A New Method for Estimating Water-Quality Flows

Water-quality flow (WQF) is used as the basis for designing some stormwater control measures. The conventional method for determining the WQF incorporates several assumptions that do not reflect reality. A method is presented for determining the WQF that does not require the main limiting assumptions. Specifically, the proposed method utilizes measured durations of the water-quality rainfall, local rainfall statistics, and representative infiltration characteristics of the catchment. The proposed method is demonstrated at two locations in south Florida. Defining the 90-percentile rainfall depth as the water-quality rainfall (WQR), the results show the WQR to be in the range of 3.4-3.9 cm, with corresponding average event durations in the range of 9.4-9.7 h. Most importantly, the results show that the actual WQF can be substantially greater than the WQF calculated using the conventional method, with correction factors in the range of 1.9-9.6 for a range of practical conditions. Some stormwater control measures (SCMs) such as biofiltration swales and vegetated filter strips are designed based on WQFs. The conventional method of determining the WQF includes certain assumptions that result in an underestimation of the WQF, and hence an underdesign of SCMs. The proposed method for determining the WQF does not require the limiting assumptions while maintaining the underlying physics of the runoff process. Utilization of the proposed method for calculating design WQFs should lead to more effective water-quality control by SCMs.