20050785 All Versions_Stormwater Info_20060907

STORMWATER MANAGEMENT PLAN
WASHINGTON BYPASS
R-2510B
PROJECT DESCRIPTION
This document is to serve as the Stormwater Management Plan for the
Washington Bypass (TIP R-2510B). The Washington Bypass consists of improvements
to US 17 from south of SR 1149 (Price Road) and US 17 to north of SR 1509 (Springs
Road) in Beaufort County. This project consists of two new interchanges (NC 33 and US
264) and includes approximately 6.0 miles of new location and 0.7miles of existing road
widening. The highlight of this project is a 2.9-mile bridge that spans the Tar River and
adjoining wetlands.
CONTACT INFORMATION
Contractor:
Flatiron/United Joint Venture
Attn: Paul Newman
8826 Highway 64 East
Knightdale, NC 27545
Telephone: [phone]
Fax: [phone]
Engineer: For any questions about this Stormwater Management Plan, please contact:
Sungate Design Group, P.A.
Attn: Josh Dalton, P.E.
915 Jones Franklin Road
Raleigh, NC 27606
Telephone: [phone]
Fax: [phone]
IDENTIFY PROJECT INVOLVEMENT
The Washington Bypass is located in the Tar-Pamlico River Basin, which is
subject to the Tar-Pamlico Buffer Rules. All streams located within the project area are
classified as C, Sw (Swamp Waters), and NSW (Nutrient Sensitive Waters).
The North Carolina Coastal Area Management Act (CAMA) provides for
jurisdictional review of impacts associated with Areas of Environmental Concern (AEC)
in 20 designated coastal counties, which includes Beaufort County. The impacts to
AEC's will require a Major Development Permit from NCDENR, Division of Coastal
Management (DCM). A CAMA consistency determination will be required from
NCDENR-DCM as a part of the permit process. A State Stormwater Permit for CAMA
Major Permits is required for this project.
The following commitments related to stormwater management have been made
in the Record of Decision for the Environmental Impact Statement:
NCDOT's "Stream Crossing Guidelines for Anadromous Fish Passage" will be
adhered to for this project.
A construction moratorium for anadromous fish from February 15 through June
15 will be adhered to for in-water work.
These project commitments are in addition to the General Nationwide Permit
conditions, Section 404 Individual Permit Special Conditions, CAMA Permit, State
Stormwater Permits for CAMA Major Projects, Section 401 Water Quality Certification
Conditions, Regional Conditions, State Consistency Conditions, NCDOT'S Best
Management Practices for Protection of Surface Waters, NCDOT's Guidelines for for
Best Management Practices for Bridge Demolition and Removal, and General
Certification Conditions.
EVALUATE POTENTIAL IMPACT
All stormwater outfalls were analyzed for BMP's based on the following criteria:
¦ The proximity of the discharge point to the receiving stream. Is this a direct
discharge or is there sufficient area for dilution?
¦ The pre and post construction runoff volumes.
¦ The ratio of the impervious surface of contributing highway area to the total
watershed of the receiving stream.
¦ Preventive BMP's that are employed.
¦ Value of the water resource.
¦ The 2-year velocity for stormwater treatment.
¦ The 10-year velocity at stream and wetland outfalls for non-erosive velocities.
SELECT AND IMPLEMENT BMP'S
Grassed Swales
BMP selection for this project mainly consisted of grass swales. According to
Natural Resource Conservation Service Soil Survey Maps, the roadway corridor mainly
consists of well-drained sandy, loamy sand, and sandy loam soils. Due to the high
content of sandy, well-drained soils and the relatively flat topography, it was determined
that infiltration rates using grassed swales would be high. Flow velocities in the 2-year
storm were determined for all grass swales to verify that runoff velocities were less than
2.0 feet per second which would provide adequate nutrient removal before reaching
outfalls. The 10-year storm velocities were also calculated at all stream and wetland
outfalls to ensure that non-erosive velocities were achieved. Grass swales were also
designed to provide adequate length for treatment based on NCDENR-DWQ's
guidelines.
At a few storm drainage system outlets located just south of the NC 33
interchange, it was determined that non-erosive velocities could not be achieved and due
to elevation constraints the systems could not be day lighted outside of the wetland
boundaries. At these locations, all of the stormwater receives adequate treatment via grass
swale ditches. Therefore, it was decided that the pipe systems should be tied to the box
culvert. This will allow the dissipation of energy within the box culverts and save the
wetlands from potential damage.
Preformed Scour Holes
At outlets where grass swale treatment was not an option, Preformed Scour Holes
(PSH) were specified to provide diffuse flow. All of the PSH's are located outside of the
riparian buffer zones. PSH's were only used on pipe sizes 18-inches or less and have
been designed per NCDENR-DWQ's guidelines.
Tar River/Wetland Bridge
The largest benefit to the environment is the construction method proposed for the
Tar River Bridge. The bridge will be constructed using a steel cantilever gantry crane
system that will enable the bridge to be built in a top-down fashion. This will eliminate
the need for any work bridge to be located in the wetlands or surface waters.
Deck drains will be required on the bridge to prevent stormwater from spreading
into the travel lanes during the design storm. For the portion of the bridge over the Tar
River to approximately 150 feet outside of the stream banks, an enclosed drainage system
will be installed where it will be discharged to an 8-foot square riprap pad. The riprap
pads will consist of gabions filled with class `B' riprap and will be placed in the wetlands
on filter fabric to dissipate energy from the falling stormwater. The remainder of the
bridge will include deck drains that free discharge to the wetlands. These deck drains are
all located at least 12 feet above the wetlands and should provide for diffuse flow.
CONCLUSION
This Stormwater Management Plan represents a low maintenance, environmentally
sensitive, and effective plan for handling the stormwater runoff from the project area.
Calculations for grass swales and BMP's have been included.
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