How "Critical" is Critical Duration in Determining Flood Risk, Flood Damages, and Stormwater Management Solutions?

The accurate assessment of flood risk and flood damages, as well as the sizing and design of flood control projects in any watershed, relies on the peak flood discharges, the peak flood stages, and the maximum runoff volumes generated for that watershed. Peak discharges, stages, and runoff volumes can be determined through application of rainfall runoff and hydraulic methodologies that utilize "critical" design rainfall events (volume, duration, and temporal distribution) to generate runoff hydrographs from design rainfall events. FEMA guidance states that the critical storm is a design storm which provides the highest flood discharges/water surface elevations for the flooding source. In practice, it is often assumed that the 100-year, 24-hour duration storm produces the critical or highest flood discharges, stages, and runoff volumes throughout the watershed. However, the actual critical duration design storm may be something other than the 24-hour design event, and there may be more than one critical duration storm that produces critical flood discharges, stages, and storage volumes across the watershed. A critical duration analysis is required to estimate representative peak discharges and flood stages and to properly size flood control projects. A critical duration analysis can be performed to determine the critical duration storm specific to each watershed ^and each study reach across the watershed. Typically, flood discharges and stages for the 1-, 3-, 6-, 12-, 24- and 48-hour storm durations for the 100-year frequency event are generated to identify the critical duration which produces the highest peak flows and stages for all study locations within the watershed. Performing this analysis and using the resulting critical duration events helps to ensure that the hydrologic/hydraulic models produce the highest flood discharges and stages across the watershed. Stormwater management planning conducted for the Little Calumet River Watershed in Chicago, Illinois, by the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC), provided the opportunity to assess the impacts of critical duration analyses on establishing 100-year peak flows, defining floodplain boundaries, and sizing flood control facilities. Two 20+ square mile watersheds from this effort will be presented. This paper will present an overview of the critical duration analysis procedure and will demonstrate the importance of proper selection of the critical duration event by assessing the impact of critical duration on peak flows and the sizing of regional flood management facilities in two sub-watersheds.