This paper describes the philosophy and application of risk-based analysis by the U.S. Army Corps of Engineers in the formulation and evaluation of flood damage reduction projects. Types of projects evaluated using risk-based analysis include dams and reservoirs, levees and flood walls, diversions, channel modifications, bypass channels, non-structural measures and combinations thereof. The application of risk-based analysis principles is considered a significant step forward in improving the basis for flood damage reduction project formulation. The risk-based analysis approach explicitly incorporates the uncertainty inherent in key engineering and economic parameters and functions into project formulation, estimation of benefits, and performance analyses. Monte Carlo simulation is used to assess the impact of the uncertainty on the discharge-probability, elevation-discharge, and elevation-damage functions that represent existing conditions and the flood damage reduction effects of proposed protective works. The risk-based analysis provides important information on the expected project performance, not available with traditional methods of evaluation, that is of great value to decision makers in comparing alternatives. The paper briefly summarizes the risk-based analysis approach, contrasts it to traditional project development study methods, and presents the results of a recent application of the methodology in studies conducted for the American River, near Sacramento, California, USA. Alternative project measures for the American River that were evaluated included new upstream storage, modification of an existing dam and present storage allocations and operation rules, raising and strengthening downstream levees, and various combinations of these alternative measures. Topics addressed in the paper include technical issues related to methods and data, communication of risk-based analysis results to local officials and the public, and project design implications.
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