Including a water qualtiy flow (WQF) in the design standards would give designers greater flexibilty to design innovative BMP layouts and would provide a consistent standard for manufactured BMP designs.
Section 4VAC50-60-1184. Water quality states that "Design standards and specifications for the BMPs in Table 1 that meet the required target pollutant removal efficiency will be available at the department." A water quality storm flow rate associated with 1xWQV, 2xWQV, etc. should be made part of the available design standards and specifications. Manufactured BMPs and innovative "offline" designs of traditional BMPs found in the storm water handbook should have an official and standardized basis of design.
One method would be to use the SCS routing of the 0.7 inch storm to produce the WQV as discussed on page 5-36 of the handbook:
Routing the water quality volume depends on the ability to work backwards from the design runoff
volume of 0.5 inches to find the rainfall amount. Using SCS methods, the rainfall needed to
generate 0.5 inches of runoff from an impervious surface (
RCN=98) is 0.7 inches. The SCS designwater quality storm using SCS methods is
ConnDOT and the State of Maryland use a curve number based method, the ConnDOT and MD DEQ methods can be accessed from the links below.
http://www.ct.gov/dot/lib/dot/documents/ddrainage/11.C.pdf
http://www.mde.state.md.us/assets/document/sedimentstormwater/Appnd_D10.pdf
Joseph Battiata and Leinhart published an article that developed a method based on the Santa Barbara unit hydrograph that provides athird method for WQF calculation. I am not sure if this article is available online at this time.
