500-Year Flood - Can It Be Reliably Estimated?

The "500-Year" flood has been proposed for a national urban flood standard (Galloway 2005). This recommendation overlooks the problems associated with estimating the "500-Year" flood. The approved pdf for making these estimates in the United States is the LP III pdf. The approved methodology provides erratic and highly unreliable estimates of the "500-Year" flood in several California watersheds. A case study of the American River flood estimates is presented to demonstrate the issues. A comparison of flood flow estimates for several probability distribution functions and the associated Confidence Intervals is presented. The paper finds that extrapolation of pdfs should be avoided and the use of "Confidence Intervals" for project formulation and levee certification should be abandoned. It is recommended to use physically based design floods rather than statistically-based design floods and/or Risk Analysis.