The percentage of storm-water runoff volume, or of the number of runoff events captured is a key factor when designing storm-water quality control facilities. Design criteria originally developed for sizing flood control detention are not appropriate for sizing detention for storm-water quality control. This paper presents a simple technique that can be used to estimate the needed storm-water detention volume of structural water quality best-management facilities. This volume is based on the point where diminishing returns begin to occur in either the long-term runoff volume capture and treatment or in the number of runoff events captured and treated. This volume is defined as the ''maximized'' volume. The procedure presented is intended for preliminary planning or design when detailed local investigations have not been performed. It is not intended to replace detailed, site-specific investigations. Its accuracy, however, was verified by comparing its results against similar findings that emerged from several detailed local investigations.
