A stochastic stormwater quality volume-sizing method with first flush emphasis.

A Monte Carlo simulation technique was applied to assess the effect of stormwater quality volume captured by best management practices (BMPs) on the frequency of discharging concentrations of constituents above certain designated threshold limits. The method used an assumption of a power law relationship between the cumulative load and flow to incorporate the first flush effect. The exponent of this relationship was considered a random variable and its frequency distribution was obtained from 78 measured pollutographs from three urban highway sites in West Los Angeles, California. Although the effect of rain depth captured by BMPs is site-specific, the method offered here provides a systematic approach to evaluate the effect of selecting various regulatory guidelines for controlling urban stormwater pollution on the overall discharge of pollutants into waterways. This allows selecting the requirements for capturing runoff volume by BMPs based on the tradeoff between the probability of concentration criteria violation and economic factors.