CH 3
flowing from the intersection with CH 1, CH 2, and CH 7 southeast toward the intersection
with CH 4. This evidence included consistent orientation of vegetation and debris in the
downstream direction and debris collection upstream of obstructions. Such evidence may
indicate increased velocities along these portions of the channel. Similar evidence of flow
was typically noted in CH 4 and CH 5 as they flow northeast along the eastern edge of the
site to their intersections with CH 5, and CH 9 and CH 12 respectively. From here, flow
' indicators reveal prevailing flow along CH 12, CH 13, CH 14, CH 17, CH 19, and CH 22,
generally trending north.
' Al
h
h
l
ong ot
er c
anne
s within the site network (CH 1, CH 6, CH 7, CH 8, CH 9, CH 10, CH
11, CH 15, CH 16, CH 20, and CH21), little or no evidence of previous flow was observed.
' Although surface water gauges clearly indicate increases in water depth at these locations.,
the lack of physical flow indicators suggest that relatively low flow velocities occurred in
these portions of the site channel network.
'
Based on observed evidence of flow, we have concluded that the dominant channel reach
within the site channel network is along the path shown in Figure 3. For this reason, our
' channel and contributing watershed modeling efforts for existing conditions were focused
on these portions of the site. Significant flow has also been observed, recorded, and
evidenced along the northernmost sections of the natural reaches in the vicinity of the
intersection of channels 17, 18, and 19. In this vicinity, during periods of high water,
significant discharge can be observed to leave the channel banks and follow the more
natural path overland toward the site outlet beneath the bridge at Catfish Lake Road
(POI 3).
2.3.2 Long Lake
During various site visits, we observed surface water conditions in the vicinity of Long
' Lake. The outlet control device along the northeast side of Long Lake (POI 1) appears to
be in poor, if not unusable, condition. For that reason, operation of the device was not
attempted. The corrugated metal outlet barrel was in poor condition, with severe corrosion
' noticeable on exposed portions. Only limited evidence of overtopping or surface flow was
observed in the vicinity of the outlet control structure.

-7-
We also conducted a reconnaissance of the shoreline of Long Lake by boat with emphasis
on the vicinity of the project site. Shoreline observations revealed a fairly consistent sand
rim or bank elevation around the lake perimeter. Low points were observed in the rim at
' several locations along the northeast portion of the lake. These locations revealed
evidence of water flow indicating potential lake discharge locations. Due to thick
vegetative cover, weather, and associated water conditions, it was not possible to obtain
t locations and dimensions of all flow locations. However, no single location was
identifiable as a predominant contributor to surface water flow within the project site. The
many relic shorelines and flotsam windrows make consistent discharge at point locations
unlikely. Rather, it is judged that discharge occurs over a relatively large portion of the
lake rim adjacent to the site when the water surface of the lake is elevated.
2.3.3 Additional Site Data Collection
During site visits we also collected data necessary for the development of stage-discharge
relationships in the vicinity of SG 7 and SG 16. This data included the measurement of
flow depth and velocity at established intervals across the flow paths. These data would
' later be used in our analyses of site hydrology. Select portions of our collected data used in
site analyses are presented in Appendix C.

2.4 Survey Data
ESI provided us with aerial topographic mapping data prepared by NCDOT, supplemented with
ground survey data. A finished aerial topographic map was not available prior to completion of
this report. In some cases, significant differences exist between aerial topographic and ground
survey data. Cross-section data used in our analysis was based on provided ground survey data
and, in some instances, the aerial topographic map. Watershed delineation was performed using
data from the USGS 7.5-minute quadrangle maps, including "Catfish Lake," "Hadnot Creek,"
"Havelock," and "Masontown," and site observations.
2.5 On-Site Rainfall and Surface Water Gauge Data
ESI also provided us with rainfall and surface water gauge data collected on the project site
between the period June 2, 1998, through February 4, 2000. Gauge readings were taken on an
hourly basis from a single rain gauge (RG), an RDS-brand rain gauge and a series of seven
surface water gauges (SG) located across the site including six channel surface water gauges and
one lake level gauge (LG) positioned at various locations on the project site until February 1999.
On February 3, 1999, three additional rain gauges, all Infinity-brand gauges, were added to the
site. One of the new gauges was placed next to the original rain gauge for data comparison
purposes. On February 25, 1999, eight additional channel surface water gauges were added
across the site. On March 25, 1999, two additional channel surface water gauges were added for
a total of 16 channel surface water gauges, four rain gauges, and one lake level gauge.
Approximate locations of the rain, lake, and surface water gauges are shown in Figure 2.

-8-
During the period of data collection, gauge malfunction or limitation in gauge data memory
caused gaps in the data. Unfortunately, in numerous instances, these gaps occurred during
significant rainfall events which otherwise could have been used in model development or
verification. In some cases, based on a review of recorded data, individual surface water gauges
experienced overtopping, and as such, peak water surface elevations were not accurately
recorded. Rainfall data collection before the installation of the Infinity rain gauges in February
1999 is not an accurate representation of the actual rainfall.
Two significant storms, which occurred during the collection period, were Hurricanes Dennis
and Floyd. Hurricane Dennis occurred September 3-4, 1999 and consisted of two major rainfall
events dropping an estimated total of 16.9 inches of rainfall over the project site. Hurricane
Floyd occurred September 15, 1999 dropping an estimated total of 6.9 inches of rainfall.
' 2.6 Off-Site Rainfall Data
Additional rainfall data from neighboring locations were used to supplement the on-site rainfall
' data. ESI additionally provided supplemental daily rainfall data from the Weather Service of the
U.S. Marine Corps Air Station at Cherry Point, NC, for the months of August and September
1998. We obtained additional daily data from the State Climate Office of North Carolina at
' North Carolina State University. Additional sites included Trenton, New Bern Airport, and
Morehead City.
' 2.7 Evaporation
Monthly evaporation data used in our analyses was collected from the State Climate Office of
' North Carolina, for the nearest station in Aurora, NC. Aurora is located approximately 32 miles
north-northwest of the site. This was the best available data that could be obtained for the site.
' Because onsite rainfall data was available, rainfall data from Aurora was not used in our
analyses.
t 2.8 Evapotranspiration
Potential evapotranspiration (ET) values used were generated from a worksheet created by
' WxSystems for estimating potential ET from the heat index and latitude of the site. Another
estimation of potential ET was generated by the Drain Mod program run by ESI in their analysis
of the site.
2.9 Analysis Period
' Much of our analyses for this report focused on the one year period from mid February 1999, to
mid February 2000, enabling us to evaluate a full year of data, as well as monthly data segments.

-9-
This period was selected, in part, due to the date on which the additional surface water gauges
were installed.
' 2.10 Focal Locations of Analysis
Our initial analysis focused on two key site locations for evaluating surface water characteristics
' of the site. The first of these locations was at the corrugated metal pipe culvert in Channel 12
located immediately downstream of SG 7 along the eastern edge of the site. This location is
' identified as POI 2 in Figure 2. The second key location was at the bridge crossing at Catfish
Lake Road (POI 3) at the northern end of the site, immediately downstream from SG 16. These
locations were selected for two important reasons, first as described in our previous report, these
' points are located along the dominant channel reach within the site channel network. Second,
because both locations incorporate a flow control structure, through the collection of various
field data, we have the ability to develop a stage-discharge relationships at these locations. These
' relationships were later used to estimate discharge at a given point in time based on measured
water surface elevation. Development of the stage-discharge relationships is discussed later in
this report.
' We elected to focus our attempt to model surface water flows on the watershed outlet at the
bridge on Catfish Lake Road. Accordingly, future discussions focus on this location. Graphical
' representation of measured data from rain gauges (on-site only) and water surface elevation (at
SG 16), during the period February 1999, to February 2000, is provided in Appendix B of this
report.

I

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3.0 SUMMARY OF PRELIMINARY PHASE I ANALYSES
Preliminary hydrologic and hydraulic analyses were conducted to model surface water runoff
potential and estimate flows for watersheds and flow networks on and around the Phase I portion
of the project site. In December 1998, Eddy Engineering, P.C. submitted a Preliminary
Hydrologic and Hydraulic Analysis Report for Phase I of the CWMB project. This section
presents a summary of our preliminary analyses and findings for Phase I of the CWMB project.
For more detailed information the interested reader is referred to Appendix A, which contains a
complete copy of our report.
As described in the sections below, the preliminary hydrologic and hydraulic analyses completed
for Phase I were beneficial in understanding the overall scope of services needed to extend the
analyses to the remainder of the site and complete conceptual designs. All preliminary analyses
centered on the watersheds that contribute flow to the dominant channel reach (CH 3, CH 4, and
CH 5) which terminates in the vicinity of SG 4.
3.1 Synthetic Hydrologic Models
A synthetic hydrologic analysis was attempted for the Phase 1 portion of the site, including
delineation of the watershed, soil and soil moisture conditions, land use, stream channel and
cross-section location and size, rainfall depth-duration-frequency, stage-storage and
stage-discharge. The watershed was analyzed as a distributed element model with two separate
model element configurations. The first model divided the watershed into several sub-
watersheds considered to be hydrologically similar. In the second model, the watershed was
analyzed as one catchment for the entire drainage area contributing flow to the outlet of CH 5 in
the vicinity of SG 4. The combined watershed size was estimated at 2.48 square miles. Because
of the difficulty encountered in model calibration, the second, less complex, distributed element
model was used for all preliminary calibration and modeling.
3.1.1 Estimation of Runoff Potential
Soils, cover, and land use data were used to assign Soil Conservation Service (SCS) runoff
Curve Numbers (CN) to the various watershed elements. Initial evaluation of site soils and
coverage yielded a weighted SCS CN of 68 for the sub-watersheds in and around the Phase
I, when Antecedent Moisture Conditions I (AMC I) is assumed. This CN is indicative of a
relatively low, direct runoff potential. Normally, AMC II (normal conditions) and the
resulting higher CN of 84 would be used for design. An important point noted was that
many of the soils within the contributing watersheds may be categorized into two
Hydrologic Soil Groups (HSG), depending on drainage conditions. The HSG's of D, A/D
and B/D are the most prevalent on the site, with paired groups representing "drained" /
"undrained" conditions. Because the site is drained by ditching and conditions preceding
our calibration storm were relatively dry, the argument could be made that the use of HSG's
with the higher infiltration capacity, or the "drained" condition, may be appropriate for

-11-
' calibration. For future site evaluations, the "undrained" CN or some other means of
adjustment are needed.
' 3.1.2 Rainfall Data
Rainfall data used during model calibration attempts were obtained from gauge data
t collected on-site. Later findings that the on-site rain gauge provided erroneous readings
precludes direct use of the preliminary report results.
' 3.1.3 Stage-Discharge Curve Development
' A preliminary U.S. Army Corps of Engineers, Hydrologic Engineering Center - River
Analysis System (HEC-RAS) computer model was constructed to obtain a preliminary
estimate of water surface elevations within selected existing channels for a range of
' discharge values. From this water surface profile model, stage-discharge curves were
developed to estimate channel discharge at select cross-sections for comparison with
surface water gauge data. As with our later analyses, these relationships were used to
' estimate discharge values from measured channel water surface elevations (from surface
water gauges).

I
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3.1.4 Model Calibration
As discussed above, the simplified distributed element model was used in an attempt to
model the watershed contributing flow to the outlet of CH 5 in the vicinity of SG 4. This
model was recreated for two different types of dimensionless unit hydrographs. The
dimensionless unit hydrographs used in our U.S. Army Corps of Engineers, Hydrologic
Engineering Center - Flood Hydrograph Package (HEC-1) computer models were the
Snyder Unit Graph method and the Clark Unit Graph method. In each case trial solutions
were conducted to find unit hydrograph parameters which would produce an outflow
hydrograph similar to outflow data measured in surface water gauges. We were unable to
develop a dimensionless unit hydrograph model that could reasonably reproduce the
observed hydrograph at SG 4. Peak flow values were generally higher than observed, with
peaks occurring later in time than observed. We concluded that possible reasons for this
may include a nonlinear runoff response, large watershed storage capacity, increased
infiltration capacity, inaccurate rainfall data, and/or incorrect watershed delineation. The
effect of storage in particular, may not be linear, especially for the drier periods, thus
making unit hydrograph models potentially inappropriate. Continuous simulation models
may be more appropriate, but calibration will require substantially more data.

-12-
' 3.2 Site Specific Unit Hydrograph Development
Since synthetic hydrologic models using standard dimensionless unit hydrographs did not agree
' with gauge data, we attempted to develop a site specific unit hydrograph for the same point of
interest.
3.2.1 Base Flow Separation
' Development of a direct runoff unit hydrograph from stream gauge data required that base
flow be separated from the direct runoff hydrograph. We chose the straight line method of
base flow separation, which assumes a linear return of base flow.
' 3.2.2 Estimation of Direct Runoff
We attempted to estimate the direct runoff by two independent means. First, having
' developed a SCS CN representative of the soils and land use in the watershed, direct runoff
was computed based on the CN. The second method used to estimate direct runoff was
' numerical integration of the discharge hydrograph. We found a significant difference
between direct runoff computed by these independent means. We concluded that the
difference can be attributed to one or more of the following possibilities:

1. There is much greater storage within the watershed than assumed in the SCS CN
Method.
' 2. Infiltration capacity is greater than assumed in the SCS CN Method.
3. The rain gauge data is incorrect.
4. The rain gauge data is not representative of the entire watershed.
' 5. The estimated drainage area of the watershed is greater than the actual area, due to
errors in connectivity and/or delineation from available topographic data.
6. The stage-discharge curve, used to convert stream gauge stage readings to discharge,
' is incorrect due to data limitations or errors, or due to channel roughness assumptions.
Without more data to confirm or revise direct runoff, we opted at the time to use the direct
' runoff computed by numerical integration for derivation of site specific unit hydrographs.
Our future calibration also used this method.
' 3.2.3 Unit Hydrograph Deconvolution
' A description of the deconvolution method and unit hydrograph synthesis is presented in
Appendix A. Unit hydrograph deconvolution is the inverse operation of unit hydrograph
synthesis, also known as convolution. We accomplished deconvolution for the direct
' runoff hydrograph at SG 4 by use of a spreadsheet program developed by us for this
purpose. Upon deconvolution of the unit hydrograph, we performed a unit hydrograph
synthesis on the same excess rainfall to check the validity of the unit hydrograph. By trial,
1

1
13-
we found that a least squares optimized unit hydrograph, forced to positive ordinates, was
able to reproduce the observed hydrograph fairly well. Another significant rainfall event
was still needed for further calibration and validation. Again, problems with the single
' rainfall gauge at the site rendered this work useless for predicting flood flows, but
invaluable for the insight gained for future model development and calibration.
' 3.3 Flood Frequency Analysis
' A regional flood frequency analysis was performed during our Phase I study. This analysis was
conducted to develop an independent estimate of peak flows for 2-, 10- , and 100-year flood
flows in and around the project site. To perform this analysis, we gathered stream flow and
' watershed area data for nine streams and watersheds in southeastern North Carolina. The U.S.
Army Corps of Engineers, Hydrologic Engineering Center - Flood Frequency Analysis (HEC-
FFA) program was used to statistically determine the peak discharges of the selected watersheds
' for several return periods of interest. From the peak discharges estimated for each gauged
watershed by the HEC-FFA program, we determined the specific discharge in cubic feet per
second per square mile (cfs/mil) for each return period of interest. A regional curve was
' established that can be used to interpolate and extrapolate the specific discharge of watersheds
within and around the project site. Accurate watershed delineation was deemed critical to
appropriate use of these curves.
' 3.4 Preliminary Conclusions and Recommendations
' Based on our analyses, we drew preliminary conclusions about certain aspects of the hydrology
of the project site. We concluded that:
1. The runoff potential from watersheds at this site are very dependent on antecedent
soil conditions, recency of rainfall, and groundwater conditions.
' 2. The percentage of rainfall that is translated to direct surface runoff may be relatively
small.
3. A larger percentage of rainfall than would be predicted by typical hydrologic models
may be lost to evaporation, evapotranspiration, depression storage, or infiltration
with the site.
4. Some of the infiltrated rainfall may also eventually reach the ditch system as base
' flow, depending on hydrogeologic conditions.
5. After closing of the ditches that drain the site, runoff potential would likely increase
with a much higher percentage of rainfall being translated into direct runoff and as
' such, there would be a significant increase in the frequency and severity of surface
water flows.
6. Modeling results, site observations, and water balance evaluations indicated that
Long Lake did not contribute significant direct surface water runoff into the site
during or after Hurricane Bonnie.

1
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-14-
7. Significant direct surface water discharge did appear possible, if the lake level was
relatively high at the beginning of a storm event. Such discharge might become
more frequent if groundwater levels were increased and lake levels generally remain
higher because of reduced groundwater gradients in the vicinity of the lake.
8. Closing of the ditches could effectively increase the watershed area contributing
direct surface runoff draining through the site. This, too, would significantly
increase the potential severity and possibly the frequency of surface water flows.
9. Since on-site rainfall and resulting peak water surface elevations were only captured
for one significant storm, Hurricane Bonnie, another significant rainfall event would
be needed to independently confirm model calibration.
10. There are inconsistencies identified in the rainfall data when compared to the daily
rainfall totals of nearby gauge locations, and other on-site indicators.
11. Flood frequency analysis results could be used to conservatively estimate peak
flows for design of water control structures at this site, assuming that direct runoff
from Long Lake does not occur and that accurate estimates of watershed area are
used.
12. The main limitation at the time of our preliminary report was the lack of complete
and consistent topographic data for watershed delineation.
We recommended data collection efforts be continued on this Phase and extended into future
' Phases of the site in order to confirm or improve existing models, and to calibrate and confirm
future model extensions, or entirely new models. To extend the rainfall and stream flow
gauging, additional stream flow recording gauges and at least two rainfall recording gauges were
' proposed. We recommended using a recording rain gauge similar to those used by the National
Weather Service. Additional survey data was also recommended.

1

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-15-
4.0 WATER BALANCE ANALYSES
In order to better predict the existing and future hydrologic conditions, it is essential to
understand the gains and losses of water to and from the project site. One way to understand this
relationship is through the development of a site water balance utilizing the theory of mass
conservation, water in equals water out. During our preliminary analysis, one question which
remained unanswered was what, if any, role Long Lake plays in the contribution of surface water
to the project site. In order to draw more specific conclusion to this end we elected to perform a
water balance on the watershed containing the CWMB. In this analysis we first developed a
stage-discharge relationship which allowed for the conversion of measured water surface
elevation data to volumetric discharges through numerical integration. Then, using this
relationship, we evaluated six specific measured storm events during the analysis period. We
also developed a comprehensive water balance model for the evaluation of water gains and losses
over the project watershed. This section presents our analyses of water processes on the project
site. Monthly water balance data can be found in Appendix E.
4.1 Numerical Integration of Runoff Data
To convert measured water surface elevation data to volumetric discharges through numerical
integration, we needed to developed a stage-discharge relationship. This relationship could then
be used to estimate discharges from surface water gauge stage versus time data in a spreadsheet
computer program. Because discharge multiplied by time yields volume, we were able to
numerically integrate the data into runoff volumes for any time period of interest. Gaps in
collected data existed in various instances, so the mean monthly discharge was used in
determining monthly volumes where gaps were present. Additionally, based on field
observations, we estimated an "effective flow" water surface elevation at each location below
which flow was observed to be negligible. In cases where the measured water surface elevation
lower than this "effective flow" elevation, the discharge was considered to be zero. Summary
data from the numerical integration of runoff are shown in Appendix D. Unless identified as
being developed by other means, all runoff volumes used in our analyses were developed from
numerical integration of runoff.
4.1.1 Measured Discharge
Using measured velocities and cross-sectional flow areas over a range of water surface
elevations, we estimated a channel discharge at the bridge on Catfish Lake Road using the
continuity equation. Then, plotting these discharges against depth of flow (or head), we
developed a stage-discharge curve. Next, using linear regression techniques, we
determined an equation for each curve. These equations could then be used to calculate
discharge for any given water surface elevation. These curves and their equations are
shown in Appendix C.

11
' -16-
4.1.2 Hydraulic Model Estimates
In order to check the validity of the stage-discharge regression equations, we developed a
simple HEC-RAS model to approximate water surface elevations within the existing
channel near the bridge for a range of discharge values. Channel cross-section data used in
our analysis were based on the provided ground survey data. Manning's n-values for
channel and overbank flow were selected based on site observations, standard references,
and experience.
r 4.1.3 Composite Stage-Discharge Relation
' When comparing the two stage-discharge curves described above, we found that our
original equation estimated discharges quite well for the lower range of water surface
elevations, those within the range of measured values. However, at higher elevations where
extrapolation beyond the measured data was required, discharges appeared to be
overestimated. In order to correct this, we used the stage-discharge equation from the
HEC-RAS model output to develop a composite stage-discharge curve. By overlapping the
two equations over the same scale and range, a composite, two-stage curve was developed
to more accurately predict discharge over the range of possible water surface elevations.
The original equation developed from measured stream data was used over the lower
portion of the curve, and the equation developed from the HEC-RAS model was used over
the upper portion of the curve. The composite curve and the equations of each segment are
shown in Appendix C.
' 4.2 Watershed Delineation
' The watershed water balance was analyzed at the entire watershed outlet at the bridge at Catfish
Lake Road, near SG 16. The drainage area was approximated using the assumed ridge line
boundary. Drainage catchments were initially delineated using topographic data from USGS
maps, and then confirmed by visual observation during site visits. Aerial topographic survey
data was not used in watershed delineation, because of the discrepancies between it and field
' survey data and because coverage was not complete. Watershed delineation is difficult on this
site because elevations vary as little as five feet over horizontal distances of up to 10,000 feet.
As such, minor differences in elevation could result in significant differences in watershed area.
' At the bridge at Catfish Lake Road, the contributing watershed was estimated at approximately
10,000 acres (15.63 square miles), not including Long Lake and its surrounding area. The
' surface area of Long Lake is approximately 1,200 acres (1.87 square miles). The area
surrounding Long Lake, which may contribute runoff to the lake, is estimated at approximately
900 acres (1.40 square miles). With the addition of this approximately 2,100 acres (3.28 square
' miles), the entire watershed consisted of approximately 12,100 acres or 18.9 square miles at SG
16. In our preliminary analyses (see Section 3 and Appendix A), one unknown was what, if any,

u

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role Long Lake played in peak runoff from the site. For this reason the site water balance was
conducted both including and excluding the lake and its surrounding area.
' 4.3 Rainfall Versus Runoff Comparison for Six Specific Storms
In reviewing collected rainfall data, we found that an estimated 59 inches of rainfall fell over the
project site during our analysis period, the one year period from mid February 1999, to mid
February 2000. During this period, well over 50 rainfall events occurred, ranging from a few
tenths of an inch up to almost 7 inches of rainfall. Unfortunately, many of these events occurred
so close in time to each other that the resulting outflow hydrographs overlapped greatly, making
a rainfall to runoff correlation for individual storms difficult.

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We found that six storm events, labeled Storm 1 through Storm 6, were separated by enough time
to be considered discrete events. Details of these rainfall events are shown in Appendix B. The
six events occurred as follows:
Storm
Event
Number
Date Total
Rainfall
(in.)
1 April 15, 1999 1.33
2 May 12, 1999 0.18
3 September 6, 1999 4.08
4 September 15 & 16, 1999
(Hurricane Floyd) 6.91
5 October 16-18, 1999 6.32
6 April 28-May 2, 1999 2.21
Because of our interest in determining whether Long Lake contributes surface water to the
project site, we analyzed the six storms on a rainfall to runoff basis. In evaluating these six
events, we compared rainfall volumes (see Section 4.4.1), both with and without Long Lake and
its surrounding drainage area, with estimated runoff volumes by numerical integration of
outflow. A plot of rainfall volume versus runoff volume for all six storms for both cases (with
and without Long Lake and its surrounding area) can be seen in Figures 4 and 5 respectively.
Both plots appear to reflect two distinct line segments. The first segment, comprised of Storms
1, 2, and 6, has a very slight slope indicating a significantly smaller runoff volume than rainfall
volume. This may be due to initial abstractions (infiltration and localized or depression storage),
evaporation (E) or evapotranspiration (ET), or some combination of these or other factors.
Because these three storm events are of a smaller magnitude than the other storms, we would

I

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' expect that a larger fraction of these storms would be lost to initial abstractions. The second
segment, comprised of Storms 3, 4, and 5, indicates an increase in runoff more typical of
conditions where the initial rainfall abstractions are met and a larger percentage of rainfall is
' converted into runoff. Here again, with these storm events of a larger magnitude, we would
expect that a smaller fraction of these storms would be lost to initial abstractions.
When the additional drainage area for Long Lake and its surrounding watershed is included
(Figure 4), in all but one case, the volume of rainfall exceeds the volume of runoff. This is what
we would expect because some rainfall will be lost to evaporation, evapotranspiration, and
possibly groundwater infiltration. In the one instance where runoff volume exceeds rainfall
volume, the difference is not quite 100 acre-feet, a relatively insignificant volume over a site this
size and well below a value considered to be hydrologically significant. We find that when the
additional drainage area for Long Lake is not included (Figure 5), runoff volumes are almost
equal to or greater than the rainfall volume. This would not be possible without an additional
source of water entering the system.
Based on these findings, we concluded that Long Lake and its surrounding drainage area does
provide direct runoff to the drainage system. Additionally, as concluded in the preliminary
analyses, infiltration, and local or depression storage likely play a significant role in the site
hydrology as evidenced by the two distinct line segment in the plots. With this understanding of
' site hydrology and the contribution of Long Lake, we have included the drainage area for Long
Lake in all future analyses.
4.4 Comprehensive Site Model Development
A comprehensive site water balance model was also developed to estimate water gains and losses
over the entire contributing drainage area. Our modeling of the site water balance considered site
water losses to consist of runoff (RO), infiltration (INF), evaporation (E), and evapotranspiration
(ET). Site water gains consisted solely of rainfall (RF). Comparisons of measured and estimated
site volume data were made using the basic equation:
Rainfall (RF) = Runoff (RO) + Infiltration (INF) + Evaporation (E) + Evapotranspiration (ET)
For comparison purposes, all volume quantities were estimated in units of acre-feet. A spread
' sheet computer program used for comparison of monthly and annual site water balance
calculations can be found in Appendix E.
t 4.4.1 Rainfall Volumes
Measured data from rain gauges (on-site only) and water surface elevation (at SG 16),
' during the analysis period February 1999 to February 2000, provided in Appendix B, were
used in our analyses. Because a three gauge network was used on site to collect rainfall
(RF) data, we elected to use the Thiessen method to estimate watershed average Rainfall

11
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' depths. This method calculates the average rainfall by weighting gauge measurements by
the area of the assumed watershed for each gauge. In this method the total watershed area
is divided into subareas, with each subarea using a rain gauge as a hub of a polygon. If we
' connect two adjacent gauges with straight line, and then by drawing a second line which
bisects the first line we can determine the subarea polygon for which each particular gauge
data applies. Although typically used when a large number of gauges are used for data
' collection, the Thiessen method is still applicable for use on our site. The subarea
boundaries used in weighting rainfall data are depicted in Figure 2. Each of the subareas
are used to determine subarea to total area ratios. These ratios are then multiplied by the
Rainfall on the particular subarea, and summed to get the average watershed Rainfall depth
over the contributing watershed.
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For months where the only available on-site Rainfall data came from the RDS rain gauge
(RG 1), supplemental data was gathered from surrounding sites and used in conjunction
with the RDS gauge to normalize the Rainfall. This was done due to apparent anomalies in
Rainfall data from the RDS gauge. For months with information available from the Infinity
rain gauges (RG2, RG3, and RG4), only the Infinity gauge data was included in the Rainfall
averaging. During the majority of the analysis period, all of the infinity gauges were
functional. Accordingly, the Thiessen method was used to estimate average Rainfall
depths. To determine the volume of water falling as rain over the site each month in acre-
feet, the average monthly Rainfall was multiplied by the total site acreage.
4.4.2 Runoff
Using the numerical integration methods described previously, monthly runoff (RO)
volumes (in acre-feet) were estimated using surface water elevation data collected during
the analysis period. Gaps in collected water surface elevation data existed in various
instances. During these periods, the mean monthly discharge was used in determining
monthly RO volumes. Significant data gaps were present during the months of November
1999 through January 2000, including no measured water surface elevations for the month
of December 1999. Since Rainfall data was available for the month of December, we
found that an approximately equal volume of rain fell in the month of February 1999. As
such, we assumed that December's RO volume would have been approximately equal to
that of February.
4.4.3 Infiltration
The term infiltration (INF), as used herein, is surface water which passes into the soil and
does not return later as surface flow or escape via evapotranspiration (ET). Water does
enter the soils at this site, but does not appear to be lost to or gained from deeper aquifers.
Water entering the soils on this site appear to be translated into subsurface and groundwater
flows, eventually becoming surface flow in a stream, ditch, or spring on the site. Based on
our discussions with ESI, it is our understanding that infiltration varies over the site and

Ci

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over the year. Surface waters are sometimes lost, through infiltration, into deeper aquifers
at some locations within site ditches. During periods of drier weather, these same locations
may provide water as exfiltration from deeper aquifers. However, these volumes are
typically a small portion of the water balance equation. Because data collected and
analyzed by ESI indicates that infiltration, as defined herein, is not significant, we elected
to ignore its role in the overall site water balance.
4.4.4 Evaporation
The monthly evaporation data used in the site water balance was for the nearest station in
Aurora, NC. The evaporation data is a pan evaporation value, thus meaning it is
essentially a potential evaporation. Each pan evaporation value is then multiplied by a
correction factor, in this case 0.7, to convert it to actual evaporation. To create a volume of
evaporated water from the site in acre-feet, the actual evaporation data was multiplied by an
assumed acreage of land covered by surface water. For the existing site condition, we
assumed this to be approximately two percent of the total drainage area, not including Long
Lake.
4.4.5 Evapotranspiration
Actual ET data was not available for the project site. For this reason three alternative
methods of estimating potential ET were used. The first set of potential ET values used
were generated from a worksheet created by WxSystems for estimating potential ET from
the heat index and latitude of the site. Another estimation of potential ET was generated by
the Drain Mod program run by ESI in their analysis of the site. This estimation takes into
account differing soil types, rooting depth, and site temperatures. This estimated ET is then
multiplied by a correction factor based on the month of the year and typical variations
associated with monthly readings. These correction factors varied monthly and ranged
from 0.91 to 2.11. We also considered the use of an average of the ET values generated by
the other two methods.
' In all cases the estimated ET value was then multiplied by a second correction factor which
considered the effect of cloud cover on site ET. These factors ranged from 0.37 to 0.65. A
comparison of the resulting ET values estimated by each method is provided in
' Appendix E.
For our modeling purposes we elected to use the ET values estimated by the WxSystems
method. These corrected or actual monthly ET values are then converted to a water volume
in acre-feet by multiplying by the watershed area, less the two percent of area assumed to
be covered by water.

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1 4.4.6 Cumulative Water Volume Comparison
For the one year analysis period, we performed a cumulative water volume comparison. A
cumulative comparison was made in order to smooth the monthly comparisons which are
complicated by the relatively long time of concentration of the watershed. Rainfall might
occur in one month, but the runoff from that rainfall could occur in the next month.
Cumulative comparison eliminated this potential problem.
By summing site water losses consisting of RO, E, and ET by month and then performing a
cumulative plot over the analysis period, we can compare volumes to that of cumulative
site water gains, specifically rainfall (RF). A comparison of the cumulative water gains, or
"Inflow", versus cumulative water losses, or "Outflow", is shown in Figure 6. In reviewing
Figure 6 we see a close correlation between the inflow and outflow curves. The largest
disparity occurred during the three month period, August through October 1999, in which
the most rainfall was measured on the site.
This cumulative plot also provides an independent check on our previous conclusion that
the contributing watershed drainage area includes Long Lake. If the additional acreage of
the lake and its surrounding area were not included, we would see a more apparent monthly
' and annual gap in the curves, with the two lines diverging.

11

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5.0 FLOOD HYDROLOGY
This section presents our analyses of collected site data and the development of methods used in
predicting peak flows on the project site. More detailed information on our analysis can be found
in Appendices F and G. It is necessary to estimate peak flows for sizing of structures in and
around the site. We evaluated probable peak flows for existing site conditions by several means,
compared observed peak flows to peak discharge estimates, compared observed rainfall with
regional frequency data, and evaluated probable changes in peak flows for proposed site
conditions.
5.1 Observed Flood Recurrence Intervals
Estimates of observed flood recurrence intervals from recorded rainfall data are possible because
rainfall data can be compared to published rainfall depth-duration-frequency data and because we
have developed a relationship between stage and flow. The underlying premise is the same as
that used in all of synthetic hydrology. Namely, that a rainfall of a given recurrence interval will
produce a flood of the same recurrence interval. This requires that the antecedent moisture
conditions be accounted for when performing synthetic hydrologic analysis or estimating flood
recurrence intervals from observed rainfall.
As discussed in Section 4, only six storm events, labeled Storm 1 through Storm 6, captured by
on-site gauges proved to be separated by enough time to be considered discrete storms. These
are the same six storms considered in Section 4. Again, more detail on these storms is available
in Appendices B and F. The six events occurred as follows:
Storm Event
Number Total
Rainfall
(in.) Storm
Duration
(hrs.)
1 1.33 8
2 0.18 7
3 4.08 11
4
(Hurricane
Floyd) 6.91 36
5 6.32 30
6 2.21 118
To estimate the recurrence interval or frequency of these six storms we calculated a moving
average of rainfall over a series of standard time durations. Durations, or moving average bin

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sizes, were selected to coincide with standard storm durations reported by the National Oceanic
and Atmospheric Administration (NOAA) in the HYDRO-35 report and the National Weather
Service (NWS) in the TP-40 report. Durations of 1, 2, 3, 6, 12, 24, 48, and 96-hour durations
' were evaluated. Because the duration of the six evaluated storms varied from 7 to 118 hours, the
durations over which each of the storms was evaluated varied. From the rainfall data for each
applicable duration, a rainfall intensity (inches per hour) and a rainfall depth (inches) were
' calculated using the moving average method. Calculated depth values were then compared to
depth values from NOAA HYDRO-35 and NWS TP-40 reports from Craven County for the
applicable storm durations to approximate the storm recurrence interval or recurrence range of
each of the six evaluated storms. The following tables present analyses results for each storm.

n

Storm 1 -April 15, 1999 16:00-23:00
Duration Intensity (in./hr.) Depth (in.) Frequency
1-hour 0.437 0.437 < 2-Year
2-hour 0.407 0.814 < 2-Year
3-hour 0.310 0.930 < 2-Year
6-hour 0.221 1.326 < 2-Year
Storm 2 - May 12,1999 09:00 - 15:00
Duration Intensity (in./hr.) Depth (in.) Frequency
1-hour 0.073 0.073 < 2-Year
2-hour 0.062 0.124 < 2-Year
3-hour 0.053 0.159 < 2-Year
6-hour 0.029 0.174 < 2-Year
Storm 3 -September 6, 1999 05:00-15:00
Duration Intensity (in./hr.) Depth (in.) Frequency
1-hour 1.717 1.717 < 2-Year
2-hour 1.340 2.680 2-Year to 5-Year
3-hour 0.981 2.943 2-Year to 5-Year
6-hour 0.638 3.828 2-Year to 5-Year
1

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Storm 4 -September 15,1999 05:00- September 16, 1999 11:00
(Hurricane Floyd)
Duration Intensity (in./hr.) Depth (in.) Frequency
1-hour 1.490 1.490 < 2-Year
2-hour 0.967 1.934 < 2-Year
3-hour 0.931 2.793 2-Year to 5-Year
6-hour 0.648 3.888 2-Year to 5-Year
12-hour 0.431 5.172 5-Year to 10-Year
24-hour 0.275 6.600 10-Year
48-hour 0.147 7.056 5-Year
Storm 5 -October 16, 1999 22:00 -October 18, 1999 03:00
Duration Intensity (in./hr.) Depth (in.) Frequency
1-hour 1.207 1.207 < 2-Year
2-hour 0.982 1.964 < 2-Year
3-hour 0.748 2.244 < 2-Year
6-hour 0.502 3.012 < 2-Year
12-hour 0.396 4.752 2-Year to 5-Year
24-hour 0.258 6.192 5-Year to 10-Year
48-hour 0.138 6.624 2-Year to 5-Year

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Storm 6 -April 28,1999 02:00 - May 2, 1999 23:00
Duration Intensity (in./hr.) Depth (in.) Frequency
1-hour 0.153 0.153 < 2-Year
2-hour 0.148 0.296 < 2-Year
3-hour 0.121 0.363 < 2-Year
6-hour 0.074 0.444 < 2-Year
12-hour 0.048 0.576 < 2-Year
24-hour 0.038 0.912 < 2-year
48-hour 0.030 1.440 < 2-Year
96-hour 0.024 2.304 < 2-Year
The tables above show that although some of the evaluated storm events, including
Hurricane Floyd, exhibited significant total rainfall when compared to predicted rainfall
depths for various recurrence intervals, they would be considered relatively high frequency
events. Based on collected rainfall data, most would be considered in the range of two to
five-year storms. Storms 4 and 5 did, however, fall into the range of five to ten- year
events, but only in longer durations.
5.2 Flood Flow Prediction Using Synthetic Hydrology
Synthetic hydrology is a process in which rainfall depth, duration, and frequency data are used in
combination with soil, land use, and other data to predict flood hydrographs. It is typically
employed where adequate stream gauge data are not available for statistical analysis and when
hydrographs, rather than just peak flows, are needed for design. The basis of synthetic hydrology
is that a rainfall of a given recurrence interval will produce flows of the same recurrence interval,
if factors such as previous rainfall and infiltration are included. While we have collected on-site
stream gauge data, its duration is not long enough for reliable statistical prediction of peak flows.
As such, we must either use synthetic hydrology or regional statistical analyses. We were
hesitant to apply regional statistical relationships to this watershed without confirmation by
synthetic hydrologic analyses verified by measured site rainfall to runoff response.
As described in our preliminary report, we experienced difficulty using synthetic hydrologic
models using standard dimensionless unit hydrographs such as the SCS Unit Hydrograph. For
this reason, we again attempted to develop a site specific unit hydrograph. As with the previous
analysis, outflow at the point of interest was computed from the stage discharge relationship
estimated with our two-stage curve combining results of our measured field data and our HEC-
RAS analysis.
I
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5.2.1 The Unit Hydrograph Method
A unit hydrograph is a simple linear model which can be used to estimate watershed flows
from excess rainfall runoff. The unit hydrograph for a given watershed represents the direct
runoff of some unit (typically taken as one inch) of runoff uniformly distributed over the
entire watershed at a constant rate for some duration of time. Once a unit hydrograph has
' been developed for a given watershed, we can then use this unit hydrograph to synthesize
the direct runoff hydrograph for any rainfall upon that watershed that produces runoff. The

' use of a unit hydrograph for synthesizing subsequent runoff hydrographs assumes that all
storm segments have the same duration, the distribution of rainfall and its losses are
uniform over the entire watershed, and only the total amount of runoff varies from segment
' to segment.
A unit hydrograph can be developed by deconvolution from rainfall and stream gauge data.
Standard unit hydrographs are also available which can be fitted to individual watersheds
' by analysis of watershed characteristics and storage.
5.2.2 Development of a Site Specific Unit Hydrograph
We initially attempted to use the process of deconvolution of the direct runoff hydrograph
' to develop a site specific unit hydrograph. Upon deconvolution of the unit hydrograph, we
performed a unit hydrograph synthesis (or convolution) on the same excess rainfall to check
the validity of the unit hydrograph. Spreadsheet computer programs were developed by us
' to perform deconvolution and unit hydrograph synthesis. For a detailed explanation of the
deconvolution-synthesis process the interested reader is referred to Appendix A (Section
4.3 of our previous report). Because results of the deconvolution-synthesis process were
' inconsistent and difficult to validate using other storm events, we opted to pursue an
alternative method using a pattern unit hydrograph. Some of the steps in our attempt to
develop a site specific unit hydrograph are the same as those in our pattern unit hydrograph
' analyses. To prevent repetition, these steps are described in the following sections.
Additional information on the deconvolution method and associated analyses can be found
t in Appendix A.
5.2.3 Pattern Unit Hydrographs
' Based on analysis results to this point, we adopted the use of a step-function pattern unit
hydrograph. The step-function was developed at North Carolina State University (NCSU)
' by H. Rooney Malcom, Ph.D., P.E., et al. The step-function unit hydrograph is scalable,
unlike the SCS Dimensionless Unit Hydrograph as implemented in the HEC-1 Computer
Program. We used the Snyder's watershed timing coefficient Ct, the known relationship
' between the time to peak, the peak flow, and the volume of the step-function, and the
observed rainfall and surface flow records to scale the step-function unit hydrograph for our
analysis.
t

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To perform our analyses, a spreadsheet model was developed to analyze watershed runoff
for existing conditions. Again, we chose to use a spreadsheet model because the less
complex data manipulation facilitates more trial solutions and graphing than a similar
analysis using HEC-1. This model required input of rainfall data, main channel length,
centroid location, Snyder's C,, watershed areas, and SCS curve numbers (CN). We also
allowed for input of the percentage of the watershed which was to be considered
impervious.
Of six discrete storm events analyzed and described in the previous section, we selected
Storm 3 for development of a step-function pattern unit hydrograph. Storm 3 was a
significant rainfall event with a total of 4.08 inches of rainfall over a nine-hour period. The
peak intensity of Storm 3 was approximately 1.7 inches per hour, and although not ideal in
its distribution, the storm did produced a more "typical" runoff hydrograph than some of
the other five storms. As described in the previous section, Storm 3 fell between the two-
year and five-year frequency for almost all of the evaluated durations.
Secondly, Storm 3 was preceded by a fairly dry period, and although almost 6.5 inches of
rainfall was measured during the month prior, flow depths, and subsequent estimated
discharges were insignificant, as such base flow was minimal. This allowed for a good
starting condition for analysis of watershed runoff, in this case Antecedent Moisture
Condition (AMC) I (dry).
Finally, shortly after Storm 3 occurred, two other rain events also occurred which were seen
to effect the overall runoff hydrograph. Storm 3, identified as Event 1 in our analyses,
consisting of 4.08 inches of rainfall, was followed by two smaller rainfall events, Events 2
and 3, of depths of 0.78 inches and 0.60 inches respectively. These smaller subsequent
events, although disruptive to analysis of the primary runoff hydrograph from Event 1
(Storm 3), allowed for the analysis of changing watershed conditions, such as increased
percent impervious area and a transition in Antecedent Moisture Conditions.
Although variations in soil type and land cover do exist over the site, the effect of their
distribution on the modeling of the site is relatively insignificant. For this reason we
elected to focus our analyses on the site watershed outlet at the bridge on Catfish Lake
Road. The development of our step-function pattern unit hydrograph model focused on this
' location with the assumption that modeling of other site locations, with smaller
contributing drainage areas, would be similar enough hydrologically that the developed
model would apply anywhere on the project site.
5.2.3.1 Base Flow Separation
Development of a direct runoff unit hydrograph from stream gauge data required that
base flow be separated from the direct runoff hydrograph. Although several methods
for base flow separation are available, we chose the simpler straight line method,
1

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which assumes a linear return of base flow. A linear interpolation was used for points
between the first rise and the inflection point of the hydrograph. Subtraction of the
base flow ordinates from the observed hydrograph ordinates yielded the direct runoff
hydrograph used in scaling of the step-function pattern unit hydrograph. The
observed hydrograph, base flow hydrograph, and resulting direct runoff hydrograph
are graphically depicted for Storm 1 through Storm 6 in Appendix F.
5.2.3.2 Rainfall-Runoff Estimation
Soils, cover, and land use data were used to assign Soil Conservation Service (SCS)
runoff Curve Numbers (CN) to the various watershed elements. Soils data were
obtained from the 1989 U.S. Department of Agriculture Soils Survey for Craven
County, NC, as provided by ESI. Cover and land use data were taken from USGS
topographic maps, vegetation maps provided by ESI (compiled from 1997 NCDOT
Photogrammetry), and visual observation from site visits.
The SCS CN is used to account for initial abstractions and infiltration when
converting rainfall to direct runoff. The higher the curve number, the larger the
portion of rainfall that appears as direct runoff. Direct runoff is considered to be that
portion of rainfall that flows overland to a watershed outlet. It does not include stored
surface water or infiltrated surface water, even though these may reach the point of
watershed discharge as later base flow.
From our preliminary report, our evaluation of site soils and coverage yielded a
weighted SCS CN of 84 for the site watershed for Antecedent Moisture Condition
(AMC) II (normal conditions). For the site in a "dry soil condition," AMC I, a CN of
68, indicative of a relatively low direct runoff potential, was selected. As previously
noted, many of the soils within the contributing watersheds may be categorized into
two Hydrologic Soil Groups, depending on drainage conditions. Hydrologic Soil
Groups are used in combination with cover conditions to estimate CN. The HSG's of
D, A/D and B/D are the most prevalent on the site, with paired groups representing
"drained" / "undrained" conditions. Because the site is drained by ditching, and
conditions preceding Storm 3 (the storm used in our step-function pattern unit
hydrograph scaling) were relatively dry, the argument could be made that the use of
HSG's with the higher infiltration capacity, or the "drained" condition, may be
appropriate for model development and validation depending on site conditions. The
use of these soil groups would result in significantly lower Curve Numbers of 47 and
68 for AMC I and AMC II, respectively. Lower Curve Numbers would reduce
predicted peak runoff values; however, experience has shown that relatively low
Curves Numbers may not be appropriate for hydrologic design. For future site
evaluations, the "undrained" Curve Numbers would be most appropriate.

I
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5.2.3.3 Model Calibration
Using the spreadsheet model described above we attempted to synthesize a
' hydrograph similar in shape and magnitude to the observed runoff hydrograph. The
model allows variation of Snyder's C„ CN, and the percentage of impervious area.
The Snyder's watershed timing coefficient, C, was varied within the model ranging
from 4.0 to 12.0 based on experience with watersheds in eastern North Carolina and
Virginia. Rainfall data from Events 1, 2, and 3 for Storm 3 were input, as well as
other required data discussed above. The resulting hydrograph was graphically
compared to the estimated runoff hydrograph from actual gauged channel water
surface elevations, and adjustments to C, were made to match observed hydrograph
' timing. By varying C, we were able to obtain a hydrograph similar in shape to the
observed hydrograph. After various trials, we selected a Ct of 5.50.
' Then varying CN and the percent impervious area within the watershed, peak
discharges were made to be similar in magnitude to those of the observed hydrograph.
' Through this trial process we found that an AMC I CN of 63 (equivalent to an AMC
II CN of 80) with an initial impervious percentage of approximately 12 percent
(including Long Lake), would produce an outflow hydrograph very similar to the
L observed hydrograph for Event 1 of Storm 3. The period preceding Storm 3 was a
fairly dry period, so the lower CN appears to be justified. By modifying both CN and
impervious percentage based on estimated changes in site conditions, we could
' reproduce hydrographs similar to those observed for Events 2 and 3 of the same
storm.

1
As discussed earlier, the observed hydrograph for Storm 3 was a result of not only the
original rainfall event, Event 1, but two additional rainfalls as well, Events 2 and 3.
Using the theory of superposition, these storm events were similarly analyzed with
overlapping synthesized hydrographs offset to coincide with the beginning of the
subsequent rainfall events. By varying CN's and percent impervious area to estimate
watershed conditions at the onset of each event, we were able to obtain a composite
hydrograph similar to the observed hydrograph. Based on our model calibration
efforts, the following estimates of percent impervious area and curve number were
used for modeling of site conditions.
AMC I AMC II AMC III
(Dry) (Normal) (Wet)
Percent 12 23 35
Impervious
Curve Number 63 80 90
1

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In cases where site conditions preceding a modeled event were estimated to be
between two Antecedent Moisture Conditions, an average of percent impervious and
curve number were applied. A comparison of the synthesized hydrograph and the
observed hydrograph from Storm 3 is shown in Figure 7. Detailed results of our
analysis are presented in Appendix F.
t 5.2.3.4 Model Validation
To validate the unit hydrograph model we produced, we used the model to predict a
' runoff hydrograph for other storms occurring over the watershed for comparison with
the observed hydrographs. Using the spreadsheet model described above, we
attempted to synthesize an outflow hydrograph for two other rainfall events, Storm 4
and Storm 5. Rainfall data from these two storms were individually entered into the
model spreadsheet. Judgement of watershed conditions was necessary to match
model parameters to conditions preceding the storms.
Accordingly, Storm 4, based on estimated preceding watershed conditions mid-way
' between AMC I and AMC II, was modeled with a CN of 72 and an impervious area
of 17.5 percent. Similarly, Storm 5 was modeled with a CN of 63 and an impervious
area of 12 percent based on preceding site conditions of AMC I. Both of these
' validating storms had smaller rainfall events occurring some time into the primary
runoff hydrograph, which influenced the observed runoff hydrograph. These later
events, however, were considered minor in comparison to the primary event, occurred
' at a point well past the observed hydrograph peak, and were judged not to be
significant for validation of the model.
' A comparison of the synthesized hydrograph and the observed hydrograph from
Storms 4 and 5 is shown in Figure 8. For Storm 4 an estimate of peak discharge must
be made because during the recording of water surface elevations following Storm 4.
' the surface water gauge overtopped, consequently, no peak water surface elevation
was recorded. By extrapolating the rising and falling limbs of the observed
' hydrograph, we can conservatively estimate a peak discharge. In doing so, we see that
the predicted peak discharge for Storm 4 would is similar to the observed peak
discharge. The predicted peak discharge for Storm 5 is also similar to the observed
peak discharge. While the predicted and the observed hydrographs do not match
exactly, they can be considered hydrologically similar. That is, they match within the
precision expected from hydrologic analyses and would be reasonable for hydraulic
' design. Detailed results of our model calibration efforts are presented in Appendix F.
5.2.4 Application of the Unit Hydrograph Method
'
Once we were able to validate our hydrograph model, peak discharges from our model
output could then be used to size required water control structures. Since the model

'
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' requires input of main channel length, centroid location, and watershed area the model
could then be used for any location around the project site. The Snyder's watershed timing
coefficient, Ct, remains the same without regard to location on-site. Because we are
designing structures for "Normal" site conditions, we use the SCS curve number (CN)
associated with AMC II (Normal) conditions, in this case 80. Additionally, we use a
percent impervious of 23 for "Normal" site conditions. Rainfall input, the key to predicting
flood flows for given recurrence intervals using our synthetic hydrologic model, is
presented in the next section.
5.2.5 Design Storm Rainfall Data
Typically, NCDOT requires structures at road crossings on secondary roads, open for
public access, to be sized to allow for passage of the 25-year discharge. Crossing sites on
less trafficked roads, such as those on the project site, are commonly designed for the ten-
year peak discharge through a culvert or culverts, without erosion in the vicinity of the
structure. As is often the case in coastal areas, limited topographic relief restricts structure
size, crossings may be designed to meet the capacity of the upstream channel or ditch,
rather than a particular event frequency. According to the North Carolina Sediment
Pollution Control Act of 1973, Erosion and Sediment Control measures must be designed
to provide protection from a rainfall event equivalent in magnitude to the ten-year peak
' runoff. Accordingly, we selected the ten-year design event for the sizing of crossing
culverts.
At the project site, however, not only do we want to allow for safe passage of the ten-year
peak discharge, but we also want to safely pass the 100-year discharge over the road, again
without erosion. As such, in addition to the development of a ten-year design discharge,
the 100-year design storm discharge was also required. For comparison purposes we used
two different methods to develop the a design storm hyetograph, or distribution pattern in
which the rain would fall over the project watershed. Details of this work can be found in
' Appendix G.
Based on our watershed time of concentration, we selected the use of a 12-hour design
' storm duration for developing the required design rainfall. Predicted precipitation depths
for the 10- and 100-year design storms were developed from Depth Duration Frequency
(DDF) values. These values were obtained from DDF data developed using the NOAA
HYDRO-35 and the NWS TP-40 reports. The 12-hour design storm depths for the 10-year
and 100-year storms are 5.77 inches and 8.55 inches of rainfall respectively.
' We first used the "Alternating Block" Method to develop our design storms. In this
method we separated our rainfall into "n" successive blocks of rainfall each of a set
' duration "t", resulting in a total storm duration, Td, equaling the product of "n" and "t". In
our design storm, "n" equals 12, since we considered a 12-hour design storm, and "t"
equals one hour, since we analyzed the event in one hour segments. Using available DDF

' - 33 -
' characteristics to basin characteristics in a generalized least-squares regression analysis. Using a
watershed area of 18.9 square miles at the site outlet at the bridge at Catfish Lake Road in the
regression equations, we calculated the peak discharges for various recurrence intervals.
' 5.4 Flood Flow Prediction using Regional Flood Frequency Analysis
' The second method used for comparison of peak discharge estimates were the regional curves
developed from the Flood Frequency Analysis (HEC-FFA) program in our preliminary report
analyses. Using HEC-FFA we determined the specific discharge in cubic feet per second per
' square mile (cfs/mil) for each return period of interest. Using a watershed area of 18.9 square
miles at the site outlet at the bridge at Catfish Lake Road, multiplied by the specific discharge,
we calculated the peak discharge for various recurrence intervals. A more detailed explanation
of this method and background data can be found in Appendix A.
5.5 Comparison of Predicted Floods with Observed Floods
As described previously, each of the six analyzed storm events was roughly classified into a
' particular storm frequency or recurrence interval (based on measured depth when compared to
expected depth values from NOAA HYDRO-35 and NWS TP-40 reports from Craven County,
NC). We then compared observed peak discharges for each of the six evaluated storms with
' estimated peaks from both the USGS Regression Equations and the HEC-FFA Specific
Discharge curves, for the applicable recurrence intervals. Again, it should be emphasized that
accurate watershed delineation is critical to appropriate use of all methods of flood flow
' prediction. Results of our analyses are shown below:

Recurrence Synthetic Hydrologic USGS Flood Measured Storm
Interval Model Regression Frequency Events
Equations Analysis
Existing Proposed
(Drained)
2-Year 129 cfs 1520 cfs 445 cfs 470 cfs Storml - 13 cfs
Storm 2 - 20 cfs
Storm 6 - 28 cfs
5-Year 1189 cfs 2144 cfs 824 cfs 865 cfs Storm 3 - 815 cfs
Storm 5 - 1070 cfs
10-Year 1520 cfs 2584 cfs 1163 cfs 1598 cfs Storm 4 - 1085 cfs
(Gauge
Overtopped)
100-Year 2857 cfs 4249 cfs 2813 cfs 6390 cfs N/A

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The results above show a fairly close correlation in resulting peak discharges for lower frequency
storms to included ten-year and 100-year design storms. The table results also show that high
frequency flows, such as the two-year storm, are significantly affected by existing site drainage
' conditions.
Frequent storm events (less than the 2-year event) currently produce smaller peaks at the
' watershed outlet than would be predicted by any of the methods considered. However, with the
projected changes in site conditions, peaks experienced as a result of more frequent storms may
be more typical of those predicted by synthetic methods. Because the predicted results of our
' synthetic hydrologic model are considered reasonable, when compared with those predicted by
the USGS Regression Equations and the Flood Frequency Analysis, and have been validated
' against on-site rainfall and runoff observations, we recommend their use in the prediction of peak
discharges at this and other locations on the project site.
1

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6.0 PROPOSED SITE TREATMENTS
Our design goal is to return the site to a more natural hydrologic condition, similar to site
conditions prior to disturbance. This section presents a description of existing features and their
effect on site hydrology, as well as site modifications and proposed constructed features which
will assist in achieving restoration goals. Details on the design of proposed site treatments is
provided in Appendix H.
6.1 Existing Conditions
As described previously, an extensive ditch network covers the project site. Relic channels are
present in the northern portion of the site.
6.1.1 Existing Ditch and Road Network

0

Based on site observations and our understanding of construction practices previously used
on the site, the existing ditches were excavated with the spoil material placed off to one
side creating the roads which currently exist parallel to the ditches. Depths of on-site
ditches vary from one to over six feet, with a typical width being about 12 to 15 feet. In
areas of higher ground elevation, the ditches are smaller, as is the case along the ridge line
which extends from the south end of CH 7 to just north of the intersection of CH's 8, 9, and
10. Much of CH 6 is similarly small in dimension. A typical cross-section of an existing
ditch is shown in Figure 9.
Site ditches appear to influence site hydrology in three main ways. First, they serve to
collect surface runoff and allow for transport off-site. Second, some of these ditches serve
to intercept groundwater movement on site, allowing its transport from the site. Third,
according to ESI's soils data, a confining layer exists over portions of the site, striating the
groundwater into shallow and deep layers. Areas where the confining layer has been
penetrated during excavation of ditches may serve as a two-directional drain depending on
groundwater conditions. During dry periods, these drains allow shallow groundwater to
move downward beneath the confining layer. During wetter periods deep groundwater is
allowed to move upward through the confining layer into the ditches and eventually off-
site. This third effect of site ditching is thought to be the least significant and may balance
over long time periods to result in no net loss or gain.
Roads also have influenced site hydrology. Although not directly confirmed by site
' investigation, we would expect that a zone of consolidated soils exists beneath the road fill
based on soil mechanics theory. This consolidated zone is created by the weight of fill
materials on top of the original ground. Consolidated soils will have a lower permeability
' than the surrounding relatively undisturbed soils. As such, the roads serve to act as water
bars or dams, which divert the natural flow of both surface and groundwater across the site.

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' This diversion may result in either the redirection of water into the ditch network or the
retention of water behind the road, or both.
Although typically earth surface, some sections have been repaired or patched with gravel
where standing water and vehicular traffic have created difficult passage. Numerous
culverts exist on the site allowing for conveyance of channel flow under existing roads. A
variety of materials have been used on site to include, corrugated metal pipe (CMP),
reinforced concrete pipe (RCP), and corrugated plastic pipe. Many of the existing pipes are
in poor condition, allowing for a condition of changing flow regime on the site. A typical
' existing road cross-section and road profile are shown in Figures 9 and 10.
' 6.1.2 Relic Channels and Natural Drainage Features
From a review of the USGS 7.5-minute series quadrangle maps, "Catfish Lake," "Hadnol
' Creek," "Havelock," and "Masontown," three prominent natural drainage features can be
seen on the project site. Two of these features appear to start near the property boundary
with Long Lake, running roughly parallel to each other on either side of the ridge line
described above. These two features appear to join near the point where they intersect
CH13. From here the feature travels northward leaving the project property and then
turning northwest towards the bridge at Catfish Lake Road, becoming East Prong Brice
Creek. The third feature begins off-site near the northwest edge of the site, and travels
north paralleling CH's 17 and 18, and eventually joining East Prong Brice Creek near the
bridge. These natural drainage features are roughly depicted in Figure 3.
6.2 Proposed Site Treatments
' In order to assist in a return of site hydrology to a more natural condition, sections of the existing
road and ditch network will require removal or modification. However, to allow continued
access to a large portion of the project site, some site roads must remain in place. As such,
various "treatments" or methods must be applied across the site to meet hydrologic goals. These
various "treatments" are described below.
6.2.1 Site Ditch Removal and Modifications
Existing roads and ditches must allow for conveyance of both surface and groundwater
along the desired "natural" flow path. At first glance, one obvious solution might be to
return existing road materials into the ditches with the hope that historic hydrology would
' return. Unfortunately, although some benefits might be realized through this method, some
problems exist in this approach. First, existing zones of compressed soil and the fill
material returned to the ditch will serve to restrict groundwater movement perpendicular to
' the road/ditch axis. Second, it is likely that a significant volume of soil was lost due to
erosion, oxidation, and consolidation. It is therefore unlikely that sufficient local soil
materials exist to completely fill the ditches, and as such the potential exists for surface

' -37-
water to eventually become concentrated such that ditches are reformed. More significant
to restoration success is the potential for the ditches to reform through erosion. This is
particularly true where post restoration flow will parallel the alignment of the existing
ditches. Another problem of simply returning the road fill to the ditches is that the site
would no longer have road access. Thus additional work is required to restore and enhance
hydrologic function while avoiding these problems. The additional work is discussed in the
following sections.
6.2.1.1 Ditch Plugs
In order to eliminate flow in the existing ditch network, earth plugs could be installed
' in selected site ditches. Two types or sizes are proposed. The first is a "Point" ditch
plug, serving to stop longitudinal flow of water in an existing ditch while limiting the
volume of fill required. These plugs would be constructed from compacted fill
' material placed to the top of bank elevation over a discrete segment of the ditch.
Details for proposed "Point" ditch plugs are shown in Figure 20. The second type of
plug is a "Reach" ditch plug again serving to stop longitudinal flow of water in an
' existing ditch, but requiring significantly more fill. Fill requirements could exceed
that available from the adjacent road. These plugs would be constructed similarly to
"Point" plugs except that they would extend over an substantial ditch lengths,
' possibly hundreds or even thousand of feet. Details for proposed "Reach" ditch plugs
are shown in Figure 21.
Placement of "Point" plugs would typically occur at locations where the surface water
gradient is generally perpendicular to the longitudinal axis of the ditch. Placement of
"Reach" plugs would typically occur at locations where the surface water gradient is
generally parallel to the longitudinal axis of the ditch. These plugs are placed along
such reaches since, due to their orientation, a greater potential exists for surface
waters to become concentrated leading to ditch reformation.
Locations of "Point" and "Reach" plugs should be selected based on a localized
' assessment of surface conditions and local topography. An initial estimate of plug
location, based on guidance by ESI and reaches deemed critical to the restoration of
natural hydrology by Eddy Engineering, P.C., is identified on Figure 13. ESI has
identified locations where the confining clay layer, previously discussed, has been
penetrated causing it to be desirable from a groundwater control standpoint to plug
these ditch locations or reaches. These reaches to be plugged include all of CH's 14,
t 16, 20, and 21, as well as portions of CH's 13 and 18. Similarly, we have identified
sections which will also require "Reach" plugs along select portions of the other
reaches deemed critical to the restoration of natural hydrology. These reaches to be
' plugged include the remaining portion of CH 18, as well as CH's 2, 4, 5, and a
portion of CH 12. Other "Reach" plug locations may be identified during final
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design. "Point" plug locations, occurring in many of the remaining channels, will be
evaluated and identified during final design.
6.2.1.2 Surface Water Diversions
To assist in the return of more natural surface water movement across the site,
particularly along ditches where the potential exists for surface waters to become
concentrated, surface water diversions are proposed. These earthen berms,
approximately 2 feet in height would be constructed perpendicular to, or at a slight
angle to, the longitudinal axis of the ditch. Diversions could be as single entities, or
as groups, and although they would be constructed to a standard height and depth,
their length would be allowed to vary such that they could be tied to local topography
to better serve their intended purpose. These diversions should be placed based on a
localized assessment of surface conditions and local topography and may be field
adjusted during construction. Details for proposed surface water diversions are
shown in Figures 20 and 21.
6.2.1.3 Scarification of Consolidated Soils
Due to road construction, zones of consolidated, and therefore less pervious, soils are
present beneath existing road segments. With ditch filling, fill materials may also be
less permeable than surrounding soils. To what extent these zones limit the flow of
groundwater is unknown and will likely vary across the project site. However,
scarification (ripping) of all such areas perpendicular to the expected flow will create
a greater horizontal permeability. This will aid in returning groundwater conveyance
to a more natural condition. Where roads are to be completely removed, scarification
will be the least costly means of increasing horizontal groundwater conveyance.
Scarification depth should be controlled to reduce the potential for penetrating
confining layers. Unfortunately, for roads to remain in service, scarification is not a
good alternative.
6.2.1.4 Removal of Existing Conveyance Structures
To assist in the achievement of site hydrology goals and better reduce the potential of
surface flow returning to the existing channel network, select existing conveyance
structures should be remove. Each existing structure should be evaluated on a case by
case basis from a localized assessment of surface conditions and local topography and
removed if it is deemed detrimental to site hydrologic goals. This assessment would
occur during final design.
As described in our preliminary report an outlet control device does exist along the
northeast side of Long Lake (POI 1). The structure appears to be in poor, if not
unusable, condition, with the corrugated metal outlet barrel showing severe corrosion

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on exposed portions. Although no evidence of overtopping or surface flow was
observed in the vicinity of the structure we recommend that this structure should be
removed, or preferably grouted, to ensure it does not serve as an outlet for Long Lake.
6.2.2 Site Road Modifications
' To allow continued access to a large portion of the project site, some roads or road sections
must remain in place, and as such steps must be taken to allow for conveyance of both
surface and groundwater along the desired "natural" flow path. Details of which roads are
' to remain and which roads are to be removed are shown in Figure 11.

L

6.2.2.1 Improving Road Surface Course
Because virtually all of the ditches on site will be experiencing some type of
modification. At some locations on the project site it is desired to completely remove
the existing road. In these cases, road material would be used to fill ditches and the
entire cross-section would be scarified. A typical cross-section where the ditch is
filled and the road removed is provided in Figure 16. As a result of proposed site
modifications, conditions where surface water exists within close vertical proximity
of the remaining road surface, are likely to occur more frequently. One result of this
increased local water surface elevation is the degradation of existing subsurface
conditions beneath the road, and in turn destabilization of the road surface itself.
Accordingly, sections of road which are to remain open will require improvements for
stability under these new conditions. We propose the placement of a new compacted
aggregate surface course. Details of an improved road cross-section are shown in
Figure 19.
6.2.2.2 Surface Water Conveyance Measures
Using our synthetic hydrologic model, we developed peak discharges to be used to
size required surface water conveyance structures. Since the model requires input of
main channel length, centroid location, and watershed area the model could then be
used for any locations around the project site. The Snyder's watershed gauge
coefficient, Ct, remains the same regardless of the location on site. Because we are
designing required structures for "Normal" site conditions, we use the SCS curve
number (CN) associated with AMC II (Normal) conditions, in this case 80.
Additionally, we use a percent impervious of 23 for "Normal" site conditions.
As described previously, structures at road crossings, such as those at the project site,
are sized to allow for passage of the 10-Year peak discharge through a culvert or
culverts, without erosion in the vicinity of the structure. In this case, at select natural
drainage features, not only do we want to allow for safe passage of the 10-Year peak
discharge, but we also would like to be able to safely pass the 100-Year discharge

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over the road, again without erosion. Accordingly we used the developed 12-hour
design storm depths for the 10-Year and 100-Year storms of 5.77 inches and 8.55
inches of rainfall respectively. The resulting storm event hyetographs from our
' modified SCS Type III Distribution were used.
Details of locations for permanent road crossings at natural drainage features which
' were evaluated for conveyance structure requirements are shown in Figure 12.
Results of our peak discharge analyses at specific locations are shown below:

C
0

Road Crossing Number
Recurrence
Interval RC1 RC2 RC3 RC4
10-Year 750 cfs 1085 cfs 670 cfs 815 cfs

100-Year
F
1230 cfs
1780 cfs
1090 cfs
1335 cfs
Each road crossing was then evaluated to determine alternative conveyance measures
suitable for passing predicted peak discharges. We considered two likely alternatives
for construction at locations where roads to be maintained cross existing natural
drainage features. At these crossings, differences in existing top of road elevation and
existing natural ground elevation at the crossings site ranged from approximately 1 to
3 feet. Because of the limited elevation change at the crossings, constraints are
encountered on the maximum diameter of pipes which can be used.
If we consider our ability to meet conveyance requirements critical such that the
potential for water to backup at crossings is small, then sufficient culvert capacity
may necessitate an increased road elevation to obtain sufficient pipe cover. Typically,
minimum pipe cover is considered 1 foot. We evaluated reinforced concrete pipe
culverts ranging in size from 12- to 36-inches in diameter in various configurations,
with various combinations of pipe diameters. These pipes could be installed in
conjunction with subsurface conveyance measures such as aggregate drains, if
needed. Culvert analyses were conducted for the 10-year peak discharge only. We
found that even using 30- and 36-inch diameter culverts, the number of pipes required
was large and would require an increase in road elevations to meet minimum cover
requirements. Additionally, the use of culverts of smaller diameter would require the
installation of too many culverts to be practical. Even with the larger culverts,
crossings would still require design for overtopping of the existing road surface to
safely pass the 100-year discharge.
The second alternative for such road crossings is the installation of one or more
smaller culverts at topographically low points along the road crossing to allow for the
passage of daily discharges. These culverts would be located such that they coincided

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with the apparent low points in local topography to reduce the potential for standing
water in the vicinity of the crossing. The crossing itself would consist of an improved
and hardened road surface which would allow for vehicular traffic, while
' simultaneously allowing for the passage of peak discharges over the road surface. An
example of such a crossing configuration alternative is shown in Figures 17 and 18.
' Preliminary analyses on conditions where allowable depth of head over road surface
is varied between 6 inches up to as much as two feet indicate weir or ford lengths
' ranging from a around 150 feet to many hundreds of feet. These crossing sites can be
constructed in conjunction with subsurface conveyance measures such as aggregate
drains, if needed. Improvements and armoring of the road surface at these locations
' may be required depending on design velocities.
6.2.2.3 Subsurface Aggregate Drains
' As described previously, due to road construction significant consolidation of
subsurface soils is likely beneath the existing road network. This has the effect of
' reducing horizontal conveyance of groundwater. Scarification is not compatible with
roads that are to remain in service. For cases where groundwater conveyance is
needed and the road must remain in service, aggregate drains would be a better
' choice. The aggregate drains can be sized and spaced such that the effective
conveyance of the combined fill and drain section can be made equal to or greater
than the undisturbed soils. Aggregates such as open graded sands and gravels have a
much higher permeability than on-site soils so only a small area of aggregate drain
would be needed to dramatically increase effective conveyance. These drains would
be installed in conjunction with surface conveyance measures such as culverts and
' lowered road crossings or ford sites as would be the case where roads are to remain as
shown in Figures 17 and 18.
' 6.3 Locating Site Treatments
' As shown in Figures 11, 12, and 13, approximate locations have already been identified for some
specific site treatments. The type of treatment for roads and ditches is dependent on many
factors. Because of the size of the site and the small variation in elevations over the entire site,
' the locating of all site treatments as described above should be conducted through localized on-
site evaluation during final design to be followed by confirmation during construction. Such
evaluation includes, but is not limited to, local surface topography, existing road or ditch
conditions, and expected future drainage conditions. In order to assure consistency in application
of treatments and to estimate what features are likely to be applied at a given location on the
project site the flow chart provided in Figure 22 can be used.

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7.0 WATER SURFACE PROFILE MODELS
Hydraulic computer models were developed to describe existing and post-construction conditions
' and to quantify the effects of filling in the ditches. This section presents a summary of our
findings and conclusions with regard to site water surface elevations. Detailed results of our
water surface profile analysis are presented in Appendix I of this report. To prevent repetition,
the interested reader is referred there for more detailed information.

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7.1 Hydraulic Models
Two computer models were constructed to evaluate migration of water from the southern end of
the site near Long Lake to the watershed outlet at the bridge on Catfish Lake Road. Models were
developed using the U.S. Army Corps of Engineers River Analysis System (HEC-RAS)
computer program. The first model was developed for existing conditions, the second for
revised or post-restoration conditions. Flow data was obtained from the Step-Function Pattern
Unit Hydrograph synthesis model previously discussed for various return periods. Manning's n-
values for channel and overbank flow were selected based on site observations and standard
references.
7.1.1 Existing Conditions Model
As described previously, an extensive ditch network covers the project site. Various
channels within the project site regularly show strong flow or physical evidence of previous
flow, while portions of other channels appeared stagnant, and no discernible direction of
flow can be observed. Channel geometry for the existing conditions model was based on
existing site cross-sections taken from topographic survey data provided by NCDOT.
Available site data on culverts, roads, and the bridge at Catfish Lake Road were
incorporated into the model. Locations of cross-sections used in our analyses are presented
on a schematic of the existing conditions model provided in Appendix I. At locations
where limited or no topographic data were available, cross-sections were simulated by
copying cross-sections from either upstream or downstream and adjusting inverts and bank
elevations based on reach slopes.
From a combination of stage and discharge data collected during site visits and from
' surface water gauges, we were able to calibrate the existing conditions model to measured
site conditions. The model was then validated using a series of measured discharges at
discrete times and locations, producing water surface elevations similar to those measured
for the estimated discharge.
7.1.2 Proposed Conditions Model
' As a part of restoration and enhancement efforts to return the project site to more natural
hydrology, the majority of the existing site ditches will be filled or plugged. In so doing,

-43-
natural drainage features and relic stream channels on-site will serve to transport surface
water from the site. This model incorporated survey data provided by NCDOT for various
site transects which cross the natural drainage features being modeled. In some cases,
because of the limited variation in elevation over the length of a given transect,
identification of the natural drainage feature was difficult. In such instances, information
provided by USGS topographic data, as well as site observations, were used to assist in the
identification of the "channel" section. This was particularly the case at the south end of
the site where elevation differences were slight over the length of the transect. Cross-
sections used in the proposed conditions model were typically much longer than those used
in the existing conditions model. A less defined channel tends to distribute flows over a
wider cross-section. Additionally, because no definition exists between the channel proper,
and left and right overbank sections, a constant Manning's coefficient was applied over the
entire cross-section.
7.2 Comparison of Water Surface Elevations
A series of key locations (KL) were selected for comparison of generated existing and post-
restoration water surface elevations. Locations of these six points are identified in Figure 23.
These locations were selected for two important reasons. First, both of the models described
above incorporated these six locations, along existing channels in the existing conditions model,
and along natural drainage features in the post-restoration model. Therefore, a direct comparison
of modeled water surface elevations can be made between existing and future conditions under
some flow conditions. Second, based on their distribution across the site, these six locations
allowed for a comparison of overall site water surface elevations, both before and after
restoration, for some flows.
' 7.2.1 Development of Discharge Estimates
The flow regime across the site will change dramatically between the existing and future
' conditions. After site restoration treatments are complete, not only will the "channel" in
which surface runoff is being conveyed past each of the key locations change, but the area

' which drains to each of these key locations may change as well. Resulting differences in
contributing drainage area, L, and L.
, are reflected in changes to peak discharges between
existing and future site conditions.
' Using the modified SCS Type III rainfall distribution and synthetic hydrologic model, we
estimated peak discharges at each of our key locations for the 2-, 5-, 10-, and 100-Year
rainfall events, as well as for a one-inch rainfall. Flow data from our analyses is provided
in the following tables.

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KL 1 - Estimated Peak Discharges
Storm
Event Existing Conditions
(cfs) Future Conditions
(cfs)
1-inch 8 53
2-Year 68 444
5-Year 97 629
10-Year 116 752
100-Year 191 1230
KL 2 - Estimated Peak Discharges
Storm
Event Existing Conditions
(cfs) Future Conditions
(cfs)
1-inch 19 78
2-Year 166 638
5-Year 236 906
10-Year 283 1085
100-Year 468 1781
KL 3 - Estimated Peak Discharges
Storm
Event Existing Conditions
(cfs) Future Conditions
(cfs)
1-inch 15 48
2-Year 121 393
5-Year 172 557
10-Year 206 667
100-Year 339 1089

11

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KL 4 - Estimated Peak Discharges
Storm
Event Existing Conditions
(cfs) Future Conditions
(cfs)
1-inch 117 59
2-Year 939 479
5-Year 1334 681
10-Year 1593 814
100-Year 2606 1334
KL 5 - Estimated Peak Discharges
Storm
Event Existing Conditions
(cfs) Future Conditions
(cfs)
1-inch 16 126
2-Year 128 1021
5-Year 183 1451
10-Year 219 1734
100-Year 359 2841
KL 6 - Estimated Peak Discharges
Storm
Event Existing Conditions
(cfs) Future Conditions
(cfs)
1-inch 186 191
2-Year 1496 1538
5-Year 2124 2185
10-Year 2537 2610
100-Year 4151 4276
The peak discharges tabulated above were developed considering CN of 80 and percent
impervious area of 23 associated with AMC II. Accordingly, at KL 6, peak discharge
values may differ from those previously presented in Section 5.5.
1

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7.2.2 Estimated Water Surface Elevations
When using the developed discharge data, our modeling indicated that overtopping of the
' channel banks occurred in all storm events, with the exception of the one-inch rainfall, in
turn making water surface elevation comparisons difficult. Because the predicted
discharges associated with the one-inch rainfall did not overtop the channel banks, these
' predicted water surface elevations are used for comparison purposes.
At the six key locations across the site, the resulting water surface elevations predicted by
' the existing and future conditions models using a one-inch rainfall event are presented in
the table below.

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11

1-inch Rainfall Event

Location Existing Conditions
Model WSE
(ft.) Future Conditions
Model WSE
(ft.)
KL 1 36.97 35.33
KL 2 31.82 31.00
KL 3 33.05 32.10
KL 4 20.80 19.08
KL 5 28.33 27.51
KL 6 17.34 17.37
7.2.3 Travel Time
By evaluating expected velocities, resulting from a one-inch storm event, for existing and
proposed conditions models, over reach lengths we can develop approximate travel times,
Tt. Travel time is estimated by the average velocity from the upstream end of the
watershed to the watershed outlet at the bridge, assuming instant translation of flow
through the lake. A comparison of estimated travel times from the one-inch storm is shown
in the following table.
Existing Conditions Restored Conditions
Storm Event Travel Time Travel Time
(hrs.) (hrs.)
1-inch 11.7 hrs. 8.9 hrs.

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7.3 Discussion of Results
In reviewing the predicted discharge results above, we see that in all locations except KL 6,
' significant changes in predicted peaks occur. These changes are primarily a result of changes to
the contributing watershed area draining to each of these key locations. Since the channel
network will be removed, the area contributing drainage to all locations except KL4 and KL 6 is
' increased. At KL 4 the contributing drainage area actually decreases, resulting in decreased peak
discharges. At KL 6, the watershed outlet at Catfish Lake Road, since the drainage area
remained the same, only minor increases were predicted. These increases at KL 6 can be
' attributed to changes in L and Lc between existing and future site drainage conditions.
' Predicted water surface elevations from the future or post-restoration conditions model are lower,
although not significantly lower, than water surface elevations in the existing conditions model.
This may be explained in part, by examining the effect of the road and ditch systems on the
relatively large flood flows. The roads act like dams and the ditches are completely inundated in
larger flood flows. After the site modifications, surface waters will be able to flow over a
significantly larger cross-sectional area. There is more shallow flow across a larger portion of
' the site than in the existing conditions model where roads, acting as dams, and low capacity
culverts tend to keep water surface elevations somewhat high for the larger flows. We can
conclude that while the existing ditch system is effective at draining near surface groundwater
from a large portion of the site, it does not have adequate capacity for larger flood flows. This is
confirmed by our previous results discussion in which we found that, except for the one-inch
rainfall, larger storm events typically experienced overbank flow.
' In comparing the estimated travel times in the restored condition, we can draw the following
conclusions. First, for high-frequency storms, more frequent than the two-year storm event, the
' existing ditch network serves to elongate the outflow hydrograph to some extent, since
significant surface runoff is being forced to travel a circuitous route through the site. In relative
terms, the decrease in travel time, and the associated time of concentration, due to the change in
' site conditions will not likely be evidenced as any significant increase in the peak discharge
resulting at the watershed outlet. There will only be an earlier peak relative to the beginning of
' the rainfall. Both pre- and post-restoration condition peaks are only a few hundred cubic feet per
second, a relatively small discharge difference.
' In general, direct comparison of the results of a site modification as drastic as those proposed for
the project site is difficult, at best. Because existing channel banks are overtopped for storm
events of lower frequency (2-, 5-, 10-, 100-year events), model validity is in question and water
' surface elevations are imprecise. However, in future conditions, because water is being
conveyed over a larger cross-section, one in which the flow is greatly disrupted by trees, brush,
and variations in local topography, the application of higher Manning's coefficients would be
' appropriate. Higher Manning's n-values tend to increase flood elevations and decrease
velocities, which will partially offset the drop in water surface elevations expected from road
removal.

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8.0 CONCLUSIONS AND RECOMMENDATIONS
Based on our analyses, we have drawn conclusions about certain aspects of the hydrology of the
' project site in both existing and future conditions. This section provides as summary of those
conclusions, as well as recommendations for final design, and future data collection and site
monitoring.
' 8.1 Watershed Characteristics
' We found that Long Lake and the surrounding area does appear to drain predominantly through
the site. A comparison of the cumulative water gains versus cumulative water losses over the
analysis period revealed that Long Lake and its surrounding area contribute direct surface water
' runoff to the site in larger, less frequent storm events. Similar to depression storage in its effect,
Long Lake will store initial rainfall if the lake level is at normal or lower level before the storm
event. If the lake level is relatively high at the beginning of a storm event, direct discharge
equivalent to impervious area is to be expected. Because proposed site modifications will likely
increase lake levels, discharge from the lake will be come more frequent, thus increasing
apparent flood discharges through the site. This again reinforces our conclusion that proposed
site modifications could significantly increase the severity and frequency of surface water flows.

8.2 Flood Flow Prediction
We developed a means to predict flood flows within and around the site for both existing
conditions and after the proposed modifications. In our analysis we developed a synthetic
hydrologic model to serve as a means of estimating peak discharges at various locations on the
project site. With the validation the model's ability to reproduce other storm events on the
watershed, we recommend that this model be used for hydraulic design within the watershed.
8.3 Recommended Site Modifications
Locations and types of site modifications to the existing road and ditch system needed to enhance
or restore wetland hydrology are proposed. These will be developed into construction plans as
part of the Final Design Phase. The proposed site modifications include elimination of virtually
all prominent site ditches, including those paralleling the existing road network, improved
crossings at select locations where remaining roads cross natural drainage features, and removal
of multiple site roads and culverts. Conceptual designs based on these analyses are proposed to
support restoration and enhancement of wetland function.
Because continued access to a large portion of the project site is required for project monitoring,
some site roads must remain in place and, as such various "treatments" or methods must be
applied across the site to meet hydrologic goals which allow for conveyance of both surface and
groundwater along the desired "natural" flow path. Treatment methods include ditch plugs,
surface water diversions, disruption of existing zones of compressed soil along existing road

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beds, removal of existing conveyance structures, installation of conveyance measures for both
surface and sub-surface flow, and the improvement of select site roads and natural drainage
feature crossing points. Select locations have already been identified for specific site treatments,
but because of the size of the site and the small variation elevation over the entire site, the
locating of all site treatments should be conducted through localized on-site evaluation during
project final design.
In general, it appears that construction of the proposed modifications can be most expediently
accomplished on an upstream to downstream basis. This should keep the work area as well
drained as is possible.
8.4 Effect of Site Modifications
Our analyses show that increased peak flood flows are likely after site modifications, because of
the increase in soil moisture and the large percentage of the drainage area that will be wetlands.
This watershed, as it currently exists in a drained condition, exhibits significant storage effect
along channels, in soils, and in depressions. After the proposed site modifications, channels will
be filled, soils will be more nearly saturated, and depressions may be partially filled with water
before the onset of a rainfall event, resulting in more of the rainfall being translated to direct
runoff. As such, we expect peak flows to increase after the proposed modifications are
completed.
In reviewing modeled water surface elevations for various recurrence intervals we find that while
' the existing ditch system is effective at draining small rainfall events and near surface
groundwater from a large portion of the site, it does not have adequate capacity for larger flood
flows as evidenced by ditch bank overtopping. For smaller storm events water surface elevations
' from the future or post-restoration conditions model are slightly lower than those predicted by the
existing conditions model, even though flows are greater. For relatively large flood flows, the
existing roads act like dams, ditches are completely inundated, and existing low capacity culverts
tend to keep water surface elevations somewhat higher than they will be in the future.
' When the existing ditch network is removed, it is likely that groundwater levels will increase.
Additionally, this in combination with more natural overland flow patterns is likely to create
conditions where local depression storage is seen to collect and pond water more frequently, and
for longer duration than is currently being experienced on site. In turn, this decrease in available
soil and surface storage will likely result in a higher percentage of rainfall being translated into
direct runoff.
' Roads that remain may not be passable at all times. Roads will be designed to be inundated
during periods of higher flow. As presented earlier, the frequency of significant flood flows is
' likely to increase.

1
1

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-50-
Because groundwater levels outside the lake are expected to rise, less lake water will be lost to
groundwater which now finds its way into the ditch system. Additionally, smaller, more frequent
rainfall events will produce surface flow, a portion of which may flow to the lake. The effect of
these conditions may be that the normal level of Long Lake rises.
Potential off-site effects to the west of the site are unknown because of the lack of topographic
data in this area; however, we do not expect a large increase in flood levels at any location within
the site. In fact, as mentioned previously, flood elevations may decrease. Groundwater levels
and soil moisture are expected to increase in all areas of the site.
8.5 Future Data Collection
Additional survey data will be collected as part of the construction effort and development of
record Drawings to establish the location and elevation of various site treatments. Complete and
consistent topographic data is not available at this time and it may not be practical to obtain
additional data before construction. The size of the site combined with the dense vegetation and
limited land slopes make topographic data collection and depiction difficult. At this point, it may
be most feasible to focus only on those areas where construction will be performed.
We recommend that hydrologic data collection efforts be continued on the project site. From a
surface water hydrology perspective, future data collection for surface water and rain gauges
should continue with collection readings changed to a daily basis. Although this will not be
detailed enough for analyses similar to those included herein, it will be adequate to confirm the
function and response of the proposed modifications. Data collection frequency needs for other
purposes may be more demanding than those for surface water hydrology and the most
demanding should govern.

-51 -
t 9.0 LIMITATIONS
This report was prepared subject to acceptance of our proposal, which includes our "Standard
' Terms for Engagement." Our evaluations, conclusions, and recommendations are based on
project and site information available to us at the time of this report and may require
modification, if there are any changes in the project or site conditions, or if additional data about
' the project or site becomes available in the future. Our professional services for this project
have been performed in accordance with generally accepted engineering practices; no warranty,
expressed or implied, is made. This report is intended for use by ESI and NCDOT, on this
project. These findings are not intended or recommended to be suitable for reuse on extensions
of the project or on any other project. Reuse on extensions of the project or on any other project
' shall be done only after written verification or adaptation by EDDY ENGINEERING, P.C., for
the specific purpose intended.

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40 MINXW 11010 a ¦=IC =9 aNa-M Maq a-= Project No. 98014-2 August 2000 Figure 1
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2000 4000 6000
Volume of Rainfall (acre-ft)
Volume Comparison (with Lake Area)
1 .3
g ENVIRONMENTAL SERVICES, INC. CROATAN WETLAND
e Raleigh, North Carolina MITIGATION BANK
EDDY ENGINEERING, P.C. Craven Co., North Carolina
«samn" curta am it M1 IN" *4-M WagIN-M Project No. 98014-2
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VOLUME COMPARISON
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WITH LONG LAKE
August 2000 Figure 4

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ENVIRONMENTAL SERVICES, INC. CROATAN WETLAND
Raleigh, North Carolina MITIGATION BANK
EDDY ENGINEERING, p.C Craven Co., North Carolina
Nsa?aoan ams woaR:Ia, a»aN-ae raayaw-rM Project No. 98014-2
8000
VOLUME COMPARISON
RAINFALL VS. RUNOFF
WITHOUT LONG LAKE
August 2000 Figure 5

?1
WETLAND MONTHLY WATER BALANCE
Raleigh, North Carolina MITIGATION BANK INFLOW VS. OUTFLOW
EDDY ENGINEERING, P. CCraven Co., North Carolina
C.
40WUs„?W SM W &=KK M hhO,_= „aOROp,_,p, Project No. 98014-2 August 2000 Figure 6
Monthly Water Balance
Cumulative Flow w/ WxSys. ET
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Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb
Month
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ENVIRONMENTAL SERVICES INC CROATAN
= = = = M = = = = M iii• ? M i «iiiiiiiii• ? ?
STORM 3 ALL EVENTS HYDROGRAPH
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ENVIRONMENTAL SERVICES, INC. CROATAN WETLAND COMPARISON OF SYNTHESIZED
o Raleigh, North Carolina MITIGATION BANK HYDROGRAPH VS. OBSERVED

EDDY ENGINEERING
P C Craven Co., North Carolina HYDROGRAPH FOR STORM 3
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MR= MW me Ems ana-M w ana-M Project No. 98014-2 August 2000 Figure 7
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e TYPICAL EXISTING
Q DITCH CROSS-SECTION
g ENVIRONMENTAL SERVICES, INC.
CROATAN WETLAND TYPICAL EXISTING
Raleigh, North Carolina MITIGATION BANK
Craven County, NC DITCH CROSS-SECTION
EDDY ENGINEERING, P.C.
w ao aorta w=ic m, p„li,-m ra milou-, Project No. 98014-2 August 2000 Figure 9

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PROFILE VIEW
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ROAD REMOVAL AT
NATURAL DRAINAGE
FEATURE
ENVIRONMENTAL SERVICES, INC. CROATAN WETLAND
Raleigh, North Carolina MITIGATION BANK
EDDY ENGINEERING, P. C. Craven County, NC
wstasuc AD aorta Y1=K VW pp w-= Mpill 64- , Project No. 98014-2
ROAD REMOVAL AT
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FEATURE
August 2000 Figure 14
Zone of Compressed
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Approximate Original
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Fill Material

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Zone of Disturbed Soils
Scarification
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ROAD REMOVAL AT
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g ENVIRONMENTAL SERVICES, INC. CROATAN WETLAND ROAD REMOVAL AT
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Former Road Surface
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31
TYPICAL DITCH FILLING
AND ROAD REMOVAL
ENVIRONMENTAL SERVICES, INC. CROATAN WETLAND TYPICAL DITCH FILLING
Raleigh, North Carolina MITIGATION BANK
C.
EDDY ENGINEERING, P. C Craven County, NC AND ROAD REMOVAL
?? ? •?• w" Ic vw a"Im-m moil Im-,w Project No. 98014-2 August 2000 Figure 16
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Former Road Surface
New Road Surface
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Fill to be Removed
Drains
Zone of Compressed
and Disturbed Soils
PROFILE VIEW
LOWER ROAD ELEVATION AND
a
STABILIZE ROAD SURFACE AT
NATURAL DRAINAGE FEATURE
a
ENVIRONMENTAL SERVICES, INC. CROATAN WETLAND LOWER ROAD ELEV. AND
D
Raleigh, North Carolina MITIGATION BANK STABILIZE ROAD SURFACE AT
Craven County, NC NATURAL DRAINAGE FEATURE
EDDY ENGINEERING, P. C.
44zolowy99?ROwo SUItE209-9A1E10N,NC n61e (919)959-18M FAX(919)VA-1801 Project No. 98014-2 August 22000000 Figure 1177

SECTION VIEW
41
LOWER ROAD ELEVATION AND
STABILIZE ROAD SURFACE AT
NATURAL DRAINAGE FEATURE
ENVIRONMENTAL SERVICES, INC. CROATAN WETLAND LOWER ROAD ELEV. AND
e Raleigh, North Carolina MITIGATION BANK STABILIZE ROAD SURFACE AT
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Existing Rood Surface
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Fill Material, Ditch Plugs
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Zone of Compressed Soils
TYPICAL DITCH FILLING
AND ROAD STABILIZATION
ENVIRONMENTAL SERVICES, INC. CROATAN WETLAND TYPICAL DITCH FILLING
Raleigh, North Carolina MITIGATION BANK
EDDY ENGINEERING, P.C Craven County, NC AND ROAD STABILIZATION
.
NO Lamm IN am= W=K MG 04 a-= Man rm-M Project No. 98014-2 August 2000 Figure 19

Tie to Existing
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to Top of Bank
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PROPOSED POINT DITCH PUGS 8c
SURFACE WATER DIVERSION

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during Fin
Plan View
Tie to Existing
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Compacted Fill
Surface Water
Diversion Berm,
Orientation Varies
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to Top of Bank
Profile View
PROPOSED REACH DITCH PLUGS &
SURFACE WATER DIVERSIONS
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Existing Channel
Banks
Compacted Fill Compacted Fill

Section View
Compacted Fill
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PROPOSED TREATMENT FLOW CHART
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1A
See Figures 14 and 15
1A1. Fill in Ditches with Road
Material.
1A2. Remove Fill Material to
Allow for Natural Surface
Water Flow Along Drainage
Feature.
1A3. Scarify with Deep Rippers
in Direction Transverse to
Ditch/Road or Parallel to
Proposed Flow.
1A4. Install Surface Water
Diversions Depending on
Site Conditions.
1B
See Figure 16
1131. Fill in Ditches with Road
Material.
1 B2. Scarify with Deep Rippers
in Direction Transverse to
Ditch/Road or Parallel to
Proposed Flow.
1 B3. Install Surface Water
Diversions Depending on
Site Conditions.
Note: In Cases Where Surface
Flow is Perpendicular to
Existing Channel Flow,
Point Filling to be Used.
2A
See Figures 17 and 18
2A1. Lower Road Elevation and
Stabilize Road Surface
Course,
2A2. Fill Ditches.
2A3. Install Conveyance
Measures, Pipes, and
Fords.
2A4. Install Aggregate Subsurface
Drains.
2B
See Figure 19
2131, Stabilize Road Surface
Course and Raise Road
Elevation.
2132. Fill Ditches.

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APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
PRELIMINARY HYDROLOGIC
AND HYDRAULIC ANALYSIS
CROATAN WETLAND
MITIGATION BANK, PHASE I
CRAVEN COUNTY, NORTH CAROLINA
Prepared for:
Mr. Kevin Markham
Environmental Services, Inc.
1100 Wake Forest Road, Suite 200
Raleigh, NC 27604
December 18, 1998
Project 98014
EDDY ENGINEERING, P.C.
Post Office Box 61367
Raleigh, North Carolina 27661
[phone]
Fax [phone]
HN11?;o1"
oplii;•7'r l'.i;?C1.
CAR
04. q
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A
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APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
' APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
t EXECUTIVE SUMMARY
' This report resents the findings of our preliminary hydrologic and hydraulic analysis presents for the
Croatan Wetland Mitigation Bank (CWMB) project. The project site is located in southern
Craven County, North Carolina, adjacent to Croatan National Forest. The area being considered
for the CWMB consists of approximately 4100 acres. Our preliminary analysis and this report
' address only the first portion (Phase I) of the project site, approximately 1500 acres. Preliminary
hydrologic and hydraulic analyses were conducted to model surface water runoff potential and
estimate flows for watersheds and flow networks on and around the Phase I portion of the project
site. The ability to predict surface water runoff potential is necessary for future design of water
control and other features at this site in addition to understanding site hydrology.
Data used in the analysis came from public record documents and data collected on-site by
Environmental Services, Inc., and the North Carolina Department of Transportation (NCDOT).
Public record data included topographic maps, soils maps, stream flow records, and rainfall data.
' On-site data included stream gauge data, topographic data, and rainfall data. On-site data
collection efforts captured one significant storm, Hurricane Bonnie. Rainfall and peak water
surface elevations resulting from Bonnie were the primary means for developing a rainfall-runoff
' relationship and for the calibration of our hydrologic models. Some gaps in the data do exist, and
there are questions about the accuracy of some instruments. Another significant rainfall event is
needed to perform model calibration. As such, we recommend that data collection efforts be
' continued and extended into future portions of the site. This data is critical for the success and
economy of future water control structures and a complete understanding of site hydrology.
' Hydrologic and hydraulic analyses were conducted using the US Army Corps of Engineers Flood
Hydrograph Package (HEC-1), River Analysis System (HEC-RAS), and Flood Frequency
Analysis (HEC-FFA) computer programs and in-house spreadsheet programs. Comparisons
between the various models and predictions of flow are made.
' Conclusions are drawn regarding watershed characteristics, model validity, and the preliminary
status of the results of this study. Most significantly, we conclude that with the site in its current
condition, the percentage of rainfall that is translated to direct surface runoff is relatively small.
' Runoff potential will increase, if the existing ditch system is closed. It does not appear that Long
Lake contributed much direct surface runoff into the site for the period of data collection. This
may change with ditch closing. Lake water levels may remain higher if groundwater levels rise
on the perimeter.
Recommendations are made for future data collection. Data collection efforts should extend over
the entire future site (all Phases) rather than just the next Phase. Long-term data collection will
provide more useful data for model calibration.
1 We consider this work to be preliminary because calibration with another storm event is needed.
Additionally, complete topographic data is needed to confirm assumptions that were made
' necessary by the current lack of complete topographic data. We recommend that the models

APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
presented in this report not be used without calibration with additional data. These models
should be considered work in progress. Additional data will result in modification to the models
' or development of new models.

I

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11

ES-2
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
11

1

1-1

u

APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
TABLE OF CONTENTS
TITLE PAGE
EXECUTIVE SUMMARY
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF APPENDICES
1.0 INTRODUCTION ....................................................... 1-
1.1 Purpose .......................................................... -1-
1.2 Authorization ..................................................... - 1 -
1.3 Scope of Services .................................................. - 1 -
1.3.1 Preliminary Data Collection ................................. - 1 -
1.3.2 Project Coordination ...................................... - 2-
1.4 Project Personnel .................................................. - 2-
2.0 PROJECT AND SITE DATA ............................................. - 3 -
2.1 Project Location ................................................... - 3 -
2.2 Project Description ................................................. - 3 -
2.3 Survey Data ...................................................... .3-
2.4 Rainfall and Surface Water Gauge Data ................................ - 3 -
2.5 Site Visits and Visual Observations .................................... - 4-
2.5.1 Channels and Streams ..................................... - 5-
2.5.2 Long Lake .............................................. .6-
3.0 SYNTHETIC HYDROLOGIC MODELS ..................................... - 7-
3.1 Watershed Elements ............................................... -7-
3.2 Estimation of Runoff Potential ....................................... - 8-
3.3 Rainfall Data ..................................................... - 9-
3.4 Stage-Discharge Curve Development .................................. - 9-
3.5 Model Calibration ................................................. - 9-
3.6 Calibration Results ................................................ - 10-
4.0 SITE SPECIFIC UNIT HYDROGRAPH DEVELOPMENT ..................... - 11 -
4.1 Base Flow Separation .............................................. - 11 -
4.2 Estimation of Direct Runoff ......................................... - 11 -
4.3 Unit Hydrograph Deconvolution ..................................... - 12-
4.4 Unit Hydrograph by Least Squares ................................... - 13-
5.0 FLOOD FREQUENCY ANALYSES ....................................... - 15-
5.1 Gauged Streams .................................................. - 15-
5.2 Statistical Analysis ................................................ - 15-
5.3 Regional Discharge Curves ......................................... - 16-
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
6.0 CONCLUSIONS AND RECOMMENDATIONS ............................. - 17-
6.1 Watershed Characteristics .......................................... - 17-
6.2 Model Validity ................................................... - 18-
6.3 Preliminary Status of Results ........................................ - 18-
6.4 Future Hydrologic Data Collection ................................... - 19-
6.5 Future Topographic Data Collection .................................. - 19-
7.0 LIMITATIONS ........................................................ .21-
FIGURES
APPENDICES
ii
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
LIST OF FIGURES
1. VICINITY MAP
2. SURFACE WATER, LAKE, & RAIN GAUGE LOCATIONS
3. CHANNEL LOCATIONS & POINTS OF INTEREST
4. FLOOD FREQUENCY ANALYSIS STREAM GAUGES
LIST OF APPENDICES
A. GAUGE DATA
B. SYNTHETIC HYDROLOGIC MODELS
C. UNIT HYDROGRAPH DEVELOPMENT
D. FLOOD FREQUENCY ANALYSIS
iii
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I

I

I

APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
1.0 INTRODUCTION
This report presents our preliminary hydrologic and hydraulic analysis of Phase I of the Croatan
Wetland Mitigation Bank (CWMB) project. This section presents the purpose of these services,
authorization, scope, and project personnel.
1.1 Purpose
Preliminary hydrologic and hydraulic analyses were conducted to model surface water runoff
potential and estimate peak flows for the contributing watersheds and flow networks on and
around the Phase I portion of the project site. The ability to predict surface water runoff potential
is necessary for future design of water control and other features at this site and for a complete
understanding of site hydrology.
1.2 Authorization
These services were authorized by acceptance of Eddy Engineering, P.C. Proposal 98014-R2 ,
dated May 4, 1998, by Mr. Richard G. Harmon of Environmental Services, Inc. (ESI). ESI has
been retained by the North Carolina Department of Transportation (NCDOT) as a consultant for
restoration of wetlands for the CWMB project.
1.3 Scope of Services
Eddy Engineering, P.C., assisted ESI with the analysis of the CWMB site by providing the
following professional services:
1.3.1 Preliminary Data Collection
1. We visited the site to observe characteristics of the site and surrounding area, verify
surface, lake, and channel flow conditions, and observe both natural and man-made
water control structures.
2. We estimated from available mapping, areas contributing surface water to the site.

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3. We estimated soil, cover, and usage conditions for existing conditions.
4. We assisted in the development of a conceptual connectivity diagram of the
ditch/stream/lake network.
5. We evaluated similar regional historic rainfall and stream flow conditions for use in
hydrologic analysis.
6. Using available topographic data we developed typical cross-sections for channels
within the project site.
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' 7. A regional flood frequency analysis was performed to estimate peak flood flows for a
range of flood frequencies. We used the U.S. Army Corps of Engineers HEC-FFA
program and USGS stream flow records to perform this analysis.

8. Using the channel cross-sections, we developed a U.S. Army Corps of Engineers

t HEC-RAS computer model to estimate water surface profiles for selected channels on
the site
.
9. We developed a series of preliminary hydrologic models of the contributing
watershed using U.S. Army Corps of Engineers computer software and in-house
spreadsheet programs. The Flood Hydrograph Package (HEC-1) was used to model
runoff potential and peak flow estimates for the watershed/stream/ditch/lake network
based on the watershed soils, ground cover, and usage characteristics. In-house
spreadsheet programs were used for unit hydrograph derivation, unit hydrograph
synthesis, and runoff volume comparisons.
10. We used these hydrologic models to draw conclusions about site conditions and
¦ evaluate the need for additional field data.
1.3.2 Project Coordination
' 1. We attended three meetings at your office and one on-site meeting to discuss project
details and our analysis.
2. We prepared this report containing our analysis, conclusions, and recommendations.
t Our scope of services did not include evaluation or identification of drainage easements, local
drainage ordinances, or,other issues not specifically described above.
1.4 Project Personnel
Analyses and report preparation were performed by John L. Eddy, P.E., Project Manager, and
Patrick K. Smith, E.I., Project Engineer.

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2.0 PROJECT AND SITE DATA
This section presents our understanding of the project, as planned, and site data that was
' available to us at the time of this report.
2.1 Project Location
' The Croatan Wetlands Mitigation Bank (CWMB) project site is located in southern Craven
' County, NC, adjacent to the Croatan National Forest. The project location is shown in Figure 1.
The site appears on the USGS 7.5-minute series quadrangle maps, "Catfish Lake," "Hadnot
Creek," "Havelock," and "Masontown," portions of which were used in the preparation of this
' report. The project site is bounded roughly by Secondary Road 1100 (Catfish Lake Road) in the
northwest, State Highway 70 in the east, and Great Lake and Little Lake in the south. The
project site drains generally into East Prong Brice Creek in the north. Approximate project site
' boundaries are presented on Figure 2.
2.2 Project Description
t In an effort to provide wetland mitigation options for highway related impacts in southeastern,
North Carolina, the North Carolina Department of Transportation (NCDOT) is considering the
' development of a wetlands mitigation bank. The parcel will serve to mitigate wetlands related to
the Havelock Bypass, as well as other future NCDOT projects. The area being considered for the
CWMB consists of approximately 4100 acres, which has been divided into three parcels or
"phases." Phases I, II and III consist of areas of 1500, 1600 and 1000 acres respectively. Our
analysis and this report address only the first parcel (Phase I) of the project site.
1 2.3 Survey Data
Environmental Services, Inc. (ESI) provided us with aerial topographic mapping data prepared by
t NCDOT, supplemented with ground survey data, on-site surface water gauge locations and data,
and rainfall gauge data. A finished topographic map was not available prior to completion of this
report. In some cases, significant differences exist between aerial topographic and ground survey
data. Cross-section data used in our analysis was based on provided ground survey data.
Watershed delineation was performed using data from the USGS 7.5-minute quadrangle maps,
' including "Catfish Lake," "Hadnot Creek," "Havelock," and "Masontown," and site
observations.
2.4 Rainfall and Surface Water Gauge Data
ESI provided us with rainfall and surface water gauge data collected on the project site between
the period June 2, 1998 through November 11, 1998. Gauge readings were taken on an hourly
basis from a single rain gauge (RN) and a series of seven surface water gauges, including six
channel surface water gauges (SG) and one lake level gauge (LG) positioned at various locations
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on the project site. Approximate locations of the rain gauge, and surface water gauges are shown
in Figure 2.
' In select instances, surface water and rainfall data were unavailable due to gauge malfunction or
limitations in gauge data memory. In particular, SG 1 provided inconsistent data collection. The
on-site rainfall gauge (RN) ceased data collection during Hurricane Bonnie, subsequently,
' portions of the storm event went unrecorded. Rainfall data may not be representative of actual
rainfall because of gauge configuration. Rainfall was collected through a PVC pipe and
channeled to a storage chamber where water levels were recorded. The collection entrance of the
' PVC pipe had a blunt end rather than one sharpened to an edge at the inside diameter. Even
though extensive calibration efforts were made to correlate the rainfall entering the pipe with
t storage chamber water level, the blunt pipe end has an unknown effect on the accuracy of the
data. Based on the configuration of the pipe end, all rainfall entering the gauge was assumed to
be applied over an area calculated from the outside diameter of the collection pipe.
' ESI additionally provided supplemental daily rainfall data from the Weather Service of the U.S.
Marine Corps Air Station at Cherry Point, NC, for the months of August and September 1998.
We obtained additional daily data from the State Climate Office of North Carolina at North
Carolina State University. Additional sites included Trenton, New Bern Airport, and Morehead
City. Only the Trenton data was complete during Bonnie. Unlike the hourly rainfall data
' provided by the on-site rain gauge, the rainfall data from other stations was collected on a daily
interval often ending at different times.
' Graphical representation of measured data from both rain gauges (on-site and Cherry Point) and
all surface water gauges, including the lake level gauge, during the period August 20, 1998
through November 11, 1998 is presented in Appendix A. Total rainfall for Bonnie from the on-
site gauge was about 16.7 inches before gauge malfunction. Trenton received only 6.4 inches.
However, Morehead City recorded 10.5 inches after an initial unavailable reading. It therefore
seems possible, but unlikely, that the on-site gauge accurately recorded actual rainfall. Because
' Hurricane Bonnie represented the only significant storm event collected and no better data source
is available, the on-site rain gauge data were used in our analysis and the preparation of this
' report.
2.5 Site Visits and Visual Observations
' We visited the project site on May 19, 1998, and again on November 11, 1998, to observe
characteristics of the site and surrounding area, verify surface, lake, and channel flow conditions,
and observe both natural and man-made water control structures. For ease of explanation,
channel sections discussed in this section have been identified as Channels (CH) 1 through 12.
Locations of these channels are depicted in Figure 3. Select Points of Interest (POI) have also
been identified in the same figure.

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2.5.1 Channels and Streams
During our first visit, channels within the project site showed limited evidence of flow.
Portions of some channels appeared stagnant, and as such, no discernible direction of
flow was observed. The outlet control device along the northeast side of Long Lake
(POI 1) appeared to be in poor, if not unusable, condition, and for that reason, operation
of the device was not attempted. The corrugated metal outlet barrel was in poor
condition, with severe corrosion noticeable on exposed portions. No evidence of
overtopping or surface flow was observed in the vicinity of the outlet control structure.
During our second visit, after Hurricane Bonnie, strong evidence of previous flow was
observed along CH 3 flowing from the intersection with CH 1, CH 2, and CH 7 southeast
toward the intersection with CH 4. Similar evidence of flow was noted in CH 4 and CH 5
as they flow northeast from their intersections with CH 3 and CH 6, to their intersections
with CH 6, and CH 9 and CH 12 respectively. This evidence included consistent
orientation of vegetation and debris in the downstream direction and debris collection
upstream of obstructions. Such evidence may indicate increased velocities along these
portions of the channel. Some evidence of flow was also observed in CH 9; however, this
was primarily along its northern section in the vicinity of the intersection with CH 5 and
CH 12.
Along other channels within the site network (CH 1, CH 6, CH 7, CH 8, CH 10, and CH
11), little or no evidence of previous flow was observed. Although surface water gauges
(SG's 5 and 6) clearly indicate increases in water depth at these locations, the lack of
physical flow indicators suggest that relatively low flow velocities occurred in these
portions of the site channel network.
Due to apparent, operational difficulties with SG 1, it is impossible to draw conclusions
about water levels within CH 2 (downstream from the outlet device on Long Lake).
However, since evidence of only minor flows were observed, it does not appear to
perform as a significant conduit in transporting discharges from Long Lake.
Some spring flow was observed at two points along the western bank of CH 3 and eastern
bank of CH 4 in the vicinity of SG 2. This flow may be subsurface water seepage coming
from Long Lake; however, without evaluation of groundwater conditions in the area, the
source cannot be confirmed.
Based on observed evidence of flow, we have concluded that the dominant channel reach
within the site channel network is along CH 3, CH 4, and CH 5. For this reason, our
channel and contributing watershed modeling efforts were focused on this portion of the
site. Additionally, the observation of relic stream channels, combined with limited
changes in relief and localized depressions, indicate large potential storage capacity of
select portions of the site.
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2.5.2 Long Lake
We also conducted a reconnaissance of the shoreline of Long Lake by boat with emphasis
in the vicinity of the project site. Shoreline observations revealed a fairly consistent sand
rim or bank elevation around the lake perimeter. Low points were observed in the rim in
several locations along the northeast portion of the lake. These locations revealed
evidence of water flow indicating potential lake discharge locations. Due to thick
vegetative cover, weather and associated water conditions, locations and dimensions of
all flow locations were not obtained. No location was identified as a major contributor to
surface water flow within the project site.
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3.0 SYNTHETIC HYDROLOGIC MODELS
A synthetic hydrologic analysis of the contributing watershed was attempted, including
delineation of the watershed, soil and soil conditions, land use, stream channel and cross-section
location and size, rainfall depth-duration-frequency, stage-storage and stage-discharge, to
develop computer models of the watershed using the U.S. Army Corps of Engineers HEC-1
' computer program. Based on observations made during site visits, we have concluded that the
dominant channel reach within the site channel network is along CH 3, CH 4, and CH 5. For this
reason our channel and contributing watershed modeling efforts were focused on this portion of
the site. All analyses centered on the watersheds which contribute flow to the channel reach
(CH 3, CH 4, and CH 5) which terminates in the vicinity of Stream Gauge 4.
' 3.1 Watershed Elements
' The watershed was analyzed as a distributed element model with two separate model element
configurations. Drainage catchments were initially delineated using topographic data from USGS
maps. This delineation was then confirmed by visual observation during site visits. Aerial
topographic survey data was not used in watershed delineation, because of the discrepancies
between it and field survey data and because coverage was not complete. Watershed delineation
is difficult on this site because elevations vary as little as five feet over horizontal distances of up
t to 10,000 feet. As such, minor differences in elevation could result in significant differences in
watershed area.
' The first model divided the watershed into several sub-watersheds considered to be
hydrologically similar. In some cases, divisions were made based on stream gauge locations.
Channel and reservoir elements were included to connect the catchment elements and route their
' outflow to the appropriate points downstream. The terminal point was the downstream end of
CH 5 in the vicinity of SG 4. Sub-watersheds (or catchments) ranged in size from 0.17 to 1.87
square miles. Individual sections of stream channel ranged between approximately 3,000 and
r 8,000 feet in length with average longitudinal slopes of approximately 0.00055 ft./ft. This model
included the surface area of Long Lake as a catchment and the area immediately surrounding the
' lake as a separate catchment. The surface area of Long Lake is approximately 1,200 acres (1.87
square miles). The area surrounding Long Lake, which could contribute runoff to the lake, is
estimated at approximately 885 acres (1.38 square miles). The model indicated significant
' storage capacity within Long Lake. Although the contribution from these two catchments is
significant in terms of potential runoff, the majority of the runoff is likely stored, thus attenuating
peak runoff to the downstream channel network. The lake itself was modeled as a reservoir, with
' an assumed normal water surface elevation at 38.0 feet. This assumption was generally
confirmed by readings from the lake gauge. A weir outlet for the reservoir was incorporated into
the model using an approximate weir length of 100 feet at an estimated elevation of 38.5 feet. A
' weir coefficient, C,, of 2.63 was used, which is a typical value for broad-crested weirs.

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1 a
' In the second model, the watershed was analyzed as one catchment for the entire drainage area
contributing flow to the outlet of Channel 5 in the vicinity of SG 4. The combined watershed
size was estimated at 2.48 square miles. In this model, however, the contribution of runoff from
' Long Lake and the immediately surrounding area were discounted based on results of the first
model and site observations. Because of the difficulty encountered in model calibration, the
second, less complex, distributed element model was used for all calibration and modeling.
' 3.2 Estimation of Runoff Potential
t Soils, cover, and land use data were used to assign Soil Conservation Service (SCS) runoff Curve
Numbers (CN) to the various watershed elements. Soils data were obtained from the 1989 U.S.
t Department of Agriculture Soils Survey for Craven County, NC, as provided by ESI. Cover and
land use data were taken from USGS topographic maps, vegetation maps provided by ESI
(compiled from 1997 NCDOT Photogrammetry), and visual observation from site visits.
' The SCS CN is used to account for initial abstractions and infiltration when converting rainfall to
direct runoff. The higher the curve number, the larger the portion of rainfall that appears as
' direct runoff. Direct runoff is considered to be that portion of rainfall that flows overland to a
watershed outlet. It does not include stored surface water or infiltrated surface water, even
though these may reach the point of watershed discharge as later base flow. Antecedent Moisture
' Condition (AMC) I (dry soil conditions) was selected because there was no rainfall in the five
days prior to Hurricane Bonnie, the storm used in model calibration. Soils within the project site
are distributed among all four Hydrologic Soils Groups (HSG), A, B, C, and D. Initial evaluation
of site soils and coverage yielded a weighted SCS CN of 68 for the sub-watersheds in and around
the site, when AMC I is assumed. This CN is indicative of a relatively low, direct runoff
potential. Normally, AMC II (normal conditions) and the resulting higher CN of 84 would be
' used for design.
It is important to note that many of the soils within the contributing watersheds may be
categorized into two Hydrologic Soil Groups, depending on drainage conditions. The HSG's of
D, A/D and B/D are the most prevalent on the site, with paired groups representing "drained" /
' "undrained" conditions. Because the site is drained by ditching and conditions preceding our
calibration storm were relatively dry, the argument could be made that the use of HSG's with the
higher infiltration capacity, or the "drained" condition may be appropriate for calibration. The
use of these soil groups would result in significantly lower Curve Numbers of 47 and 68 for
AMC I and AMC II, respectively. Lower Curve Numbers such as these would most certainly
lower predicted peak runoff values; however, experience has shown that relatively low Curves
Numbers may not be appropriate for hydrologic design. For future site evaluations, the
"undrained" Curve Numbers would be most appropriate.

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3.3 Rainfall Data
Rainfall data used during modeling were obtained from gauge data collected on-site. Predicted
' precipitation for the 2-, 10- and 100-year design storms were developed from Depth Duration
Frequency (DDF) values. DDF values were taken from the National Oceanic and Atmospheric
Administration (NOAA) HYDRO-35 and the National Weather Service (NWS) TP-40 reports.
' 3.4 Stage-Discharge Curve Development
' A preliminary HEC-RAS model was constructed to obtain a preliminary estimate of water
surface elevations within selected existing channels for a range of discharge values. Channel
' cross-section data used in our analysis were based on the provided ground survey data.
Manning's n-values for channel and overbank flow were selected based on site observations,
standard references, and experience.
From this water surface profile model, stage-discharge curves were developed to estimate
channel discharge at select cross-sections for comparison with surface water gauge data. A
Stage-Discharge Curve for the channel in the vicinity of SG 4 is presented in Appendix C. The
Stage-Discharge Curve was then used to estimate discharge values from measured channel water
surface elevations (from surface water gauges).
3.5 Model Calibration
' As discussed above, the simplified distributed element model was used in an attempt to model
the watershed contributing flow to the outlet of CH 5 in the vicinity of SG 4. This model was
recreated for two different types of dimensionless unit hydrographs. The dimensionless unit
' hydrographs used in our HEC-1 models were the Snyder Unit Graph method and the Clark Unit
Graph method. In each, case trial solutions were conducted to find unit hydrograph parameters
which would produce an outflow hydrograph similar to outflow data measured in surface water
gauges. Six-hour time increments were used for HEC-1 models.
The first type of HEC-1 input file was constructed to model the watershed based on Snyder's Unit
Graph method. Snyder's unit hydrograph is based on two key parameters/coefficients: the
standard basin lag coefficient, tp, and the storage coefficient, Cp. The Snyder method relates
basin lag time and peak watershed discharge to various physiographic watershed characteristics
to include basin slopes, basin storage, main channel length, and the location of the basin centroid
with respect to the main channel. The model was developed to analyze watershed runoff for
' existing conditions. These models required input of rainfall data, channel length, slope and
cross-section data, Snyder parameters, as well as sub-basin drainage areas and SCS curve
numbers. Snyder parameters were varied within the model ranging from 4.69 to 6.31 for tp, and
' 0.4 to 0.8 for Cp, based on experience with watersheds in eastern North Carolina and Virginia.
Rainfall data from Hurricane Bonnie were used to allow for the comparison of model runoff
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estimates with actual runoff values estimated from channel water surface elevations (from
surface water gauges).
The second type of HEC-1 input file was constructed to model the watershed based on Clark's
unit hydrograph method. Clark's unit hydrograph is based on two key parameters: the time of
concentration, T, and the storage coefficient, R. The Clark time of concentration is the time
' from the end of a rainfall segment to the inflection point on the receding or downward portion of
the outflow hydrograph. An R value can be estimated by taking the discharge at the same
' inflection point and dividing it by the slope of the curve at that point. Clark parameters were
varied within the model ranging from 60 to 70 hours for T, and 45 to 60 for R. These results
were also compared with actual runoff values obtained from surface water gauges.
1 3.6 Calibration Results
Using the dimensionless unit hydrograph models described above, we were not able to develop a
model that could reasonably reproduce the observed hydrograph at SG 4. Peak flow values were
generally higher than observed with peaks occurring later in time than observed. Possible
' reasons for this may include a nonlinear runoff response, large watershed storage capacity,
increased infiltration capacity, inaccurate rainfall data, incorrect watershed delineation, and/or
incorrect drainage connectivity.
The effect of storage in particular, may not be linear, especially for the dryer periods, thus
making unit hydrograph models potentially inappropriate. Continuous simulation models may be
' more appropriate, but calibration will require substantially more data. Conclusions about
modeling efforts are discussed in detail later in this report.

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t 4.0 SITE SPECIFIC UNIT HYDROGRAPH DEVELOPMENT
Upon finding that the synthetic hydrologic models using standard dimensionless unit
hydrographs did not agree with gauge data, we attempted to develop a site specific unit
hydrograph. As with the previous analysis, outflow at the point of interest was computed from
the stage discharge relationship estimated with HEC-RAS. In this case a three-hour duration was
used, whereas six-hour unit dimensionless hydrographs were used in the preceding calibration
attempt. More detailed information about our site specific unit hydrograph development attempts
can be found in Appendix C.
4.1 Base Flow Separation
' Development of a direct runoff unit hydrograph from stream gauge data required that base flow
be separated from the direct runoff hydrograph. Several methods for base flow separation are
' available. We chose the straight line method, which assumes a linear return of base flow. A
linear interpolation was used for points between the first rise and the inflection point of the
hydrograph. Subtraction of the base flow ordinates from the observed hydrograph ordinates
' yielded the direct runoff hydrograph used in development of the site specific unit hydrograph.
The observed hydrograph, base flow hydrograph, and resulting direct runoff hydrograph are
graphically depicted on Page 7 of Appendix C.
4.2 Estimation of Direct Runoff
' Derivation of a site specific unit hydrograph requires an estimate of direct runoff for the subject
watershed so that the unit hydrograph can be normalized to one unit of runoff. We attempted to
estimate the direct runoff by two independent means. First, having developed a SCS CN
representative of the soils and land use in the watershed, direct runoff was computed based on the
CN. For the portion of Bonnie captured by the on-site rain gauge (about 16.7 inches), and a SCS
CN of 68, about 12.1 inches of direct runoff would be expected. For an SCS CN of 47, about 8.1
inches of direct runoff would be expected. Again, we have concern that the rainfall amounts
recorded by this gauge are greater than the actual rainfall, but no other suitable rainfall data was
available.
The second method used to estimate direct runoff was numerical integration of the discharge
' hydrograph. Assuming that the watershed area contributing direct runoff to SG 4 is about 1600
acres, the direct runoff from the first peak of the hydrograph after separation of base flow is
estimated to be about 2.1 inches.
The discrepancy between the direct runoff computed by the two methods above can be attributed
to one or more of the following possibilities:
' 1. There is much greater storage within the watershed than assumed in the SCS CN
Method.

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2. Infiltration capacity is greater than assumed in the SCS CN Method.
3. The rain gauge data is incorrect.
4. The rain gauge data is not representative of the entire watershed.
5. The estimated drainage area of the watershed is greater than the actual area, due to
errors in connectivity and/or delineation from available topographic data.
6. The stage-discharge curve used to convert stream gauge stage readings to discharge is
incorrect due to data limitations"or errors, or due to channel roughness assumptions.
' Without more data to confirm or revise direct runoff, we opted to use the direct runoff computed
by numerical integration for derivation of site specific unit hydrographs.
' 4.3 Unit Hydrograph Deconvolution
A unit hydrograph is a simple linear model which can be used to estimate excess rainfall runoff.
' The unit hydrograph for a given watershed represents the direct runoff of some unit (typically
taken as one inch) of excess rainfall uniformly distributed over the entire watershed at a constant
rate for some duration of time. Once a unit hydrograph has been developed for a given
watershed, we can then use this unit hydrograph to synthesize the direct runoff hydrograph for
any excess rainfall upon that watershed. The use of a unit hydrograph for synthesizing
subsequent runoff hydrographs assumes that all storm segments have the same duration, the
distribution of rainfall and its losses are uniform over the entire watershed, and only the total
amount of runoff varies from segment to segment.
Unit hydrograph deconvolution is the inverse operation of unit hydrograph synthesis, also known
as convolution. To understand deconvolution, synthesis must be understood. To develop or
synthesize a direct runoff hydrograph from a unit hydrograph and excess rainfall, each excess
rainfall segment is multiplied by each ordinate of the unit hydrograph. A time delay, equal to the
duration of the excess rainfall segment (also called the unit hydrograph duration) multiplied by
the number of prior excess rainfall segments, is applied to each set of products. The resulting
t sets of products, so delayed, are summed by time increment. The resulting time series of sums is
the direct runoff hydrograph. To obtain the complete hydrograph, base flow must be added to the
direct runoff hydrograph. Unit hydrograph synthesis is the method HEC-1 uses to develop runoff
hydrographs from excess rainfall and unit hydrographs. In the case of our previously described
HEC-1 models, the unit hydrographs are standard dimensionless unit hydrographs that have
specific shape assumptions. The difference here is that by deconvolution, we are attempting to
find what the unit hydrograph must be in order to obtain the observed direct runoff hydrograph.
' Therefore, deconvolution is the separation of the observed direct runoff hydrograph into unit
pulses. Deconvolution can be accomplished by writing equations for convolution, then isolating
unknown variables and successively solving for them. This works because there are more
' equations than unknowns.

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' We accomplished deconvolution for the direct runoff hydrograph at SG 4 by use of a spreadsheet
program developed by us for this purpose. Optimization could be performed within HEC-1, but
we chose to use the spreadsheet. The less complex data manipulation within the spreadsheet
facilitated more trial solutions and graphing. Printouts of the spreadsheet program tabulations
with results is presented in Appendix C. Because the resulting unit hydrograph exhibited erratic
variations and negative ordinates on the first trial, we forced all ordinates to be positive. The
resulting unit hydrograph is presented in tabular form on Page 9 of Appendix C.
Upon deconvolution of the unit hydrograph, we performed a unit hydrograph synthesis on the
' same excess rainfall to check the validity of the unit hydrograph. A spreadsheet program was
developed to perform unit hydrograph synthesis. As expected, this synthesis produced a direct
' runoff hydrograph with a peak much greater than the observed hydrograph. Forcing all the
ordinates to be positive increased the area under the unit hydrograph and made it no longer equal
to one inch of runoff.
We then performed another deconvolution, this time allowing negative ordinates. A printout of
the spreadsheet program tabulation with results is presented on Page 10 of Appendix C. As can
be seen on the synthesized direct runoff hydrographs presented on Pages 13 and 14 of Appendix
C, this unit hydrograph reproduces the direct runoff hydrograph very well. Due to the erratic
nature of the unit hydrograph, it is unlikely that it would be applicable to other storm events.
' However, the fact that the hydrograph is reproduced does validate the spreadsheet programs,
which are independent except for rainfall data.
4.4 Unit Hydrograph by Least Squares
Derivation of a unit hydrograph that would be less erratic, but still closely reproduce the direct
runoff hydrograph, was attempted by a least squares approach. We used the same spreadsheet
synthesis program used to validate the deconvolution unit hydrographs. Squares of the difference
between the computed (synthesized) direct runoff hydrograph and the observed direct runoff
hydrograph were summed. An internal optimization routine within the spreadsheet program was
used to find a minimum of the sum of the squared differences by varying the unit hydrograph
ordinates. This least squares approach yields a best fit between the observed and computed direct
runoff hydrographs. A printout of the spreadsheet program tabulation with results is presented
on Page 11 of Appendix C. The least squares synthesized runoff hydrograph is presented on
Page 12 of Appendix C. The least squares unit hydrograph is less erratic than the deconvolution
unit hydrograph.
' As can be seen in the synthesized direct runoff hydrograph presented on Page 12 of Appendix C,
the least squares unit hydrograph does reproduce the direct runoff hydrograph better than those
obtained by deconvolution. Had we allowed negative unit hydrograph ordinates in either the
' deconvolution or least squares approach, a closer approximation of the direct runoff hydrograph
would be obtained; however, the unit hydrographs may not be compatible with some hydrologic
programs. The least squares unit hydrograph forced to positive ordinate does not have the same
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problem with volume that was observed with the deconvolution, positive ordinate only unit
hydrograph.
' Observation of the direct runoff hydrograph synthesized by the least squares unit hydrograph
reveals a prominent peak following the initial peak (see Page 12, Appendix Q. Upon tracing the
source of this peak back through the various operations, we see that it is a direct result of a later
rainfall spike in the rainfall hyetograph (see Page 3, Appendix C). Whether this second peak is a
local or gauge anomaly is not clear. What is clear, however, is that the observed hydrograph
shows no evidence of a second peak corresponding to the time of the second rainfall peak. This
r can be clearly seen by observation of peak times for graphs on Pages 3 and 6 of Appendix C.
The hydrograph predicted by our least squares unit hydrograph clearly shows that the second
peak at around reading 180 should be higher than the first peak at around reading 160 (see Page
12, Appendix C). The second peak would be higher, because the rainfall is nearly the same and
the hydrograph from the first rainfall peak has not yet diminished back to base flow. In any
event, the rainfall captured at the on-site rain gauge is clearly not representative of the entire
watershed, even if the recorded values are accurate for the gauge location.
' Assuming that the second rainfall peak around reading 180 is spurious, we cropped the rainfall
hyetograph with a linear decay eliminating the second peak. While realizing that this is
speculative data manipulation, it does serve to demonstrate a point. With the altered hyetograph,
' we were able to produce a least squares unit hydrograph that was not erratic and could reproduce
the observed hydrograph fairly well (see Pages 15 and 16 of Appendix Q. Another significant
rainfall event is still needed for further calibration and validation.

APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I

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-15-
5.0 FLOOD FREQUENCY ANALYSES
Because long-term stream/channel flow data for the project site is not available, a regional flood
frequency analysis was conducted. This analysis was conducted to develop an independent
estimate of peak flows for 2-, 10- , and 100-year flood flows in and around the project site. A
summary of these analyses is presented in this section.
5.1 Gauged Streams
We gathered stream flow and watershed area data for the nine streams and watersheds in
southeastern North Carolina listed below. The locations of these stream gauges are shown in
Figure 4.
1. Browns Creek near Elizabethtown
2. Big Swamp near Roseboro
3. Buckhead Creek near Owens
4. Hood Creek near Leland
5. Mill Branch near Tabor City
6. Reese Creek near Fayetteville
7. Tenmile Swamp near Lumberton
8. Turnbull Creek near Elizabethtown
9. Wet Ash Swamp near Ash
The above watersheds range in size from 2.62 to 60.10 square miles with a median of 16.00
square miles and a mean of 19.38 square miles. Project site watersheds analyzed as part of this
project range from 0.08 to 5.65 square miles in area. The selected gauged watersheds are
considered conservative for use in predicting flood flows in and around the project site because
the gauged watersheds are similar in terms of location, size, slope, land use, and geology to the
project site watersheds, but exhibit somewhat less storage and slope.
5.2 Statistical Analysis
The U.S. Army Corps of Engineers Flood Frequency Analysis (HEC-FFA) program was used to
statistically determine the peak discharges of the selected watersheds for several return periods of
interest. This program uses the Log Pearson III distribution with regional skew correction to
estimate flood flows for a range of exceedence probabilities. Each set of stream gauge data was
entered into the HEC-FFA program with the same regional skew coefficient of 0.35. The
regional skew coefficient varies significantly over this area of North Carolina and would be
different for each of the watersheds. However, for this analysis, the regional skew coefficient
was selected based on the location of the project site.
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
' -16-
5.3 Regional Discharge Curves
From the peak discharges estimated for each gauged watershed by the HEC-FFA program, we
determined the specific discharge in cubic feet per second per square mile (cfs/mil) for each
return period of interest. These were plotted against watershed area on a log-log scale, with a
separate plot for each return period. A regional curve was established that can be used to
' interpolate and extrapolate the specific discharge of watersheds within and around the project site
assuming that direct runoff from Long Lake does not occur. Future conditions and low frequency
storm events with high antecedent rainfall could result in higher flows than would be predicted
' by the regional curves. Accurate watershed delineation will be critical to appropriate use of these
curves. The regional discharge curves are presented in Appendix D.

I

APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
I APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
-17-
6.0 CONCLUSIONS AND RECOMMENDATIONS
Based on our analyses, we have drawn conclusions about certain aspects of the hydrology of the
' project site. Specific areas of concern are addressed, and recommendations for site data
collection modifications and monitoring are presented.
6.1 Watershed Characteristics
Generally, watersheds similar to those found on this site will exhibit significant storage effect
' along channels and in depressions depending on recent rainfall history and available storage. For
a significant portion of the year, depressions may be partially filled with water before the onset of
a rainfall event, resulting in more of the rainfall being translated to direct runoff than at drier
times. However, at the other extreme, during periods of little to no rainfall in the recent past, as
was the case prior to the onset of Hurricane Bonnie, storage and infiltration have the potential to
' play a large role in attenuation of discharge peaks and total direct runoff volume. Therefore, we
can conclude that the runoff potential from watersheds at this site are very dependent on
antecedent soil conditions, recency of rainfall, and groundwater conditions. The effect of storage
' in particular may not be linear, especially for the drier periods, thus making unit hydrograph
models potentially inappropriate. Continuous simulation models may be more appropriate, but
calibration will require substantially more data that may not become available during the
conceptual design period.
We can also conclude that with the site in its current condition, the percentage of rainfall that is
' translated to direct surface runoff is relatively small. A larger percentage of rainfall than would
be predicted by typical hydrologic models will be lost to evaporation, evapotranspiration,
depression storage, or infiltration, with the site in its existing condition. Some of the infiltrated
t rainfall may also eventually reach the ditch system as base flow, depending on hydrogeologic
conditions. After closing of the ditches that drain the site, runoff potential will likely increase
with a much higher percentage of rainfall being translated into direct runoff. The percentage
could increase to approach values predicted by typical hydrologic models. As such, there would
be a significant increase in the frequency and severity of surface water flows.
t Modeling results, site observations, and water balance evaluations indicate that Long Lake did
not contribute significant direct surface water runoff into the site during or after Hurricane
Bonnie. Significant direct surface water discharge appears possible, if the lake level is relatively
high at the beginning of a storm event. Such discharge may become more frequent if
groundwater levels are increased and lake levels generally remain higher because of reduced
' groundwater gradients in the vicinity of the lake. Therefore, closing of the ditches could
effectively increase the watershed area contributing direct surface runoff draining through the
site. This too would significantly increase the potential severity and possibly the frequency of
surface water flows.

APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
I APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
-18-
' 6.2 Model Validity
Based on the comparison of results from our synthetic hydrologic and storage HEC-1 model
' hydrographs with the outflow hydrographs developed from surface water gauges, a significant
quantity of precipitation appears to be removed from direct runoff into infiltration and basin
storage. A portion of the infiltrated and stored water returns as base flow into the ditch system
with the remainder lost to evaporation, evapotranspiration, and groundwater recharge.
Our HEC-1 model results, when compared to the outflow hydrographs developed from surface
' water gauges, lead us to conclude that storage effects within these models do not adequately
represent the storage and infiltration losses experienced within the watershed during Hurricane
Bonnie. Peak flow estimates from the flood frequency analysis curves also do not accurately
represent probable peak flows for this site because of the storage and infiltration losses
experienced within the watershed.
On-site data collection efforts only captured one significant storm, Hurricane Bonnie. Rainfall
and peak water surface elevations resulting from Bonnie were the primary means for developing
' a rainfall-runoff relationship and for the calibration of our modeling technique. Another
significant rainfall event is needed to independently confirm model calibration.
' There are inconsistencies in the rainfall data when compared to the daily rainfall totals of nearby
gauge locations, and other on-site indicators. The on-site rain gauge recorded more than twice
the rainfall that was recorded at nearby gauge locations, even though the on-site rain gauge
ceased data collection before the end of rainfall recorded other sites. Also, the level of Long
Lake did not rise as high as would be expected from the rain gauge data. The surface of the lake
would be expected to rise nearly the same as the total rainfall over a short duration. This would
hold true even considering possible seepage losses and direct surface water discharge of
relatively large magnitude. If there are areas contributing runoff to the lake, the water surface
would be expected to rise to a height greater than the rainfall.
Estimates of stage-discharge relationships were made using the HEC-RAS model, estimated
' channel roughness, and available channel cross-section data. Velocity measurements during a
significant storm event could be used to calibrate channel roughness. The need for topographic
data is discussed in a later section of this report.
' 6.3 Preliminary Status of Results
' We consider this work to be preliminary because confirmation of model calibration by another
storm event is needed. Models only apply to SG 4. We recommend that the hydrologic models
presented in this report not be used without such calibration with additional data. These models
should be considered work in progress. The additional data will result in modification to the
models.

APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I

1

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APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
-19-
The flood frequency analysis results could be used to conservatively estimate peak flows for
design of water control structures at this site assuming that direct runoff from Long Lake does
not occur. It would be necessary to have accurate estimates of watershed area for the regional
curves to be of use. The main limitation currently is the lack of complete and consistent
topographic data for watershed delineation. It should be noted that if groundwater levels rise,
more direct surface water runoff can be expected, and the curves may become somewhat less
conservative. Also, as noted above, increased groundwater levels could effectively increase the
watershed area contributing direct runoff to the site by increasing lake levels. If surface runoff
from Long Lake occurs, the flood frequency regional curves would not be appropriate for
estimating peak flows.
6.4 Future Hydrologic Data Collection
We recommend that data collection efforts be continued on this Phase and extended into future
Phases of the site. The purposes would be to confirm or improve existing models, and to
calibrate and confirm future model extensions or entirely new models.
To extend the rainfall and stream flow gauging, additional stream flow recording gauges and at
least two rainfall recording gauges are needed. We recommend using a recording rain gauge
similar to those used by the National Weather Service. A larger opening and sharp opening edge
are critical to accurate measurements. Velocity measurements during a flood event would also
be helpful in calibrating the stage discharge relations at gauge locations. Additionally,
temperature and pan evaporation data would be useful for continuous simulation modeling.
Base flow measurement should be made with the aid of a V-notch weir below SG 4 and by
detailed cross-section at the Catfish Road Bridge. A stream gauge should be placed immediately
upstream of each of these locations. The weir below SG 4 should be placed so as to not
influence readings at SG 4.
Future data collection should continue with collection readings on an hourly basis. Although
more time consuming during the collection phase, continuous hourly monitoring of channel
water gauges and rainfall gauges allow for a more thorough evaluation of project site hydrology.
6.5 Future Topographic Data Collection
Complete and consistent topographic data is needed to confirm assumptions that were made
necessary by the current lack of complete and consistent topographic data. Such topographic
data will also be necessary to determine the extent of areas influenced by future site
modifications for wetland restoration, or enhancement. While aerial topographic mapping is
suitable on many sites for large scale site mapping efforts, aerial topographic survey methods
may not be suitable for use for these specific purposes on this site.
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I

-20-
As discussed earlier, significant differences exist between aerial topographic and ground survey
data provided to us. On a site where elevations vary as little as five feet over a distances of up to
10,000 feet, minor inaccuracies in elevation may potentially result in significant differences in
' watershed delineation or the estimated influence of future site modifications. Often there will be
a significant difference between the aerial photo topography, which must focus on the visible
surface, usually vegetated, and that based on ground survey shots, which can penetrate the
' vegetation. A difference of one-tenth of one foot could be significant on this site.
Typical cross-section data used was developed from field survey data provided to us. Most
' cross-sections consisted of a series of eight to ten survey points, of which, four, at most, defined
the channel cross-section. Without refined cross-section data, at least at key locations along
' select channels, flow estimates will be made from rough trapezoidal sections, and could vary
significantly from actual flow values. Additional data collection along portions of the perimeter
of Long Lake will also be required to confirm assumptions made about lake discharge.
I

11

t APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I

-21-
7.0 LIMITATIONS
This report was prepared subject to acceptance of our proposal, which includes our "Standard
' Terms for Engagement." Our evaluations, conclusions, and recommendations are based on
project and site information available to us at the time of this report and may require
modification, if there are any changes in the project or site conditions, or if additional data about
' the project or site becomes available in the future. Our professional services for this project
have been performed in accordance with generally accepted engineering practices; no warranty,
' expressed or implied, is made. This report is intended for use by ESI and NCDOT, on this
project. These findings are not intended or recommended to be suitable for reuse on extensions
of the projector on any other project. Reuse on extensions of the project. or on any other project
shall be done only after written verification or adaptation by EDDY ENGINEERING, P.C., for
the specific purpose intended.
I

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APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
EDDY ENGINEERING, P.C.
FIGURES
Post Office Box 61367 [phone]
Raleigh, North Carolina 27661 Fax [phone]
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
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Raleigh, North Carolina Mitigation Bank STREAM GAUGES
EDDY ENGINEERING, P.C. Craven County, NC
P.1 a axe N= K = (eq 51e-10 M (so) 511-IM Project No. 98014 December 1998 Figure 4
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
' APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
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Project No. 98014
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December 1998 Figure 3
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I APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
' APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
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SURFACE WATER, LAKE, &
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December 1998 Figure 2
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PA IX 93R ounce, W UN pnq 511-IM M (NO III-IM Project No. 98014 December 1998 Figure 1
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
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APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
EDDY ENGINEERING, P.C.
APPENDIX A
GAUGE DATA
Post Office Box 61367
Raleigh, North Carolina 27661
[phone]
Fax [phone]
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I

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APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
CROATAN WETLANDS MITIGATION BANK
Project 98014
HEC-1 Model, First Model
Typical Example, using Snyder's Unit Hydrograph Method
ID CROATAN WETLAND MITIGATION BANK PROJECT 98014
ID Phase I - Flow from Perimeter of Long Lake to
ID Stream Gauges #4 (Eastern Model)
ID
ID JOHN EDDY AND PATRICK SMITH
ID EDDY ENGINEERING, P.C.
ID P.O. BOX 61367
ID RALEIGH, NC 27661
ID [phone]
ID --------------------------------------------------------------------------
ID FILE: E02.DAT (EXISTING 02)
ID PURPOSE: EVALUATION OF EXISTING CONDITIONS w/ Flow from Bonnie
ID 12-98
ID ---------------------------------------------------------------------------
ID NOTES:
ID PLAN 1
ID EVENT Hurricane Bonnie
ID ---------------------------------------------------------------------------
ID
*FREE
*DIAGRAM
* PROVIDE 10*300/60=50 HOURS SIMULATION TIME
IT 360
IO 0,2
JP 1
KK Al
KM Al = DRAINAGE AREA AROUND Long Lake (885 acres)
* AREA 1
IN 360,26AUG98,0600
PI 0,0,0,0,0.28,1.96,3.85,2.66,2.73,4.83,0,
BA 1.38, ,1
LS 0,68,5
US 2.25,0.38
KM
KK A2
KM A2 = Long Lake DRAINAGE AREA (1194 acres)
* AREA 2
BA 1.87„1
LS 0,100,100
US 2.25,0.38
KM
KK JX1
KM Combine flow from Al and A2
* JUNCTION 1
HC 2
KM
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I APPENDIX C
PAGE 4
CLIENT: ENVORONMENTAL SERVICES, INC./NCDOT
ROJECT: CROATAN WETLAND MITIGATION BANK
NO.: 98014
DATE: 12/98
BY: J. EDDY
FILE: JLE LIH ANALYSIS.wb3
Computed
Elev. Discharge, cfs In(x) In(y) Regression Output: Discharge, cfs
29.21 20 3.374511 2.995732 Constant -69.173 21.1
29.97 40 3.400197 3.688879 Std Err of Y Est 0.063371 36.6
31.11 80 3.437529 4.382027 R Squared 0.99578 81.4
31.72 120 3.456947 4.787492 No. of Observations 5 123.4
32.09 160 3.468544 5.075174 Degrees of Freedom 3 158.1
X Coefficient(s) 21.40273
Std Err of Coef. 0.804449
y=Cx"b
C= 9.1E-31
b= 21.40273
Stage-Discharge Relation
Stream Gauge 4
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Elevation, feet
--aHEC-RAS -f- Regression
EDDY ENGINEERING, PC
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
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APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I APPENDIX C
PAGE 8
CLIENT: ENVORONMENTAL SERVICES, INC./NCDOT
PROJECT: CROATAN WETLAND MITIGATION BANK
NO.: 98014
DATE: 12/98
BY: J. EDDY
FILE: JLE UH ANALYSIS.wb3
1587.2 ACRES, WATERSHED AREA
3.0 HOUR TIME STEPS
INCREMENTAL CUM. VOL.,
READING DIRECT, CFS VOLUME, CF CF N
154.0 0.0
157.0 20.1 108476.6 108476.6 1
160.0 51.4 386270.9 494747.5 2
163.0 64.4 625706.2 1120453.7 3
166.0 74.5 750039.5 1870493.2 4
169.0 77.3 819328.9 2689822.0 5
172.0 79.0 844005.1 3533827.2 6
175.0 76.4 839418.1 4373245.3 7
178.0 74.4 814180.7 5187426.0 8
181.0 70.3 780959.0 5968385.1 9
184.0 66.3 737353.7 6705738.8 10
187.0 59.6 679967.8 7385706.7 11
190.0 56.0 624561.1 8010267.7 12
193.0 52.5 586302.1 8596569.8 13
196.0 46.3 534020.1 9130589.8 14
199.0 43.2 483538.8 9614128.7 15
202.0 39.1 444183.8 10058312.5 16
205.0 33.8 393264.9 10451577.3 17
208.0 30.1 344844.9 10796422.2 18
211.0 26.6 306095.8 11102518.1 19
214.0 22.1 263160.9 11365679.0 20
217.0 19.0 222313.0 11587991.9 21
220.0 16.8 193519.6 11781511.5 22
223.0 13.7 164772.1 11946283.6 23
226.0 11.0 133314.0 12079597.6 24
229.0 8.4 104612.8 12184210.4 25
232.0 5.1 72987.9 12257198.2 26
235.0 4.1 49951.2 12307149.4 27
238.0 1.7 31218.0 12338367.4 28
241.0 0.0 8952.8 12347320.2
EDDY ENGINEERING, PC
12347320.2 CF, TOTAL VOLUME OF DIRECT
283.5 AC-FT, TOTAL VOLUME OF DIRECT
3401.5 AC-IN, TOTAL VOLUME OF DIRECT
2.1 INCHES, DIRECT RUNOFF (VOL./AREA)
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I
APPENDIX A -PRELIMINARY HYDROLOGIC AND HYDRAULIC ANALYSIS REPORT, PHASE I APPENDIX C
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EDDY ENGINEERING, P.C.

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c
0.4 w
0.2
0
800

b
a
a
d
b
J
wr w w w? ww w ?w wr ww ww w? ww ww w w w ?¦. w .?.
d
d
z
z
r?
r?
z
G?
b
n
60
50
40
.12
U
N
°) 30
Ca
C-
0
CO
0 20
10
0
0

110
00
2
fl,
N
200 400 600
Time (hours)
July 1999 - Discharge at Bridge
1
0.8
0.6
4---
c
0.4 CU
0.2
800

b
b
CD
a
d
b
QrQ
M
00
?r ?r r ?r r r? r r? r? r r ? ? ? ? ?¦r . ? ar.
d
d
z
c?
z
r?
z
b
n
a
0
N
60
50
40
v
L 30
CU
U
Cn
0 20
10
0
0
200 400 600
Time (hours)
August 1999 - Discharge at Bridge
1
0.8
0.6
c
0.4
0.2
0
800
Y
b
b
C
d
?D

m
d
d
K
z
z
r?
z
b
n
td
1?c
00
0
N

IC

b
b
C7
a
d
b
00
O

M
t7
t7
z
z
z
G?
b
n
1200
1000
200 400 600
Time (hours)
October 1999 - Discharge at Bridge
2
800
Co
U
N
600
U
N_
E 400
2,00
0
0

00
0
N
1.5
C
1
C
CU
0.5
800

b
b
a
as'
d
b

M
tv
d
z
z
z
b
n

00
0
N
1400
1200
1000
U
800
N
CU
600
Cn
E
400
200
0
0
200 400 600
Time (hours)
November 1999 - Discharge at Bridge
2
1.5
171
C
1
C
0.5
800
Y
V
b
d
a
d
b
N
M M M M M M M M M M M M M M M M M ? ?
C?1
d
d

60
r?
z

z

50

z
G7
b 140
n
U
N
2" 30
M
Cn
0 20
10
0
0
n
td
110
00
2
N
200 400 600
Time (hours)
December 1999 - Discharge at Bridge
1
0.8
0.6 c
cc
4--
0.4 w
0.2
800
Y
b
b
C
a
d
QtQ
W
M M M M M M M M i M M M M M M i M M M
d
d
r?
z
G?
z
t?
z
G?
b
n
n
Cd
00
0
A
N
1400
1200
1000
U
800
U
CU
v 600
CO
0
400
200
0
0
200 400 600
Time (hours)
January 2000 - Discharge at Bridge
2
1.5
C
1
c
Q!
0.5
800
Y
It
b
a
d
ro
oa
A
M M i M M M M M M M M M M M M M M M M
d
d
z
z
z
ro
n
1400
1200
1000
800
N
L
600
Cn
0
400
2
1.5
171
c
1
LOT
o?
D.5
200
0
0
Gd
0
N
February 2000 - Discharge at Bridge
200

b
b
p
k
d
b
50 100 150
Time (hours)

DO
, Q
S

O ?
a
i
w
O ,a

S?
N
v
?. u

G

z
n

A A .
00 Qo
O O
M M M M M M M M M M ? ? ? M M A???
M? 03/22/00
V
d CROATAN WETLAND WATER BALANCE
CLIENT: ENVIRONMENTAL SERVICES, INC./NCDOT
PROJECT: CROATAN WETLAND MITIGATION BANK
NO.: 98014 -2
z DATE: 02129/00
^ : CGP
4 J FILE: Water Balance.wb3
1-1
z
Drainage Area for Stream Gauge 16 (@ Bridge)
Consider Equation: RF(rainlalf) = RO(run-ofl) + INF(inffltratlon) + ET(evapotranspiration) + E(evaporation)
z
P
Ib Analysis of Site using Theissen Polygon Method:
Site Entire Drainage Area Lake Area
Rainfall percentage percentage
Thessien Polygon Areas: square miles acres of whale square miles acres of whole Ste (acres) Entire Area (acres)
RG2 • RDS 2.21 1414.00 0.35 3.78 2408.00 0.24 2100 2100
RG3 2.40 1534.00 0.38 5.80 3711.00 0.37 (rainfall)
RG4 1.72 1103.00 0.27 6.06 3881.00 0 39
Total 6.33 4051.00 15.63 10000.00
Monthly Rainfall Totals

Croatan
Nat. Forest New Bern Cnery Point Kinston Trenton
Gages Rainfall Rainfall Rainfall Rainfall Rainfall
Month RDS RG2 RG3 RG4 (in) (in) (in) (in) (in)
Jura - 98 181 -• - - 4.03 -
July • 98 10.68 541 5.16 - 4.78 -
August - 98 25.20 10 - 13.48 6.03 6.60
September • 98 1.29 2 3.40 6.64 145 -
October - 98 0.77 0 006 - 0.51 -
November • 98 1.63 1.65 1.64 - 2.24 -
December - 98 0.00 8.1 5.03 - 5.07 -
January - 99 8.26 3,89 2.69 - 5.50 -
February • 99 3.27 0.91 1.23 1.49 IS 1.79 - 2.59 -
March - 99 8.08 3.13 3.37 3.29 2.31 2.68 - 2.09 -
ApAl - 99 3.53 4.27 3.98 3.61 3 3.31 - 3.55 -
May - 99 0.52 2.02 2.67 2.38 - 2.84 - 2.73 -
June .99 4.90 3.33 2.71 3.11 - 2.15 - 4.91 -
July - 99 3.70 2.62 3.27 2.66 1.75 4.62 - 3.68 -
August - 99 11.52 5.24 7.36 6.81 - 7.67 - 3.24 -
September - 99 13.33 18.85 20.91 18.84 - - - - -
October-99 732 7.55 7.93
A November - 99 2.18 2.28 2.31 - - - - -
December - 99 04 0.97 0.99
January. 00 5.12 596 5.77 - - - - -
7
F February - 00 0.10 0.00 0.00 - - - - -
? Total 55.07 61.65 58.44 Averages for 2nd Half of Feb '99 to 1st Half of Feb '00
?D
00 Notes:
1. New Bern Rainfall Data Inaccurate for August 1998 due to gaps in information. Nearly all of June 1908 is missing from New Bern Data.
N Unable to kxate Data for December 1998 for RDS raftage. Also missing last two weeks of November 1998 and one Week from and of April to beginning of May 1999.
2
A .
1
N
Monthly Rainfall Data for Site ONLY
Weighed
Average Weighed Volume of Rainfall
Rainfal Average Rainfall per Morin
(in) (in) (aae•rt)
3.61 3.61 1851.45
8.63 8.63 3397.66
12.86 12.66 6488.07
3.10 310 1588.98
0.33 0.33 170.95
1.79 1.79 918.04
5.40 5.40 2767.95
5.08 5.05 2805.97
1.21 1.19 580.87
3.28 3.26 1649.77
3.95 3.98 2090.99
2.38 2.36 1151.60
3.05 3.04 1607.42
2.05 2.88 1429.74
6.47 6.47 3101.25
19.53 19.63 9924.61
7.73 7.71 3954.71
2.28 2.25 1142.18
1.00 1.00 519.54
5.62 5.62 2791.55
0.03 0.03 29.28
58.72 58.83 29672.98

PC
b
fD
Cy
y4
b
fv
7Q
i--t
M r = = = = M ? ? ? ? ? ? . ? ? ? ?
M 03122/00
r
d
r?
z
z
r?
z
b
Pan Evaporation Data Factor 0.70 % Site Covered by Water 2.00 %
(Not Including Lore Lake)
Monthly Rainfall Data for TOTAL DRAINAGE AREA
Weighted
Average Weighted Volume of Rainfall
Rainfall Average Rainfall per Month
(in) (in) (acre'rt)
3.61 3.61 3642.10
6.63 6.63 6683.74
12.66 12.66 12763.08
3.10 3.10 3121.80
0.33 0.33 33628
1.79 1.79 1805.93
5.40 5.40 5445.00
5.08 5.08 5126.37 Half of the Month
1.21 1.25 1204.13 602.06
3.26 3.28 3282.05
3.95 3.91 4002.45
2.36 2.40 2354.28
3.05 3.01 3094.86
? 2.85 2.88 2855.78
t 6.47 6.64 6447.04
19
53 19.61 19640.90
.
7.73 7.74 7799.68
2.26 2.27 2271.14
1.00 0.99 1010.85
110 5
62 5.68 5632.66
00 .
03
0 0.02 37.57
O .
Fr
58.72 59.06 59031.31
N1
Evaporation Data
SCONC Pan Percentage Drainage Area
Aurora Correction Corrected of Drainage Evaporation
Data Facter From Data Area Controlled Long Lake Drainage Area Combined
(in) Chow, et at. (in) by Evaporation (aae'8) (acre'ff) (acne'ft)
June 809 0.7 5.66 0.02 991.03 94.38 1085.41
July 8.08 0.7 5.66 0.02 989.80 94.27 1084.07
August 6.84 0.7 4.79 0.02 837.90 79.80 917.70
Sept 6.15 04 4.31 0.02 753.38 71.75 825.13
October 434 0.7 3.04 0.02 531.85 50.63 582.26
Nov 2.72 0.7 1.90 0.02 333.20 31.73 364.93
Dec 2.00 0.7 1.40 0.02 245.00 23.33 268.33
Jan 2.68 01 1.88 0.02 32810 31.27 359.57
Feb 2.96 0.7 2.07 0.02 362,60 34.53 397.13
Mar 4.42 0.7 3.09 0.02 541.45 51.57 59102
Apr 6.65 0.7 4.66 0.02 814.63 77.58 892.21
May 6.45 0.7 4.52 002 790.13 75.25 865.38
June 6.79 0.7 4.75 0.02 831.78 79.22 910.99
July 7.60 0.7 5.32 0.02 931.00 88.67 1019.67
August 5.30 0.7 3.71 0.02 649.25 61.83 711.08
Sept 3.53 0.7 2.47 0.02 432.43 41.18 473.61
October 4.03 01 2.82 0.02 493.68 47.02 540.69
Nov 2.85 0.7 2.00 0.02 349.13 33.25 382.38
Dec 1.28 0.7 0.88 0.02 154.35 14.70 169.05
Jan 1.15 0.7 0.81 0.02 140.87 1342 154.29
Feb 1.23 0.7 0.86 002 150.68 14.35 165.03
Yearly
sum 6460.65 615.30 7075.95
Had of the Montt
196.57

V
b
C7
b
A7
CrQ
l9
N
M M M M i M M M ? ? ? ? ? ? ? ? ? ? ?
?.yJ 03/22100
V
d
z
0-4
z
r?
r?
z
b
Max. Percentage of Month ET Applies 0.70
Evapotranspiration Data
bd
NIo
Do
0
lv

t
e
S
Drain Mod
Average
Drain Mod
Average
Drain Mod
wxsystems

t
P
Average
Drainage Area Mod
Drain rain Area
ea WaSys"M
Dra4lapa Area
in
ea
em
Wx
ys
Potential ET Potential ET Correction Potential ET Potential ET Potential ET Potential ET Potential ET Potential ET
t ercen
age
of Orainage Evapobanspiration ation
Evapobanapr Evap
anapir
Data From Data From fader From Corrected Corrected to
Actual Actual ET
to to
Actual ET ET
o
Aqu E o9od
Area , (+de•8)
WaSyctems Drain Mod Drain Mod Data Data
i io
Correction (in) (irt) (in) as
C ) D (tee B) I
B)
lbl (am) lbl (
n) 32
08
6 80
5 011 3.69 3.57 3.16 0.98 3013.12 292915.69
15
0 2826.
2872.13
June 5.71
6
71 5.35
16
5 1.1
0. 95 .
S.81 .
5.19 0.52 3.02 2.69 3.52 0.98
0
98 2161.27
1791
13 .
1809.01 1918.83
July
A
t .
1
80 .
4.9 0 LIt 1.16 0.50 2.19 2.22 2.39
10
2 .
0
98 .
1976.77
2054.22 11758.07
ugus
Se
t .
3.66 3.96 0.97 370 3.81 0.65 2.12
71
1 2.52
1
95 .
1.34 .
098
1]99.26
1591
.19 1091.81
93
627
p
October 2.05 2.71 1.1 262 2.98 0.65
0
63 .
25
1 .
U6 0 0.77 77 0.96 101707 1192.11 .
289
68
Nov 1.22 1.73 134
155 198
12
1 2.32
1.19 .
OA7 .
0.68 0.80 0.35 0.98 551.6/
12
599
Hag month 651.27
689.36
Hag more .
261.50
Dec 0.75
0
71 1.09
98
0 111, .
1.18 1.70 0.50 0.73 0.81 0.35 0.88
98
0 .
22
11N
570.81
1561.62
700'81 .62
3
/
Jan
Feb .
67
0. .
1.15 2.11 2.21 3.06 0.63 140 1.91
19
7 0.12
0
51 .
0.98 .
1121.50
?
? j
j7. 6
Mar 0.1
9 2.81 2.01 3.73 5.65 0.66
58
0 2.11
50
3 .
1.62 .
1.35
0.98
2860.28 : 110857
1125
80
Apr 2.32 4.5 1.76 6 7.92
77
6 .
54
0 .
2,98 3.67 1.75 0.98 2431.60 299747 .
1908.56
May 3.23 5.17 1.31
1
1 5150
5
21 .
5.89 .
0.56 2.93 3.30 231 098 2395.41 2691.41
32
2391 3121.58
June 4.17
8
77 5.35
5
46 .
0.95 .
S.Bt 5.19 0.56 3.26 2.93 3.52 0.98
0
98 2678.55
61
2153 .
2220.15 2223.18
July
August .
1.61 .
4.9 0.91 1.33 4 0.61 2.61 2.72
43
1 2.81
1
22 .
0.98 .
1067.61
1171.11 997 ?
93.69
Sept 3.26 3.96 0.97 360 3.84 0.31
0
61 1.31
68
1 .
1
62 .
1.10 0.98 1369.60 1181.25 1
69140 * 85
80
8
October 2.29 2.71 1.1 2.75 2.98
32
2 .
63
0 .
1.30 .
1.46 0.65 0.98 1063.22 e92.? .
]59.81
Nov 1.35 73 1.31 2.07
38
1 .
69
1 .
0.63 0.88 1.07 0.11 0.98 715.01 192.10
Dec 0.70 1.09 1.55
1
77 .
2D
1 .
1.70 0 .47 0.61 0.81 0.21 0.98 499.0] , 192.10
Jan 0.50 0.96
725
0 .
11
2 .
1.29 1.53 0.17 0.61 0.73 021 ew /88'87 Yearly Yearly
Feb 0.50 . . 31
96 Y
SUM 20027.92 SUM 23428.95 SUM 14023.15
sums 56.91 66.98 7243 81.05 40.91 45.70 .
Ha/ Monty
170.81

b
b
b
a
?e
?b
tic
W

03/22/00
d
d
z
c?
z
r?
z
c?
b
Runoff Data
Monthly Discharge
(ac(e'h)
June 0.00
July 0.00
August 0.00
Sept 0.00
October 000
Nov 0
Dec 000
Jan 000
Feb 491.02
Mar 1238.75
Apr 825.58
May 1170.48
June 238.57
July 72.38
August 17.32
Sept 15443.09
October 11076.45
1??..r1 Nov 4141.46
W Dec 500.00
1?0 Jan 3843.65
00 Feb 1978.76
O
I?
? 41037.52
r
N Note: Due to limited discharge data
available for Decemeber 1999. the
shown discharge Is estimated based
on similar rainfall data collected for
the month of February 1999.

00
10
P9
b
a
W
Oq
fD
m m m m m m ! m ? ? ? ? ? ? ? ? ? ? ?
51..?1
d
d
L.,?yyy
l?
^^z
?z..I
l ?
rM?
/z
1441
V
n
l l
try
110
00
0
1
N
012700
CROATAN WETLAND WATER BALANCE
CLIENT. ENVIRONMENTAL SERVICES, INC.IN000T
PROJECT' CRGATAN WETLAND MITIGATION BANK
NO.' 90014 -2
DATE: 02rA=
BY: COP
FILE: W««B.Ir¢•.r.b3
a•1«w.w• C-A w
R•6V.9 ET E R-." Tar O? 091Or Q46Or
Av«•9. 0. nMOC W.S9.WO. A-." D.. MOO W.S'.W Av«p• •n M.O W.37.Wn.
60206 57061 70001 17001 19651 49102 126019 111039 66039 602.06 12W Is ••••• 06039
3202 05 172150 2503 90 417 K 593 02 1236 75 355326 035 65 224923 3004 Ill 2520 30 310962
4002 45 2660.20 3773.00 1106 51 592 21 025 50 457000 5490 79 2624 37 7006 56 607364 593390
2354 20 2434 60 299717 020 00 665.30 100.46 4470 46 503333 346466 10240 63 10651 72 9396 as
3091.66 239541 .'41 190656 910.99 23057 35097 3610.91 305612 1333570 IS122.18 12456.77
2655 70 2616.55 2391.32 312110 10967 72.36 377060 3"3 37 421393 15191.46 10661.15 1667069
6017.04 2153 61 2220.15 229349 71100 1732 255202 2910 56 3021.90 22610.52 22431 75 19602 59
1964090 106761 1171.14 99309 47361 1544309 1690130 1700703 1691050 4227942 2531977 36603.16
7799 66 1369.60 1401.25 114066 510 69 11076 45 17906 75 1310140 12750 01 50079.10 42301.07 19361.10
2271 14 106322 1192.71 69660 39230 414146 5507 05 571655 5220 64 52350 24 55790 62 51501.62
101065 715.04 612.36 359 61 169 05 50000 136409 1541 41 102006 53361.09 6007/.67 ••••. 55610.69
5632 fib 4" 03 656.03 192.10 154 29 3643 65 "96 97 465597 419005 50993 74 67161.96 50000.74
37.57 49007 592.11 19210 16503 1976.76 264266 2736.19 2335.09 59031.31 6675093 6213562
skims 20027.92 2312095 1402315 707595 41037.52 6014139 1151242 6213662 390074.10 30MO44 366216.92
SUM CHECK 20027.92 23420.95 14023 15

b
a
x
t?
b
c?
u0
co

CrJ
d
d
z
r?
z
b
Monthly Water Balance
Cumulative Flow w/ WxSys. ET
70000
60000
50000
d
L
40000
4) E 30000
_n
O 20000
10000
0

bd
00
0
A
N
Inflow Outflow
b
b
a
X
M
b
s?
va
CD

rep mar Apr may Jun Dui Aug Sep Uct Nov Dec Jan Feb
Month
? 1' ? ? ? !? ? !' !? ? ? I? ? ? ? !? ? III ?
M
Cv
z
c?
CrJ
z
b
n
7A
7
110
00
O
r-+
N
UYlONO
70000
60000
50000
m
40000
E 30000
0 20000
10000
0
Dec Jan b
Monthly Water Balance
Cumulative Flow w/ WxSys. ET
70000 - ----- -- --
60000 - ------ - ---- ----
-
50000
40000 - - ------ ---'--
E 30000 - - ----------------
20000 - ---- ------- ------------
10000
0 -- f-+---{---F---+---1--f----?+-----+-----F---f----+--
Fab Mar Apr May Jun Jul Aup Sep Oct Nov Dec Jan Feb
Month
•- Inflow .- Outflow
Monthly Water Balance
Cumulative Flow w/ Avg. ET
Monthly Water Balance
Cumulative Flow w/ Drain Mod. ET

70006
60000
50000
A
40000 - ---- - -
m
30600 . --. ----
> 20000 - - - - - ?
10000 - ----- ----------
0 --+- i?-f-i---+---f
Apr May Jun Jul Aup sip
Month
-•- Inflow .- Outflow

b
b
b
b
UU
!D
J
-•- Inflow Outflow

r?
d
d
k
z
z
b

00
0
.A
N
------------
Comparison of Monthly ET Estimates
WxSy. ET N/s. Drain Mod. ET vs. Avg. ET
6 -____-__5._...._.
oowm%
4
L
V
E
? 2
`?
0
Feb
ar
Apr May Jun Jul Aug Sep Oct Nov D"a"b
Month
--s-- WxSystems -a.-- Drain Mod by ESI --o-- Average

b
b
a
b
aCD
00

o `° ?'?
k
n
W
? O /1
N ?? FBI

N
q
a
ti
?
n

V
1

C ro

Z z
y C7
a ?
y
r
?C

b b

b b

? v
OO Op
.N
m m m m w m m w m m m? A? m ? ? ? .
m
d -
d
z
G?
z
r?
z
b
25
20
015
N
rn
L
0 10
Q
5
041
350

110
00
0
N
400
450 500 550
Reading (hourly)
STORM EVENT ANALYSIS
600 650

b
b
C
Q
b
s r== m=== m m m ? m m m ? ? IMI
m
d
d
z
z
r?
z
b
n
35
30
25
N
a--
20
a)
rn
L
v 15
U)
0
10
5

?0
00
0
N
0
300
OBSERVED - BASE DIRECT

Storm Event #2 - 13 May 1999
Total Event Rainfall = 0.18 inches

350 400 450 500 550
Reading (hourly)
STORM EVENT ANALYSIS
600

b
b
b
k
b
tv
CD
N
m m m r it = = = m m m = = m ?. ?
h
t
t
M
r
.
G
r
.
F
1000
800
G

/1
N
v 600
a?
v?
L
N 400
0
200
0
100

?z ,
00 ..
0
N

b
b
b
?D
W
150 200 250 300 350
Reading (hourly)
STORM EVENT ANALYSIS

m
t7
d
K
Cri
z
G?
z
z
G?
ro
n
1400
1200
1000
800
rn
L -
cu
600
ch
0
400
200
0
300

00
0
N
350 400 450 500 550
Reading (hourly)
STORM EVENT ANALYSIS
600 650
Y
b
b
C
a
b
A
m r m m m m' m m m m m m = m m m ¦¦s ?
m
d
d
z
G7
z
z
b
n
1400
1200
1000
800
a)
rn
cm
v 600
ch
400
200
0
350

00
0
A
N
b
C
Q
b
CD
LA
400 450 500 550 600 650
Reading (hourly)
STORM EVENT ANALYSIS
m m m m m r m== m m? m m m ? ? s
m
d
K
z
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z
z
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ro
n
20
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0.4
0.3
42
4-
c
0.2 If
0.1
0
353

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00
2
fl,
N
4U4 455 506
Reading (hourly)
DIRECT RUNOFF HYDROGRAPH
0.5
557
b
b
b
a
b
J

m

d

z 20 -
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t? -

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n

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a)
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w
s
U
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0
5
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274

00
0
N
25 376 427
Reading (hourly)
DIRECT RUNOFF HYDROGRAPH
0.04
c

b
b
CD
p
a
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b
00
m m i i m== m m ? ? m m ? ? ??
m
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m
U) 400
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c
1 9
c
200

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00
0
A
i
N
0
126 147 168 189 210 231 252 273 294 0
Reading (hourly)
DIRECT RUNOFF HYDROGRAPH

b
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m
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z
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800
U
(D
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cu
s
U
U
0 400
200
0
337
n
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00
O
A
N
388 439 490 541
Reading (hourly)
DIRECT RUNOFF HYDROGRAPH
592
1.6
1.4
1.2
1 ?.
C
0.8
w--
c
cu
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0.4
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800
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10 400
200
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0
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0
36
DIRECT
1.4
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1
0.8
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0.6
0!
0.4
0.2
n

b
b
C
a
tDIRECT RUNOFF HYDROGRAPH
434 485 536 587
Reading (hourly)
3 b
m = = = S = = = = = m m ? m m ? ? i ?
m
d
d
G?
z
z
b
n
30
25
20
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2) 15
cu
s
U
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0 10
5
A
00
0
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N
0
651
/U1
753 804 855 906
Reading (hourly)
DIRECT RUNOFF HYDROGRAPH
957
0.16
. 0.14
0.12
0.1
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0.08
w-
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0.02
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STORM 5 HYDROGRAPH COMPARISON
Predicted vs. Observed
1500 - - -
1000
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Synthesized Hydrograph Q, cfs -- Observed Storm Hydrograph Q, cfs b
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CLIENT: ENVIRONMENTAL SERVICES, INC./NCDOT
PROJECT: CROATAN WETLAND MITIGATION BANK
NO.: 98014 -2
DATE: 02/22/00
BY: PKS
FILE: Regional Flood Relations.wb3
Drainage Area (ac.): 12090 acres
18.89 square miles
Table 2. - Regional flood relations for rural basins
Flood discharge, Qt
for recurrence Sandhills
interval T years
Coastal Plains
Formula Discharge
2 Q2 = 29.7 DA 0.733 256.00
5 Q5 = 48.8 DA 0.738 426.86
10 Q1o = 64.4 DA 0.740 566.64
25 Q25 = 86.2 DA 0.751 783.37
50 Q5o = 105 DA 0.757 971.20
100 Q1Do = 126 DA 0.763 1186.17
500 Qsoo = 180 DA 0775 1755.35
Drainage Area, DA, in square miles
Formula Discharge
Q2 = 69.4 DA 0.632 444.58
Q5 = 149 DA 0.582 824.07
Q1o = 225 DA 0.559 1163.07
Q25 = 362 DA 0-532 1728.51
Q5o = 490 DA 0514 2219.15
Qsoo = 653 DA 0.497 2813.25
Qsoo = 1130 DA 0.477 4590.37
Blue Ridge-Piedmont
Formula Discharge
Q2 = 144 DA 0.691 1097.11
Q5 = 248 DA 0.670 1776.38
Q1o = 334 DA 0-665 2357.49
Q25 = 467 DA 0.655 3200.80
Q5o = 581 DA 0.650 3924.07
Q1oo = 719 DA 0643 4757.24
Qsoo = 1070 DA 0.636 6935.48
Tabel and Formulas as provided in USGS Water Resources Investigations Report 87-4096
Magnitude and Frequency of Floods in Rural and Urban Basins of North Carolina," Dated 1987

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d Period: February 1999 to January 2000

CLIENT: ENVIRONMENTAL SERVICES, INC./NCDOT
z PROJECT: CROATAN WETLAND MITIGATION BANK
NO.: 98014 -2
?.y DATE: 02/22/00
z BY: CGP
FILE: Site Monthly Rainfall Data Feb 99 through Feb OO
wb3
.

z

2 Hour 3 Hour 6 Hour 12 Hour 24 Hour 2 day 4 day 7 day 10 day
b Max. Intensity (in/hr) 1.340 0.981 0.648 0.431 0.275 0.153 0.098 0.061 0.058
Total Depth (in) 2.680 2.943 3.890 5.173 6.597 7.337 9.427 10.672 13.923
From 9:00:00 AM 8:00:00 AM 2:00:00 PM 12:00:00 PM 7:00:00 AM 11:00:00 AM 7:00:00 AM 7.00:00 AM 3:00:00 PM
06-Sep-99 06-Sep-99 15-Sep-99 15-Sep-99 15-Sep-99 04-Sep-99 03-Sep-99 03-Sep-99 27-Aug-99
To 10:00:00 AM 10:00:00 AM 7.00:00 PM 11:00:00 PM 6:00:00 AM 10:00:00 AM 6:00:00 AM 6:00:00 AM 2:00:00 PM
06-Sep-99 06-Sep-99 15-Sep-99 15-Sep-99 16-Sep-99 06-Sep-99 07-Sep-99 10-Sep-99 06-Sep-99

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m DESIGN STORM DEVELOPMENT
tV CLIENT: ENVIRONMENTAL SERVICES. INC./NCDOT
U' PROJECT: CROATAN WETLAND MITIGATION BANK -
NO.: 9W14 .2
DATE: 03/09100
BY: PKS
? FILE: Design Som Develo ment
^^
41
Consider Alternating Block Method as shown on p. 466, Applied Hydrology, Chow.

l Storm Duration 12 hours
Rainfall Increment 1 hours
Frequency of concern 10 year
'
-
y
f
rr Depth by IDF Function
/
ma
ly l 1 2 3 4 5 8 7 8 9
Duration Intensity Cumulative Incremental Rank Rearranged Precipitation Cumulative Time
b 10-Year Depth Depth Order Order Depth

J Design Storrs Depth (in.)= 5.77 (lours) (in /hr.) (in.) (in) (N) (N) (in.) (in.) (fours)
1 2.93 2.93 2.93 1 11 0.10 0.10 1
2 1.67 3 34 0.41 3 9 0.20 0 30 2
3 1.25 3.75 0.41 4 7 0.30 0.60 3
4 1.05 4.20 0.45 2 5 0.31 0.91 4
5 0.90 4.50 0.30 8 3 0.41 1.32 5
B 0.80 4.80 0 30 7 1 2.93 4.25 6
7 0.70 4.90 0.10 11 2 0.45 4.70 7
6 0.65 5 20 0.30 8 4 0.41 5.11 6
9 0.60 5.40 0.20 9 8 0.30 5.41 9
10 0.53 5.30 -0.10 12 6 0.30 5.71 10
11 0.51 561 0.31 5 10 0.15 5.86 11
12 0.48 5.76 0.15 10 12 0.00 5.86 12
Depth by DDF Function
1 2 3 4 5 6 7 8
Duration Depth Incremental Rank Rearranged Precipitation CumWalive Time
10-Year Depth Order Oder Depth
Design Storm Depth (In.)= 5.77 (hours) (In.) (in) (N) (N) (in.) (in.) (hours)
1 2.93 2.93 1 11 0.10 0.10 1
2 3.34 0.41 4 9 0.20 0.30 2
3 3.76 0.42 3 7 0.23 0.53 3
4 4.20 0.44 2 5 0.30 0.83 4
5 4.50 0.30 6 3 0.42 1.25 5
6 4.82 0.32 5 1 2.93 4.16 e
7 5.05 0.23 7 2 0.44 4.62 7
6 5.25 0.20 a 4 0.41 5.03 8
9 5.45 0.20 9 6 0.30 5.33 9
10 5.55 0.10 11 6 0.20 5.53 10
11 5.65 0.10 12 10 0 12 565 11
12 5.77 0.12 10 12 0.10 5.75 12
n
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IM DESIGN STORM DEVELOPMENT
tvy CLIENT: ENVIRONMENTAL SERVICES, INCJNCDOT
v PROJECT: CROATAN WETLAND MITIGATION BANK
NO.: 98014 •2
y?y DATE: 03/09/00
L BY: PKS
^Z^ FILE: Design Storm Development
1?1
Z Consider Alternating Block Method as shown on p. 466, Applied Hydrology, Chow.
Storm Duration 12 hours
yy Rainfall Increment 1 hours
M/??+1 Frequency of concern 10 year

Depth by IDF Function
1 2 3 4 5 6 7 8 9
b Duration Intensity Cumulative Incremental Rank Rearranged Precipitation Cumulative Time
100-Year Depth Depth Order Order Depth
Design Storm Depth (in.) = 8.55 (hours) (in./hr.) (in.) (in.) (N) (0) (iii) (in) (hours)
1 4.20 4.20 420 1 11 0.25 0.25 1
2 2.42 4.84 0.64 4 9 0.27 0.52 2
3 1.83 5.49 0.65 3 7 0.33 085 3
4 1.55 6.20 0.71 2 5 0.55 140 4
5 1.35 6.75 0.55 5 3 065 2.05 5
6 1.18 7.08 0.33 7 1 4.20 6 25 6
7 1.05 7.35 0.27 8 2 071 696 7
8 0.95 7.60 0.25 10 4 0.64 760 8
9 0.85 7.65 0.05 12 6 0.35 795 9
10 0.80 8.00 0.35 6 8 0.27 8.22 10
11 0.75 8.25 0.25 11 10 025 847 11
12 0.71 6.52 0.27 9 12 0.05 8.52 12
Depth by DDF Function
1 2 3 4 5 6 7 8
Duration Depth Incremental Rank Rearranged Precipitation Cumulative Time
100-Year Depth Order Order Depth
Design Storm Depth (inJ= 855 (hours) (in.) (in.) (0) (0) (in.) (in.) (hours)
1 4.20 4.20 1 11 0.20 0.20 1
2 4.84 0.64 2 9 0.25 0.45 2
3 5.48 0.64 3 7 0.35 0.80 3
4 6.05 0.57 4 5 0.55 1.35 4
5 6.60 0.55 5 3 0.64 1.99 5
6 7.10 0.50 6 1 4.20 6.19 6
7 7.45 0.35 7 2 0.64 6.83 7
8 7.70 0.25 8 4 0.57 7.40 8
9 7.95 025 9 6 0.50 7.90 9
10 8 20 0.25 10 8 0.25 8.15 10
11 8.40 0.20 11 10 0.25 8.40 11
12 8.55 0.15 12 12 015 8.55 12
n
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100-Year Design Storm Hyetograph
Craven County, NC
4
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s
12
1 2 3 4 5 It 9 10 It 12
Dspih by DDF Funam 0 D.pth by OF Funcim

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DESIGN STORM DEVELOPMENT
M
tV CLIENT: ENVIRONMENTAL SERVICES, INC/NCDOT
(J" PROJECT: CROATAN WETLAND MITIGATION BANK
NO.: 98014 -2
DATE: 03/09/00
BY: PKS
?y FILE: Design Storm Devebpment

I-I IDF-DDF Information based on: NOAH Hydro 35
USWB TP 40
L?J
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f?.rN
.y1
iy IDF Information
10 yr 25 yr 100 yr
4 l Duration liruty) (nmr) (am)
1 min 213 3.43 4.20
V 2 111 1.67 1.96 2.42
3 hr 1.25 1.46 1.83
• 8 hr 0.80 0.95 1.18
12 hr 0.48 0.57 0.71
DDF Information
10 yr 25 yr 100 In
Duration (in) (in ) (in.)
1 min 2.93 3.43 4.20
2 hr 3.34 3.93 4.64
3 hr 3.76 4.43 5.48
6 hr 4.82 5.72 7.10
12 M 5.77 6.86 8.55
1 1
a
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Depth-Duration-Frequency Functions
Craven County, NC
------ ------
0 2 4 6 0 10 12
T-dD.-(h)
100 250 0 1000
WWI wks 2-68, Tabtualed Chris G. Ply
02722100
TABLE 1 INPUT
Location. Craven County (Cioatan National Forest). NC
Owauon 2 yr 100 yr Sowce
(n) (in)
5 min 0.49 079 NOM Hydro 35
15 min 1.00 1.79 NOM Hydro 35
60 min 2.1 4 2 NOM Hydra, 35
24 hr 45 10 USWB TP 40
TABLE2 DEPTH - DURATION - FREQUENCY
Duration 2 yr 5 yf to In 25 In 50 in 100 yr Yearly Max
H-/' (n) (in) (in) (in) (in) (in)
5 min' 0.49 0.55 0.60 0.67 0.73 0.79
10 min 0.84 0.95 1.04 1.17 1.27 1.38
15 min' 1.08 1.23 1.34 1.54 1.65 1.79
30 min 1.58 1.89 2.12 2.45 2.71 217
60 nut 2.10 2.55 2.93 3.43 3.01 4.20
2 M ' 2.36 2.94 3.34 3.93 4.38 4.64
3 N ' 2.63 3.29 3.76 4.43 4.95 5.46
If N ' 3.30 420 4.62 572 6.41 7.10
12 M ' 3.90 5.01 5.77 6.86 7.71 8.55
24 N ' 4.50 5 81 8.72 0.01 9.01 10.00
2 day 5.50 7.00 7.90 9.50 10.50 12.00
4 day 6.20 8.00 9.50 11.50 13.80 15.00
7 day 7.00 9.50 10.50 13.00 14.00 16.00
10 oar 8.00 10.00 1200 . 14.00 15.50 17.00
From Eqs. III-( .3), p.IIH4.5), Storm«ater Management Vol I
Urban Hydrobgy. B.H.Bradford, N.S.GM. L.S. Tucker
TABLE 3- INTENSITY - DURATION - FREQUENCY
Duration 2 N 5 yr 10 yr 25 y/ 50 yr 100 in
l ir) (nrte) (nmr) (int) WWI (trlmr)
5 min 5.88 6.60 7.17 6.06 8.77 9.48
10 in 5.03 5.69 6.21 7.01 7.65 628
IS in 4.32 4.90 5.36 6.06 6.61 THI,
30 mn 3.16 3.78 4.24 4.90 5.42 5.94
60 min 2.10 2.58 2.93 3.43 3.81 4 20
2 hr 1.18 1.47 1.67 1.96 2.19 2.42
3 far 0.68 1.10 1.25 1.48 1.65 IIA3
61r 0.55 0.70 0.60 0.95 1.07 1.16
12 IV 0.33 0.42 0.46 0.57 0.64 0.71
24 hr 0.19 0.24 0.28 0.33 0.36 0.42
2 day 0.115 0.146 0.165 0.198 0.219 0.250
4 day 0065 0083 0,099 0.120 0.144 0.156
7 day 0.042 0.057 0.063 0.077 0.063 0.095
10 day 0.033 0.042 0.050 0.058 0.065 0.071
kdensay • Depth / Time

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b
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Intensity-Duration-Frequency Functions
Craven County, NC
5
4
171
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c
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c
1
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by
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00
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N
2 4 6 8
Time of Duration (hr.)
o 10 yr w 100 yr

' ol 0 ?, D, 1 Q.41

0111
10 12

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Depth-Duration-Frequency Functions
Craven County, NC
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Time of Duration (hr.)
10 yr w 100 yr

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DESIGN STORM DEVELOPMENT
M
tZI CLIENT: ENVIRONMENTAL SERVICES, INCJNCDOT
?.y
CI PROJECT: CROATAN WETLAND MITIGA TION BANK

hC NO.: 98014 4
DATE: 03109100
BY: PKS
1.wI
l FILE: Design Storm Development
z
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Z SCS Rainfall Distributions
1.
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y?? From Table 14.3.1. SCS Rain all Distributions
l?J p. 461463, Applied Hydrology. Chow

?y 2 ow Storm

P. I Prt Act-I
How U24 Type 1 Type 1A Type U Type 111

0.00 0.000 0.000 0.000 0.000 0.000
2.00 0.083 0.035 0.020
' 4.00 0.161 0.076 0.043
6.00 0.250 0.125 0.072
7.00 0.290 0.156 0.089
8.00 0.333 0.194 01115
8.50 0.354 0.219 0.130
9.00 0.375 0.254 0.148
9.50 0.396 0.303 0.167
915 0.406 0.362 0.178
10.00 0.417 0.515 0.189
10.50 0.438 0.583 0.216
11.00 0.459 0.624 0.250
11.50 0.479 0.654 0.296
11.75 0.489 0.669 0.339
12.00 0.500 0.682 0.500
12.50 0.521 0.706 0.702
13.00 0.542 0.727 0.751
13.50 0.563 0.748 0.785
14.00 0.583 0167 0.811
18.00 0.667 0.830 0.886
20.00 0.833 0.926 0.957
24.00 1.000 1.000 1.000
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tOYear 100-year
Design Design
Storm Storm
5.77 ir. 8 55 A.
10Year 10-Yew 100-year 100-Year
Type III Type III Type W Type 01
Cumulative twerneraal Cumulative Inuemeraal
Hour Interpolated Depot Depot Depot Depot
Type 111 Ile.) (in.) (in.) (in.)
0.00 0 000 0.00 0.00 000 0.00
1.00 0 010 0.06 0.06 0 09 0.09
200 0 020 0.12 0.06 0.17 0.09
300 0032 0.18 0.07 0.27 0.10
4.00 0 043 025 0.07 0 37 0.10
500 0.058 0.33 0.08 0.49 0.12
6.00 0 072 0 42 0.06 0.62 0.12
1.00 0089 051 0.10 0.76 0.15
8.00 0.115 0.66 0.15 048 0.22
9.00 0.148 0.85 0.19 1'27 0.26
1000 0.189 1.09 0.24 162 0.35
11 00 0 250 1.44 035 2.14 0.52
1200 . 0.500 2.89 1.44 4.28 2.14
13.00 0.751 4.33 1.45 8.42 2.15
14.00 0.811 4.68 0.35 6.93 0.51
15.00 0.849 4.90 0.22 7.25 0.32
16.00 0 886 5.11 0.22 7.58 0.32
17.00 0.904 521 0.10 7.73 0.15
18.00 0.922 5.32 0.10 7.88 0.15
1900 . 0.939 5.42 0.10 8 03 0.15
20.00 0957 5.52 0.10 8.18 0.15
21.00 0.968 5.58 0.06 8.27 009
2200 . 0 979 5.65 006 8.37 0.09
23.00 0.989 5.71 0.06 8.46 0.08
24.00 1.000 5.77 0.06 8.55 0.09
t t
0.8 0.5
0.6 0.6
o
5 0.4 0.4
LL LL
0.2 0.2
o 0
03691215182124
Time (MS.)
10-Year & 100-Year Design Storm
Type 11124-Hour Storm Distribution
25
0.6 ....
0 . ru ai r r
/ li.ati.?p..) e e
tar.. ¦ 1aaY..
1 1
011 08
t
1 0.6 0.6
0i0, 0.4
LL LL
02 02
0 o
03691215182124
Time (has.)

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SCS 24-Hour Rainfall Distribution
Actual
SCS 24-Hour Rainfall Distribution
Hourly Interpolated
m m m m m m ' m m m m m ? ? m m m m m
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pe ll
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FIGURE 14.3.3
Location within the United States for application of the SCS 24-hour rainfall hyetographs. (Source: U. S. Dept. of Agriculture, Soil
Conservation Service, 1986).

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DESIGN STORM DEVELOPMENT
m
CLIENT: ENVIRONMENTAL SERVICES, INC./NCDOT
ty PROJECT: CROATAN WETLAND MITIGATION BANK
NO: 98014 -2
tv DATE: 03/09/00
???wwwrrr By: PKS
FILE: Design Storrs Development
^^
41
~
x SCS Rainfall Distributions
11.
,,..11
y From Table 14.3.1, SCS Rainlan Distributions
1?1 P. 461-463. Applied Hydrology, Chow

our form
v
?i Pr/Pn
Actual
A
Hour V12 Type 1 Type to Type 11 Type 111
0.00 0.000 0.000 0.000 0.000 0.000
1.00 0.0113 0.020
2.00 0.167 0.050
3.00 0.250 0,080
4.00 0.333 0.130
5.00 0.417 0.200
6.00 0.500 0.590
7.00 0.583 0390
8.00 0.887 0.860
9.00 0.750 0.920
10.00 0.833 0.950
11.00 0.917 09W
12.00 1.000 1.000

1-Inch
Design
Storm
100 n
Cropped Condensed
I-Inch 1-Inch 1-inch 1-Inch
Cropped Condensed Type III Type 111 Type 19 Type 91
Distribution Distribution Cumulative Incremental Ctaratalive Incremenal

N Interpolated Interpolated
oun Depth Depth Depth Depth
Type III Type 111 (n) (n I (n.) (n )
0.00 0.000 0.000 0.00 000 0.00 0.00
1.00 0.020 0.020 0.02 0.02 0 02 0.02
2.00 0.050 0.043 0.05 0.03 0.04 0.02
3.00 0 080 0.072 0.09 0.03 0.07 0.03
4.00 0.130 0.115 0.13 0 05 0.12 0.04
5.00 0.200 0.189 0.20 0 07 0.19 0.07
6.00 0.590 0.500 0.59 0.39 0.50 0.31
7.00 0.790 0.811 0.79 0 20 0.81 0.31
8.00 0.890 0.898 0.86 0.01 0.89 0.07
9.00 0.920 0.922 0.92 0 06 0.92 0.04
10.00 0.950 0.957 0.95 0.03 0.98 0.04
11.00 0.980 0.979 0.99 0.03 0.98 0.02
12.00 1.000 1000 1.00 0.02 1.00 0.02

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PROJECT CROATAN WETLAND MITIGATION BANK
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CUE NT ENVIRONMENTAL SERVICES, INC /NCDOT
PROJECT CROATAN WETLAND MITIGATION BANK
NO 116014 -2
DATE. O2JDN0
BY. JLE REVISED PKS
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