Appendix D – Water Quality Flow Calculation Method

The Water Quality Flow (WQF) is the peak rate of discharge associated with the water quality storm or Water Quality Volume (WQV). This section describes the recommended method for calculating the WQF when designing flow diversion structures for off-line stormwater Best Management Practices (BMPs). The WQF is also used for the design of stormwater BMPs that are sized based on flow rate rather than volume, including grass channels and proprietary stormwater BMPs such as hydrodynamic separators, catch basins inserts, and media filters.

The WQF should be calculated using the design WQV and a modified Runoff Curve Number for small storm events. This method is used to estimate peak discharges for small storm events based on the approach described in Claytor and Schueler (1996). This method is more appropriate than: 1) the traditional Natural Resources Conservation Service (NRCS) Curve Number methods, which are valuable for estimating peak discharge rates for storms greater than 2 inches but can significantly underestimate runoff from small storm events, and 2) the Rational Formula, which should only be used with reliable intensity, duration, and frequency (IDF) tables or curves for the storm and region of interest (Claytor and Schueler, 1996).

The design WQV (either 100% or 50% of the WQV depending on the applicable retention/treatment requirement, as described in Chapter 4), converted to watershed inches, should be substituted for the runoff depth (Q) in the NRCS TR-55 Graphical Peak Discharge Method.