Benchmarking DARF, Design Storms, and Temporal Distribution Procedures for Hydrologic Design

Current processes for developing Depth Area Reduction Factors (DARF), design storms, and temporal rainfall distributions from approximately 20 agencies as well as procedures identified in U.S. National Oceanic and Atmospheric Administration (NOAA) rainfall atlases were reviewed and benchmarked for the Contra Costa County Flood Control and Water Conservation District (District) in Martinez, CA. Most of the agency procedures reviewed use some variation of the NOAA Atlas DARF curves. Modified NOAA Atlas 2 procedures often follow the longer DARF duration (e.g., 3-24 hr) storms of NOAA Atlas 2 but use NOAA processes to create new DARFs for short durations (e.g., 5 min to 1 hr). Two agencies, the City of Colorado Springs and the Metropolitan Sewer District of St. Louis, are utilizing the growing database of radar rainfall estimates to develop DARF curves using a storm-centered approach. Most counties evaluated in this study rely on a rational method approach for small drainage areas, which requires an estimate of rainfall intensity. Each county or agency has a unique method for obtaining the rainfall intensity, but the differences among them appear minor. Only three of the counties or agencies reviewed have design storm procedures with additional steps for adjusting point rainfall estimates beyond applying a DARF factor. Typically, one of three general approaches is used in determining the temporal distribution of a design storm. The first is called a nested or balanced storm pattern. The second approach is the use of synthetic curves such as SCS types I, IA, II, and III. The third approach is the use of local historical data to develop a temporal distribution.