HomeMy WebLinkAbout19970722 Ver 1_Mitigation Plans_20010209DETAILED WETLAND MITIGATION PLAN
RANDLEMAN RESERVOIR WATER SUPPLY
HICKORY CREEK MITIGATION SITE
GUILFORD COUNTY, NORTH CAROLINA
Prepared for:
PIEDMONT TRIAD REGIONAL WATER AUTHORITY
Prepared by:
EcoScience
EcoScience Corporation
612 Wade Avenue, Suite 200
Raleigh, North Carolina 27605
November 2000
DECEIVED
N.C. Dept. of EHNR
FEB E 9 2009
Winston-Salem
Regional Oftice
TABLE OF CONTENTS
Paqe
LIST OF FIGURES...... ......... ............................... iii
LIST OF TABLES .. ....................... .................. iv
1.0 INTRODUCTION ..... ............ .. .. .................. 1
1.1 Purpose . ........... ............. . .... .... 1
1.2 Objectives of Wetland Restoration ... ...... ........... 1
1.3 Primary Methods for Wetland Restoration ..................... 3
1.4 Mitigation Site Selection ........... ... ... ......... ... 4
2.0 METHODS ........................ .... ................. 8
3 0 EXISTING CONDITIONS ....... ............... ............ . 11
3.1 Physiography, Topography, and Land Use . ................... 11
3.2 Soils ...... ........... . .... .......... 13
3.3 Plant Communities .......... .......................... 15
3.4 Hydrology ................ .... ................ 17
3.5 Water Quality .. ....... ............................... 21
3.6 Jurisdictional Wetlands ................ ................ 21
4.0 WETLAND RESTORATION STUDIES .. ........................... 24
4.1 Restoration Alternatives Analyses ................ ........ 24
42 Reservoir Pool Level and Sedimentation Analysis 26
43 Surface Water Analyses ...... 33
4.4 Groundwater Modeling ............ ... . ....... 36
4.5 Reference Greentree Impoundments ............ ............ 39
4.6 Reference Plant Communities ............ .... ........... 40
4.7 Reference Physiography and Surface Topography ................ 47
5.0 WETLAND RESTORATION PLAN .... .. .... ... 49
5 1 Passive Saturation / Inundation from the Reservoir Pool .. 49
5.2 Sediment Control Check Dams . ........... . ..... ...... 50
5.3 Impoundment / Weir Construction ....... 50
5.4 Surface Scarification / Waste Debris Removal ................. 52
5.5 Woody Debris Deposition . .. ...... I . .... . ........ 52
5.6 Wetland Community Restoration . ......................... 52
60 MONITORING PLAN ........... . .... ....... ....... 57
6 1 Hydrology ..... .. . . . ..... . 57
6 2 Hydrology Success Criteria .. ........ ........ 57
6.3 Soil........... .................. .. .....59
6.4 Soil Success Criteria ....................... ............ 59
6.5 Vegetation .... . .......... 59
6.6 Vegetation Success Criteria ............ ........ ........ 60
6.7 Report Submittal . .......................... . .... 61
70 IMPLEMENTATION SCHEDULE ....... ......... ... ........... 62
8.0 MANAGEMENT PROGRAM . ... ............................... 63
9.0 DISPENSATION OF PROPERTY . .. ........ .................. 65
100 WETLAND FUNCTIONAL EVALUATION ........... .............. 66
10 1 Existing Conditions ...... ............................... 66
10.2 Projected Post - Restoration Conditions ........................ 66
11.0 REFERENCES ................. .............................67
LIST OF FIGURES
Page
Figure 1
Site Location: Randleman Reservoir .....
2
Figure 2
Site Location: Hickory Creek Mitigation Site .....
7
Figure 3:
Aerial Photograph (1999) .. ...............................
9
Figure 4•
Physiography, Topography, and Land Use ..................
12
Figure 5
Soil Map Units ................ .............. .....
14
Figure 6:
Plant Communities ............. ................ ......
16
Figure 7•
Flood Frequency Analyses . ...............................
20
Figure 8:
Jurisdictional Wetlands ... ...............................
22
Figure 9:
Pool Level and Sedimentation. Reference Site 1 .................
29
Figure 10
Pool Level and Sedimentation. Reference Site 2 .................
30
Figure 11
Lake Pool and Sedimentation: On -Site Lake Pool . ...............
32
Figure 12:
Conceptual Impoundment Design ...........................
41
Figure 13.
Reference Lake Shoreline . ........ . ....................
42
Figure 14•
Reference Greentree Impoundment
43
Figure 15:
Reference Site: Plan View and Cross - Sections ...... ... . ...
48
Figure 16:
Hydrology Restoration ........... . . ...... ... ....
51
Figure 17:
Planting Plan .......... ...............................
53
Figure 18.
Monitoring Plan / Mitigation Design Units ............ ... .
58
LIST OF TABLES
im
Page
Table 1:
Estimated Acreage of Mitigation Design Units Based on Preliminary Studies
for 10 Potential Mitigation Sites Associated with the Randleman
Reservoir
..... ........... ... ................ .. ......
5
Table 2•
Water Surface Elevation Estimates for Various Flood Frequencies
34
Table 3•
Modeled Groundwater Discharge Zone of Influence
on Wetland Hydroperiods: Congaree Soils . ................
38
Table 4•
Reference Forest Ecosystem Plot Summary .................
45
Table 5:
Reference Forest Ecosystem Plot Summary . . ............ 46
Table 6:
Planting Plan .. .... .... ........ .......
... 55
im
DETAILED WETLAND MITIGATION PLAN
RANDLEMAN RESERVOIR WATER SUPPLY
HICKORY CREEK MITIGATION SITE
GUILFORD COUNTY, NORTH CAROLINA
1.0 INTRODUCTION
1.1 PURPOSE
The Piedmont Triad Regional Water Authority (PTRWA) proposes development of the
Randleman Reservoir in Randolph and Guilford Counties, North Carolina (Figure 1). The
purpose of this project is to develop a safe and dependable water supply source for North
Carolina's Piedmont Triad region that will satisfy the projected water demand for a period
of 50 years. The proposed 3,000 -acre reservoir will unavoidably impact approximately 121
acres of wetlands through impoundment and establishment of an open water system.
These jurisdictional wetlands are subject to regulation under Section 404 of the Clean
Water Act (CWA) (33 U.S.C. § 1344).
For unavoidable wetland impacts, compensatory mitigation is required to facilitate no net
loss of wetland functions in the region. Compensatory mitigation is typically performed
to replace similar wetland types and wetland functions as those impacted (ex: forested,
stream -side wetlands). Wetland restoration, creation, enhancement, or preservation
represent typical methods designed to offset wetland impacts. The North Carolina
Division of Water Quality (DWQ) has instituted a policy that prefers a minimum of 1 acre
of wetland be restored or created for every acre of wetland impacted. Subsequently,
remaining wetland functional replacement needs may then be off -set through wetland
enhancement and /or preservation.
1.2 OBJECTIVES OF WETLAND RESTORATION
The primary objectives for wetland restoration include:
1) Restore or create 121 acres of wetlands as required under regulatory
guidance.
2) Assist in protecting the drinking water supply from pollutants discharged
from the developing watersheds. Excess nutrients, fecal coliform,
sediments, and chemical contaminants (metals, etc.) represent the primary
water quality concerns for the reservoir.
3) Maximize benefits to water quality through establishment of functioning
wetlands above the reservoir pool.
4) Replace habitat for wetland dependent wildlife displaced by establishment
of open water.
5) Maximize the acreage of wetland restoration or creation achieved at the
Hickory Creek mitigation site.
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Water quality benefits are maximized by reducing the capacity for sediment to reach open
water within the reservoir pool. Entrenched streams in the region have abandoned
adjacent floodplains and tend to discharge large quantities of sediment within water
supply reservoirs (Section 4 2). Therefore, wetland restoration for water quality should
be designed to reduce sediment transport capacity of streams immediately above the
reservoir and to entrap sediment within vegetated wetland surfaces As a result, sediment
would be deposited on floodplain surfaces that periodically dry out in areas outside of the
reservoir pool. Wetland vegetation would serve to provide nutrient uptake and recycling
functions within deposited sediment. Using this rationale, entrenched stream and terrace
systems would be converted into alluvial wetland fans or greentree impoundments. A
highly sinuous (E) to braided (D) stream system would be developed within the alluvial
deposition area (Rosgen 1996). Wetland vegetation would be established on the alluvial
deposition to stabilize the sediment and provide for pollutant recycling through oxidation
(drying) and reduction (wetting) processes. Within reservoir pools immediately
downstream of entrenched streams, sediment from the watershed is deposited directly
into the water supply, in a permanently inundated, reducing environment. Without
periodic oxidation processes, pollutants generally dissolve within the water column and
consequently reduce drinking water quality.
Establishment of the reservoir will effectively stop sediment transport capacity within each
tributary that flows into the lake. The primary objective of mitigation is to extend the
sediment deposition wedge in the upstream direction of each valley, prior to confluence
with the lake. The reservoir will passively develop wetlands within a certain area in each
valley, immediately above the reservoir pool. This project is intended to assist that
process through active measures, including extension of wetlands in the up- valley
direction and establishment of wetland vegetation on active alluvial surfaces.
1.3 PRIMARY METHODS FOR WETLAND RESTORATION
Two primary methods for wetland restoration have been proposed to extend the sediment
wedge into design wetlands above the reservoir pool, restore 121 acres of riverine
wetlands, and to provide suitable habitat for wetland dependent wildlife. Primary
methods include: 1) in- stream structures designed to reduce sediment transport capacity;
and 2) greentree impoundments designed to allow management of water levels and
sediment deposition patterns.
In- Stream Structures
In- stream structures are proposed primarily along dredged or entrenched stream corridors
on relatively low -slope valley floors (<0.009 rise /run) supporting forest vegetation.
Adjacent floodplains have been abandoned by the incised stream and converted to
elevated terraces not regularly exposed to overbank flooding or wetland hydrodynamics.
Properly designed in- stream structures are expected to reduce the degree of channel
incision, reduce the rate of groundwater discharge from the floodplain into the channel,
3
increase overbank flooding from the channel onto the floodplain, reduce sediment
transport capacity, and provide greater sediment deposition within vegetated wetlands.
Greentree Impoundments
Greentree impoundments are proposed on more steeply sloped floodplains and stream
terraces (>0.008 rise /run) or pastured sites where relatively severe stream channel
degradation and steepening has occurred above the reservoir pool. In general, a greentree
impoundment comprises a floodplain levee and controllable outlet structure that is
modified periodically to promote the development of forested wetlands. Functioning
greentree impoundments above the lake reservoir are expected to provide for significant
nutrient uptake, recycling, and management benefits.
The elevation of the outlet is typically raised during winter months to promote ponding,
sediment deposition, and waterfowl habitat. Subsequently, the elevation of the outlet
is lowered in early spring to allow for vegetation growth, nutrient uptake, and seedling
establishment. Levee systems are typically constructed to provide for less than 2 to 3 feet
of inundation during winter months, to prevent over - topping, and to allow for survival of
tree seedlings. The winter depth is generally dependent upon the height of seedlings
The raising and lowering of outlet structures requires regular monitoring and maintenance
by qualified personnel to facilitate the growth and survival of tree species. The time for
outlet adjustment may vary annually and is dependent upon localized conditions within
the watershed. Seedling mortality is typically tracked on an annual basis, and the date
of spring lowering of water levels is modified periodically to maximize the rate of forest
regeneration. Tree or shrub species selected for planting will vary based on expected
hydrologic regimes and other site - specific conditions. In general, sites lacking substantial
forest vegetation and primarily used as pasture land were targeted for this mitigation
Option.
1.4 MITIGATION SITE SELECTION
During the environmental impact assessment, project planners identified and evaluated a
total of 25 potential mitigation sites within stream corridors extending above the reservoir
pool. A description of mitigation potential for each of these sites was prepared in
previous documents (ESC 1998a, ESC 1998b, ESC 1999).
Of these 25 sites, 10 sites were determined to support wetland restoration potential on
floodplains encompassing up to 121 acres of restorable land area. Table 1 and Figure 1
depict the location of each site and projected acreages potentially available for wetland
restoration use.
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This document details restoration and enhancement procedures for riverine and lake
shoreline wetland restoration and creation along Hickory Creek, one of the 10 mitigation
sites (Figure 2). The Hickory Creek mitigation site (Site) includes 29.4 acres that
encompass the stream and adjacent floodplain servicing a watershed of approximately 8.7
square miles immediately above the reservoir. In- stream structures are proposed within
extensively dredged stream channels at or immediately below the reservoir pool elevation
to reduce sediment transport capacity into the water supply. A greentree impoundment
is also proposed immediately above the reservoir pool to extend the sediment deposition
wedge and wetland extent in the up- valley direction. Modifications along the dredged
and entrenched Hickory Creek corridor are designed to reduce the rate of groundwater
discharge from the floodplain into the channel, increase overbank flooding from the
channel onto the floodplain, and increase deposition of sediment on vegetated wetland
surfaces above the reservoir pool.
This summary document includes the following: 1) descriptions of existing conditions; 2)
surface and groundwater hydraulic analyses; 3) reference lake shoreline and greentree
impoundment studies; and 4) reference soil and forest ecosystem investigations.
Subsequently, detailed plans are provided for in- stream structures, wetland
restoration /creation, vegetation planting, site monitoring, and success criteria. Remaining
mitigation sites that follow the format of this mitigation plan will be attached as appendix
documents to this summary mitigation plan.
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MAF FIGURE
EcoSC1e11CC RANDLEMAN RESERVOIR MITIGATION PROJECT Ckd by
JWN
corporattc)n HICKORY CREEK SITE Date
RaiMh, North Carolina Guilford County, North Carolina NOV 2000 2
Protect 00 -010
2.0 METHODS
Natural resource information for the Site was obtained from available sources, including
U. S. Geological Survey (USGS) topographic mapping (USGS High Point East and High
Point West 7.5 minute quadrangles), U. S. Fish and Wildlife Service (USFWS) National
Wetlands Inventory (NWI) mapping, and Natural Resource Conservation Service (NRCS)
soil survey (USDA 1977). These resources were utilized for base mapping and evaluation
of existing landscape and soil information prior to on -site inspection. Current (1999) aerial
photography was obtained and utilized to map relevant environmental features (Figure 3).
Characteristic and target natural community patterns were classified according to
constructs outlined in Schafale and Weakley's, Classification of the Natural Communities
of North Carolina (1990). North Carolina Natural Heritage Program (NCNHP) data bases
were evaluated for the presence of protected species and designated natural areas which
may serve as reference (relatively undisturbed) wetlands for restoration design.
On -site reference (relatively undisturbed) stream and wetland sites were selected to orient
restoration design and to provide baseline information on target (post - restoration) wetland
conditions. A regional vegetation reference data base and on -site inventory were used to
characterize target, post - restoration species composition. Topographic maps of the basin
floor were also prepared to show the gradation from permanently to semi- permanently
inundated conditions throughout the lower half of the Site. A concurrent pool elevation
and sedimentation study provided information on sedimentation and wetland development
around the point of stream inflow and conservation pool levels. Topographic data were
overlaid on wetland restoration areas to establish methods for construction and restoration
of wetland communities within the Site.
Detailed topographic mapping to 1 -foot contour intervals was developed by ground survey
paneling and aerial photogrametric methods. Additional land surveys were performed to
establish channel cross - sections and measure reference wetland surface topography.
Field investigations were performed in the Summer of 2000 including soil surveys, on -site
resource mapping, land surveys, and landscape ecosystem classifications. Existing plant
communities and jurisdictional wetlands were described and mapped according to
landscape position, structure, composition, and groundwater analyses.
Wetland boundaries were obtained from a delineation performed in 1998. NRCS soil map
units were ground truthed by licenced soil scientists to verify units and to map inclusions
and taxadlunct areas. The revised soils maps were used as additional evidence for
predicting natural community patterns and wetland limits prior to human disturbances.
L-11
Groundwater conditions were modeled using DRAINMOD, a computer model for
simulating withdrawal rates for shallow soils with high water tables. The model was
utilized to predict historic hydroperiods, the extent of wetland degradation due to channel
entrenchment, and the potential for wetland restoration through stream modification.
Surface water analyses for the Hickory Creek site were completed using standard study
methods of the U.S. Army Corps of Engineers (USACE) and NRCS. Flood events of a
magnitude which are expected to be equaled or exceeded once on average every 1 -, 2 -,
5 -, 10 -, or 100 -year period were selected for use. These flood events are commonly called
the 2 -, 5 -, 10 -, and 100 -year floods. Floods with a return frequency of less than 2 years
are usually not studied in engineering applications. These analyses reflect either existing
or proposed conditions at the Site. The projected frequency and extent of overbank
flooding were used to determine potential for rivenne wetland restoration in floodplain
portions of the Site.
This Site has been selected for wetland restoration use to promote a reduction in
sediment, nutrients, and pollutants flowing into Randleman Reservoir. Mitigation
activities are intended to provide sediment deposition, and pollutant and nutrient cycling
from surface waters within created and restored wetland areas. Recycling functions are
designed to reduce elevated nitrogen and phosphorus loads from the watershed towards
background (forest) levels, prior to discharge into the reservoir.
10
3.0 EXISTING CONDITIONS
3.1 PHYSIOGRAPHY, TOPOGRAPHY, AND LAND USE
The Site is located in the Piedmont Physiographic Province of North Carolina.
Physiography is characterized by moderately hilly terrain with interstream divides
intermixed with steeper slopes along well - defined drainage ways. The Site is situated in
the Hickory Creek floodplain within the Cape Fear River Basin (Hydrologic Unit #03030003
[USGS 19741, DWQ Sub -Basin 03- 06 -08). The Site is located approximately 7.5 miles
southeast of High Point and approximately 10.5 mi southwest of Greensboro. Annual
precipitation in the region averages 42 inches per year with June and August representing
the months that support the highest average rainfall (4.21 inches and 4.36 inches
respectively) (USDA 1977).
The Site contains an approximately 3400 -foot reach of Hickory Creek (Figure 4). Hickory
Creek supports a primary watershed of approximately 8.7 square miles and flows into the
Deep River 1.8 miles downstream. Three auxiliary tributaries flow into lower reaches of
the Site within a relatively broad floodplain area located immediately north of SR 1132
(Figure 4). Above the confluence with these tributaries, the Hickory Creek valley steepens
and the floodplain narrows as the main -stem channel is enclosed by moderate to steeply
sloped escarpments along adjacent uplands.
The Hickory Creek watershed supports a mixture of forest, agricultural, residential,
commercial, and infrastructural uses. The headwater region extends into urban areas
associated with the City of Greensboro, including the Interstate 85 (1 -85) highway
corridor. Lower portions of the watershed, south of 1 -85, primarily support residential
communities, pasture land, forest, and limited commercial development. Land use within
the watershed surrounding the Site includes recently timbered tracts, nearby residential
homes, and abandoned and active agricultural lands situated within the creek floodplain
(Figure 4).
The Site encompasses one primary physiographic landscape area for restoration planning
purposes; elevated stream terrace (Figure 4). The primary variables utilized to segregate
wetland landscape units include land slope, overbank flood frequencies (Section 4.3), and
the rate and direction of groundwater flow (Section 4.4).
The stream terrace encompasses 29.4 acres along both sides of the stream channel. The
main channel averages approximately 30 feet in width and 6 feet in depth through the
area The main -stem channel and auxiliary tributaries were dredged and straightened
during conversion of the floodplain to agricultural land. Construction of stream -side
levees during dredging activities and down - cutting into the channel bed have effectively
eliminated the presence of an active floodplain within the Site. Under historic conditions,
the stream terrace is expected to have included active floodplains and a low stream terrace
supporting natural communities such as Piedmont bottomland hardwood forest and mixed
mesophytic hardwood forest (Schafale and Weakley 1990).
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Down - cutting in the stream has reduced the frequency of overbank flooding within the
primary floodplain from an estimated 1 -year return interval to a 2- to 10 -year return
interval under existing conditions. Due to extensive dredging, the constructed canals have
also induced a relatively steep groundwater gradient across the floodplain and relatively
rapid groundwater discharge into the stream channel. As a result, wetland functions
(sediment retention, nutrient cycling, energy dissipation, etc.) have been effectively
eliminated from the physiographic area by stream alterations.
3.2 SOILS
Surficial soils have been mapped by NRCS (USDA 1977). Soils were verified in the
summer of 2000 by licenced soil scientists to refine soil map units and locate inclusions
and taxadjunct areas. Systematic transects were established and sampled to ensure
proper coverage. Refined soil mapping is depicted in Figure 5. Primary soil types include
the Chewacla series, Wehadkee series, and Congaree series.
Chewacla soils (Fiuvaquentic Dystrochrepts) encompass approximately 13.8 acres (47 %)
of the 29.4 -acre Site. Chewacla soils are somewhat poorly drained, nonhydric soils which
have been formed primarily by fluvial activity. Chewacla soils, located in the broad,
relatively flat valley floor, contain numerous inclusions of hydric soils (i.e. Wehadkee
[Typic Fiuvaquents]) in depressions, ephemeral channels, and swales which are more likely
to support wetland hydrology. Chewacla soils generally exhibit broad, inter- layered
variability in texture and permeability dependent upon energy dissipation and sediment
deposition patterns associated with each stream overbank flood event. Soil texture
generally ranges from coarse sandy loam to silt loam of moderate to moderately rapid
infiltration. Important factors in the formation and maintenance of wetland systems as
hydric inclusions in the Chewacla map units include: 1) microtopography and variability
in fluvial deposition across the landscape; 2) groundwater and surface water movement
from adjacent uplands along the outer edge of the floodplain; and 3) groundwater
discharge rates from the interior floodplain into the stream channel. Stream dredging,
straightening, and conversion to agricultural lands has likely increased the extent of
Chewacla (nonhydnc) soils and concurrently decreased the extent of Wehadkee (hydric)
map units in the Site.
Hydric soils are defined as "soils that are saturated, flooded, or ponded long enough
during the growing season to develop anaerobic conditions in the upper soil layer" (USDA
1987). Hydric soils comprise the Wehadkee series (Typic Fiuvaquents), located primarily
within relict backwater sloughs, depressions, ephemeral channels, and swales which
remain within the secondary floodplain Under existing conditions, the Wehadkee series
comprises approximately 2.9 acres (10 %) of the Site.
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developed in loamy alluvium. The soil is located primarily along relatively narrow
floodplains and the more steeply sloped valley comprising 11 .5 acres in upper reaches of
the Site. The seasonal high water table ranging from 2 to 4 feet below the surface.
Congaree soil map units are not expected to have supported wetlands historically with the
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sloughs.
3.3 PLANT COMMUNITIES
Plant communities are influenced by logging, dredging activities and Site conversion to
agricultural lands. Five communities have been identified for descriptive purposes,
including: 1) bottomland hardwood forest; 2) Piedmont swamp forest; 3) agricultural land;
4) levee /stream -side forest; and 5) early successional / cut areas (Figure 6).
Bottomland Hardwood Forest
Bottomland forest accounts for approximately 13.9 acres in central reaches of the Site.
The bottomland forest assemblage has experienced prolonged degradation from past
logging, watershed diversion, agricultural usage, and ditch networks. The forest canopy
includes box elder (Acer negundo), sycamore (Platanus occidentabs), green ash (Fraxinus
pennsylvanica), tulip poplar (Liriodendron tulipifera), American elm (Ulmus americana), and
black willow (Salix nigra). Under -story species distribution is variable along hydrologic
gradients and includes muscadine grape (Vitis rotundifoha), beggar's ticks (Desmodium
sp.), Japanese honeysuckle (Lonicera japonica), poison ivy (Toxicodendron radicans),
jewel -weed (Impatiens capensis), Chinese privet (Ligustrum sinense), Virginia creeper
(Parthenocissus quinquefoha), and blackberry (Rubus sp.).
Piedmont Swamp Forest
Swamp forests persist within isolated, relict backwater areas along the eastern Site
boundary. This community, covering approximately 2.8 acres, appears to have been
affected by reductions in drainage area, loss of surface hydrodynamics, reductions in
hydroperiod, and past conversion to crop land. The canopy includes species listed in
bottomland hardwood forest, with inclusion and increasing frequency of swamp chestnut
oak (Quercus michauxii), ironwood (Carpinus caroliniana), swamp cottonwood (Populus
heterophyllus), and sweetgum (Liquidambar styraciflua). The understory is dense to
sparse, depending on the openness of the overstory. Under open canopies, the understory
is dominated by a dense layer of graminoids and forbs, including sedges (Carex spp.),
rushes (Juncus spp.), lizard tail (Saururus cernuus), false nettle (Boehmeria cyhndrica),
seedbox (Ludwegia spp.), and jewel -weed.
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Levee / Stream -Side Forest
Approximately 3.1 acres of levee forest are found in a narrow fringe, on the elevated
deposits (natural and man -made) along Hickory Creek These moderately well drained
areas support plant species characteristic of levee forest communities (Schafale and
Weakley 1990). Canopy species include box elder, sycamore, river birch (Betula nigra),
green ash, black walnut (Juglans nigra), sweetgum, ironwood, and red maple. The mid -
story is dominated by eastern cedar (Juniperus virginiana), black cherry (Prunus serotina),
Chinese privet, and honey locust (Gleditsia triacanthos). Understory species include virgin
bower (Clematis virginiana), Japanese honeysuckle, muscadine grape, mouse ear
chickweed (Cerastium glomeratum), common greenbrier (Smilax rotundifolia), and poison
ivy.
Agricultural Land
Approximately 7.1 acres of pasture land is located primarily in upper reaches of the Site.
Hay was last harvested in June of 2000. Pasture land is dominated by a variety of grasses
and herbs. The predominant species is fescue (Festuca spp.). Other characteristic species
include asters (Aster spp.), goldenrods (Sohdago spp.), horseweed (Erigeron canadensis),
pigweed (Chenopodium album), ragweed (Ambrosia artemisiifolia), common morning glory
(lpomoea purpurea), clover (Trifohum spp.), crabgrass (Digitana spp.), and love grass
(Erogrostis sp.).
Adjacent border and forest edge communities support young sweetgum, black cherry, box
elder, and red maple (Acer rubrum). Thickets containing blackberry, winged sumac,
Canada elder (Sambucus canadensis), switch cane (Arundinaria gigantea), muscadine
grape, and common greenbrier are also present.
Early Successional / Cut Areas
Approximately 1 6 acres consists of recently clear -cut forest land along the eastern
property boundary This community is characterized by disturbance adapted species such
as sapling elements of eastern red cedar (Juniperus virginlana), box elder (Acer negundo),
and red maple (Acer rubrum) Understory species include a dense thicket of common
greenbrier (Smilax rotundifolia.), poison ivy (Toxicodendron radicans), Japanese
honeysuckle (Lonicera japonica), blackberry (Rubus sp.), and common ragweed (Ambrosia
artemisufolia) .
3.4 HYDROLOGY
The Site is located within the Piedmont hydrophysiographic province, which encompasses
the entire drainage basin for the East and West Forks of the Deep River. The region is
characterized by moderately hilly terrain with interstream divides exhibiting dendritic
drainage patterns and moderately steep slopes along valley floors (0.005 -0.015 rise /run).
The region is characterized by moderate rainfall. In Guilford County, precipitation averages
42 inches per year with precipitation evenly distributed throughout the year (USDA 1977).
17
Large floods (20 -100 year return interval) typically correspond to large thunderstorms and
tropical events in the region.
Bed load material supplied by the region consist primarily of silts, sands, and weathered
bedrock (very coarse sand and small gravel). Bedrock outcrops are considered common
within incised streams in more steeply sloped valleys. Suspended load consists primarily
of easily eroded clays and silts, which transport attached nutrients into downstream
waters. Erosion and suspended sediment loads have been linked to nutnfication problems
within the Piedmont hydrophysiographic province, including the Randleman Reservoir
region (DWQ 2000).
Surface Water
The Site encompasses two valley types in association with the 3400 -foot reach of Hickory
Creek and the confluence with three unnamed tributaries (Drainage Area = 8.7 square
miles). The lower half of the Site consists of a relatively broad, abandoned floodplains
extending along a 1400 -foot reach of Hickory Creek. In this section, the valley slope
measures approximately 0.004 rise /run, suggesting the presence of a relatively flat valley
floor relative to typical conditions in the Piedmont Province. The floodplain ranges from
500 feet to 900 feet in width. A majority of this floodplain area typically resides between
680 feet to 684 feet above mean sea level and will be directly affected by the established
reservoir pool (to 682 feet above mean sea level). Capacity for surface water runoff in this
area is reduced due to the relative lack of land slope and broad width of the floodplain
floor. Ponded conditions have been observed along the outer floodplain fringe for more
than 1 week after significant rainfall events in August 2000.
In upper reaches of the Site, the valley floor narrows and steepens along an approximately
2000 - linear foot reach. The floodplain slope steepens to an average, 0.007 rise /run
including floodplain widths of less than 200 feet in most areas. The elevation of these
floodplains typically ranges from 685 feet to 687 feet above mean sea level and outside
the influence of the reservoir pool. Surface water runoff within this floodplain appears
relatively rapid in both the down - valley and across - floodplain direction. Ponding was
absent in this area immediately after significant rainfall events with rapid return flow
observed into the main -stem channel.
The main -stem and auxiliary channels were dredged and straightened in the last several
decades with low -lying levees constructed adjacent to agricultural areas. Due to past
dredging and /or active down - cutting, the cross - sectional area of within the Hickory Creek
channel measures approximately 180 square feet. However, according to regional curves,
a stable Hickory Creek channel is projected to support cross - sections of approximately 90
square feet (Harman et a/. 1999, Rosgen 1996). The incised channel supports a sinuosity
(channel length /valley length) of 1.1. Substrate within the channel is composed of
unconsolidated sand, small gravel, and bedrock outcrops exposed by incision and localized
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bank erosion. The channel is classified as a G4 (gravel dominated gully) based on fluvial
geomorphic features (Rosgen 1996).
Stream discharge and flood elevations under existing conditions have been predicted
based on hydraulic models (Section 4.2). Figure 7 provides model predictions for the 1-
(projected bankfull), 2 -, 5 -, and 100 -year storm (Section 4.3). The study suggests that
entrenchment has confined the 1- to 2 -year flows within the eroding channel banks,
effectively bypassing floodplain functions associated with pollutant removal and
maintenance of wildlife habitat for overbank flood dependent species.
Groundwater
Surface water hydrodynamics, such as periodic overbank floods, fluvial sediment
deposition, and hydraulic energy dissipation, represent important attributes of floodplains
and bottomland hardwood forest in the region. However, streams in the region typically
function as groundwater withdrawal features throughout most of the year. Therefore,
groundwater inputs from auxiliary watersheds and upland slopes abutting the floodplain
represent the primary hydrologic input resulting in the development and maintenance of
riverine wetlands at this Site.
Groundwater gradients in June 1998 and May 2000 indicate that the groundwater table
typically resides from surface saturation to 6 feet below the land surface. In lower reaches
of the Site, the groundwater gradient is relatively flat along the outer floodplain and
steepens within distances of 200 to 300 feet from the dredged stream channels In upper
reaches of the Site, the groundwater gradient remained more than 2 feet below the
surface throughout the relatively narrow floodplain with a relatively steep gradient
induced by the stream channel.
A majority of the Site outside of the proposed reservoir pool is expected to support limited
groundwater storage potential typically associated with maintenance of wetland surfaces.
Although adjacent escarpments supply riparian inflow of groundwater, this flow appears
steeply inclined with relatively rapid discharge towards the stream channel. Entrenchment
of Hickory Creek has further accelerated groundwater discharge to depths of greater than
6 feet below the surface near the stream channel. Restoration of a shallower (less incised)
stream network will generate a flatter groundwater gradient. However, groundwater
models (Section 4.4) suggest that groundwater tables will continue to remain more than
1 foot below the surface. Therefore, restoration of wetlands within upper reaches of this
Site may require establishment of a backwater (surface water induced) wetland
immediately above the reservoir. Backwater wetland conditions may be induced by
establishment of a greentree impoundment across the valley floor.
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3.5 WATER QUALITY
Hickory Creek maintains a State best usage classification of WS -IV CA ((Stream Index No.
17 -8) (DWQ 2000). Class WS -IV waters are protected as water supplies which are in
moderate to highly developed watersheds. Point source discharges are generally required
to meet stringent pre- treatment standards, to maintain pre- treatment failure (spill
prevention) plans, and to perform point source monitoring for toxic substances. Local
programs to control nonpoint source and stormwater discharge of pollution are also
required. CA signifies a Critical Area designation for watershed areas within 0.5 miles of
a water supply intake for the reservoir.
The Site consists primarily of existing and former agricultural land adjacent to the stream
channel. Fertilizers, pesticides, and nutrients associated with land uses may have
influenced water quality in the vicinity. Restoration of wetland hydrology and diversion
of area runoff onto restored wetland surfaces will provide local water quality benefits,
including important functions such as particulate retention, removal of elements and
compounds, and nutrient cycling.
Historically, the floodplain provided water quality benefits to the 8.7 square mile
watershed associated with Hickory Creek. However, runoff from this land area effectively
bypasses wetland floodplains as the entrenched channel transports flow directly through
the Site. Restoration of wetland hydrology and diversion of watersheds onto restored
wetland surfaces will provide for restoration of overbank flooding and associated water
quality benefits above the Randleman Reservoir.
Because the Site is located at the elevation of the reservoir conservation pool, sediment
transport capacity of the stream system will be generally stopped at elevations of 681 feet
above MSL. A sediment wedge will develop within the reservoir pool and incrementally
extend further into the open water area. Research suggests that up to 300,000 cubic feet
(0.7 acre -feet) of sediment will be deposited annually within the reducing aquatic
environment (Simmons 1976). Associated pollutants will not generally be degraded,
assimilated, or recycled through periodic wetting (reduction) and drying (oxidation).
Therefore, wetland restoration has been designed to reduce sediment transport capacity
within the 3,400 -foot reach of Hickory Creek. As a result, the sediment wedge will be
extended in the upstream direction outside of the reservoir pool (above elevation 682),
where wetland functions will potentially provide for greater pollutant processing.
3.6 JURISDICTIONAL WETLANDS
Jurisdictional areas are defined using the criteria set forth in the U.S. Army Corps of
Engineers, Wetlands Delineation Manual (DOA 1987). Approximately 2.9 acres of
jurisdictional wetlands were delineated on -site and confirmed by the U.S. Army Corps of
Engineers Figure 8 depicts the boundary location of existing jurisdictional wetland
systems.
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stream oxbow in lower reaches of the Site. Discharge from adjacent groundwater slopes
and the relatively flat valley floor provide for periodic surface water expression during early
portions of the growing season. Wetland extent was most likely more extensive prior to
stream dredging and Site conversion to agricultural lands. Approximately 0.4 acres of
these jurisdictional wetlands will be inundated by the reservoir conservation pool (2.5 net
acres of jurisdictional wetlands will remain after filling of the reservoir).
23
40 WETLAND RESTORATION STUDIES
This section summarizes studies performed to orient restoration design. Studies include:
1) Restoration Alternatives Analyses: Alternatives for wetland restoration
relative to stream, floodplain, and reservoir functions were assessed.
2) Reservoir Pool Sedimentation and Wetland Vegetation Study: Sedimentation
patterns and extent of wetlands were assessed on existing reservoirs in the
region.
3) Surface Water Analyses: Overbank flooding frequency and extent was
estimated based on the selected wetland restoration method.
4) Groundwater Modeling: The effect of drainage features on groundwater
wetland hydroperiods was modeled.
5) Reference Plant Communities: Reference wetland communities were
sampled to predict the target distribution of vegetation to be established in
restoration areas.
6) Reference Physiography and Surface Topography: Reference wetland
surfaces were measured within an existing greentree impoundment to
characterize long term, projected conditions at the Site.
4.1 RESTORATION ALTERNATIVES ANALYSES
The objectives of this project include:
1) Assist in protecting the drinking water supply from pollutants discharged
from the developing watersheds. Pollutants attached to sediment
represents the primary water quality concern for this project.
2) Maximize benefits to water quality through establishment of functioning
wetlands above the reservoir pool.
3) Replace habitat for wetland dependent wildlife displaced by establishment
of open water.
4) Maximize the area of wetland restoration achieved by the project.
Restoration alternatives suggested by project participants are briefly described below.
Stable Channel Construction
Reconstruction of a potentially stable stream system was assessed as a replacement for
the existing, dredged and incised channel. The new channel would be designed to mimic
referenced, stable attributes including the geomorphic dimension, pattern, and profile
needed to transport water and sediment produced by the watershed. The restored
channel would reduce the rate of groundwater withdrawal from adjacent floodplains,
potentially resulting in wetland hydrology restoration in certain areas.
24
This alternative would facilitate transport of sediment generated by the watershed into
the Randleman Reservoir. Within the lower half of the Site, this capacity to transport
sediment would be unavoidably stopped by the conservation pool associated with the
reservoir. In addition, facilitating transport of sediment and pollutants into water supply
waters is considered contrary to project objectives. Therefore, this option was discarded.
Alluvial Wetland Fan Development
This option is designed to elevate water tables and reduce sediment transport into the
reservoir. Alluvial fan development entails placement of fixed, in- stream weirs within the
dredged channel. The in- stream modifications are expected to reduce the degree of
channel incision, increase overbank flooding, reduce stream sediment transport capacity,
and provide greater sediment deposition within vegetated wetlands. The system would
progress towards an alluvial wetland fan where the channel actively migrates across fluvial
material. During the interim period, in- stream structures will sustain significant energy
during flood events; therefore, the potential exists for development of channel by- passes
(shoot cut -offs) around the structures. As such, risk of wetland restoration failure exists.
The structures must be designed to avoid short - circuiting and provide for sediment
deposition in the incised channel. Over a relatively long period of time, the shallower
channel would invariably abandon the structures and begin to actively migrate across the
restored floodplain. At this point, the system would need to be monitored for evidence
of head - cutting immediately above the reservoir or migration back into the abandoned,
dredged channel. Additional structural modifications may be required in the future, after
the migrating stream abandons the existing dredged channel.
Within lower reaches of the Site, passive development of an alluvial fan immediately
above the reservoir represents a viable option for this Site. This option will reduce initial
disturbance in the floodplain associated with greentree impoundments, reduce
implementation cost relative to greentree impoundments, and will require less need for
seasonal management of the structures. Based on alternatives analyses, placement of in-
stream structures and alluvial wetland development has been selected as the preferred
mitigation alternative within the lower valley and Chewacla soil map units depicted in
Figure 5.
Based on groundwater models (Section 4.4) passive development of an alluvial wetland
fan within upper reaches of the Site (Congaree soil map units, Figure 5) does not
represent a viable option for this Site. Due to soil characteristics, across terrace slope,
and down - valley slope, the groundwater model suggests that groundwater migration will
continue into the low -lying channel corridor, with development of shoot cut -offs
considered likely during storm events. In essence, the floodplain floor is not wide enough
or flat enough to induce adequate groundwater retention or to provide adequate energy
dissipation functions in the vicinity of the in- stream weirs. Therefore, this option was
discarded in favor of greentree impoundments.
25
Greentree Impoundments
This alternative is similar to alluvial fan development described above. However,
Greentree impoundments include a series of floodplain levees and controllable outlet
structures that are modified periodically throughout the year to induce backwater flooding
and promote the development of forested, shrub - scrub, and emergent wetlands.
Greentree impoundments have been constructed above other water supply reservoirs in
the region for wetland, wildlife, and sediment retention functions. These structures can
be controlled to regulate the depth and frequency of inundation based upon objectives
of the system. In this case, the structures would be used to establish vegetated wetlands
and limit transport of pollutants into the reservoir.
In general, the levee system is constructed to provide for less than 2 to 3 feet of
inundation during winter months, to prevent over - topping, and to allow for survival of tree
seedlings. The winter depth is generally dependent upon the height of seedlings. The
raising and lowering of outlet structures requires regular monitoring and maintenance by
qualified personnel to facilitate the growth of tree species. The actual date that the
outlet is modified may vary annually and is dependent upon localized conditions within
the watershed. Seedling mortality is tracked on an annual basis and the date of spring
lowering is modified to maximize the rate of forest regeneration. Tree species selected for
planting may also be modified based upon collected data. Greentree impoundments
designed for forested wetland restoration have failed in the past, due primarily to lack of
resources for long term monitoring, management, and manipulation.
Based on alternatives analyses, Construction of a greentree impoundment across the
Hickory Creek floodplain represents the preferred option in upper reaches of the Site.
Construction activities would occur primarily within maintained pasture areas. Impacts
to existing forest stands will be limited. In addition, the structures would allow pro - active
control of wetland development and function behind each impoundment.
4.2 RESERVOIR POOL LEVEL AND SEDIMENTATION ANALYSIS
Reservoir sediment quality is an important environmental concern because sediment may
act as both a sink and a source of water - quality constituents to the overlying water
column and biota. Sedimentation rates are dependent on existing land use, land use
modifications, soils, and basin topography. Alternatives for sediment reduction potentially
include up- stream land management techniques, sluicing of sediment through the
reservoir, or interception of the sediment prior to entry into the reservoir. Even with strict
land management techniques in an urbanizing watershed, a certain amount of
"background" sediment will continually be moved by erosion processes. In a stable river
bed, erosion, sediment transport and deposition are in equilibrium. Dam construction
disrupts this balance, raising the base level of streams above the reservoir, and
encouraging the deposition through reduction of the gradient. As a result, the stream
deposits sediment as a delta at the confluence with the conservation pool. The
deposition process, otherwise known as warping or shoaling, advances at varying rates
26
dependent on the site and rate of sediment supply. Slope angles and extension patterns
on the delta are dependent on the particle size of the prevailing sediment deposited, with
courser sediment associated with higher angle slopes.
The Hickory Creek wetland restoration project resides immediately above the normal pool
elevation (i.e. conservation pool) for the Randleman reservoir (Elevation = 682 feet above
MSL). Currently, the stream channel is incised to depths which induce a groundwater
withdrawal gradient (rate) that does not allow for wetland presence on the adjacent
primary floodplain. After the reservoir is constructed, sediment accretion is likely to occur
within the Site, immediately upstream and downstream of the conservation pool. This
accretion will form a "sediment wedge" within the floodplain, stream channel, and delta
area inducing passive wetland conditions for an unknown distance (or elevation) in both
the upstream and downstream direction.
Obiectives
The objective of this study is to collect the following information on existing reservoirs
in the region:
1) Determine the elevation range above the conservation pool that supports
relatively contiguous wetlands apparently resulting from the conservation
pool, groundwater mounding, and /or sediment accretion. The data reported
will include the elevation of the conservation pool, the average period that
the reservoir water surface resides at the conservation pool elevation, the
elevation range for relatively contiguous, forested wetland formation, and
elevation range for emergent wetland formation.
2) Determine the elevation range of the stream bed above the conservation
pool that has sustained sediment accretion to the extent that the channel
supports an average depth of 1 foot or less.
3) Determine the range of channel depths present in the wetland elevation
range.
4) Prepare a profile and plan view that depicts typical conditions relative to the
conservation pool
Methods
Field reconnaissance of several reservoirs and impoundments in the central Piedmont was
conducted in order to establish baseline information on sedimentation and wetland
development around the point of stream inflow and conservation pool levels. Two sites
were chosen that best exhibited long term sedimentation processes (> 10 years) for large
and small watersheds. One site is located at the point of inflow of Little Briar Creek into
Crabtree Lake, Wake County. Little Briar Creek services a watershed of approximately
13.5 square miles. Land use for this sub -basin is predominantly forested, but is part of
27
a rapidly developing Research Triangle Park, including the Raleigh- Durham International
Airport. The second site is located at the inflow of an unnamed tributary of Horse Creek
into Falls Lake, Wake County. The tributary services a watershed of approximately 0.3
square miles. Land use for this watershed is predominantly forested, though residential
development is encroaching upon the upper reaches of the area.
Topographic maps of the alluvial wetland fans were prepared to 0.5 -foot contour intervals
by laser level and Global Positioning System (GPS) measurements. Embankments,
shorelines, and wetland boundaries were measured and surveyed relative to the reservoir
water surface elevations. For Falls Lake, conservation pool and daily water surface
elevations were obtained for a 16 -year period from USACE (1984- 1999). Pool elevation
for Crabtree Lake was obtained from the elevation of the low -flow spillway crest tied to
a local USGS benchmark. Cross - sections and profiles were generated for the local stream
reach and floodplain area. Sediment borings were sited at various points along the
boundary of wetlands to determine depth of groundwater table relative to reservoir pool
elevations.
Results
Figures 9 and 10 provide a plan view and cross section for the sites at Crabtree Lake and
Falls Lake, including the groundwater elevations along the wetland fringe. Within the
conservation pool, a majority of the sediment deposition area resides under approximately
0.5 feet of water. Sediment deposition is greatest below the pool elevation contour with
inundated sediment bars generally extending into the lake over time. As the deposition
wedge elevates within central reaches of the inundated floodplain, the channel tends to
migrate towards the toe of adjacent slopes, incrementally filling the inundated valley floor.
On average, emergent wetland vegetation is present on accreted sediment to within 1 foot
below the normal pool. Forested and shrub -scrub wetland vegetation is typically present
to 2 feet above the normal pool in the upstream direction. These wetlands appear be
induced by an up- welling in the down - valley migration of groundwater, prior to inflow
with the conservation pool.
A review of lake level data for these reservoirs suggests that the water surface that
induces these wetlands resides within 0.5 feet of the conservation pool elevation, on
average, for 66% of the year. In Falls Lake, data indicate that the sediment wedge is
generated by water surfaces within 2 feet of the conservation pool elevation for 82% of
the time over the 1 5 -year monitoring period'.
1 For Falls Lake, the conservation pool elevation has been held fixed at 250.1 feet above MSL
over the 15 -year monitoring period
28
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or Just below the conservation pool elevation. As the sediment surface approaches the
elevation of the conservation pool, the wedge begins to extend further into the reservoir.
This accreting surface passively develops emergent wetland vegetation on surfaces within
1 foot of the conservation pool. This accreting wetland surface will continue to enlarge
over the life of the reservoir.
Based on the two reference sites studied, forested and shrub - scrub - wetlands typically
develop passively on floodplain surfaces that reside up to 2 feet above the conservation
pool. The area sustains sediment deposition in the active channel, including channel
migration across the developing delta over time.
Application
For this application, the water surface within the filled, Randleman Reservoir is assumed
to remain within 0.5 feet of the conservation pool (682 feet above MSL) for a minimum
of 66% of the year and within 2.0 feet of the conservation pool for a minimum 82% of
the time over a 15 -year period. Figure 11 depicts mapping of resulting passive wetland
development projected to occur on the Site as a result of the Randleman Reservoir.
The Site includes approximately 5.3 acres of open water (below 681 elevation) that are
projected to support future accretion and development of emergent wetlands. The 5.3-
acre area includes backwater conditions from the reservoir that will extend up a majority
of the on -site reach of the Hickory Creek channel under existing conditions.
Between elevations of 681 and 682 feet above MSL, emergent and submerged aquatic
vegetation is projected to develop. This area will be semi- permanently saturated or
inundated up to 1 foot in depth immediately after filling the reservoir. Based on reference
studies, the area is expected to support 3.5 acres of wetlands upon project completion.
In upper reaches of the lake effect area, shrub -scrub and forested wetland systems are
expected to be passively restored, encompassing approximately 11 0 acres on lands from
682 to 684 feet above MSL.
In total, approximately 14 5 acres are expected to support wetlands as a result of
hydrological modifications associated with the reservoir. Active wetland restoration
measures will include placement of sediment control structures (check dams) within
entrenched channels along the immediate periphery of the reservoir pool and construction
of a green tree impoundment in areas above the reservoir pool. Additional active
restoration measures will include preparation of pastured surfaces and planting of wetland
adapted vegetation designed to laterally extend the sediment wedge and wetland extent
in the up- valley direction.
31
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4.3 SURFACE WATER ANALYSES
Surface drainage on the Site and surrounding area was analyzed to predict the effects of
diverting existing surface drainage into wetland restoration areas along the primary and
secondary floodplains. Several alternatives were evaluated to determine surface water
modifications that maximize wetland acreage and water quality benefits, while providing
for increased wetland habitat for dependent species.
Hydrologic and hydraulic analyses were completed using standard study methods of
USACE and NRCS. Flood events of a magnitude which are expected to be equaled or
exceeded once on average every 1 -, 2 -, 5 -, 10 -, or 100 -year period were selected to
characterize existing and proposed conditions at the Site.
Hydrologic Analyses
Hydrologic analyses were carried out using the USACE HEC -1 model to establish the peak
stream discharge for the 1 -, 2 -, 5 -, 10 -, and 100 -year flood events.
Input for the HEC -1 model consisted of synthetic storm precipitation data, drainage area,
NRCS curve numbers, and drainage basin lag time. Table 2 summarizes the total, 24 -hour
precipitation event for each storm that was analyzed. Precipitation data was obtained
from U.S. National Weather Service documents (NOAA TP -40 and Hydro -35). The
drainage area was delineated on 7.5- minute USGS topographic maps and then subdivided
into subbasins based on land use or location of tributaries. The drainage area for each
subbasin was estimated using a planimeter. The NRCS curve numbers were estimated
using methods described in NRCS TR -55. The subbasin lag times were estimated using
Snyder's method.
Because there were no onsite gage data, the HEC -1 computer models could not be
calibrated. The models were validated by comparing the 100 -year peak discharges
estimated from the HEC -1 models with peak discharges estimated by regional formulas for
the Piedmont region of North Carolina in the USGS Water- Resources Investigations Report
87 -4096. NRCS curve numbers for the HEC -1 models were adjusted until the HEC -1 peak
discharges were within 25 - 30% of the regional formula values. Table 2 summarizes peak
discharges estimated by the validated HEC -1 model and the regional equations.
Hydraulic Analyses
Water- surface elevations of the 1 -, 2 -, 5 -, 10 -, and 100 -year floods of Hickory Creek were
estimated using the USACE HEC -2 computer program. Channel cross sections for the
hydraulic analyses were obtained from the most currently available HEC -2 computer runs
completed for Federal Emergency Management Agency (FEMA) Flood Insurance Studies
(FIS). Additional cross - sections that were needed to model proposed structures were
taken from digital orthophoto maps with a contour interval of 1 foot.
33
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Roughness coefficients (Manning's "n ") in the channels and on the overbank areas were
taken from the HEC -2 computer runs previously mentioned and verified with field
inspections of the sites. Roughness coefficients in the main channel were 0.06 and 0.12
for overbank areas.
Starting water surface elevations and energy slope for existing conditions were estimated
from the published flood insurance studies for Guilford County. The slope -area option
provided by the HEC -2 model was used to estimate the true value of the water surface
elevation at the beginning cross - section. For future conditions, the starting water surface
elevation at the first cross - section was 688.21 feet (100 -year reservoir pool elevation) for
the 100 -year flood and 682 feet for the 1 -, 2 -, 5 -, and 10 -year floods (normal reservoir
pool elevation).
Tables 2 summarizes the water surface elevations for existing and proposed conditions.
Figure 7 (Section 3.0) depicts modeled flood elevations for the 1 -, 2 -, 5 -, 10- and 100 -
year, 24 -hour storm event. The model indicates that overbank flooding has been
effectively eliminated until the 2 -year storm along the entrenched channel under existing
conditions.
Restoration methods are designed to reduce the channel from 6 feet in depth below the
floodplain to saturated / inundated conditions in the vicinity of the reservoir pool. In
addition, a greentree impoundment is proposed immediately above the reservoir with
water surface elevations fluctuating seasonally between 682 feet and 685 feet above
mean sea level, representing surface saturated / inundated within a majority of the former.
dredged channel.
Establishment of the reservoir pool and greentree impoundment will elevate flood levels
to elevations conducive to wetland restoration. Therefore, post - project conditions were
modeled to evaluate the potential increase in flood levels if the greentree impoundment
controllable weirs were lowered to the maximum extent feasible. Water surface elevations
range from 2 9 feet 0 -year storm) to 9.1 feet (100 -year storm) feet higher for proposed
conditions in downstream reaches of the Site, due primary to the reservoir pool. As the
cross - sections progress upstream away from the reservoir, the difference between
proposed and existing conditions decreases with the 100 -year flood elevation increasing
by an average of approximately 1.8 feet (starting upstream of cross - section 1376).
Potential impacts to above ground structures were not observed during field reviews
within the post - project 100 -year floodplain area. However, immediately below the Site,
SR 1 132 appears to reside under the projected 100 -year flood elevation induced by the
reservoir pool (688.2 feet above mean sea level).
35
4.4 GROUNDWATER MODELING
Groundwater modeling was performed to characterize water table elevations under historic
(reference), existing, and post - restoration conditions. The groundwater modeling software
selected for simulating shallow subsurface conditions and groundwater behavior at the
Site is DRAINMOD. This model was developed by R.W. Skaggs, Ph.D., P.E., of North
Carolina State University (NCSU) to simulate the performance of water table management
systems.
Model Description
DRAINMOD was originally developed to simulate the performance of agricultural drainage
networks on sites with shallow water table conditions. DRAINMOD predicts water
balances in the soil -water regime at the midpoint between two drains of equal elevation.
The model is capable of calculating hourly values for water table depth, surface runoff,
subsurface drainage, infiltration, and actual evapotranspiration over long periods
referenced to climatological data. The reliability of DRAINMOD has been tested for a wide
range of soil, crop, and climatological conditions. Results of tests in North Carolina
(Skaggs, 1982), Ohio (Skaggs et a/. 1981), Louisiana (Gayle et a/. 1985; Fouss et a/.
1987), Florida (Rogers 1985), Michigan (Belcher and Merva 1987), and Belgium (Susanto
et a/. 1987) indicate that the model can be used to reliably predict water table elevations
and drain flow rates. DRAINMOD has also been used to evaluate wetland hydrology by
Skaggs et a/. (1993). Methods for evaluating water balance equations and equation
variables are discussed in detail in Skaggs et a/. (1993).
DRAINMOD has been modified for application to wetland studies by adding a counter that
accumulates the number of events wherein the water table rises above a specified depth
and remains above that threshold depth for a given duration during the growing season.
Wetland hydrology is defined as groundwater within 12 inches of the surface for 28
consecutive days (12.5% of the growing season), and 11 consecutive days (5% of the
growing season). Wetland hydrology is achieved in the model if target hydropenods are
met for more than one -half of the number of years modeled (i.e., 16 out of 31).
Groundwater drainage contours are established on available mapping for various durations
of saturation within 1 -foot of the soil surface (i.e. saturation contour for 0 -5 %, 5- 12.5 %,
and 12.5 -20% of the growing season).
Model inputs for DRAINMOD simulations were obtained as follows: the United States
Department of Agriculture (USDA) soil texture classification, number of days in the
growing season (defined as March 26 - November 6), and hydraulic conductivity data
were obtained from the NRCS soil survey for Guilford County (USDA 1977). Inputs for
soil parameters such as the water table depth /volume, drained /upflux relationship, Green -
Ampt parameters, and water content /matric suction relationship were obtained utilizing
the MUUF computer software developed by NRCS. Precipitation and temperature files
were obtained for the years 1930 through 1980 for Charlotte, North Carolina.
DRAINMOD simulations were run for Congaree soils, the dominant series within upper
reaches of the Site. Because Chewacla and Wehadkee map units are inundated by the
reservoir in lower reaches of the Site, DRAINMOD simulations were not considered
applicable within these future surface water driven wetlands.
Forested and maintained pasture conditions (evapotranspiration rates) were modeled. The
simulations were run for six channel inverts (0, 1, 2, 4, 6, and 8 feet) and at various target
hydroperiods during the growing season. Table 3 provides a depiction of the groundwater
discharge zone of influence by invert depth (elevation below floodplain). For example, a
stream channel invert 4 feet below the forested floodplain elevation is modeled as
reducing surface hydroperiods below 5 % of the growing season at a distance of 145 feet
from the channel in a Congaree soil. A forested floodplain surface 6 feet in elevation
above the channel invert and greater than 195 feet from the channel is projected to
support wetlands.
ESC interpreted the groundwater drainage model based upon channel depth (based on
measured cross - sections), and floodplain elevation (based on topographic maps). Model
parameters were set to predict the average annual duration in which groundwater remains
within 1 -foot of the soil surface at assigned elevations above a channel invert or in- stream
structure. The floodplain elevations outside of the groundwater drainage contour and at
the modeled channel depth were fudged to be wetlands. Draft mapping of areas
potentially supporting wetlands was prepared based on available topographic mapping.
DRAINMOD simulations were designed to predict the transition zone from Congaree soils
to Wehadkee soils based on groundwater drainage conditions within a relatively flat
floodplain surface. Congaree soils represent a nonhydric (nonwetland), well drained soil
that is common on stream terraces immediately adjacent to streams. The Wehadkee series
comprises hydnc (typically wetland), poorly drained soils that are typical in backwater
floodplain areas situated further from drainageways.
Post - Restoration Model Applications and Results
For groundwater wetland restoration, the primary objectives of this project include: 1)
reduce channel incision along Hickory Creek and associated tributaries; 2) elevate the
groundwater gradient into the rooting zone for developing vegetation; and 3) establish
minimum wetland hydroperiods encompassing 5% of the growing season, which are
typical for nvenne wetlands in the Piedmont hydrophysiographic province. Therefore, the
effective post - project depths of the Hickory Creek channel will be reduced from an
average of 6 feet under existing conditions to gradients between 1 to 3 feet below the
floodplain.
37
TABLE 3
Modeled Groundwater Discharge Zone of Influence on Wetland Hydroperiod
Congaree Soil
Floodplain
Groundwater
Number of
Groundwater
Number of
Elevation Above
Discharge Zone of
Years
Discharge Zone of
Years
Channel Invert /
Influence'
Wetland
Influence
Wetland
Weir Height'
(feet)
Criteria Met
(feet)
Criteria Met
(feet)
(Surface
(Surface
Hydropenods
Hydropenods <
<5% of the
12 5% of the
growing season)
growing season)
Forested Conditions
(relatively high surface water storage and rooting functions)
0
- - - --
27/31
- - - --
19/31
1
55
16/31
UA3
14/31
2
90
17/31
UA
14/31
4
145
16/31
UA
14/31
6
195
16/31
UA
12/31
8
215
16/31
UA
12/31
Fallow Field /Pasture Conditions
(relatively low surface water storage and rooting functions)
0
- - - --
25/31
UA
6/31
1
120
16/31
UA
3/31
2
165
17/31
UA
3/31
4
240
16/31
UA
2/31
6
280
16/31
UA
2/31
8
305
16/31
UA
2/31
"Weir Height" is assumed to represent the effective depth (invert) of the drainage feature
Discharge Zone of Influence is equal to % of the modeled drainage spacing
UA = wetland criteria unachievable
DRAINMOD simulations modeled the zone of influence of the post project channel on
wetland hydroperiods within the primary floodplain. The maximum zone of influence may
be used to predict the area of groundwater wetland hydrological influence that may result
due the elevation of stream flow within the channel. In addition, the model provides an
estimate of the area that may continue to be affected in perpetuity by the stream channel
at a depth of 1 to 3 feet below the floodplain elevation.
Based on these simulations, the post - restoration channel is expected to continue to
effectively drain groundwater from the Congaree soil map unit which encompasses the
entire Site. Model simulations indicate that a series of in- stream weirs placed to within
1 foot of the adjacent floodplain elevation will not restore significant areas of wetlands.
Based on NRCS -MUUF parameters, Congaree soils exhibit relatively low vertical hydraulic
conductivity and rapid lateral flow within the upper soil profile. A channel invert 1 foot
below the adjacent stream terrace continues to effectively drain an area 120 feet adjacent
to the drainage feature. Gradual slopes in remaining portions of the outer floodplain are
projected to continue draining towards the modified stream channel. Therefore, in- stream
weirs do not provide a viable option for wetland restoration use based on the groundwater
model. To create wetlands within upper reaches of the Site, greentree impoundments will
be required to elevate the groundwater surface above the floodplain elevation
(immediately adjacent to the channel) periodically throughout the year.
4.5 REFERENCE GREENTREE IMPOUNDMENTS
Established greentree impoundments within the Piedmont of North Carolina were visited
to measure wetland attributes, review various structural designs, and to discern
management strategies employed. Reference systems include the Beaver Creek greentree
impoundment above Jordan Lake in Wake and Chatham Counties, the Little Creek
impoundment to Jordan Lake in Durham County, the Rocky Branch impoundment above
Falls Lake in Wake County, and the Country Line Creek impoundments in Caswell County.
These impoundments have typically been located above water supply reservoirs in the
region primarily to replace wetland habitat inundated by the reservoir, provide waterfowl
habitat, and to control sedimentation.
Controllable weirs range from concrete dams and electronic sluice gates on larger
tributaries to corrugated metal pipe using flash -board risers on smaller systems. The
associated dams typically consist of an earthen causeway with rip- rapped emergency
spillways and erosion control areas. Dams likely to be overtopped within watersheds
greater than 10 square miles have often been reinforced with concrete materials placed
on the earthen dam
39
Figure 12 provides a conceptual depiction of a typical weir and dam for greentree
impoundments within watersheds ranging from 2 to 7 square miles. The weir consists of
two, 4 -foot wide slots with wooden flash -board risers used to control the water surface
elevation. For this application, the flash boards could be completely removed to provide
for existing channel flows during summer months, planting periods, or for other
management purposes. Subsequently, the boards may be installed during the winter
and /or early part of the growing season to establish wetland hydrology behind the'\
impoundment. The type or size of the weir used at the Hickory Creek Site may be'�
modified during the engineering design phase based upon watershed size, projected
discharge /velocities on the structure, potential for stability, cost, constructability, and /or
management concerns.
A profile of the Country Line Creek impoundment in Caswell County was measured(to
evaluate wetland development relative to the dam height, typical winter weir height,
summer weir height, and valley slope. Figures 13 and 14 provide a depiction of the
greentree impoundment and lake shoreline reference characteristics, including vegetation
development patterns relative to water surface elevations. Within the reference greentree
impoundments, stream channels have been obscured due to alluvial sediment deposition
and vegetation development patterns. The stream channel has been altered to the extent
that wetland characteristics typically occupy the entire impoundment land area, up to the
water surface elevation established during winter months.
4.6 REFERENCE PLANT COMMUNITIES
In order to establish a forested wetland system for mitigation purposes, a reference
community needs to be established. According to Mitigation Site Classification (MIST)
guidelines (EPA 1990), the area of proposed restoration should attempt to emulate a
Reference Forest Ecosystem (RFE) In terms of soils, hydrology, and vegetation. In this
case, the target RFEs were composed of steady -state woodlands In the region that have
sustained loading of fluvial sediments on floodplains In the past. However, forest
canopies have developed on these reference sites which support soil, landform, and
hydrological characteristics that restoration will attempt to emulate.
All of the RIFE sites have been Impacted by sediment deposition, selective cutting or high-
grading, channel migration /disturbances, and relatively high energy flood events.
Therefore, the species composition of these plots should be considered as a guide only.
Reference forest data used In restoration was modified to emulate steady state community
structure as described In the Classification of the Natural Communities of North Carolina
(Schafale and Weakley 1990).
40
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Two regional reference sites were selected within floodplains along the Rocky River in
Cabarrus County, North Carolina Floodplains associated with this river system have
aggraded over the past century, inducing braided channel configurations and accelerated
sediment deposition within reference feeder tributaries. However, the 16 plots have been
placed within relatively mature bottomland hardwood /swamp forests that have developed
on accreted sediment.
The reference vegetation samples are designed to characterize the plant communities
proposed for restoration. Circular, 0.1 -acre plot sampling was utilized to establish base -line
vegetation composition and structure in reference areas. Species were recorded along
with individual tree diameters, canopy class, and dominance. From collected field data,
importance values (Brower et al. 1990) of dominant canopy and mid -story trees were
calculated. The composition of shrub /sapling and herb strata were recorded and identified
to species (Table 4 and Table 5).
At Site 1 (Table 4), the forest canopy is dominated by green ash, (Importance value [IV]
28 %), sweetgum (IV 19 %), American elm (Ulmus amencana) (IV 11 %), box elder (IV 8 %)
and red maple (IV 7 %). Canopy species with lesser importance include black willow,
slippery elm (Ulmus alata), river birch, tulip poplar, and water oak (Quercus nigra).
Understory trees include flowering dogwood, ironwood, and sugarberry (Celtis laevigata).
A developed shrub layer is not generally present. Herbs include Nepal microstegium,
violets (Viola spp.), asters (Aster sp.), and river oats (Chasmanthium latif6hum).
At Site 2 (Table 5), the forest canopy is dominated by green ash, (IV 39 %), box elder (IV
22 %), American elm (IV 12 %), and swamp chestnut oak (IV 6 %). Portions of the canopy
at RFE locations were also dominated by ironwood, overcup oak (Quercus lyrata),
sugarberry, sweet gum, red maple, black willow, slippery elm, water oak, and river birch.
The shrub /sapling layer is characterized by the non - native Chinese privet (Ligustrum
chinensis), paw -paw (Asimrna triloba), and shade tolerant' canopy species. Herbaceous
species include Japanese honeysuckle (Lonicerajaponica), blackberry, muscadine, common
greenbriar, sedges (Carex spp ), and poison ivy.
Piedmont swamp forests are communities located in depressional areas, along toe slopes,
and at the confluence of alluvial valleys, where lateral flow is restricted. These sites are
hydrologically influenced by upland seeps and drainages, and by occasional nverine
flooding. Overstory species are dominated by flood - tolerant bottomland elements such
as sweetgum, American elm, willow oak (Quercus phellos), swamp chestnut oak, green
ash, overcup oak, and swamp cottonwood (Populus heterophylla). Wetter sites may
provide a broken to open canopy providing enough light for development of a dense
herbaceous /shrub layer. Species found on these sites may include button -bush
(Cephalanthus occidentalis), elderberry (Sambucus canadensis), silky dogwood, false
nettle (Boehmena cylindrica), sedges (Carex spp), rushes (Juncus spp.), and lizard's tail
(Saururus cernuus). Giant cane (Arundinarla gigantea) is prevalent in places.
44
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4.7 REFERENCE PHYSIOGRAPHY AND SURFACE TOPOGRAPHY
Surface features were mapped within reference Piedmont' swamp /bottomland hardwood
forest in order to establish base -line topographic conditions for restoration planning use.
This community lies within a seasonally to semi- permanently inundated area that has
supported sediment accretion in the past with inundation from stream flows occurring on
a frequent basis. The channel is actively migrating across aggrading floodplain, similar to
conditions that may be induced in a greentree impoundment over time.
Topographic maps of the accretion area were prepared to 0.5 -foot contour intervals by
laser level and tape measure. Abandoned stream channels were mapped along with
approximate jurisdictional wetland extent relative to the, water surface within stream
channels. A plan view, cross - sections, and profiles were generated for the channel and
adjacent alluvial surface. The channel dimension, rate of channel migration, and slope of
the floodplain floor represent the primary features extrapolated for use in restoration
planning. The objective of restoration is to reduce the effective size of the Hickory
channel to induce sediment deposition, channel migration, braiding, ponding, and /or
anastomosed stream types above the water supply reservoir.
Figure 15 depicts a plan view and cross - sections of the alluvial fan, including locations of
abandoned channels that have developed over the last several years. The channel exhibits
active migration across the valley floor as aggradation processes elevate isolated portions
of the floodplain. The active channel is classified as an E5 (highly sinuous) stream type
in upper reaches of the reference site. Subsequently, the channel transitions into an
anastomosed (DA5 -6) channel and subsequent braided (D6) channel immediately prior to
the confluence with a near - permanently inundated section of the reference reach. This
braided reach and near - permanently inundated area represents projected conditions within
the transition zone into the reservoir conservation pool. The floodplain throughout this
reference reach continues to support forest vegetation, including shrub -scrub dominated
communities within the potentially inundated areas.
47
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5.0 WETLAND RESTORATION PLAN
This restoration plan has been designed to establish wetlands along the periphery of the
reservoir pool through planting of lake effect wetland areas and through construction of
a greentree impoundment immediately above the reservoir pool. The lake effect wetlands
are projected to reside between floodplain elevations of 681 feet to 684 feet above mean
sea level.
Immediately above the lake effect wetlands, a greentree impoundment is proposed to
extend contiguous wetland presence to floodplain elevations ranging from 684 feet to 687
feet above mean sea level. The greentree impoundment is expected to facilitate the
establishment of emergent, shrub, and forested wetlands. In general, a green tree
impoundment comprises a floodplain levee and controllable outlet structure The
elevation of the outlet is raised during winter months to promote ponding, sediment
deposition, and wetland habitat. Subsequently, the elevation of the outlet is lowered in
early spring to allow for vegetation growth, nutrient uptake, and seedling establishment.
A management plan has been prepared (Section 8.0) for long term maintenance of these
systems over the life of the Randleman Reservoir. Management techniques for greentree
impoundments surrounding the reservoir will be managed according to constructs outlined
in the U.S. Army Corp of Engineer's, Greentree Reservoir Management Handbook (Mitchell
and Newling 1986).
Components of this plan have been established based on reference wetland studies
described in Section 4.0. This effort will be performed by: 1) passively
saturating /inundating surfaces associated with the reservoir conservation pool; 2)
installing sediment control check dams within dredged channels at the reservoir pool
elevation; 3) installing a controllable weir and dam; 4) scarifying pastured surfaces for
reforestation, 5) distributing woody debris into formerly cleared and pastured areas, and
6) planting of target wetland tree species in the area. Monitoring of wetland development
will be performed to track successional characteristics of the Site and to verify wetland
restoration success.
5.1 PASSIVE SATURATION / INUNDATION FROM THE RESERVOIR POOL
Based on reference studies within existing water supply reservoirs in the region, wetlands
are projected to develop on surface elevations ranging from a minimum of 1 foot below,
to 2 feet above the conservation pool (681 -684 feet above MSL). As depicted in Figure
11 , shrub /scrub and submerged aquatic vegetation is projected to develop on surfaces
ranging between 681 and 682 feet above MSL, encompassing approximately 3.5 acres of
land area. This land area is expected to expand into open water portions of the Site over
time as sediment accretion occurs within the reservoir pool (below existing elevations of
681 feet above MSL). However, restoration plans are designed to delay this accretion in
favor of alluvial deposition within a greentree impoundment.
H We
Based on reference studies, elevations between 682 and 684 feet above MSL will passively
develop shrub - scrub, bottomland hardwood, and bottomland swamp forest wetlands.
This area encompasses approximately 11 acres within central to lower portions of the Site
(Figure 11). These areas will be planted with tree species likely to replace existing
elements as wetland hydrology begins to dominate the area (Section 5 6) Active wetland
restoration will be initiated immediately above wetlands passively restored by the
conservation pool. Because restoration design is dependent upon initial establishment of
backwater conditions in Hickory Creek, construction activities associated with controllable
weirs and dams should be performed concurrent with filling of the reservoir.
5.2 SEDIMENT CONTROL CHECK DAMS
Rock check dams will be installed within the dredged and straightened stream channel to
reduce sediment transport capacity and to increase sediment storage potential within the
Site. As depicted in Figure 16, four rock check dams are proposed within the lake effect
wetland area (D1 to D4) with top elevations ranging from 680 feet to 682 feet above
mean sea level. These check dams will reside at or under the reservoir pool when water
surfaces reside at the conservation pool elevation of 682 feet. However, these check
dams will reduce erosional forces and sediment transport potential when reservoir pool
levels are below 682 feet. The sediment and associated pollutants deposited within the
dredged stream system will exhibit greater potential to remain on -site during drought
periods and low lake levels.
The check dams will be constructed of relatively large rip -rap material placed to within 1-
foot of the adjacent floodplain elevation. The dam will span the 10 -foot to 30 -foot width
of the dredged channels with a minimum 2.1 side slopes in the upstream and downstream
direction. The check dams will be installed after forest clearing activities have been
completed within the reservoir conservation pool and immediately prior to filling of the
reservoir.
5.3 IMPOUNDMENT / WEIR CONSTRUCTION
One greentree impoundment structure will be constructed within the Site, as depicted in
Figure 16. The impoundment has been placed within a confined location on the valley
floor at the 684 -foot contour depicted on topographic mapping. A more accurate
placement of this structure can be achieved by observing actual water surface elevations
in the channel resulting from the established conservation pool. Subsequently, the weir
would be accurately placed 2 feet above the conservation pool water surface.
Figure 12 provides a conceptual depiction of the proposed impoundment structure
including target elevations for the winter water surface and embankment height The
design or placement of these impoundments may be modified during the engineering
design phase based on potential stability, constructability, cost, or other constraints.
50
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5.4 SURFACE SCARIFICATION
Before wetland community restoration is implemented, pasture lands will be scarified
(Figure 16). The scarification will be performed as linear bands directed perpendicular to
land slope (surface water flows). After scarification, the soil surface will exhibit complex
microtopography ranging to 0.5 feet in vertical asymmetry across local reaches of the
landscape. Restored microtopographic relief is considered critical to short term hydrology
restoration efforts. Therefore, multiple passes along each band are recommended to
ensure adequate surface roughing and surface water storage potential across the Site.
Subsequently, community restoration will be initiated on scarified wetland surfaces.
5.5 WOODY DEBRIS DEPOSITION
Re- introduction of woody debris represents an important component of wetland
restoration on pastoral lands. Woody debris, including downed trees, tip mounds, snags,
and decomposing material represents important habitat elements for wetland dependent
wildlife. However, the restoration areas on pasture land will not be capable of providing
coarse, woody habitat elements for up to 50 years after re- planting. Therefore, woody
material generated from embankment construction or other activities on the property will
be distributed across future wetland surfaces to the extent feasible. The material may be
lifted or pushed from adjacent windrows or forest areas as well.
5.6 WETLAND COMMUNITY RESTORATION
Restoration of wetland forested communities provides habitat for area wildlife and allows
for development and expansion of characteristic wetland dependent species across the
landscape. Ecotonal changes between community types contribute to diversity and
provide secondary benefits such as enhanced feeding and nesting opportunities for
mammals, birds, amphibians, and other wildlife.
RIFE data, on -site observations, and ecosystem classification has been used to develop the
species associations promoted during community restoration activities. Target plant
community associations include: 1) bottomland hardwood / swamp forest and 2) scrub -
shrub / swamp forest
Planting Plan
The planting plan consists of- 1) acquisition of available wetland species; 2)
implementation of proposed surface topography improvements; and 3) planting of
selected species on -site. Figure 17 depicts the locations targeted for planting,
encompassing 18 6 acres of land area. Planted species names by community are listed
below.
52
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Bottomland Hardwood / Swamp Forest
1 .
Cherrybark Oak (Quercus pagoda)
2
Overcup Oak (Quercus lyrata)
3.
Willow Oak (Quercus phellos)
4.
Swamp Chestnut Oak (Quercus michauxii)
5.
Swamp Cottonwood (Popu/us heterophylla)
6.
Shagbark Hickory (Carya ovata)
7.
Bitternut Hickory (Carya cordlformis)
8.
Green Ash (Fraxinus pennsylvanica)
9
American Elm (Ulmus amencana)
10
Winged Elm (Ulmus alata)
1 1 .
Tulip Poplar (Liriodendron tulipifera)
Scrub -Shrub / Swamp Forest
1 .
Black Willow (Salix nigra)
2.
Possum -haw (flex decidua)
3.
Carolina holly (flex ambigua)
4.
River Birch (Betula nigra)
5.
American Sycamore (Platanus occldentalis)
6.
Green Ash (Fraxinus pennsylvanica)
7.
American Elm (Ulmus amencana)
8.
Swamp Cottonwood (Populus heterophylla)
9.
Overcup Oak (Quercus lyrata)
10.
Swamp Chestnut Oak (Quercus michauxii)
1 1 .
Silky Dogwood (Corpus amomum)
12.
Button -bush (Cephalanthus occidentalis)
13.
Elderberry (Sambucus canadensis)
14.
Tag Alder (Alnus serrulata)
Species selected for planting will be dependent upon availability of local seedling sources
Advanced notification to nurseries (1 year) may facilitate availability of various non-
commercial species. In full planting areas (existing agricultural land), bare -root seedlings
of tree species will be planted within specified areas at a density of 680 stems per acre
(8 -foot centers). Existing forest areas will receive supplemental planting to re- introduce
diagnostic tree elements at a density of 170 stems per acre (16 -foot centers). The total
number of stems and species distribution within each planting area are depicted in Table
6.
54
TABLE 6
Planting Plan
Vegetation Association
(Planting area)
Shrub - Scrub/
Swamp Forest
Bottomland
Hardwood/
Swamp Forest
(full planting)
Bottomland
Hardwood/
Swamp Forest
(supplemental
planting)
TOTAL STEMS
PLANTED
Stem Target (trees /ac)
680
680
170
- - - --
Area (acres)
5 8
42
86
186
SPECIES
# planted
(% total)
# planted
(% total)
# planted
(% total)
# planted
M total)
River Birch
400 (10)
400
Silky Dogwood
400 (10)
400
Button -bush
400 (10)
400
Elderberry
400 (10)
400
Tag Alder
400 (10)
400
Black Willow
200 (5)
200
Possum -haw
200 (5)
200
Carolina Holly
200 (5)
200
American Sycamore
200 (5)
200
Swamp Cottonwood
400 (10)
300 (10)
150 (10)
850
American Elm
200 (5)
150 (5)
100 (5)
450
Green Ash
200 (5)
150 (5)
100 (5)
450
Swamp Chestnut Oak
200 (5)
300 (10)
150 (10)
650
Overcup Oak
200 (5)
300 (10)
150 (10)
650
Cherrybark Oak
300 (10)
150 (10)
450
Willow Oak
300 00)
150 (10)
450
Shagbark Hickory
300 (10)
150 (10)
450
Bitternut Hickory
300 (10)
150 (10)
450
Winged Elm
300 (10)
150 (10)
450
Tulip Poplar
300 (10)
150 (10)
450
TOTAL
4000
3000
1550
8550
Some non - commercial elements may not be locally available at the time of planting
The stem count for unavailable species should be distributed among other target
elements based on the percent 1 %) distribution One year of advance notice to forest
nurseries will promote availability of some non - commercial elements However,
reproductive failure in the nursery may occur
Scientific names for each species, required for nursery inventory, are listed in the
mitigation plan
Planting will be performed between December 1 and March 15 to allow plants to stabilize
during the dormant period and set root during the following spring season. Opportunistic
species, which typically dominate early- to mid - successional forests have been excluded
from initial plantings on interior floodplains. Opportunistic species such as sweetgum, red
maple, and loblolly pine may become established. However, to the degree that long term
species diversity is not jeopardized, these species should be considered important
components of steady -state forest communities. Planting of opportunistic species such
as black willow will be targeted as stabilization elements in erosion control areas
immediately adjacent to the creek.
The planting plan is the blueprint for community restoration The anticipated results
stated in the regulatory success criteria (Section 6.0) may reflect vegetative conditions
achieved after steady -state forests are established over many years. However, the natural
progression through early successional stages of floodplain forest development will prevail
regardless of human interventions over a 5 -year monitoring period. In total, approximately
8,550 seedlings will be planted during wetland community restoration efforts.
56
6.0 MONITORING PLAN
The Monitoring Plan will entail analysis of the restoration area according to jurisdictional
wetland criteria (DOA 1987) Monitoring will include the observation and evaluation of
three primary parameters including hydrology, soil, and vegetation. Monitoring of
restoration efforts will be performed for 5 years or until success criteria are fulfilled.
6.1 HYDROLOGY
After hydrological modifications are performed, surficial monitoring wells will be designed
and placed in accordance with specifications in U.S. Corps of Engineers', Installing
Monitoring Wells /Piezometers in Wetlands (WRP Technical Note HY- IA -3.1, August 1993).
Monitoring wells will be set to a depth of up to 24 inches below the soil surface to track
water surface elevations in the impoundment relative to the weir height. All screened
portions of the well will be buried in a sand screen, filter fabric, and /or a bentonite cap
to prevent siltation during floods. Recording devices (if used) will be placed five feet
above the ground elevation. Placement of recording devices at this height should guard
against over topping for a projected 50 -year flood elevation. The well will be stabilized
from flood shear by reinforcing steel bar (re-bar).
Six monitoring wells will be installed in restoration areas to provide representative
coverage throughout the Site. Approximate well locations are depicted in Figure 18.
Hydrological sampling will be performed during the growing season (March 26 to
November 6) at intervals necessary to satisfy the hydrologic success criteria. In general,
the wells will be sampled weekly through the spring and early summer and intermittently
through the remainder of the growing season, if needed to verify success.
6.2 HYDROLOGY SUCCESS CRITERIA
Target hydrological characteristics have been evaluated using regulatory wetland
hydrology criteria. The regulatory wetland hydrology criterion requires saturation (free
water) within one foot of the soil surface for 5% of the growing season under normal
climatic conditions.
Success Criteria
Under normal climatic conditions, hydrology success criteria comprises saturation (free
water) within 1 foot of the soil surface for a minimum of 5% of the growing season. This
hydroperiod translates to saturation for a minimum, 1 1 -day (5 %) consecutive period during
the growing season, which extends from March 26 through November 6 (USDA 1977).
If wetland parameters are marginal as indicated by vegetation and hydrology monitoring,
a jurisdictional determination will be performed in the questionable areas.
57
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6.3 SOIL
Mitigation activities are expected to increase the deposition and transport of stream
sediments during overbank flood events. As a result, soils (Fluvaquents) are continuously
reworked by fluvial processes. Because iron reduction rates (gleying) are not spatially or
temporally uniform on recent alluvial deposits, soil color or other visual, hydric soil
properties are not considered suitable for quantitative wetland soil monitoring /success
criteria on active floodplains.
Soil monitoring will entail measurement of sediment accretion /reduction (aggradation/
degradation) at the location of each monitoring well and other hydraulically active areas
as identified by Site managers. Mitigation activities are designed to provide for flood and
sediment storage from the watershed. Therefore, hydraulic and energy dissipation
patterns should be distributed throughout as much of the Site as possible. However, an
area of particularly accelerated sediment deposition may raise land surfaces above the
elevation of the primary wetland floodplain over a relatively short period of time.
Conversely, deep scour holes or head -cuts may form in locations where flow velocity or
sediment deficits exceed a "normal distribution." Soil monitoring is designed to provide
a cursory review to predict the need for additional site modifications if accelerated
deposition or scour potentially jeopardizes wetland restoration efforts.
The re -bar used to support monitoring wells will be marked upon installation and in each
monitoring year at the elevation of the existing ground surface. In addition, the height
of silt lines will be recorded to predict the depth of inundation during the flood period.
Additional re -bar will be placed and measured in high energy areas identified by Site
managers, as needed. The change in elevation of the alluvial surface and deposition /scour
patterns relative to flood elevations will be recorded and compared to previous years.
6.4 SOIL SUCCESS CRITERIA
Success criteria require that the deposition /scour rate not exceed over 1 foot change in
surface elevations in any given year. Any areas affected by this excessive deposition /scour
will be mapped in the field The area will be reviewed to determine modifications to
drainage patterns that should be implemented, if any. Changes in surface elevations of
less than 1 foot per year will meet regulatory success criteria; however, modifications to
deposition / scour patterns may also be considered in certain circumstances.
6.5 VEGETATION
Restoration monitoring procedures for vegetation are designed in accordance with EPA
guidelines presented in MIST documentation (EPA 1990) and Compensatory Hardwood
Mitigation Guidelines (DOA 1993). The following presents a general discussion of the
monitoring program.
Vegetation will receive cursory, visual evaluation during periodic reading of monitoring
wells to ascertain the general conditions and degree of competition or overtopping of
planted elements. Subsequently, quantitative sampling of vegetation will be performed
once annually during the fall (September /November) for 5 years or until vegetation success
criteria are achieved. Sampling dates may be modified to accommodate flood events and
plot inundation, as needed.
During the first sample event, a visual survey will be performed in the reference wetlands
to identify all canopy tree species represented within target communities. These reference
tree species will be utilized to define "character tree species" as termed in the success
criteria.
Permanent (on each well head) or nonpermanent, randomly placed plots will be
established at representative locations in the restoration areas. Each plot will consist of
two, 300 -foot transects extending at a randomly selected compass bearing from a central
origin. The plot width along the transect will extend 4 feet on each side of the tape,
providing a 0.11 -acre plot sample at the location (600 feet x 8 feet / 43,560 square
feet /acre). Ten plots will be established to provide a 4% sample and a depiction of tree
species available for current and future seed sources within the restoration area. In each
plot, tree species and number of stems will be recorded and seedling /sapling /tree height
measured. Tree data from all plots will be combined into one database to calculate an
average density, by species, represented in restoration areas of the Site.
In each plot, presence /absence of shrub and herbaceous species will be recorded. A
wetland data form (DOA 1987) will be completed to document the classification and
description of vegetation, soil, and hydrology.
6.6 VEGETATION SUCCESS CRITERIA
Success criteria include the verification, per the wetland data form, that each plot
supports a species composition sufficient for a jurisdictional determination. Additional
success criteria are dependent upon density and growth of "Character Tree Species "
"Character Tree Species" are identified through visual inventory of reference wetland
communities used to orient the restoration project design. All canopy tree species
identified in the reference wetland will be utilized to define "Character Tree Species" as
termed in the success criteria (Character Tree Species are generally listed in Section 4 6
and Section 5.5).
An average density of 320 stems per acre of Character Tree Species must be surviving in
the first three monitoring years. Subsequently, 290 stems per acre of character tree
species must be surviving in year 4, and 260 stems per acre of Character Tree Species in
year 5 Each individual species is limited to representing up to 20% of the 320 stem per
.•
acre total. Additional stems of a particular species above the 20% threshold are discarded
from the statistical analysis. In essence, a minimum of five different character tree species
must be present with each species representing up to 20% of the 320 stem per acre total.
If vegetation success criteria are not achieved based on average density calculations from
combined plots over the entire restoration area, those individual plots that do not support
the stem per acre requirement and the representative area will be identified. Supplemental
planting will be performed in the identified area as needed until vegetation success criteria
are achieved.
No quantitative sampling requirements are proposed for herb assemblages. Development
of a forest canopy over several decades and restoration of wetland hydrology will dictate
success in migration and establishment of desired wetland understory and groundcover
populations.
6.7 REPORT SUBMITTAL
An Annual Wetland Monitoring Report (AWMR) will be prepared at the end of each
monitoring year (growing season). The AWMR will depict the sample plot and quadrant
locations and include photographs which illustrate site conditions. Data compilations and
analyses will be presented as described in Sections 6 1 through 6.6 including graphic and
tabular format, where practicable. Raw data in paper or computer (EXCEL) file format will
be prepared and submitted as an appendix or attachment to the AWMR.
61
7.0 IMPLEMENTATION SCHEDULE
Project implementation will include performance of restoration work in three primary
stages including: 1) impoundment / weir construction and site preparation, 2) tree and
shrub planting; 3) monitoring plan implementation, and 4) management program
implementation. This mitigation plan or implementation schedule may be modified based
upon civil design specifications, permit conditions, or contractor limitations.
Stage 1: Impoundment / Weir Construction and Site Preparation
Stage 1 will be performed concurrent with or subsequent to filling of the reservoir. Rock
check dams and the greentree impoundment will be installed at the designated locations.
This work will be performed during late spring and /or early summer months to reduce
erosion hazards associated with saturated soil or large August storms. Site preparation,
including debris removal, woody debris deposition, and scarification will be performed
during the same summer period, prior to tree planting.
Stage 2: Tree Planting
Tree and shrub planting will be performed the first winter after Stage 1 is complete. The
seedlings will be planted during the winter dormant period, prior to March 1 .
Stage 3: Monitoring Plan Implementation
Monitoring wells and permanent vegetation plots will be established immediately after
construction and planting activities are completed (prior to March 26, the start of the
growing season). The Site will be visited frequently to read monitoring wells and to
evaluate wetland development during the first growing season. Vegetation sampling and
hydrology monitoring will be completed by November 6 (the end of the growing season).
The first year of monitoring would be completed upon submittal of the Annual Wetland
Monitoring Report and fulfillment of success criteria. The monitoring sequence will be
repeated as described for four additional years or until success criteria are achieved.
Stage 4 Management Program Implementation
Green tree impoundments require active management throughout the life of the wetland
facility and water supply reservoir. Therefore, long -term management programs will be
required to ensure that wetland development is established and maintained. The
management program will be implemented concurrent with the monitoring plan as
described above. Constructs of the management program are described in the next
Section.
62
8.0 MANAGEMENT PROGRAM
Greentree impoundments require modification of water surface elevations on a regular
basis. Typically, the elevation of outlets is raised and lowered at variable times each year
to provide for development of target wetland vegetation. Wetland vegetation is typically
harvested and /or planted periodically to establish target vegetation patterns for waterfowl
or other wetland dependent wildlife. Invasive species such as Kudzu (Pueraria /obata) may
require systematic removal as well. For this project, outlet controls and vegetation
maintenance must also be manipulated to promote forested wetland development within
the 29.4 -acre Site. Target hydrological goals include soil saturation or inundation for a
minimum of 5% of the growing season (March 26 to November 6). The 5% criterion must
be achieved in 50% of the years over the life of the Randleman Reservoir.
The Piedmont Triad Regional Water Authority (PTRWA) will provide the fiscal and
administrative resources necessary to maintain and manage the greentree impoundments
over the life of the water supply reservoir. PTRWA will make provisions for establishment
of an Environmental Compliance Officer (Officer) to serve as the primary administrator and
authority over the greentree impoundments. The Officer will be under control of PTRWA
while PTRWA continues to manage the property. If the property is deeded to a resource
agency as described in Section 9.0, the resource agency will provide resources necessary
for establishment and maintenance of the Officer.
The Officer will be tasked to supervise, coordinate, monitor, and manipulate the greentree
impoundments throughout the life of the water supply reservoir. The Officer will
coordinate and implement, in consultation with qualified wildlife biologists, the following
greentree impoundment management components as described in the U.S. Army Corps
of Engineer's, Greentree Reservoir Management Manual (Mitchell and Newling 1986).
1) The Officer will be responsible for raising and lowering the controllable weirs
at a frequency and duration needed to establish wetland hydrology and
maximize development of wetland vegetation Target vegetation patterns
include establishment of tree species to the maximum extent feasible.
2) The Officer will periodically visit the Site to visually assess waste debris
dumping, erosion problems, debris dams on structures, vegetation patterns,
and other aspects of wetland development. The Officer will repair identified
problems to ensure continued functioning of the wetland.
3) The Officer will provide for periodic quantitative sampling of vegetation to
ensure that target vegetation species are developing and being replaced
within the impoundments. The results of vegetation samples will be used
63
by the Officer to adjust the frequency and /or duration that the controllable
weirs are raised or lowered and to order and plant vegetation elements as
needed.
4) The Officer will submit an annual report to the responsible resource agency
summarizing the dates of weir modification, the current vegetation sample,
trends in vegetation patterns, and recommendations for weir modifications
over the next monitoring weir. The report will also include
recommendations for structural modifications or additional plantings, as
needed. These reports will be prepared and submitted on annual basis over
the life of the Randleman Reservoir Water Supply.
M
9.0 DISPENSATION OF PROPERTY
PTRWA will maintain ownership of the property until all mitigation activities are
completed and the site is determined to be successful. Although no plan for dispensation
of the Site has been developed, PTRWA may continue to manage the property or may
deed the property to a resource agency (public or private) capable of managing the
greentree impoundments over the life of the reservoir. The resource agency will be
approved by the appropriate regulatory agencies. Covenants and /or restrictions on the
deed will be included along with adequate fiscal resources to ensure adequate
management and protection of the Site throughout the life of the reservoir.
65
10.0 WETLAND FUNCTIONAL EVALUATIONS
Mitigation activities at the Hickory Creek Mitigation Site should be determined based on
wetland functions generated and a comparison of restored functions to potentially
impacted wetland resources. Therefore, an evaluation of mitigation wetlands by
physiographic area is provided to evaluate site utility for mitigation in the region.
10.1 EXISTING CONDITIONS
Under existing conditions, hydrodynamic functions have been degraded or effectively
eliminated due to stream entrenchment, bed /bank erosion, soil leveling /compaction in
pasture areas, and removal of characteristic vegetation. Features which depict
performance of hydrodynamic wetland functions such as surface microtopography,
seasonal ponding, meandering stream channels, and characteristic wetland vegetation
have been effectively eliminated by alternative land uses on the stream terrace. Reduction
or elimination of wetland hydrology has also negated nutrient cycling and biological
functions within the complex. These former wetlands do not support natural communities
adapted to wetlands or the wetland dependent wildlife characteristic in the region.
10.2 PROJECTED, POST - RESTORATION CONDITIONS
Lake Effect Wetlands
Wetland restoration objectives have been adapted for this project due to location of the
Site relative to the Randleman Reservoir. The lower half of the Site will be directly
influenced by the reservoir conservation pool, including sediment deposition within stream
channels, development of accreting sediment bars, and establishment of wetlands on
saturated /inundated surfaces. Based on reference studies, these lake effect wetlands are
projected to support a mosaic of emergent, shrub - scrub, and forested wetland
communities on land surfaces from 1 foot below to 2 feet above the conservation pool.
This land area, comprising approximately 14.5 acres (Figure 11), will provide wetland
functions such as increased habitat for wetland dependent wildlife, sediment retention
above the water supply, and pollutant processing on vegetated wetland surfaces. Open
water is also projected to encompass approximately 4.4 acres within the lake effect
wetland that will provide for sediment storage and accreting wetland shorelines (Figure
18).
Monitoring and management of these lake effect wetlands will assist in ensuring that land
surfaces exposed to long -term sedimentation are not elevated above the water table, with
capacity to support wetland functions subsequently reduced or diminished over time. In
addition, efforts to maintain vegetation within rapid sedimentation or inundated areas will
also be employed during the monitoring period.
M. M.
Greentree Impoundments
The upper half of the Site will be used to extend the sediment deposition wedge
associated with the reservoir and to establish wetland communities capable of providing
wildlife habitat and water quality benefits The greentree impoundment is projected to
provide for restoration of regular overbank flood events and filling of the entrenched
channel with sediment over time. As a result, the floodplain areas are expected to support
an array of emergent, shrub - scrub, and forested wetland communities, providing
replacement of habitat for wetland dependent species displaced by the reservoir. Water
quality benefits are projected to include sediment retention and pollutant processing of
waters generated by the 8.7- square mile, urbanizing watershed.
Pro - active mitigation within the greentree impoundments is projected to provide
approximately 8.0 acres of wetland restoration / creation above the lake effect, wetland
restoration / creation area (an additional 14.5 acres). Therefore, 22.5 acres of wetland
restoration / creation, 2.5 acres of wetland preservation / enhancement, and 4.4 acres of
open water are potentially provided by the 29 4 -acre Site (Figure 18).
67
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