ABSTRACT
In urban areas, pollutants are accumulated on impervious and pervious surfaces. During a storm event, the early runoff flow washes over surface areas and carries solids, chemicals, and heavy metals into the waterways. It is advisable to intercept the first flush of storm runoff before highly concentrated runoff flows enter the drainage system. Therefore, it is recommended that a storm water-quality control basin (WQCB) be placed upstream of a street inlet for the purpose of source control. As a rule of thumb, a water quality capture volume (WQCV) is sized for the first flush volume and its effectiveness is evaluated by its long-term storm runoff capture rate. To determine the design volume for a WQCB, the on-site long-term continuous rainfall event-depth distribution is firstly converted into a long-term runoff event-depth distribution. Next, the runoff event-depth distribution suggests that the optimal basin size is one that captures up to 80 to 85% of runoff event-depths. Over the years, the effort made to understand rainfall and runoff event-depth distributions has led to the development of two sets of empirical formulas from some 40 long-term hourly rainfall records observed in continental USA. These empirical formulas are further employed to produce the synthetic runoff volume and runoff event capture curves for the drain times of various basins. All synthetic runoff capture curves exhibit similar characteristics to the exponential non-exceedance probability distribution. In this chapter, we will review the background concept of WQCV and discuss how to use the optimal WQCV for WQCB designs.
