Determination of the Water-Quality Rainfall and Design Drawdown Time of Stormwater Control Measures.

Stormwater control measures (SCMs) are designed to treat the runoff from a water-quality rainfall (WQR). The design of SCMs also requires specification of a drawdown time which is limited by the characteristic interevent time, and it commonly is overlooked that the WQR is not independent of the characteristic interevent time. A procedure is proposed for extracting the relation between the WQR and the characteristic interevent time so that consistent specifications can be developed for WQR and design drawdown time. The proposed procedure was demonstrated for South Florida, where it is shown that, for a given minimum interevent time of 6 h, the limiting design drawdown times of SCMs decrease with increasing percentile rainfall amounts, and that design drawdown times of less than 20 h might be needed to provide a reliability of 50% that water quality basins will fully drain the water-quality volume prior to the next event. It also is shown that both the WQR and the limiting design drawdown time are sensitive to the minimum interevent time that is used to separate rainfall measurements into discrete rainfall events. [ABSTRACT FROM AUTHOR]