20170117 Ver 2_04 B-5758_HYD_SMPv2 05_20160627 (Apr 2016)_20180108

I II�;B11v��y North Carolina Department of Transportation
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,.. , Highway Stormwater Program �
STORMWATER MANAGEMENT PLAN —
(Version 2.05; Released April 2016) FOR NCDOT PROJECTS
WBS Element: 45714.1.1 TIP No.: B-5758 County(ies): Moore Page 2 of 2
Bridge to Culvert Avoidance and Minimization
Proposed Structure Summa
Sheet No. & Station Sheet No.: 4 Station: 14+75 Number of Barrels: 3
Drainage Area (ac or sq mi): Barrel Width/Diameter (ft): 14
Surface Water Body: 1 Aberdeen Creek Barrel Hei ht ft : 9
Culvert T e: Culvert Len th ft 41
Avoidance and Minimization Efforts: Offsite detour available, can maintain approximate existing horizontal and vertical profile. 2:1 fill slopes used. No ditches proposed in wetlands.
(Bridge to Culvert) Culvert barrels burried in excess of 1'-0" to minimize impacts to aquatic passage. Two low flow barrels provided approximate existing channel
dimension. Floodplain barrel with bench provided - native bed material specified to facility wildlife passage.
Stream Slope Fish and/or Aquatic Life Passage
Existing Average Stream Slope (%): 0.20 % Existing Low Flow Channel Dimensions 28 ft waters edge to waters edge. 5' water
Proposed Culvert Slope (%): 0.30 % in the Stream: depth (+/-). Variable side slopes 1.5:1 or
Culvert Burial flatter.
Proposed Culvert Burial Depth (ft): 1'-6" (Low Flow / 3'-6" (Floodplain Bench)
Existing Streambed Material: Sand, silt loam / some aquatic vegetation / organics Proposed Low Flow Dimensions 2@14' low flow barrels. 2:1 slope at bank.
Through the Culvert: 1.5"1 slope at bench.
Proposed Sills/Baffles: 1'-6" sills provided in Iow flow (left two) culvert barrels,
up and downstream, inset 2' minimum. 3'-6" sills Existing Low Flow Velocities in the
provided in floodplain bench (right) culvert barrel, up Stream (ft/s): < 1 fps
and downstream, inset 2' minimum. Floodplain sill ropose ow ow e oci ies roug
height set to approximate existing floodplain elevation the Culvert (fUs): < 1 fps
immediately adjacet to channel. Alternating Low Flow Sills/Baffles: Not required to approximate existing bed
pattern / profile.
Culvert/Stream Alignment
Stream Patterns Upstream and Downstream Stream does meander upstream of crossing, but 90 degree skew is well aligned with the floodplain and the channel at the existing crossing
of the Culvert that Could Affect Fish location. Floodplains accessed by storm flow regularly, limiting shear stress potential on the banks. Some armoring provided to prevent lateral
Passage and Bank Stability: migration and erosion up and downstream of the crossing.
Bed Forms Impacted by Culvert (riffles, Viewing the surveyed bed profile, the culvert will be located in an existing pool. Local deep bed e�evations can be deceptive. Culvert invert
pools, glides, etc.): elevations have been set to the best approximating of the overall bed profile when viewed from riffle to riffle, neglecting pools and dunes.
Low Flow Floodplain Bench Required? Yes Aberdeen creek has a very broad floodplain. Exising bridge provides flooplain relief span. Culvert designed to approximate this
(provide justification) existing condition and non-erosively pass the low flow events. Proposed level of service is the 2-year return period event.
Sharp Bends at InIeUOutlet? No Culvert is well aligned with stream at a proposed skew angle of 90 degrees.
(describe culvert alignment with stream)
Stream Realignment Necessary? (provide No Culvert will accommodate existing stream channel in its existing location.
justification)
Bank Stabilization: Class 'll' rip rap bank stabilization is recommended for those storm events which pass the culvert without accessing floodplain relief over the road
2- r event and lower
Outlet Velocities
Natural Stream Channel 2-yr Velocity (ft/s): 2.24 Natural Stream Channel 10-yr Velocity (ft/s): 2.28
Proposed Culvert 2-yr Outlet Velocity (ft/s): 3.37 Proposed Culvert 10-yr Outlet Velocity (ft/s): 2.45
Roadway Geometric Considerations
Evaluate/Describe Roadway Geometric Constraints:
Existing roadway has a less than 2-yr hydraulic level of service. Roadway overflow occurs both line ahead and line back of the existing bridge. Several insurable structures are located in the
floodplain both up and downstream of the crossing. In order to comply with all FEMA requirements, roadway overflow sections must be maintained. In order to accomodate the stream channel,
2@14' low flow barrels must be provided, requiring a 17" top slab. Through an iterative process, a roaday profile was set which approximates the existing horizontal and vertical, and limits the
lowering of existing low chord. Minimal cover has been provided (6" at worst point). Low profile also limits impacts to wetlands. Stream is well aligned with crossing.