Overflow risk analysis for stormwater quality control basins

The operational cycle of a stormwater quality control basin can be divided into the waiting period between events and the filling and draining period during an event. In this study, an inherent overflow risk is defined as the probability of having a large event exceed the basin storage capacity. Such a probability is prescribed by the basin storage capacity and the local distribution of rainfall event depths. An operational overflow risk is defined as the probability of having the basin overwhelmed by a subsequent storm event during the draining process. An operational risk is found to be dependent on watershed runoff coefficient, basin drain time, local average rainfall event depth, and average rainfall interevent time. In practice, the selection of a basin drain time is a tradeoff between the removal of suspended solids in stormwater and the overflow risk. The concept of "the longer, the better" applies to the sedimentation process, but concern for the overflow risk requires that the basin drain as fast as possible. This paper presents a design method by which the overflow risk associated with a basin storage volume can be evaluated for various drain times. The mathematical models developed to describe the distribution of rainfall interevent time and the runoff capture curve provide good agreement with the long-term continuous rainfall data recorded in seven metropolitan areas in the United States. The risk-based approach developed in this study provides a quantifiable basis for making the decision on the operation of a stormwater quality control basin.