Flow Duration Hydrograph Analyses for Assessing LID Performance

Abstract

Low impact development ( LID) and other green infrastructure that features onsite retention are now recognized as an integral component of stormwater management. Located as close to the source of runoff generation as possible, one hallmark of LID is a reduction of stormwater runoff volume and pollutant loading to the municipal stormwater management system. The sizing of LID and other stormwater management facilities across North America largely relies on event based hydrologic modeling methods, which target specific rainfall characteristics. A systems approach to stormwater management requires consideration of both the rainfall stimulus as well as the runoff response, as indicated by impacts to the receiving watercourse or waterbody. Further, continuous simulation offers a greater diagnostic tool for assessing impacts, since it can describe the full range of runoff response characteristics, compared to a limited snapshot view using design storm events. An analysis of continuous simulation results can be used to quantify downstream impacts, particularly through identifying exceedance and deficit durations with respect to threshold values of flow hydrograph ordinates (e.g. flowrate, depth or velocity). In California, a regulatory permitting process has been adopted for identifying potential hydrograph modification (known as hydromodification) based on flow-duration curves that indicate geomorphologic changes due to development. This paper presents a new methodology that extends California's hydromodification criteria for flow-duration exceedances to encompass a broader range of downstream impacts, and is demonstrated for two case studies that illustrate LID facilities applied under vastly different site conditions.