20151229 Ver 1_More Info Received_20160324

Homewood, Sue
From: Jason Kennedy <jken [e-mail]>
Sent: Thursday, March 24, 2016 11:57 AM
To: Homewood, Sue; Darling, Richard
Cc: [e-mail]; Lisa Elmore; Kimberly Hodges; Paul Smith; Randall, Mike
Subject: RE: DRAFT Haeco
Sue,
Good afternoon. Thank you for taking the time to discuss this with me over the phone yesterday. As we discussed, I
believe there has been a misunderstanding regarding the proposed permit language under condition number 4a. The
below email is lengthy but I believe it will be helpful in walking us through the needed clarifications regarding the permit
language and our recently revised report.
Draft Permit Language
The text that was proposed does not reflect the stormwater plan that was outlined in our submitted report and
plans. The proposed text reads:
a. The high flow rate bioretention pond shall treat a total of 24.61 acres of existing and proposed built
upon area. The approved high flow rate bioretention pond is proposed to treat a maximum built
upon area of 44.52 acres. In the event that Piedmont Triad Airport Authority proposes to direct new
stormwater from additional built upon areas beyond the proposed 24.61 acres they shall first submit
and receive approval for a revised stormwater management plan by the Division. An analysis of the
existing function and compliance of high flow rate bioretention pond shall be included in any request
for a revised stormwater management plan. [15A NCAC 02H .0506(b)(5)]
I understand this text to say that the bio -retention area is being built with additional capacity so that future built upon
area may be routed to it at a later date . Our report does not propose this condition. Instead, the proposed bio -
retention area is designed for its ultimate condition and there are no plans at this time to direct additional stormwater
flow from future project areas. The summary statement found in the introduction section of our report dated
November 2015 reads as follows:
"As outlined in this report, the proposed SCM will provide water quality treatment for a total of 44.52 acres of
impervious cover which exceeds the minimum required for treatment (24.61 acres). As a result, the airport is
formally requesting water quality treatment credits to offset a site development project in the future with up to
19.91 acres of impervious surface. The following table summarizes the water quality treatment credits being
requested:"
Table 1: Summary of Area Required for Treatment
Location
Impervious Cover (acres)
Proposed Hangar
5.06
Proposed Apron
5.11
Proposed Access Route
0.32
Proposed Sidewalk
0.09
Existing HAECO Site (Wet Pond)
14.03
TOTAL = 24.61 acres

The report states that the area required to receive water quality treatment is 24.61 acres which consist of the existing site
treated by the wet pond and the proposed new impervious. Our stormwater management plan will treat 44.52 acres by
also bringing in impervious areas which currently is not treated by a stormwater control measure. With this clarification in
mind, I recommend a rewording of the permit language. This recommended revision is not a change in our approach, but
is intended to make sure the permit correctly represents the stormwater plan that we are proposing. Proposed revised
language would read:
The high flow rate bioretention pond shall treat a total of 44.52 acres of existing and
proposed built upon area. The approved high flow rate bioretention pond is proposed to treat a
maximum built upon area of 44.52 acres. In the event that Piedmont Triad Airport Authority
proposes to direct new stormwater from additional built upon areas beyond the proposed 24,6-1
44.52 acres they shall first submit and receive approval for a revised stormwater management
plan by the Division. An analysis of the existing function and compliance of high flow rate
bioretention pond shall be included in any request for a revised stormwater management plan.
[15A NCAC 02H .0506(b)(5)]
Report Revision
With regard to the report revisions dated 3-9-16. These revisions are very minor in nature and consist of two elements.
1) Evaluation of downstream channel stability. As a result of your correspondence with Dave Kiker regarding
downstream channel stability, the report text was amended and a memorandum was completed and
submitted via email to you on February 24th. This memo documents that the proposed shear stresses and
velocities at the downstream receiving channel are within the permissible shear stress and velocity
ranges. This memo has been added to our report as Appendix F.
2) Minor Drainage Area revisions. The revised report also includes minor revisions to the contributing
drainage areas of the bio -retention area. This change shows that we will actually be able to capture and
treat an additional 9.05 acres with the bio -retention area. This additional area is captured from two locations,
1) additional roof area from the proposed Hangar, originally thought unable to be directed to the bio -
retention and 2) Basin 90 (existing Hangar and parking) which will now also be directed to the bio -
retention. In November 2015, best available data showed that Basin 90 drained directly to Radar Road and
could not be directed to the bio -retention area. Upon further review of as -built data and additional field
survey, we have confirmed that this area will be directed to the bio -retention.
I have prepared the table below to summarize this revision of drainage areas:
HAECO Facility Improvements - 401 Water Quality Cert. Report
Bio -retention area Contributing Drainage Area Revision Summary
3/23/2016
Description: This table summarizes the revisions to contributing drainage areas which drain to the proposed bio -retention
area as documented in the 401 Water Quality Certification report dated November 2015 and revised March 2016.
Location
November 2015 Report
Impervious Cover (Acres)
Report Revised
3/9/2016
Drainage Area
Change
Reason For
Change
Proposed Hangar
3.35
5.06
1.71
See note 1
Proposed Apron
5.11
5.11
0
n/a
Proposed Fire Access Road
0.32
0.32
0
n/a
Proposed Side Walk
0.09
0.09
0
n/a
Existing HAECO Site to the East
11.92
11.92
0
n/a

Sub -Basin 60
0.42
0.42
0
n/a
Sub -Basin 70
9.22
9.22
0
n/a
Sub -Basin 80
14.09
14.09
0
n/a
Sub -Basin 90
0
7.34
7.34
See note 2
Totals
44.52
53.57
9.05
1. Proposed Drainage System has been updated to capture all of the Hangar Roof and direct to the Bio -retention area
2. Review of Existing Facility As -built and additional field survey confirmed that Basin 90 will also be directed to the
Bio -retention area
We request that these changes be considered minor clarifications and allowed without another formal technical
review. These are the only changes associated with the March report revision. No changes regarding the overall
approach or stormwater management plan are proposed in this revision. The sizing of the proposed bio -retention area
has been evaluated to ensure the design criteria outlined in section 3.1.2 of our report will be met with the larger
drainage area. A PDF of the revised report may be downloaded at the following link. All of the revised elements in the
report are shown in red text help expedite your review.
https://transfer.wkdickson.com/message/3GS8kk6tcOlCeGkvlGeJAo
Permit Text— update to reflect the revised Report
Finally, by applying the report revisions to the recommended permit text revision, the revised text should read as follows:
a. The high flow rate bioretention pond shall treat a total of 53.57 acres of existing and
proposed built upon area. The approved high flow rate bioretention pond is proposed to treat a
maximum built upon area of 53.57 acres. In the event that Piedmont Triad Airport
Authority proposes to direct new stormwater from additional built upon areas beyond the
proposed 53.57 acres they shall first submit and receive approval for a revised stormwater
management plan by the Division. An analysis of the existing function and compliance of high
flow rate bioretention pond shall be included in any request for a revised stormwater
management plan. [15A NCAC 02H .0506(b)(5)]
Requested water quality credits
We do not wish to delay permit approval to discuss the consideration of Water Quality credits.
I hope this email is helpful and can keep us on track for the permit issuance in the next few days. I will make myself
available for phone calls or meetings as needed to assist with the final approval.
Thanks again for all your help,
Jason
Jason P. Kennedy, P.E.
WK Dickson & Co., Inc.
720 Corporate Center Drive
Raleigh, NC 27607
Office: 919-782-0495
Direct: 919-256-5615
Mobile: [phone]
Email: [e-mail]

401 WATER QUALITY
CERTIFICATION REPORT
HAECO FACILITY IMPROVEMENTS PROJECT
Submitted on Behalf of.
Piedmont-Triad International Airport
Prepared by:
WK Dickson & Co Inc.F
°. , ' {
720 Corporate Center Drive
Raleigh, North Carolina 27607
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9'
March 2016
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Table of Contents
1. Introduction.......................................................................................................................1-1
1.1. Project Description.....................................................................................................1-1
2. Hydrology and Hydraulics..............................................................................................
2-1
2.1. Methodology..............................................................................................................2-1
2.2. Hydrology...................................................................................................................
2-1
2.2.1 Drainage Areas.................................................................................................
2-1
2.2.2 Rainfall...............................................................................................................2-1
2.2.3 Land Use............................................................................................................2-2
2.2.4 Hydrograph Translation.................................................................................
2-2
2.2.5 NRCS Curve Numbers....................................................................................
2-3
2.2.6 Channel/Storage Routing................................................................................
2-4
2.2.7 Summary of Hydrologic Results....................................................................
2-4
2.3. Hydraulics..................................................................................................................2-4
2.3.1 Energy Loss Coefficients.................................................................................
2-5
2.3.2 Starting Water Surface Elevation...................................................................2-5
2.3.3 Model Run Descriptions.................................................................................
2-5
2.3.4 Hydraulic Evaluation of Radar Road............................................................
2-6
2.3.5 Evaluation of Downstream Flooding............................................................
2-6
2.3.6 Evaluation of Downstream Channel Stability .............................................
2-8
2.3.7 Closed Drainage Systems...............................................................................
2-8
2.3.8 Outfall Protection for Closed Drainage System ..........................................
2-8
3. Water Quality Compliance............................................................................................. 3-1
3.1. Overview..................................................................................................................... 3-1
3.1.1 Proposed Impervious Areas........................................................................... 3-1
3.1.2 High Flow Rate Bioretention Pond Design Criteria .................................... 3-2
3.1.3 Water Quality Volume (WQV)...................................................................... 3-2
3.1.4 Pond Design Summary................................................................................... 3-3
3.2. Conclusion...................................................................................................................3-6
HAECO Facility Improvements Project Page i
Stormwater Report

Tables
Table 1: Summary of Area Required for Treatment............................................................1-1
Table 2: Summary of Water Quality Treatment Credits.....................................................1-2
Table 3: Design Storm Rainfall Depths..................................................................................
2-2
Table 4: Summary of Hydrologic Input Data.......................................................................
2-3
Table 5: Comparison of Peak Flows at Harris Teeter Downstream Channel ..................
2-4
Table 6: Energy Loss Coefficients...........................................................................................
2-5
Table 7: Bend Loss Coefficients..............................................................................................
2-5
Table 8: Culvert Performance for at Radar Road.................................................................
2-6
Table 9: Hydraulic Summary of Harris Teeter Open Channel ..........................................
2-6
Table 10: Evaluation of Downstream Insurable Structures for Flooding .........................
2-7
Table 11: Minimum Area of Impervious Cover Required for Treatment .........................
3-1
Table 12: Proposed Impervious Cover to SCM....................................................................
3-1
Table 13: Calculated Storage Volumes..................................................................................
3-2
Table 14: Water Surface Elevations at Proposed Pond ........................................................
3-3
Table 15: Summary of Flow Splitter Design.........................................................................
3-4
Table 16: Flow Splitter Performance......................................................................................
3-4
Appendices
Appendix A Proposed Concept Plan
Appendix B Input Data for SSA
Appendix C Existing and Proposed Conditions Drainage Area Maps
Appendix D Existing and Proposed Conditions Land Use Mapping
Appendix E CD with Digital Copy of Autodesk SSA Models
Appendix F Stream Stability Evaluation Memorandum
Appendix G Outlet Protection Calculation
Appendix H Water Quality Calculation and Stage -Storage Relationship for SCM
Appendix I Anti -Floatation Calculation for Riser
Appendix J Detention Time Calculation
Appendix K Maintenance and Operation Plan
HAECO Facility Improvements Project Page ii
Stormwater Report

Section 1: Introduction
1.1 Project Description
This report supports the design of the stormwater control measures (SCMs) needed to
develop the HAECO Facility Improvements project at the Piedmont Triad International
Airport in compliance with the North Carolina Department of Environmental Quality
(NCDEQ) regulatory requirements for new development at an airport. A 0.8 -acre high
flow rate bioretention pond is being proposed to meet the regulatory water quality
requirements for NCDEQ. This bioretention pond was designed to infiltrate runoff
generated from the 1St inch of rainfall at a relatively high rate to satisfy the water quality
requirements outlined in Session Law 2012-200. As shown in the concept plans
included in Appendix A, the airport is proposing a 15.9 -acre site development project
including the construction of the following:
♦ 5.06 acres of new impervious area associated with the proposed HAECO hangar;
♦ 5.51 acres of new impervious area associated with the proposed HAECO apron;
♦ 0.32 acres of new impervious area associated with the proposed HAECO fire
lanes flanking the proposed hangar;
♦ 0.09 acres of new impervious area associated with the proposed HAECO
sidewalks;
♦ Removal of an existing fire suppression pond;
♦ Removal of an existing 1.1 -acre wet pond being used for detention and water
quality; and
♦ Construction of a new 0.8 -acre high flow rate bioretention pond that will result in
infiltration of the water quality rainfall event.
In addition to providing treatment for the proposed new impervious areas associated
with the HAECO Facility Improvements project, the SCM will replace the treatment
being provided by an existing wet pond located on the eastern side of the site. This
existing wet pond has a contributing drainage area of 15.29 acres with 13.95 acres of
impervious cover. In total, the proposed SCM will need to provide treatment for 22.51
acres of impervious cover as shown in the following table:
Table 1: Summary of Area Required for Treatment
Location
Impervious Cover (acres)
Proposed Hangar
5.06
Proposed Apron
5.11
Proposed Access Route
0.32
Proposed Sidewalk
0.09
Existing HAECO Site (Wet Pond)
11.92
TOTAL = 22.51 acres
HAECO Facility Improvements Project Page 1-1
Stormwater Report

Section 1: Introduction
As outlined in this report, the proposed SCM will provide water quality treatment for a
total of 53.57 acres of impervious cover which exceeds the minimum required for
treatment (22.51 acres). As a result, the airport is formally requesting water quality
treatment credits to offset a site development project in the future with up to 19.91 acres
of impervious surface. The following table summarizes the water quality treatment
credits being requested:
Table 2: Summary of Water Quality Treatment Credits
Description
Impervious Area (acres)
Required Area for Treatment
22.51
Provided Area for Treatment
53.57
DIFFERENCE = 31.06 acres
Also provided in this report is an evaluation of downstream flooding resulting from the
proposed site changes. The analysis showed that the proposed project will cause
increases to peak flows downstream but will not flood insurable structures, roads, or
cause damage to existing property or the existing Harris Teeter detention pond.
HAECO Facility Improvements Project Page 1-2
Stormwater Report

Section 2: Hydrology and Hydraulics
2.1 Methodology
Autodesk's Storm and Sanitary Analysis 2015 (SSA) was used to size the proposed
collection system, flow splitters, and bioretention pond with riser. This model is based
on the Environmental Protection Agency's (EPA's) Storm Water Management Model 5.0
(SWMM). SSA simulates the surface runoff response to precipitation for an
interconnected system of surfaces, channels, closed pipe systems, culverts, flow splitters,
and ponds. SSA is an ideal model for a complex drainage system such as the one seen at
the HAECO site as it combines hydrology and hydraulics and allows the user to not
only size on-site improvements but also evaluate downstream flooding. Combining
hydrology and hydraulics eliminates the need to iterate between a hydrologic model
and a hydraulic model which eliminates the potential for errors.
2.2 Hydrology
Input data for the model was developed using topographic, landuse, and soils maps in
GIS to delineate and calculate the basin areas, percent impervious, and Natural
Resources Conservation Service (NRCS) hydrologic parameters. The precipitation data
for the 24-hour duration, Type II storm was used to represent the synthetic rainfall
event. SSA estimates surface runoff for a sub -basin based on percent impervious, basin
width, basin slope, and NRCS curve number for the unconnected pervious areas. A
copy of the SSA input values for the existing and proposed conditions is provided in
Appendix B. Unit hydrographs are translated using the watershed basin and slope
parameters. This is unique to SSA.
2.2.1 Drainage Areas
Drainage area maps for the existing and proposed conditions have been included with
this report in Appendix C. Drainage areas were delineated using the following
topography:
♦ 2 -foot contour interval existing conditions topographic mapping from Guildford
County GIS;
♦ 1 -foot contour interval topographic mapping provided by Michael Baker &
Associates titled "ADP Mapping (May 2014).dwg";
♦ Inventory mapping of pipes and catch basins provided by Michael Baker &
Associates titled "ADP Mapping (May 2014).dwg"; and
♦ 1 -foot contour interval proposed conditions topographic mapping generated by
WK Dickson.
2.2.2 Rainfall
Rainfall distributions for the SSA model were derived using the NRCS Type II standard
distribution. Total rainfall depths for the modeled frequency storms were obtained
HAECO Facility Improvements Project Page 2-1
Stormwater Report

Section 2: Hydrology and Hydraulics
online from the NOAA's Nation Weather Service website. Table 3 shows the total
rainfall depths used for this study.
Table 3: Design Storm Rainfall Depths
Design Storm
Rainfall Depth (in)
2 -year, 24-hour
3.31
10 -year, 24-hour
4.77
25 -year, 24-hour
5.65
50 -year, 24-hour
6.35
100 -year, 24-hour
7.07
Source: NOAA's Nation Weather Service website
2.2.3 Land Use
Land use is the watershed cover condition as it relates to the actual type of development
within the watershed. Land use influences the runoff characteristics of a sub -basin, and
combined with other basin characteristics is used to determine the percent impervious
and NRCS curve number for the basin. Appendix D shows the existing and proposed
conditions land use mapping for this project. Input data for the existing and proposed
percent impervious values is found in Table 4.
2.2.4 Hydrograph Translation
NRCS methodologies typically use a time of concentration parameter to help calculate
the response of the watershed to rainfall. SSA uses watershed basin width and slope
parameters to create the unit hydrograph used in the model that will translate the
rainfall into runoff. The watershed width is a parameter unique to SSA that helps define
the watershed shape by taking the watershed area and dividing it by the length of the
longest flow path. Additionally, SSA requires input of a basin slope in the calculations
used to translate the hydrograph. The basin slope is the maximum grade change from
the upstream end of the watershed to the downstream end divided by the length of the
longest flow path. The sub -basin slopes and widths are included in Table 4.
HAECO Facility Improvements Project Page 2-2
Stormwater Report

Section 2: Hydrology and Hydraulics
Table 4: Summary of Hydrologic Input Data
Drainage
Basin
ID
Drainage
Area
(acre)
Existing/
Proposed
Pervious
RCN
Existing
Percent
Impervious
M
Proposed
Percent
Impervious
M
Proposed
Basin
Slope
M
Proposed
Basin
Width
(feet)
10
77.1
74
31%
31%
0.6%
1019
20
28.8
74
34%
34%
0.8%
995
30
19.3
74
40%
40%
1.0%
454
40
4.1
74
44%
44%
1.1%
176
50
3.6
74
21%
21%
1.9%
301
60
5.2
74
8%
8%
1.7%
269
70
9.2
74
100%
100%
1.3%
331
80
14.3
71
99%
100%
1.9%
568
90
8.9
74
24%
82%
2.0%
286
100
19.3
74
82%
43%
0.6%
573
110
6.1
74
43%
29%
1.3%
531
120A
13.95
74
92%
85%
1.6%
409
120B
2.43
74
92%
90%
3.3
165
130
4.5
74
92%
47%
3.7%
207
142
9.9
74
44%
32%
5.0%
608
144-A
0.84
74
2%
100%
0.5%
266
144-B
0.86
74
2%
100%
0.5%
302
144-C
0.75
74
2%
100%
0.5%
239
144-D
0.88
74
2%
100%
0.5%
252
144-E
0.51
74
2%
100%
0.5%
126
144-F
0.47
74
2%
100%
0.5%
201
144-G
0.54
74
2%
100%
0.5%
220
144-H
1.74
74
2%
42%
0.5%
320
146-A
0.81
74
2%
100%
0.5%
86
146-B
1.71
74
2%
100%
0.5%
184
146-C
1.53
74
2%
100%
0.5%
164
146-D
1.99
74
2%
100%
0.5%
193
146-E
0.67
74
2%
28%
0.5%
274
148
1.08
74
2%
10%
16.2%
171
2.2.5 NRCS Curve Numbers
The NRCS curve number approach was used in computing the runoff response in SSA.
Runoff curve numbers (RCNs) were generated for the pervious areas of the sub -basins
using the NRCS document entitled Urban Hydrology for Small Watersheds, dated June
1986 and commonly referred to as TR -55. This method relates the drainage
characteristics of soil group, land use category, and antecedent moisture conditions to
assign a runoff curve number. The runoff curve number and an estimate of the initial
surface moisture storage capacity are used to calculate a total runoff depth for a storm in
HAECO Facility Improvements Project Page 2-3
Stormwater Report

Section 2: Hydrology and Hydraulics
a basin.
2.2.6 Channel/Storage Routing
Flood peaks attenuate, or reduce, as they travel downstream due to the storage
characteristic of the channel itself. Channel routing was simulated in the hydraulic
block of SSA. Routing was modeled using dynamic wave routing. Dynamic wave
routing uses the actual shape and condition of the stream channel input into the
hydraulic model to calculate the attenuated downstream flows.
2.2.7 Summary of Hydrologic Model Results
The SSA model was used to compute peak runoff for the 2-, 10-, 25-, 50- and 100- year
design storms for the existing and proposed conditions. The results of the existing
conditions hydrologic model are summarized in Table 5. A CD containing the digital
files for the SSA model is included in Appendix E.
Table 5: Comparison of Peak Flows at Harris Teeter Downstream Channel
Condition
Storm Event
1 -year
(cfs)
2 -year
(cfs)
10 -year
(cfs)
25 -year
(cfs)
100 -year
(cfs)
Existing
84
112
187
251
312
Proposed
264
343
537
616
700
Temporary Construction
Condition
285
358
511
565
629
Temporary construction condition assumes concrete pad and building built and Phase 5 erosion control
measures in place. High flow rate bioretention pond is offline in Phase 5.
Although Session Law 2012-200 precludes the project from having to provide detention,
a detailed hydrologic and hydraulic evaluation was performed to confirm there are no
adverse impacts to downstream properties with regards to flooding. A summary of this
evaluation is found in the following Hydraulics section of the report.
2.3 Hydraulics
SSA was chosen as the hydrologic/hydraulic model because of its ability to model
complex drainage systems and to evaluate downstream flooding. The project involves
the construction of a single central high flow rate bioretention pond to provide water
quality treatment for the proposed site development. The airport desires to reduce the
potential for bird strikes by eliminating two existing wet ponds referred to in this report
as the fire suppression wet pond and the existing HAECO site wet pond. The existing
conditions SSA model attenuates peak flows through these two ponds to more
accurately determine the proposed projects effects on peak flows. To fully evaluate the
project's impacts on downstream properties, the SSA model was extended through the
HAECO Facility Improvements Project Page 2-4
Stormwater Report

Section 2: Hydrology and Hydraulics
Harris Teeter distribution site and immediate downstream open channel. In addition, a
HEC -RAS model was developed to provide a quality control measure for the changes to
water surface elevations developed using Autodesk SSA.
2.3.1 Energy Loss Coefficients
Contraction and expansion of flow produces energy losses caused by transitioning. The
magnitude of these losses is related to the velocity and the estimated loss coefficient.
Where the transitions are gradual, the losses are small. At abrupt changes in cross-
sectional area, the losses are higher. Energy losses resulting from expansion are greater
than losses associated with contraction. Energy loss coefficients used for the SSA models
are presented in Table 6:
Table 6: Energy Loss Coefficients
Type of Transition
Expansion
Contraction
None
0
0
Manhole/Inlet
0.35
0.25
Culvert
1.0
0.9 - Projecting from fill CMP
Open Channel
0.3
0.1
Additional energy losses for structures having bends were divided between the two
joining pipes. The bend losses used for this project are based on NCDOT values, and are
shown below in Table 7.
Table 7: Bend Loss Coefficients
Angle (°)
Loss Coefficient
Angle (°)
Loss Coefficient
90
0.70
40
0.38
80
0.66
30
0.28
70
0.61
25
0.22
60
0.55
20
0.16
50
0.47
15
0.10
2.3.2 Starting Water Surface Elevation
The downstream limit of the HAECO Facility Improvements study area is located near
the mouth of Horsepen Creek. The starting water surface elevations for the SSA models
were generated using the normal depth method based of the channel slope at the outfall
(0.008 ft/ft).
2.3.3 Model Run Descriptions
The Autodesk SSA model was used to compute flood elevations at each structure
located in the HAECO Facility Improvements project study area for the water quality
HAECO Facility Improvements Project Page 2-5
Stormwater Report

Section 2: Hydrology and Hydraulics
event, 2-, 10-, 25-, 50- and 100 -year storm events. A digital copy of the SSA model is
included on the CD provided in Appendix E.
2.3.4 Hydraulic Evaluation of Radar Road
The following table summarizes the performance of the twin 8.9' x 6.6' corrugated metal
pipe (CMP) arches at Radar Road:
Table 8: Culvert Performance for at Radar Road
Although there are increases to peak flows, the downstream drainage system can
accommodate these increased flows. The existing twin 9.8' by 6.6' arched CMPs pass
896 cfs when flowing full. The 114" diameter closed CMP located at the Harris Teeter
distribution center conveys 753 cfs when flowing full. The Radar Road culverts and
Harris Teeter closed pipe will be flowing approximately half full during a 100 -year
storm event therefore there are no impacts to the performance of either of these
drainage systems.
2.3.5 Evaluation of Downstream Flooding
Approximately 85 feet from the top of bank (in the left overbank) is the toe of the water
quality pond embankment for the Harris Teeter distribution center. For this reason, a
check was made to confirm that the additional flows from the HAECO Facility
Improvements project would not cause adverse impacts to the existing water quality
pond embankment. Table 9 summarizes the size, slope and hydraulic characteristics of
the channel located immediately downstream of Harris Teeter.
Table 9: Hvdraulic Summary of Harris Teeter Oven Channel
Bottom
Top
Existing Water
Proposed Water
Channel
Culvert Invert
Roadway
Depth
Flood
Surface
Surface
Width
Elevation
Elevation
Slope
Capacity
Frequency
Elevations
Elevations
(feet NAVD 1988)
(feet NAVD 1988)
(feet)
(feet)
(ft/ft)
(ft/ft)
(feet NAVD 1988)
(feet NAVD 1988)
WQ Event
831.29
840.90
831.76
832.09
2 -Year
831.29
840.90
832.38
833.74
10 -Year
831.29
840.90
832.72
834.99
25 -Year
831.29
840.90
832.89
835.38
100 -Year
831.29
840.90
833.19
835.96
Although there are increases to peak flows, the downstream drainage system can
accommodate these increased flows. The existing twin 9.8' by 6.6' arched CMPs pass
896 cfs when flowing full. The 114" diameter closed CMP located at the Harris Teeter
distribution center conveys 753 cfs when flowing full. The Radar Road culverts and
Harris Teeter closed pipe will be flowing approximately half full during a 100 -year
storm event therefore there are no impacts to the performance of either of these
drainage systems.
2.3.5 Evaluation of Downstream Flooding
Approximately 85 feet from the top of bank (in the left overbank) is the toe of the water
quality pond embankment for the Harris Teeter distribution center. For this reason, a
check was made to confirm that the additional flows from the HAECO Facility
Improvements project would not cause adverse impacts to the existing water quality
pond embankment. Table 9 summarizes the size, slope and hydraulic characteristics of
the channel located immediately downstream of Harris Teeter.
Table 9: Hvdraulic Summary of Harris Teeter Oven Channel
Bottom
Top
Side
Channel
Channel
Floodplain
Depth
Width
Width
Slopes
Slope
Capacity
Capacity
(feet)
(feet)
(feet)
(ft/ft)
(ft/ft)
(cfs)
(cfs)
10
25
4
2:1
0.001
260
2,150
Assumed Manning's 'n' value= 0.06
Floodplain capacity is the flow needed to inundate the toe of the existing Harris Teeter pond
As shown in Table 9, the existing channel can almost convey the proposed conditions
10 -year flood without overtopping its banks. The flow needed to inundate the lowest
HAECO Facility Improvements Project Page 2-6
Stormwater Report

Section 2: Hydrology and Hydraulics
toe elevation of the Harris Teeter pond is 2,150 cfs which is significantly more than the
511 cfs that will leave the proposed HAECO site.
This existing open channel extends approximately 295 feet downstream of the Harris
Teeter culvert prior to entering Horsepen Creek which is a FEMA stream with an 832 -
acre (1.3 square miles) drainage area and 100 -year peak flow of 1,598 cfs. On the
upstream side of Radar Road (along Horsepen Creek), the drainage area increases to
1,344 acres (2.1 square miles) with a 100 -year peak flow of 3,018 cfs.
The location where the drainage area at the airport becomes less than 10% of the total
drainage area of Horsepen Creek is found downstream of Ballinger Road (DA = 3.1 sq.
mi.) and a point 1200 feet downstream of Ballinger Road (DA = 5.6 sq. mi.). An aerial
map of this area was evaluated to determine if there were any insurable structures
inundated by the 100 -year storm inside this footprint where the drainage area was less
than 10% of the drainage area leaving the HAECO site (241 acres). The following table
summarizes this evaluation:
Table 10: Evaluation of Downstream Insurable Structures for Flooding
As shown in Table 10, there are no insurable structures in close proximity to the
floodplain that would be adversely impacted by a relatively small increase to 100 -year
peak flows from the proposed HAECO Facility Improvements Project.
HAECO Facility Improvements Project Page 2-7
Stormwater Report
Approximate First
100 -Year Water
Structure Description and
Approximate
Floor Elevation (Ft
Surface Elevation
Location
Freeboard (ft)
NAVD '88)
(Ft NAVD '88)
Right overbank at confluence
of Harris Teeter unnamed
839.0
824.0
15.0
tributary and Horsepen Creek
Right overbank at FEMA Cross
845.0
818.0
27.0
Section 441 (at Radar Road)
Right overbank just
818'0
800.5
17.5
downstream of I-840
About 340 feet downstream of
800.0
796.0
4.0
Ballinger Road
As shown in Table 10, there are no insurable structures in close proximity to the
floodplain that would be adversely impacted by a relatively small increase to 100 -year
peak flows from the proposed HAECO Facility Improvements Project.
HAECO Facility Improvements Project Page 2-7
Stormwater Report

Section 2: Hydrology and Hydraulics
FiLyure 1: FEMA FIRM
2.3.6 Evaluation of Downstream Channel Stability
As shown in this report, peak flows are increasing due to the loss of the two onsite
detention ponds. As a result, a detailed hydraulic evaluation of the downstream
channel stability was performed at the request of DEQ. Appendix F includes a technical
memorandum that summarizes this hydraulic evaluation.
2.3.7 Closed Drainage Systems
Closed systems were designed to pass the 10 -year flood without surcharging the pipe.
With the exception of the SCM underdrain system, all drainage pipes are reinforced
concrete (RCP).
2.3.8 Outfall Protection for Closed Drainage System
Rip -rap pads are proposed at two locations in the high flow rate bioretention. These
outfalls are located where the flows enter back into the natural drainage system or the
bioretention ponds. The NY DOT method was used to design the length, width, depth
and size of the rip -rap pads. Appendix G shows the calculation used to size the rip -rap
pads.
HAECO Facility Improvements Project Page 2-8
Stormwater Report

Section 3: Water Quality Compliance
3.1 Overview
To satisfy the water quality requirements outlined in Session Law 2012-200, a proposed
0.8 -acre high flow rate bioretention pond is being proposed. Session Law 2012-200
requires runoff generated from the 1St inch of rainfall for a development project shall be
infiltrated into the ground. There are no specific requirements to remove total
suspended solids (TSS), nitrogen, or phosphorus. In addition, there are no requirements
to detain the 1 -year or any other storm event to at or below pre -project conditions. As
shown in this report, the proposed high flow rate bioretention pond exceeds the
minimum infiltration requirements set forth in Session Law 2012-200.
3.1.1 Proposed Impervious Areas
The separately attached construction plans and concept plan provided in Appendix A
show the proposed pond, new and existing impervious areas, location of flow splitters
and overall site layout. The following table summarizes the proposed impervious areas
associated with the HAECO Facility Improvements project:
Table 11: Minimum Area of Impervious Cover Required for Treatment
Location
Impervious Cover (acres)
Proposed Hangar
5.06
Proposed Apron
5.11
Proposed Fire Access Roads
0.32
Proposed Sidewalk
0.09
Existing HAECO Site to the East
11.92
Sub -Basin 60
TOTAL = 22.51 acres
Because the existing fire suppression pond is being abandoned as part of this project, the
proposed SCM will need to be designed to accept runoff from the system currently
going to the existing fire suppression pond. The stormwater runoff generated in sub -
basins 60, 70 and 80 will be redirected into the proposed SCM. Appendix C highlights
the areas that will drain to the pond along with a breakdown for the impervious area
contributed from each sub -basin. As a result, an additional 31.06 acres of impervious
area will be infiltrated in the proposed SCM as shown in the following table:
Table 12: Proposed Impervious Cover to SCM
Location
Impervious Cover (acres)
Proposed Hangar
5.06
Proposed Apron
5.11
Proposed Fire Access Roads
0.32
Proposed Side Walk
0.09
Existing HAECO Site to the East
11.92
Sub -Basin 60
0.42
Sub -Basin 70
9.22
Sub -Basin 80
14.09
Sub -Basin 90
7.34
TOTAL = 53.57 acres
HAECO Facility Improvements Project Page 3-1
Stormwater Report

Section 3: Water Quality Compliance
In total, the proposed SCM will have a contributing drainage area of 64.67 acres with
53.57 acres of impervious cover.
3.1.2 High Flow Rate Bioretention Pond Design Criteria
The State BMP Manual does not specifically have a set of design guidelines for a high
flow bioretention pond so the following guidelines were used in the design of the
proposed high flow bioretention pond:
♦ Infiltrate 100% of the runoff generated from the 1St inch of rainfall;
♦ Side slopes shall be no steeper than 3(H):1(V);
♦ SCM shall be located in a recorded drainage easement;
♦ A bypass or internal overflow is required for bypassing storm flows in excess of
the design flow;
♦ Media permeability shall be between 6 and 10 inches per hour with a targeted
detention time of 10 to 15 hours for infiltrating the water quality volume;
♦ Ponding depth for the water quality event shall be limited to 4.0 feet;
♦ Media depth will be 2 feet for each of the two soil media zones of the
bioretention pond;
♦ An underdrain shall be located under the soil media to keep the pond dry and
prevent groundwater from entering the pond; and
♦ A rip -rap energy dissipater shall be located at the outfall of each pipe entering
the pond.
3.1.3 Water Quality Volume (WQV)
The volume of runoff generated from the 1St inch of rainfall was calculated using an in-
house spreadsheet based on the Schuler Simple Method. This spreadsheet shows the
calculated water quality volume along with proposed SCMs stage -storage sizing (see
Appendix H). The following table summarizes the minimum required volume along
with the provided volume:
Table 13: Calculated Storage Volumes
Description
Impervious Area
(acres)
Surface Runoff
(ft3)
Required Area for Treatment
22.51
81,703
Compensatory Treatment of Sub -basins 60, 70, 80 & 90
31.06
105,057
Total Provided Area for Treatment
53.57
186,761
Net Credit for WQ Treatment
31.06
105,057
As shown in Table 13, the proposed high flow rate bioretention pond will infiltrate an
additional 105,057 cubic feet of runoff and 31.06 acres of impervious cover more than
required.
HAECO Facility Improvements Project Page 3-2
Stormwater Report

Section 3: Water Quality Compliance
3.1.4 Pond Design Summary
A concrete riser structure is proposed to control flows leaving the high flow rate
bioretention pond. The primary spillway will include the following elements: a poured,
reinforced concrete box riser and reinforced concrete outfall pipe with gaskets at joints.
Because the weir length on these structures is 18.67' and the flows entering the ponds
are generally very small, there were no emergency spillways proposed for the pond.
The following is a summary of the design for the proposed high flow rate bioretention
pond (See the separately attached plan set for additional details):
♦ Surface Area: The proposed high flow rate bioretention pond is larger than the
minimum size needed to achieve the water quality goals of the project. The
surface area of the pond was achieved by targeting a pond depth of less than 4.0
feet and a detention time between 10 and 40 hours. The more well -draining the
soils the smaller the footprint of the pond needed to drain the pond in
approximately 10 hours. As shown in this report, the surface area that drains the
pond in approximately 13 hours is 27,000 square feet (0.62 acres).
♦ Primary Outfall: A concrete box riser with an outside dimension of 6'x6' is
proposed with a primary weir elevation set at 584.40 feet NAVD 1988. The total
weir length of the primary outfall is 18.67 feet (four 4.66' long weirs). The
primary weir elevation was iteratively raised inside SSA until elevation 854.40
feet NAVD '88 whereby 0 cfs was leaving the pond in the water quality rainfall
event.
♦ Top of Dam: The top of dam is set at elevation 856.25 feet which is approximately
0.6 feet above the 100 -year flood elevation. The total dam height measured from
the toe of the embankment on the downstream side is approximately 2.25 feet.
The following table summarizes the water surface elevations at the proposed pond for
the water quality event, 2-, 10- and 100 -year floods:
Table 14: Water Surface Elevations at Proposed Pond
Water Quality
Event
2 -Year Storm
(NAVD '88)
10 -Year Storm 100 -Year Storm
(NAVD '88) (NAVD '88)
854.35
855.34
855.50 855.66
HAECO Facility Improvements Project Page 3-3
Stormwater Report

Section 3: Water Quality Compliance
Riser
The riser detail provided in the separately attached plan set shows the 6'x6' concrete box
to control water surface elevations inside the proposed SCM. The primary spillway was
set at elevation 854.40 feet which is the dynamic elevation calculated inside SSA for the
water quality storm event (an NRCS Type II distribution with 1.0 inches of rainfall). The
riser has a 42" diameter RCP barrel that conveys flow from the pond to a new 48"
diameter closed drainage system. This 48"diameter closed system conveys the by-pass
flows for larger storm events from the eastern side of the existing HAECO development.
An anti -floatation calculation (See Appendix I) was performed for the pond riser
resulting in a factor of safety of 1.22. This calculation ignores the friction forces of the
underlying soil and therefore a factor of safety larger than 1.22 would be achieved in real
conditions. Because this is a dry pond and water levels will rarely reach 6" above the
crest of the weir therefore a factor of safety of 1.22 is acceptable.
Flow Splitters
Three flow splitters are proposed to divert stormwater runoff from the proposed closed
drainage system into the high flow rate bioretention pond. For water quality rainfall
event (1.0 inch of rain), 100% of the runoff generated will flow directly into the high flow
rate bioretention pond. Inside each flow splitter is a weir wall that will direct flows
generated from larger storm events into a closed by-pass pipe. The elevation of this weir
wall was calculated in Autodesk SSA by iteratively adjusting the elevation of the wall
until no flow was being diverted in the water quality rainfall event.
The splitter box located just north and west of the pond (Structure 6) will require a
special design. Flows that go over the weir wall will drop into a concrete manhole
structure and eventually into the sites main 72 inch diameter RCP. The following table
summarizes the key elevations for the three proposed concrete flow splitters:
Table 15: Summary of Flow Splitter Design
The separately attached design plans provide additional details on the size and
construction of the flow splitters being used for this project.
HAECO Facility Improvements Project Page 3-4
Stormwater Report
Pipe to Pond
Pipe Sizes
Splitter #
Invert Elevation
Weir Wall Height
Entering Splitter
Pipe to Pond
Diameter (in)
(feet NAVD 1988)
(ft)
Box (in)
1 (structure 31)
870.94
1.15
24"
15"
2 (structure 53)
855.90
0.75
48"
24"
3 (structure 6)
851.69
3.2
72" and 42"
42"
The separately attached design plans provide additional details on the size and
construction of the flow splitters being used for this project.
HAECO Facility Improvements Project Page 3-4
Stormwater Report

Section 3: Water Quality Compliance
Table 16: Flow Splitter Performance
Splitter #
Water Quality Event
10 -Year Storm Event
Flow To Pond Flow Around Pond
WS) (cfS)
Flow To Pond Flow Around Pond
(cfS) WS)
1 (structure 31)
3 0
4 18
2 (structure 53)
12 0
33 43
3 (structure 6)
32 0
60 148
As shown in Table 16, between 60% and 82% of the peak flows from the 10 -year storm
event will be diverted around the pond.
Detention Time and Soil Media for Hiah Flow Rate Bioretention Pond
Per discussions with DEQ, it was agreed that the proposed high flow rate bioretention
pond would detain the water quality event for between 10 and 40 hours. To achieve this
goal, a well -draining sand media is needed that promotes infiltration at a rate that is not
too quick (3 or 4 hours) and not too long (over 40 hours). With an assumed infiltration
rate of 10 inches per hour for this well -draining sand, a footprint was iteratively
determined until the time to drain the pond was approximately 10 hours. This area was
calculated to be 14,563 square feet. For those areas outside the well -draining sands an
infiltration rate of 2 inches/hour was assumed. As shown in Appendix J, the combined
flow rate passing through the soil media and leaving the pond is 3.9 cfs. To achieve this
infiltration rate the SSA model includes a small culvert (7.85" diameter RCP) that
conveys 3.9cfs from the pond.
For the area of well -draining sand, the construction of the high flow rate bioretention
pond will mimic the design of a PGA golf green. It is assumed that the best draining
soils that can be stockpiled from the onsite borrow area will be used for those areas
outside the well -draining sands. At a minimum this media in Zone 1 will have a
permeability of 2 inches/hour. The following is a summary of the construction for the
area of the pond that mimics the PGA golf green:
Option #1 for Zone 2 (No. 57 Stone at base)
• 12" thick base of No. 57 stone (approximately 3/4" in size)
• 4" of washed sand
• T of well -draining sand -soil mix (with a permeability of 10 inches/hour)
Option #2 for Zone 2 (Pea Gravel at base)
• 12" thick base of peak gravel (100% passage of 3/8" sieve)
• 2' of well -draining sand -soil mix (with a permeability of 10 inches/hour)
Specifications for the two soil zones will be prepared at final design.
HAECO Facility Improvements Project Page 3-5
Stormwater Report

Section 3: Water Quality Compliance
Energy Dissipation
At the outfall of each pipe entering the high flow rate bioretention pond are designed
rip -rap energy dissipaters. These energy dissipaters were designed similar to a plunge
pool where the area immediately downstream of the pipe outfall will be excavated to a
depth of 1.0 feet. As water fills the pool it will enter the upper limits of the pond by
overtopping the outer limits of the rip -rap energy dissipater which will be acting like a
"level spreader". Class 1 rip -rap is proposed to a depth of 24 inches as shown in the
separately attached plans.
Maintenance and Operation Procedures
A maintenance and operation plan for the bioretention facilities has been included with
this report as Appendix L.
3.2 Conclusion
As shown in this report, the proposed high flow rate bioretention pond is designed to
bring the HAECO Facility Improvements project at the Piedmont -Triad International
Airport in compliance with the State's requirements for water quality as outlined in
Session Law 2012-200. By diverting runoff for the water quality rainfall event from
basins 60, 70, 80 and 90 into the proposed SCM, the airport is providing treatment for
53.57 acres of impervious cover. As shown in this report, the proposed SCM is
providing treatment for approximately 31.1 acres more than the minimum required
amount. The airport would like to request a water quality credit to offset the need to
provide or minimize treatment with a future onsite development.
HAECO Facility Improvements Project Page 3-6
Stormwater Report

Appendix A
Proposed Concept Plan

WDICKSON
community Infrastructure consultants
Proposed Concept Plan - Appendix A
F High Flow Rate Bioretention Pond
Piedmont -Triad International Airport
HAECO Site Development
200 100
1 inch = 200 feet
200 Feet

Appendix B
Input Data for SSA

Project: HAECO Facility Improvement @ PTIA, Greensboro, NC
Prepared by: DJK
Date: March 9, 2016
SWMM Input Data
Appendix B
241.31
EXISTING CONDITIONS
SUBBASINS
SWMM Sub-
Basin ID
Pervious
RCN
Area
(acres)
Area (sq. ft.)
Flow
Length
ft
Width
(ft.)
Elevation
Change (ft.)
Basin
Slope (%)
Percent
Impervious
%
10
74
77.1
3356329
3294
1019
21
0.6%
31%
20
74
28.8
1253061
1259
995
11
0.8%
34%
30
74
19.3
842697
1856
454
19
1.0%
40%
40
74
4.1
180429
1027
176
12
1.1%
44%
50
74
3.6
156421
520
301
10
1.9%
21%
60
74
5.2
225908
841
269
14
1.7%
8%
70
74
9.2
401743
1215
331
16
1.3%
100%
80
71
11.2
489458
1095
447
21
1.9%
100%
85
71
6.7
293360
338
868
36
10.7%
24%
90
74
6.5
282480
1359
208
27
2.0%
88%
100
74
19.3
840092
1465
573
9
0.6%
43%
110
74
6.1
263574
496
531
7
1.3%
29%
120
74
16.3
711138
958
742
25
2.6%
92%
130
74
4.5 1
195877 1
944
207
35
1 3.7%
44%
142
74
9.4
407511
707
576
35
5.0%
26%
145
74
12.2
529689
944
561
35
3.7%
2%
150
74
1.9
81743
850
96
5
0.6%
75%
241.31
241.31
PROPOSED CONDITIONS
SUBBASINS
SWMM Sub-
Basin ID
Pervious
RCN
Area
(acres)
Area (sq. ft.)
Flow
Length
ft
Width
(ft.)
Elevation
Change (ft.)
Basin
Slope (%)
Percent
Impervious
0
�
10
74
77.05
3356329
3294
1019
21
0.6%
31%
20
74
28.77
1253061
1259
995
11
0.8%
34%
30
74
19.35
842697
1856
454
19
1.0%
40%
40
74
4.14
180429
1027
176
12
1.1%
44%
50
74
3.59
156421
520
301
10
1.9%
21%
60
74
5.19
225908
841
269
14
1.7%
8%
70
74
9.22
401743
1215
331
16
1.3%
100%
80
71
10.89
474558
1095
433
21
1.9%
100%
90
74
6.5
282480
1359
208
27
2.0%
88%
100
74
19.29
840092
1465
573
9
0.6%
43%
110
74
6.05
263574
496
531
7
1.3%
29%
120A
74
15.00
653246
1487
439
25
1.6%
88%
120B
74
2.43
105911
640
165
21
3.3%
90%
130
74
4.50
195877
944
207
35
3.7%
47%
142
74
9.36
407511
707
576
35
5.0%
32%
144-A
74
0.84
38104
143
266
0.715
0.5%
100%
144-B
74
0.86
38293
127
302
0.635
0.5%
100%
144-C
74
0.75
32703
137
239
0.685
0.5%
100%
144-D
74
0.88
38306
152
252
0.76
0.5%
100%
144-E
74
0.51
22097
175
126
0.875
0.5%
100%
144-F
74
0.47
20479
102
201
0.51
0.5%
100%
144-G
74
0.54
23709
108
220
0.54
0.5%
100%
144-H
74
1.74
75931
237
320
1.185
0.5%
42%
146-A
74
4.56
198487
849
234
4.245
0.5%
100%
146-B
74
1.71
74691
406
184
2.03
0.5%
100%
146-C
74
1.53
66657
406
164
2.03
0.5%
100%
146-D
74
1.99
86824
450
193
2.25
0.5%
100%
146-E
74
0.67
29025
106
274
0.53
0.5%
28%
148
74
1.1
47109
275
171
44
16.16%
10%
150
74
1.9
81743
850
96
5
0.59%
75%
241.31

Appendix C
Existing and Proposed Conditions Drainage Area Maps

PSWK Existing Conditions Drainage Area Map - Appendix C 500 250 0 500 Feet
W DICKJON Piedmont Triad International Airport
community infrastructure consultants HAECO Facility Improvements 1 inch = 500 feet

WW.WK Contributing Drainage Areas and Impervious Cover to SCM - Appendix C 300 150 0 300 Feet
DICKSON Piedmont -Triad International Airport
community infrastructure consultants HAECO Site Development 1 inch = 300 feet

Appendix D
Existing and Proposed Conditions Land Use Mapping

�'�l< Existing Landuse Map - Appendix D _� 500 250 0 500 Feet
W DICKSON Piedmont -Triad International Airport
community infrastructure consultants HAECO Site Development 1 inch = 500 feet

�'�l< Proposed Landuse Map - Appendix D <�v 500 250 0 500 Feet
W DICKSON Piedmont -Triad International Airport s ,
community infrastructure consultants HAECO Site Development 1 inch = 500 feet

Appendix E
CD with Digital Copy of Autodesk SSA Models

Appendix F
Stream Stability Evaluation Memorandum

M E M O R A N D U M
720 Corporate Center Drive Raleigh, North Carolina 27607
TO: Sue Homewood & Mike Randall
FROM: David Kiker, PE
DATE: February 24, 2016
�N&WK
"�r
W DICKSON
community Infrastructure consultants
919.782.0495 tel. [phone] fax
RE: HAECO Facility Improvements Project— Evaluation of Downstream
Stream Stability
This memorandum summarizes the WK Dickson evaluation of downstream stream stability as a
result of the onsite development of the HAECO Facility Improvements Project at the Piedmont Triad
International Airport in Greensboro, North Carolina. An overview map that shows the closed
drainage systems for the HAECO and Harris Teeter Distribution sites, and the unnamed tributary to
Horsepen Creek is provided as Attachment #1. The conclusions drawn in this memorandum are
based on the 2 -year storm event and a hydraulic evaluation that used the U.S. Army Corps of
Engineers HEC -RAS 4.1.1 model, AutoCAD Sanitary Sewer Analysis (SSA) model, and an in-house
excel spreadsheet. This 2 -year storm event is typically used in the industry when evaluating stream
stability given the assumption that a channel can repair itself in the less frequent storms. A check
was also made using the 100 -year flood event to confirm that the proposed downstream
improvements are sustainable for a larger storm event.
Evaluating stream stability is an inexact science given the many variables that affect stream stability.
Such factors as channel shape, channel slope, cohesiveness of bank material, riparian bank
vegetation, bank armoring, bed composition, in -stream sediment load, downstream tailwater
conditions, meander pattern and other variables can all affect a stream's stability. A common
approach to evaluating stream stability is to determine the stream's velocity and shear stress and
compare these values to published permissible shear stress and stream velocities for streams of
similar conditions. The permissible shear stress and stream velocity presented in this memorandum
are based on the U.S. Army Corps of Engineers document titled Stability Thresholds for Stream
Restoration Materials, dated May 2001. As shown in this memorandum, the post -project conditions
along the unnamed tributary to Horsepen Creek will remain stable as the post -project velocities and
shear stresses are within the permissible ranges for a stable channel.
7

Existing Downstream Open Channel Conditions
The existing downstream open channel is approximately 295 feet in length prior to its mouth at
Horsepen Creek. Horsepen Creek is a FEMA mapped stream with a drainage area of 2.1 square
miles at Radar Road. The channel invert at Horsepen Creek is approximately 1 foot below the 114
inch diameter CMP leaving the Harris Teeter site. For this reason, during a significant storm event
the entire length of the unnamed tributary to Horsepen Creek and the Harris Teeter culvert itself
will be under the backwater effect from Horsepen Creek. This high tailwater condition will have the
effect to dampen the energy of the flow leaving the Harris Teeter closed pipe system as it passes
through the open channel prior to entering Horsepen Creek. Located at the downstream limits of the
unnamed tributary to Horsepen Creek is an open channel that is partially covered with NCDOT
Class A rip -rap with a Dso of 6 inches. This material is relatively small for rip -rap protection and the
fact that it has not mobilized downstream supports that contention that the high tailwater
conditions of Horsepen Creek will help dampen the effect of erosive flows passing through the
unnamed tributary. Table 1 shows the 2 -year water surface elevation from Horsepen Creek
interpolated from output found in the duplicate effective HEC -RAS model.
Table 1: Tailwater Condition from Horsepen Creek
Harris Teeter Culvert Invert
(ft NAVD'88)
2 -Year WSEL from Horsepen
Creek (ft NAVD'88)
Tailwater Depth (ft)
820.21
823.4
3.2
WSEL - water surface elevation
As shown in Table 1, the tailwater depth inside the Harris Teeter culvert is in excess of 3 feet. This
equates to the majority of the open channel being inundated close to the channel banks in a 2 -year
flood event. As a result, the unnamed tributary to Horsepen Creek will have its stream energy
dampened by the high tailwater condition.
Also providing protection from future erosion are two grade control structures located in the
channel bottom as shown in Attachment #1. These grade controls that were constructed for the
following reasons:
• Grade Control #1: Approximately 50 feet downstream of the Harris Teeter culvert is a rip -
rap (NCDOT Class B and Class I) lined grade control that was designed to create a pool of
water at the outfall of the culvert (see photo #1 and #2).
• Grade Control #2: Approximately 35 feet from the mouth at Horsepen Creek is a City of
Greensboro sanitary sewer line. This sewer system is being protected by a series of gabion
baskets set in the channel bottom (see photo #3 and #4). The top of these baskets are
approximately 0.25 inches below the invert of the 114 inch diameter CMP leaving the Harris
Teeter site.
Although there are a series of pools located between the outfall of the Harris Teeter culvert and the
Grade Control #2 that have localized slopes, the overall channel slope is extremely flat as shown in
Table 1.
r�

Table 2: Overall Channel Slove
Culvert Invert
Channel Invert at
Existing
112
Temporary Construction Condition (Phase 5)
359
Final Recommended Proposed Conditions
339
Overall
Elevation at Harris
Sanitary Sewer
Elevation
Channel
Channel Slope
Teeter Outfall
Line Elevation
Change (ft)
Distance (ft)
(ft/ft)
(ft NAVD '88)
(ft NAVD '88)
820.21
819.96
0.25
265
0.001
The existing downstream open channel is trapezoidal in shape with the following typical
dimensions (see photo #5):
• Bottom width: 12 feet
• Top width: 15 feet
• Bank height: 3.5 to 5.0 feet
• Manning's "n" value: 0.05
• Channel capacity flowing full: 260 cfs
The riparian corridor between the Harris Teeter culvert outfall and Horsepen Creek include small
trees, underbrush and kudzu. The kudzu vines have choked out much of the trees and other
vegetation that would typically provide a root system to protect the channel banks.
Hydrologic Evaluation
Peak flows in the model were obtained from a WK Dickson prepared AutoCAD Sanitary Sewer
Analysis model that runs off the EPA SWMM engine. The model was developed to size the pipe
infrastructure and high flow rate bioretention pond proposed for the onsite HAECO Facility
Improvements Project. The following table is a summary of the peak flows for the existing (pre -
project), the temporary during construction, originally proposed, and final recommended
conditions:
Table 3: Summary of Peak Flows
Condition
2 -Year Peak Flow (cfs)
Existing
112
Temporary Construction Condition (Phase 5)
359
Final Recommended Proposed Conditions
339
Hydraulic Evaluation
In addition to the SSA SWMM model, WK Dickson developed a HEC -RAS model and an in-house
spreadsheet to calculate shear stresses and further evaluate stream stability. The HEC -RAS model
includes seven (7) cross sections based on field measured data and City of Greensboro GIS
topographical mapping generated from LiDAR data. A copy of the in-house spreadsheet can be
found in Attachment #2.
3

The following series of table summarize the findings from the WK Dickson hydraulic evaluation:
Table 4: Summary of Calculated Overall Shear Stress for 2 -Year Storm Event
Condition
2 -Year Shear Stress (lbs/sq ft)
Existing
0.17
Temporary Construction Condition (Phase 5)
0.28
Final Recommended Proposed Conditions
0.27
Shear stresses calculated using WK Dickson in-house spreadsheet for overall channel slope
The shear stress for all the evaluated conditions are extremely low for a typical open channel in the
Piedmont region. WK Dickson typically targets a shear stress value of less than 0.50 lbs/square foot
on their natural stream restoration designs and rarely achieves a value under 0.30 lbs/square foot in
Piedmont stream. Once the vegetation is established these natural stream restoration projects with
designed shear stresses of 0.50 lbs/square foot become stable fairly quickly. The following table
summarizes the calculated channel velocity for the 2 -year storm event using both HEC -RAS and the
AutoCAD SSA model (based on the EPA SWMM model):
Table 5: Summary of Calculated Overall Stream Velocity
Condition
HEC -RAS Calculated 2 -Year
SWMM Calculated 2 -Year
Channel Velocity (ft/sec)
Channel Velocity (ft/sec)
Existing
3.5
2.7
Temporary Construction
5.4
3.2
Condition (Phase 5)
Final Recommended Proposed
5.4
3.3
Conditions
Note: HEC -RAS results were averaged over the entire reach.
The 2 -year channel velocities shown in Table 5 for all the evaluated conditions are relatively low for
a typical open channel in the Piedmont region. The average velocity calculated in HEC -RAS for the
proposed conditions would have been 4.7 feet per second if the two rip -rap lined reaches of
channel been removed from the calculation. The SSA model results were considerably lower than
HEC -RAS because the model is generating a weighted velocity that includes overbank flows. A
more detailed analysis of the results presented in Table 5 is provided in the next section of the
report.
Permissible Shear Stress and Velocity
Based on WK Dickson February 5, 2016 field walk, and the U.S. Army Corps of Engineers document
titled Stability Thresholds for Stream Restoration Materials, we are proposing the following
permissible shear stress and velocity for the 2 -year flood event:
4

Table 6: Recommended Permissible Shear Stress and Velocity for 2 -Year Flood Event
Permissible Shear Stress (lb/square foot) Permissible Velocity (ft/sec)
0.5 6.0
These recommended threshold values presented in Table 6 are relatively low given that natural
stable streams with cohesive banks in the Piedmont region very often see velocities that exceed 6
feet per second for a 2 -year flood. A review of the 11 other tributaries to Horsepen Creek found in
the FEMA duplicate effective HEC -RAS model shows that the typical 2 -year velocities range from 3
to 6.5 feet per second with maximum values approaching 10 feet per second.
When evaluating the permissible shear stress and channel velocity downstream of the HAECO site,
one must consider that the open channel is relatively short in length and has two significant grade
control structures that set the overall channel slope. The gabion baskets at Grade Control #2 are set
so that they are not exposed on the upstream face and as a result will be able to handle shear
stresses that exceed 10 lbs/square foot. While the Class I rip -rap found at Grade Control #1 can
withstand a shear stress of approximately 5 lbs/square foot. To evaluate channel bank erosion, you
must understand that typically the banks become unstable as a result of the toe of the channel
becoming unstable. This typically occurs when the channel thalweg experiences degradation. The
presence of these grade control structures will limit future channel degradation, will maintain the
existing overall channel slope and as a result help minimize and future channel bank erosion.
Other Design Considerations
The culverts at Radar Road are twin 8.9 feet by 6.6 feet CMP pipe arches that when flowing full
convey approximately 896 cfs. The primary closed pipe that is located at the Harris Teeter site is a
9.5 feet diameter CMP that convey approximately 750 cfs when flowing full. It appears that the
engineer who designed these two closed drainage systems considered ultimate build out for the
landuse conditions of the upstream watershed. The landuse may have reflected an industrial
landuse not reflective of the current airport's landuse which is primarily composed of highly
impervious pockets of industrial landuse with the majority of the drainage area flat grass infields.
The peak flows found in the existing and proposed conditions model prepared by WK Dickson are
relatively low given the 241 acre (0.4 square mile) drainage area and culvert capacity of Radar Road
and the primary Harris Teeter closed drainage system. It also appears that the existing open
channel was constructed with this same "conservative" hydrologic approach used to design the
Radar Road and Harris Teeter closed pipe systems. When evaluating channel capacity alone, the
existing open channel is adequately sized to convey 260 cfs prior to overtopping its banks. The 1 -
year peak flow is a storm event that should reach approximately the bankfull elevation. The 1 -year
proposed conditions peak flow is 259 cfs which indicates that the existing channel is appropriately
sized for the post -project conditions for the HAECO Facility Improvements Project.
Conclusions
The resultant shear stresses and velocities for the unnamed tributary to Horsepen Creek are under
the permissible values found in Table 6. Grade Control Structures #1 and #2 are locking in the
channel thalweg and as a result will limit future bank erosion. For these reasons, we are not
recommending additional onsite or offsite changes to the current design.
5

AV
le
AV

AL
At
�L
ri i

WDICKSON
community Infrastructure consultants
N Stream Stability Evaluation Map
WF
s Piedmont -Triad International Airport
HAECO Site Development
100 50
1 inch = 100 feet
100 Feet

Shear Stress Analysis of Unnamed Tributary to Horsepen Creek
Project: HAECO Facility Improvement Project
Location: Downstream Channel (Below Harris Teeter Distribution Center)
Engineer: DJK
Date: 2-23-16
Mannings Equation, Q=(A)( 1.49 Ro bb S0.5
n )I
Shear Stress, T = yds
T = shear stress in Ib/sq. ft.
y = unit weight of water, 62.4 Ib/cu. ft.
d = flow depth in ft.
s = channel slope in ft./ft.
TemporaryLiners
Material
A[[ow Shea6tress
Phase 5 (During Construction Prior to Final SCM Online)
Final Recommended Proposed Site Conditions
Tacked Mulch
Existing Conditions
Jute Net
Storm
Design
Storm
Design
Chan Bot Side Side Slope
Design
Storm
Design
Chan Bot Side Side Slope
Design
Chan
Wetted
Hydraulic
Mann.
Channel
Q
Calc.
HEC -RAS
Shear
Temp.
Perm.
Event
Flow (cfs)
Width
Slope Length
Depth
Area
Perim., Pw
Radius
"n"
Slope
Allow.
Depth
Velocity
Stress
Liner
Liner
1 -Year
84
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
2.5
3.5
0.15
NA
NA
2 -Year
112
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
2.9
3.8
0.17
NA
NA
10 -Year
187
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
3.6
4.5
0.21
NA
NA
100 -Year
312
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
4.5
5.3
0.27
NA
NA
Shear Stress, T = yds
T = shear stress in Ib/sq. ft.
y = unit weight of water, 62.4 Ib/cu. ft.
d = flow depth in ft.
s = channel slope in ft./ft.
TemporaryLiners
Material
A[[ow Shea6tress
Phase 5 (During Construction Prior to Final SCM Online)
Final Recommended Proposed Site Conditions
Tacked Mulch
0.35
Jute Net
Storm
Design
Storm
Design
Chan Bot Side Side Slope
Design
Chan
Wetted
Hydraulic
Mann.
Channel
Q
Calc.
HEC -RAS
Shear
Temp.
Perm.
Event
Flow (cfs)
Width
Slope Length
Depth
Area
Perim., Pw
Radius
"n"
Slope
Allow.
Depth
Velocity
Stress
Liner
Liner
1 -Year
259
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
4.2
5.0
0.25
NA
NA
2 -Year
339
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
4.7
5.4
0.27
NA
NA
10 -Year
518
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
5.3
6.5
0.31
NA
NA
100 -Year
675
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
5.7
7.2
0.33
NA
NA
Shear Stress, T = yds
T = shear stress in Ib/sq. ft.
y = unit weight of water, 62.4 Ib/cu. ft.
d = flow depth in ft.
s = channel slope in ft./ft.
TemporaryLiners
Material
A[[ow Shea6tress
Phase 5 (During Construction Prior to Final SCM Online)
(Ib/sgft)
Tacked Mulch
0.35
Jute Net
Storm
Design
Chan Bot Side Side Slope
Design
Chan
Wetted
Hydraulic
Mann.
Channel
Q
Calc.
HEC -RAS
Shear
Temp.
Perm.
Event
Flow (cfs)
Width
Slope Length
Depth
Area
Perim., Pw
Radius
"n"
Slope
Allow.
Depth
Velocity
Stress
Liner
Liner
1 -Year
286
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
4.5
4.9
0.26
NA
NA
2 -Year
359
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
4.8
5.4
0.28
NA
NA
10 -Year
509
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
5.3
6.2
0.31
NA
NA
100 -Year
634
12
1 5.7
4
64
23
2.7
0.050
0.0009
115
5.7
6.8
0.33
NA
NA
Shear Stress, T = yds
T = shear stress in Ib/sq. ft.
y = unit weight of water, 62.4 Ib/cu. ft.
d = flow depth in ft.
s = channel slope in ft./ft.
TemporaryLiners
Material
A[[ow Shea6tress
Material
(Ib/sgft)
Tacked Mulch
0.35
Jute Net
0.45
Straw w/Net
1.45
SytheticMat
2.00
ClassA
1.25
ClassB
2.00
Class[
3.40
Class[I
4.50
Max. Permissibb Velocitiesfor Unproected Soilsin Ex. Channels
Material
Max Permissib6 Velocity(Us)
FincSand(noncollidl)
2.5
Sand Loam(noncollidl)
2.5
SiltLoam(noncollidl)
3.0
OrdinaryFirm Loam
3.5
FincGravel
5.0
Stiff Clay(verycollidal)
5.0
Graded,Silt toCobbles
5.0
Notes:
Side slope = horiz./vert.
Depth and Velocity calculated using WK Dickson generated HEC -RAS model for average overall
ax. Allow. Design V for Vegetative Channels
71Slope Soil
Grass Lining Pemnssibb
V 0t 0
5% Sands/Sill
Bemuda
5.0
Tall Fescue
4.5
KYBhregrass
4.5
Gra,s-legarrcaix
3.5
ClayMixcs
Bermuda
6.0
Tall Fescue
5.5
KY Bluegrass
5.5
Grass-legum,nix
4.5
10% Sands/Silt
Bermuda
4.5
Tall Fescue
4.0
xYBluegrass
4.0
Grass -leges -aux
3.0
Clay Mixes
Bermuda5.5
Tall Fescue
5.0
KY Bluegrass
5.0
Grass-legunvtnx
3.5
and velocity
Attachment #2

Appendix G
Outlet Protection Calculation

25
LIJ
Q
U
0 5 10 15 20 25
DIAMETER OF PIPE IN FEET
APPENDIX F
ZONE
APRON CLASS
MATERIAL OF
STONE
SIZE
OF
STONE
LENGTH
OF
APRON •
MINIMUM
THICKNESS
OF STONE
I
STONE FINE
J.
4 X D
9"
2
STONE LIGHT
6"
6 X D
12"
3
STONE MEDIUM
13"
8 X D
18"
4
STONE HEAVY
23"
B X D
30"
5
STONE HEAVY
23"
10 X 0
30"
6
STONE HEAVY I
23"
12 X D
30"
7
REQUIRES LARGER
DEVICE. DESIGN IS
PROCEDURE.
STONE OR ANOTHER TYPE OF
BEYOND THE SCOPE OF THIS
MOTH = DIAMETER f 0.4 (LENGTH)
: 6' MINIMUM
• LENGTH TO PREVENT SCOUR HOLE, MIN LENGTH 10'
NAME
WEIGHT
SIZE
SPECIFICATIONS
(LBS)
I
oma
RIP -RAP
roma
30% SHALL WEIGH AT LEAST 100
CLASS i
5 — 200
LBS EACH. NO MORE THAN 101
SHALL WEIGH LESS THAN 15 LBS.
EACH.
60% SHALL WEIGH AT LEAST 100
CLASS 2
25 — 250
LBS EACH. NO MORE THAN 59
SHALL WEIGH LESS THAN 50 LBS.
EACH.
EROSION CONTROL STONE
CLASS A
2' — 6"
10% TOP & BOTTOM SIZES.
NO GRADATION SPECIFIED.
CLASS B
15 — 300
1
NO GRADATION SPECIFIED.
EMENEENNEENNE
FR "'
DDICKSON
.. No 1607
R ej01
o,�s
oma
community infrastructure consultantssau
roma
SOURCE • 'BANK & CHANNEL LINING PROCEDURES•,
NEW YORK DEPARTMENT OF TRANSPORTATION,
DIVISION OF DESIGN AND CONSTRUCTION, 1971.
FR "'
DDICKSON
.. No 1607
R ej01
o,�s
oma
community infrastructure consultantssau
roma

Appendix H
Water Quality Calculation and Stage -Storage Relationship for SCM

Appendix H
Water Quality Volume and Stage Storage at Proposed Central High Flow Bioretention Pond
Project: HAECO Facility Improvement @ PTIA, Greensboro, NC
Prepared by: DJK
Date: March 9, 2016
Summary of Impervious Areas
NODE INVERT
Description
Impervious
Area ac
Total Drainage
Area ac
Basin 60
0.42
5.19
Basin 70
9.22
9.22
Basin 80
14.09
14.09
Basin 90
7.34
8.93
Basin 120
11.92
13.95
Proposed Apron
5.11
6.59
Proposed Hangar
5.06
5.95
Proposed Access Rd
0.32
0.66
Proposed Sidewalk
0.09
0.09
Total Provided Treatment Area
53.57
64.67
DICKSON
En;N e . Planners. S-eyors
Landscape Aahilels
R,., Runoff coefficient
The R,- is a measure of the site response to rainfall events, and in theory is calctilated as:
R,. = r/p, where rand pare the voltune of storm rttuoff and storm rautfall,
respectively, expressed as inches.
The R,, for the site depends on the nature of the soils, topography, and cover. However. the
prullary influence on the R, in a bare areas is the amount of imperviousness of the site,
hupervious area is defined as those surfaces in the landscape that cannot infiltrate rainfall
consisting of building rooftops, pavement, sidewalks, driveways, etc. In the equation R. -
0.05 + 0.009(7), "1" represents the percentage of impervious cover expressed as a whole
number. A site that is 75% impervious would use I = 75 for the purposes of calculating R,,.
Total Required 22.51 27.25 Calculate the required volume to be detained for the first 1" of runoff (new impervious area only):
Volume = 1.9 acre-feet
Calculate the runoff coefficient: Volume = 81,703 ft'
Rv=0.05+0.009(la) Calculate the required volume to be detained for the first 1" of runoff:
Rv = runoff coefficient = storm runoff (inches) / storm rainfall (inches) Volume = (Design rainfaI1)(Rv)(Drainage Area)
la = percent impervious = impervious portion of the drainage area (ac.)/drainage area (ac.) Volume = 1" rainfall * Rv * 1/12 (feet/inches) * Drainage Area
la 82.84
Rv= 0.80 (in./in.)
Stage Storage Relationship
Volume = 4.3 acre-feet
Volume= 186,761 W
112=h3 A,+Az+ A,•A,
Stage -Storage from Contours - Proposed Detention Facility - High Flow Bioretention Pond
NODE INVERT
SWMM
CONTOUR DEPTH
CONTOUR
AREA
INCREMENTAL
VOLUME
S
ACCUMULATIVE
VOLUME
S
TOTAL
VOLUME
(FT)
(FT)
(FT)
(AC) (SF)
(GAL)
(CF)
(AC*FT)
(GAL)
(CF)
(AC -FT)
(%)
848.50
848.50
0.00
0.00 1
849.00
0.50
0.00 2
6
1
0.000
6
1
0.000
0%
850.00
1.50
0.00 3
19
2
0.000
24
3
0.000
0%
Pond Bottom
851.00
2.50
0.33 14,563
4925
0.113
36,866
4,928
0.113
2%
852.00
3.50
0.56 24,328
143909
19238
0.442
180,775
24,166
0.555
9%
853.00
4.50
0.60 26,138
188715
25228
0.579
369,490
49,394
1.134
18%
854.00
5.50
0.64 28,005
202469
27066
0.621
571,959
76,460
1.755
289/6
855.00
6.50
0.69 29,929
216648
28962
0.665
788,607
105,421
2.420
38%
856.00
7.50
0.73 31,910
231254
30914
0.710
1,019,862
136,336
3.130
49%
857.00
8.50
0.78 33,948
246287
32924
0.756
1,266,148
169,259
3.8
1 61%
858.00
9.50
0.83 36,043
261745
34990
0.803
1,527,894
204,250
4.689
740%
859.00
10.50
0.88 38,196
539362
72102
1.655
2,067,256
276,352
6.344
100%
Incremental volume determined using "conic' method as described in USACE HEC -1 manual
Pond bottom
Elevation that exceeds the water quality volume (assuming static elevation with no infiltration)

Appendix I
Anti -Floatation Calculation for Riser

Riser Structure Flotation Calculation
Project: HAECO Site Development
Prepared by: DJK
Dated: 11-19-15
3ottom of pond with regards to soil (invert of underdrain system is 848.5
:alc
_alc
_alc
_alc
_alc
_alc
_alc
:alc
Design Input (Target factor of safety of 1.2)
_alc
:alc
Conservative Assumptions:
Bouyant force measured at top of structure lid
100 -year flood depth is 8.6 feet in depth (calculation went to elevation 9.5 feet)
Weight of soil on outfall pipe not accounted for in calculation
Anti-Floatation.xls
Appendix H
QC Check on Calcs
Invert Out Elev.
848.50
Primary Weir Elev.
853.80
Overflow Weir Elev.
853.80
Secondary Weir Hght (ft)
0.00
Secondary Weir Width (ft)
4.67
Primary Weir Hght (ft)
0.00
Primary Weir Width (ft)
4.67
Top of Box Elev.
853.80
Height of box (below top)
5.3
Inside Lgth (ft) (perpendicular to flow)
4.67
Inside Width (ft)
4.67
Outside Lgth (ft) (perpendicular to flow)
6.00
Outside Width (ft)
6.00
Primary weir hgth (ft) (CALCULATED)
5.30
Overflow weir hgth (ft) (CALCULATED)
5.30
Wall thickness (ft)
0.67
Top thickness (ft)
0.00
Base thickness (ft)
1.75
Weight of Concrete
20,203
Orifice diameter (in)
0.00
Orifice area (sq -ft)
0.00
Outlet pipe dia (in)
42.00
Outlet pipe area (sq ft)
9.62
15,837
Concrete weight (lbs/cu ft)
146.00
Water weight (lbs/cu ft)
62.40
Probable
Str volume (cu -yd)
4.89
Str weight (lbs)
19,267
Buoyant force (lbs)
15,837
Resultant weight (lbs)
3,430
Factor of Safety
1.22
Bearing Weight (lbs/sq ft)
535.19
3ottom of pond with regards to soil (invert of underdrain system is 848.5
:alc
_alc
_alc
_alc
_alc
_alc
_alc
:alc
Design Input (Target factor of safety of 1.2)
_alc
:alc
Conservative Assumptions:
Bouyant force measured at top of structure lid
100 -year flood depth is 8.6 feet in depth (calculation went to elevation 9.5 feet)
Weight of soil on outfall pipe not accounted for in calculation
Anti-Floatation.xls
Appendix H
QC Check on Calcs
Check on Volume
Inside width of box
4.67
Outside width of box
6.00
Area of inside box
22
Area of outside box
36
Height of box (below top)
5.3
Net Volume of Walls
75.377778 cu ft
Top Area of structure
36
Thickness of top
0.00
Volume of top
0
cu ft
Volume of base
63
cu ft
Total Volume of Concrete
138.4
5.1
Weight of Concrete
20,203
Volume of displaced water
253.8
cu ft
Unit weight of water
62.4
Force of displaced water
15,837
Factor of Safety
1.28

Appendix J
Detention Time Calculation

Appendix J
Detention Time and Design of High Flow Rate Media
Project: HAECO Facility Improvement @ PTIA, Greensboro, NC WKPrepared by: DJK + 93ICKSC3N
Date: March 9, 2016 F�'Urwr� -mmms. surnywn
IAndx.rw AmMIM3
Description
Impervious Area (ac)
Total Drainage Area (ac)
Basin 60
0.4
5.2
Basin 70
9.2
9.2
Basin 80
14.1
14.1
Basin 90
7.3
8.9
Basin 120
11.9
14.0
Proposed Apron
5.1
6.6
Proposed Hangar
5.1
6.0
Proposed Access Rd
0.3
0.7
Total Provided Treatment Area
53.6
64.7
Calculate the runoff coefficient:
Rv=0.05+0.009(la)
Rv = runoff coefficient = storm runoff (inches) / storm rainfall (inches)
la = percent impervious = impervious portion of the drainage area (ac.)/drainage area (ac.)
la 82.84
Rv= 0.80 (in./in.)
Calculate the runoff volume for the water quality event (first 1" of runoff):
Volume = (Design rainfall)(Rv)(Drainage Area)
Volume = 1" rainfall * Rv' 1/12 (feet/inches)' Drainage Area
Volume = 4.3 acre-feet
Volume = 186,761 ft3
Infiltration Zone and Assumed Infiltration Rates for Pond
Zone 1 Peak Flow (cfs)
Zone 2 Peak Flow (cfs)
Assumed Infiltration Rate
Zone Area (sq ft) (inch/hr)
Assumed Infiltration Rate
(ft/hr)
Assumed Infiltration
Rate (ft/sec)
1 (moderately draining soils) 12,403 2
0.2
0.000046
2 (well draining sand) 14,563 10
0.8
0.000231
Calculate Peak Flows and Drawdown Time for WQ Event
Zone 1 Peak Flow (cfs)
Zone 2 Peak Flow (cfs)
Total Flow (cfs)
Time to Drain Pond (sec)
Time to Drain Pond Time to Drain
(min) Pond (hours)
0.6
3.4
3.9
47,338
789 13.1

Appendix K
Maintenance and Operation Plan

Permit Number:
(to be provided by DEMLR)
Drainage Area Number:
High Flow Rate Bioretention Pond
Operation and Maintenance Agreement
I will keep a maintenance record on this BMP. This maintenance record will be kept in a
log in a known set location. Any deficient BMP elements noted in the inspection will be
corrected, repaired or replaced immediately. These deficiencies can affect the integrity
of structures, safety of the public, and the removal efficiency of the BMP.
Important maintenance procedures:
— The drainage area of the high flow rate bioretention pond will be carefully
managed to reduce the sediment load to the sand filter.
— Once a year, sand media will be skimmed.
— The sand filter media will be replaced whenever it fails to function properly after
maintenance.
The high flow rate bioretention pond will be inspected once a quarter and within 24
hours after every storm event greater than 1.0 inches. Records of operation and
maintenance will be kept in a known set location and will be available upon request.
Inspection activities shall be performed as follows. Any problems that are found shall
be repaired immediately.
BMP element:
Potentialproblem:
How I will remediate theproblem:
The entire BMP
Trash/ debris is present.
Remove the trash/ debris.
The grass filter strip or
Areas of bare soil and/or
Regrade the soil if necessary to
other pretreatment area
erosive gullies have formed.
remove the gully, and then plant a
ground cover and water until it is
established. Provide lime and a
one-time fertilizer application.
Sediment has accumulated to
Search for the source of the
a depth of greater than six
sediment and remedy the problem if
inches.
possible. Remove the sediment and
dispose of it in a location where it
will not cause impacts to streams or
the BMP.
The flow diversion
The structure is clogged.
Unclog the conveyance and dispose
structure (if applicable)
of any sediment off-site.
The structure is damaged.
Make any necessary repairs or
replace if damage is too large for
repair.
High Flow Rate Bioretention Pond O&M Page 1 of 3

Permit Number:
(to be provided by DEMLR)
BMP element:
Potentialproblem:
How I will remediate theproblem:
The bioretention cell:
Mulch is breaking down or
Spot mulch if there are only random
soils and mulch
has floated away.
void areas. Replace whole mulch
layer if necessary. Remove the
remaining mulch and replace with
triple shredded hard wood mulch at
a maximum depth of three inches.
Soils and/or mulch are
Check to see if the collection system
clogged with sediment. Water
is clogged and flush if necessary. If
is ponding on the surface for
water still ponds, remove the top
more than 24 hours after a
few inches of the filter bed material
storm.
and replace. If water still ponds,
then consult an expert.
Outlet device
Clogging has occurred.
Clean out the outlet device and
dispose sediment in a location that
will not impact a stream or the
BMP.
The outlet device is damaged.
Repair the outlet device.
The observation well(s)
The water table is within one
Contact DEMLR Stormwater
foot of the bottom of the
Permitting staff immediately at
system for a period of three
919-707-9220.
consecutive months.
The outflow pipe is clogged.
Provide additional erosion
protection such as reinforced turf
matting or riprap if needed to
prevent future erosion problems.
The outflow pipe is damaged.
Repair or replace the pipe.
The emergency overflow
Erosion or other signs of
The emergency overflow berm will
berm
damage have occurred at the
be repaired or replaced if beyond
outlet.
repair.
The receiving water
Erosion or other signs of
Contact the N.C. Division of Water
damage have occurred at the
Resources 401 Certification Program
outlet.
staff at 919-707-8789.
High Flow Rate Bioretention Pond O&M Page 2 of 3

Permit Number:
(to be provided by DEMLR)
I acknowledge and agree by my signature below that I am responsible for the
performance of the maintenance procedures listed above. I agree to notify DEMLR of
any problems with the system or prior to any changes to the system or responsible party.
Project name: HAECO Site Development Project
BMP drainage area number:
Print name:
Title:
Address:
Phone:
Signature:
Date:
Note: The legally responsible parry should not be a homeowners association unless
more than 50% of the lots have been sold and a resident of the subdivision has
been named the president.
I, , a Notary Public for the State of
County of , do hereby certify that
personally appeared before me this
day of I , and acknowledge the due execution of the
forgoing high flow rate bioretention pond maintenance requirements. Witness my hand
and official seal,
SEAL
My commission expires
High Flow Rate Bioretention Pond O&M Page 3 of 3