Runoff capture and delivery curves for storm-water quality control designs

A storm-water quality control system consists of trickle channels and water quality control basins (WQCB). To be economical, a WQCB is often designed to capture rainfall events smaller than extreme events. In current practice, the design rainfall statistics and runoff capture rates for sizing a WQCB have to be derived from the local long-term continuous rainfall record. Such a rainfall-runoff simulation process is lengthy and time-consuming. In this study, the analyses of 30- to 40-year continuous rainfall data recorded from seven metropolitan areas in the United States indicate that the frequency distribution of rainfall event-depths can be described by an exponential decay function. Applying the exponential distribution to a complete rainfall data series, the normalized runoff capture curve is derived in this study to describe the nonexceedance probability distribution of runoff depths. Similarly, the normalized runoff delivery curve is also developed to describe the nonexceedance probability distribution of runoff rates. These two curves provide necessary and important design information by which both the trickle channel and the WQCB can be sized on a consistent basis of overflow risk.