Development of a Design Storm for Water Quality Control in Urban Areas.

Quality control criteria for urban runoff are now part of many guidelines for stormwater management. Recognizing that it is not necessary to capture and treat the extreme events, the target most commonly defined in these guidelines is based on an analysis of the precipitation with a rainfall distribution plot to determine a rainfall quantity to be used for design. The basic assumption is that by capturing the runoff generated by a given quantity of precipitation (between 85th to 95th percentile usually), a significant part of the annual runoff will be treated.

Many difficulties arise in practical applications with the use of these simple analyses for design and it is not clear if the design criteria currently applied are indeed adequate to meet the basic objective to treat a given percentage of the annual runoff volumes. First of all, some Best Management Practices (BMPs) are volume based treatment systems whereas others are based on discharge. The design parameters should therefore take that into account and consider not only a rainfall quantity but also an intensity. Moreover, the actual rainfall characteristics (total volume, average and maximum intensities, duration, distribution in time and gap between events) could also have a significant impact on the overall performance.

An analysis of the available design criteria for water quality control is first given, with a comparison of long-term simulation results for typical design situations (a part of a parking lot draining towards a bioretention and a small residential area of 10 ha draining towards a retention basin). A database of 10 years of rainfall at 5 minutes interval simulated with PCSWMM is used for the evaluation. A methodology is thereafter presented to use this historical rainfall series to generate temporal distributions for design storms to be applied for different types of BMPs.