Morrisville Engineering, Design, and Construction Manual - July 2016

Section 7: Stormwater Management 7.1 Purpose 7.2.1 General

Section 7: Stormwater Management

The standards in this section supplement those in Article 7: Stormwater Management, of the UDO.

7.1. PURPOSE The purpose of this section is to provide design criteria for stormwater quantity, quality, and Best Management Practices (BMPs). This EDCM, in conjunction with the current edition of Stormwater Best Management Practices Manual (BMP Manual) published by the North Carolina Department of Environment and Natural Resources (NCDENR) shall be considered the “Design Manual” as referred to in Section 7.1.6, Design Manual, in the UDO. The goal of this section is provide additional clarification, interpretation, and detail to assist the public in preparing an adequate submittal in support of a development application.

7.2. DESIGN

General

7.2.1.

All development and re-development, including additions or expansions of existing properties, shall meet the requirements for both stormwater quantity and quality of Article 7: Stormwater Management, of the UDO. Re-development that does not propose a net increase in the amount of Built Upon Area (BUA)—as defined in Section 11.5, Terms abd Uses Defined, of the UDO—from existing development (installed or approved prior to 1999), and that provides equal or greater stormwater control as the existing development, will not be subject to the requirements of Article 7, of the UDO. When additional development occurs on a parcel that has existing development, stormwater control requriements cannot be applied retroactively to the existing development portion, unless otherwise required by state or federal law. As required by the Town’s NPDES Phase II Stormwater Permit, the Town’s Stormwater Engineer position shall be designated as the “Stormwater Administrator” (see Section 7.2.1, Stormwater Administrator, of the UDO). The “pre-development” condition shall be based on the land use and improvements made to the property that were approved and completed prior to the year 1999, as shown on available aerial imagery, and shall be considered to be in “good” condition for analysis purposes. The corresponding pre-development drainage areas must be delineated and field verified to ensure that the entire contributory area has been properly identified and that any off-site runoff has been accounted for. The corresponding runoff rate must be calculated assuming off-site properties are fully developed at each discharge point (that point at which runoff leaves the tract of land or enters a stream buffer). The analysis shall utilize point precipitation frequency estimates from NOAA Atlas 14, for the Raleigh Durham WSFO AP, NC, station for precipitation data. The pre- and post-development drainage area maps must clearly delineate the areas that are being directed to each analysis point/stormwater pond, and identify the corresponding areas, runoff values and Tc flow paths and flow types (Sheet, Shallow Concentrated, channel, etc., unless the post- developed Tc is assumed to be 5 minutes). The USDA NRCS Hydrologic Urban Hydrology for Small Watersheds (formerly the SCS Method) must be utilized to determine peak runoff rates. At the Stormwater Administrator’s descresion, the Rational Method may be used for drainage areas less than 2 acres. The corresponding C, CN, and hydrologic soil groups for soil types must utilize the information provided within the current edition of the All applicable forms and checklists are available in the Administrative Manual. Water Quantity (Hydrology)

7.2.2.

Morrisville, NC

July 2016 Page 41

Engineering, Design, and Construction Manual