79-Flood Hazard Analysis and Mitigation.

Abstract

This paper describes the procedures adopted for delineation of flood hazards and potential mitigation measures to support the development of inland communities in California, many of which are located on alluvial fans. The streams that distribute water and sediments on the fans are not fixed in location with respect to time, but instead are subject to sudden and dramatic changes of location. Consequently, it is difficult to predict flood hazards that may impact a development located on a portion of a fan. The initial step in the assessment of the flooding potential is to examine historical flood pathways and the magnitude and timing of peak flows. The latter is dictated by the geomorphology and flood control infrastructure in the intervening lands upstream of the site. Especially critical is the determination of peak flows and hydrograph at the apex of an alluvial fan. Flood routing provides the spatial and temporal information on the floodplain inundation and peak flows impacting a site. Once the peak offsite flows have been determined, onsite mitigation measures are implemented to intercept and convey the floodwaters through and out of the site in a manner that closely mimics the natural drainage distribution. Sediment transport calculations in the floodplain and onsite channels provide a means to assess whether scour and deposition affects channel function to convey sediment through the system. A flood hazard analysis for a proposed residential development in Southern California illustrates the application of some of the methodologies and models.