20220805 Ver 2_Reference Reach and TSP certification_20220715

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MEMORANDUM DICKSONr
community infrastructure consultants
720 Corporate Center Drive Raleigh,North Carolina 27607 919.782.0495 tel.
TO: Bryan Roden-Reynolds,PWS
Senior Regulatory Project Manager
U.S.Army Corps of Engineers,Wilmington District
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FROM: LIZ DINATALE,PE \\\�N C.A/Z
TSP-20-23015 `�0� • E S /�•�iy
DATE: 07/15/22 = :a SEAL _
�,: 040266 :
RE: NWP-27 ��;••F,yGINE
9
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The tributary to Davis Mine Creek is located in the back yards of houses along Poplar Glen
Drive. For this memo, and the design,the tributary is referred to as Poplar Glen Tributary.
Following a flood impact analysis completed by WK Dickson in January of 2021,WKD was
retained to develop a design that will provide relief to the houses along Poplar Glen Drive who
had been experiencing flooding from the tributary in their back yards. The tributary appears
undersized for its drainage area and is very flashy, often topping its banks in minor storm
events. For the purpose of planning,this was assumed to be a 2-year storm and modeled as
such.
The stream appears to have been historically channelized as there is no sinuosity and riffle-pool
bedforms are generally absent.Additionally,there was little to no aquatic benefit to the
tributary. The neighbors had taken it into their own hands to shore up their yards and add
stream bank armoring.The stream was filling in with sediment,blocking storm drains and
causing water to back up in multiple locations.
The existing stream is restricted in an artificial valley between the houses along Poplar Glen and
a relic soil spoils pile, likely from the construction of the houses along Poplar Glen and and
Lakewood Drive. Establishing natural meanders was not an option due to the proximity of
houses, decks and fill.
The Poplar Glen Tributary has little to no sediment load.Davis Mine Creek, downstream of
Poplar Glen Tributary,was considered as a reference for this quality of the streams in this
watershed.Davis Mine Creek also showed little to no sediment transportation but does exhibit
cobble/gravel riffles and pools.

HEC-RAS was used to model Poplar Glen Tributary to find the best solution to not only achieve
the clients' goals of nuisance flood reduction but also to enhance aquatic features for a healthier
ecosystem.
Taking reference from the existing conditions and wanting to provide relief to the homeowners,
the proposed design included lowering the invert of the stream such that the low flow channel
would carry a 2-year storm and allow for flood water dissipation by incorporating a 5 foot
bench to the north, away from the backyards of the homeowners (See Figure 1,right side of
channel).The new geometry carries the base flow without reducing efficiency and because the
stream tends to come out of its banks,the bench overflow will act as an energy dissipator as
well as wetland habitat.See example riffle cross section below.
POPLAR GLEN Plan:Designs-WKDFIaw 5/17/2022
RS•720 ..
710.
700
700
701-
100 120 100 100
S1010 tb
Figure 1-Example Riffle Cross Section
Riffle-pool sequences are included in the proposed design to provide diverse aquatic habitat.
The pool to pool spacing was based on the empirical value of 5 to 7 channel widths.Below is an
example pool cross section.
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POPLAR GLEN Plan:Designl-WKDFIow 5/17/2022
RS=710
725' 1 "
1� I .1
720'
715'
710
705'
7000
50 100 150 200 ]50
SWIM(It)
Figure 2—Example Pool Cross Section.
In both Figure 2 and Figure 3, the blue line represents the 2-year flow.
After carefully considering the benefits this project has on the aquatic and riparian habitat by
restoring natural bedform diversity where habitat has been greatly diminished or eliminated,it
is my opinion, as a qualified Technical Service Provider for the NRCS,that the Poplar Glen
Tributary project qualifies as an aquatic habitat enhancement to be authorized under
Nationwide Permit#27.
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