Normalized Runoff Capture Volumes for Low Impact Designs

The latest stormwater low impact developments encourage on-site treatment and stormwater disposal, including retentions, vegetal beds, wet lands, porous detentions, and sidewalk landscape. All of these efforts are to aim more at frequent rainfall events rather than the extreme events. In practice, a 2-yr event is considered too large for sizing on-site stormwater quality control systems. Without using the flood return period as an index, determining the design volume for a water quality control system has been a major challenge. Over the years, the concept of runoff capture volume has been derived from the analyses of tens of hundreds of individual storm events delimited from a continuous record. Rainfall event separation process can be subjective, depending on how the minimum interevent (no rain) time is chosen. In general, the longer the rainfall event separation time is, the higher the average rainfall event depth will be. To improve the consistency among the rainfall databases, this paper presents a mathematical model by which a continuous rainfall record can be directly converted into stormwater runoff capture curves. Applying the exponential distribution to a complete rainfall data series, the normalized runoff capture curve was derived in this study to describe the non-exceedence probability distribution of runoff volume population. This curve provides necessary and important design information by which the stormwater basins can be sized on a consistent basis of overflow risk.