White Paper: Analysis Of Long Term Rainfall To Develop Stormwater Quality Flow Rate Design Criteria

By B. Lee, F. Tonto, P. Tov, Rinker Materials / Concrete Pipe Division

The intent of stormwater quality best management practices (BMPs) is to preserve and/or improve the existing quality of our water resources by achieving a water quality outcome. The water quality outcome is often expressed as a desired level of annual total suspended solids (TSS) removal and by a minimum volume of runoff that must be stored called the water quality volume (WQV), (USEPA, 2004). Traditional structural BMPs such as wet ponds and wetlands, are designed based on the WQV which is defined by a depth of rainfall that represents treating the 80th to 90th percentile volume of the annual runoff. However, the design basis for the WQV is often applied generically to all types of stormwater management BMPs without considering unit process limitations to manufactured BMPs.

Manufactured BMPs are intended for water quality improvement and are designed for space constrained sites. The design criteria for manufactured BMPs are flow based and performance is dependent on a prescribed particle size gradation. Therefore, applying the WQV design methodology to manufactured BMPs would be contrary to a flow based design principle and problematic when land for large detention facilities is not available. Flow based sizing, however does not mean treatment of design storms (i.e., 2 to 100 year storms which are typically applied for quantity control design) but rather treating the flows that contribute to the majority of the average annual runoff volume.

The purpose of this paper is to propose a flow based sizing methodology for manufactured BMPs called the water quality flow (WQF). An overview of the "first principles" used to determine a WQV is discussed and a similar approach to determine a WQF is proposed. Like the WQV, the WQF is selected based on treating the 80th to 90th cumulative percentile volume of the annual runoff.

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