Estimating Probability of Extreme Events

The Corps of Engineers is currently evaluating its portfolio of dams with regard to risk and related maintenance. A draft Engineer Technical Letter "Risk Analysis and Assessment for Dam Safety" is under review. This ETL describes processes that will be used for screening projects for planning of corrective actions. One of the contributing factors that must be uniformly evaluated across all projects is the development of inflow frequency curves (peak flow and volume frequency) that define the frequency curve in the mid-range events (1 in 500 to 1 in 3000) and then extend out to the probable maximum flood level. Currently, no extension method is uniformly accepted. multiple methods exist to facilitate extension. The current effort being undertaken by the Hydrologic Engineering Center is to present the variety of methods and provide general guidance for curve extension that can be applied. A consistent method of curve extension allows for a uniform comparison of dams. This paper will provide an overview of the current status of the extension methodology.