Morrisville Engineering, Design, and Construction Manual - July 2016

Section 7: Stormwater Management 7.2 Design 7.2.2 Water Quantity (Hydrology)

Stormwater Best Management Practices Manual published by the North Carolina Department of Environment and Natural Resources (NCDENR).

The Rational Method shall not be used for BMP/pond routing hydraulic analysis.

Time of Concentration (Tc) estimates for the Rational Method should utilize the Kirpich Equation, with the following adjustments for Tc:

1.

For well-defined natural channels use Tc.

2.

For overland flow on grassy surfaces use Tc x 2.

3.

For overland flow on paved surfaces use Tc x 0.4.

4.

For concrete channels use Tc x 0.2.

Time of concentration estimates for the NRCS TR-55 shall utilize the segmental Tc approach as outlined in Worksheet 3: Time of Concentration or travel time, from the publication Urban Hydrology for Small Watersheds (TR-55) , United States Department of Agriculture (USDA), with the following parameters:

1.

Sheet Flow

a. For surface description and Manning’s roughness coefficient (n), use Table 3-1, Roughness coefficients (Manning’s n) for sheet flow from the TR-55 Manual. Pre-development wooded areas must be assumed to be woods (woods, dense underbrush).

b. For flow length, use the following:

(1) Pre-development:

(A)

Minimum of 100 feet.

(B)

Maximum of 300 feet.

(2) Post-development:

No more than 50 feet unless it can be shown that the sheet flow depth is 0.10 foot or less. c. For the 2-year, 24-hour rainfall, utilize point precipitation frequency estimates from NOAA Atlas 14, Raleigh Durham WSFO AP, NC, Precipitation depths. d. For shallow concentrated flow, use average velocities for estimating travel time from Figure 3-1 of the TR-55 manual. 2. Channel Flow: For Manning’s roughness coefficient (n), use Table 3-9, Rational runoff coefficients from the BMP Manual. Soil name and hydrologic group shall be based on the most recent printed version (1970) of the USDA Wake County, North Carolina, Soil Survey. A copy of the printed map with the subject tracts clearly identified shall be provided with the submittal documents. Rational Method analysis shall utilize Table 3-2, Rational runoff coefficients, from the NCDENR Stormwater BMP Manual, unless alternate values are agreed to by the Stormwater Administrator. TR-55 method analysis shall utilize Table 3-5, Runoff curve numbers for the SCS method, from the BMP Manual, for land cover description and CN values. Stormwater hydrologic routing and BMP routing calculations must be provided for the 1-, 2-, 10-, 25- and 100-year, 24-hour storm events. All proposed development or improvements within the Watershed Management Easement around Wake County owned lakes must be approved by the Wake County Open Space Program of the Parks & Recreation Department. (see Appendix A.1 for contact information)

January 2016

Morrisville, NC

Page 42

Engineering, Design, and Construction Manual