Virginia Administrative Code
Title 9 - ENVIRONMENT
Agency 25 - STATE WATER CONTROL BOARD
Chapter 875 - VIRGINIA EROSION AND STORMWATER MANAGEMENT REGULATION
Part V - Criteria and Requirements for Regulated Land-Disturbing Activities
Article 3 - Water Quantity and Water Quality Technical Criteria
Section 9VAC25-875-620 - Design storms and hydrologic methods

A. Unless otherwise specified, the prescribed design storms are the one-year, two-year, and 10-year 24-hour storms using the site-specific rainfall precipitation frequency data recommended by the U.S. National Oceanic and Atmospheric Administration (NOAA) Atlas 14. Partial duration time series shall be used for the precipitation data.

B. Unless otherwise specified, all hydrologic analyses shall be based on the existing watershed characteristics and how the ultimate development condition of the subject project will be addressed.

C. The U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS) synthetic 24-hour rainfall distribution and models, including TR-55 and TR-20; hydrologic and hydraulic methods developed by the U.S. Army Corps of Engineers; or other standard hydrologic and hydraulic methods shall be used to conduct the analyses described in this part.

D. For drainage areas of 200 acres or less, the VESMP authority or department as the VSMP authority may allow for the use of the Rational Method for evaluating peak discharges.

E. For drainage areas of 200 acres or less, the VESMP authority or department as the VSMP authority may allow for the use of the Modified Rational Method for evaluating volumetric flows to stormwater conveyances.