20061945 Ver 1_Stormwater Info_20070226 (2)

WITHERS RAVENEL
oU -►C� L�s
STORMWATER MANAGEMENT PLAN
PHASE 1
Edgemont Landing Subdivision
Wendell, North Carolina
Prepared For:
D.R. Horton
Attn: Scott Morrison
2000 Aerial Center Pkwy, Suite iio
Morrisville, NC 2756o
Prepared By:
WITHERS & RAVENEL, INC
ill MacKenan Drive
Cary, North Carolina 27511
February zoo?
W&R Project No. 2o6023
Stephanie A. Hall, PE

INTRODUCTION
The purpose of this study is to document the stormwater management plan for Phase 1 of the
proposed development. The plan includes the design of two proposed best management practices
(BMPs 1 and 2) in Phase 1 in order to treat stormwater runoff. The site is located just north of
Wendell Blvd (NC 97) adjacent to Buffalo Creek.
1
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Figure 1-1. Site outlined in red on 2005 Aerial Photography with phase line in yellow
The proposed BMPs for Phase 1 are constructed wetlands, designed to draw down the first inch of
runoff over a z to 5 day period and sized according to the Division of Water Quality surface area
requirements in order to achieve 85% Total Suspended Solids and 40% total nitrogen reduction.
The future Phase z of this development will employ the use of constructed wetlands and/or wet
detention basins followed by a forested filter strip in order to meet Water Quality Certification #3402
in order to manage stormwater. These BMPs will be designed and submitted at a later date.
Edgemont Landing Subdivision 1 W&R Project 2060237
Stormwater Management Plan — Phase i February 2007

METHODOLOGY
The stormwater study was conducted using the natural drainage features as depicted by the Wake
County aerial topographic information (2 -ft contours), field survey data and proposed development
within the drainage areas.
The scope of work included the following analyses:
Hydrology
❑ Simulation of the 10 -year and ioo-year rainfall events for the Wendell area
❑ Formulation of the 10 -year, and loo -year flood hydrographs for the proposed developed
drainage area
Hydraulic
❑ Routing the 10 -year and loo -year flood hydrographs for post development runoff through
the proposed stormwater BMPs
The results of the hydrologic calculations are used in the hydraulic analyses. The hydraulic design
requires the development of stage -storage and stage -discharge functions for the wet detention
basins. The rainfall/runoff hydrographs, stage -storage and stage -discharge functions have been
compiled to create a routing computer simulation model using Haestad Methods PondPack vio.o
software. This PondPack model was then used to assess the impact of the peak discharges for the
design rainfall events. The PondPack modeling results are provided as appendices to this report.
HYDROLOGY
The SCS Method was used to develop runoff hydrographs for the 10 -year and loo -year storm events.
NOAA Atlas 14 rainfall data for Raleigh-Durham International Airport was used to determine the 24-
hour duration rainfall for the 10 -year and loo -year storm events. This method requires three basic
parameters: a curve number (CN), time of concentration (tj, and drainage area.
Curve numbers were based on soil type and land use. Soil types were delineated from the Soil
Survey of Wake County, North Carolina (November 1970). Land use data was determined using aerial
photography and the most recent site plans for the proposed site. The curve numbers used in this
study are listed in the appendix of this report.
Times of concentration were estimated to have a time of concentration of 10 minutes to account for
shallow paved flow, and piping for the drainage areas to BMPs 1 and 2. The time of concentration
for the drainage area to BMP 3 was estimated to be 5 minutes due to the small drainage area.
HYDRAULICS
Computer simulated reservoir routing of the 10 -year, and loo -year design storms utilized stage -
storage and stage -discharge functions. Stage -storage functions were derived from the proposed
contours for the wet detention basins. A non-linear regression relation for surface area versus
elevation was derived for the BMPs. This relation estimates the incremental volume of the basin to
the stage or elevation of the basins. Stage -discharge functions were developed to size the outlet
structures for the wetlands.
CONCLUSIONS
Based on the routing study, the outlet proposed for each constructed wetland passes the loo -year
Edgemont Landing Subdivision 2 W&R Project 2o60237
Stormwater Management Plan — Phase i February 2007

storm through the spillway with freeboard. The results are included in the appendix of this report.
The drainage area map on Sheet 1 of the appendix includes the outlet design for each BMP. Planting
schedules and more detailed design information is included on Construction Drawings entitled
"Edgemont Landing Subdivision Phase s" submitted with this report.
Edgemont Landing Subdivision 3 W&R Project 2o60237
Stormwater Management Ptan — Phase i February 2007

APPENDIX

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