Stochastic Event Flood Model Approach for Determining Hydrologic Loading at Reclamation Facilities

The Bureau of Reclamation’s Dam Safety Program is responsible for about 370 high and significant hazard dams. Under the program’s public protection guidelines, determining the hydrologic risk at a dam may not be the most cost effective solution using traditional deterministic methods or stream gage analysis. Reclamation has recently used a stochastic events flood model (SEFM) approach to determine the hydrologic hazard for several dams. The basic concept of the SEFM approach is the computer simulation of several-thousand years of annual maximum hydrologic and meteorological parameters for a specific watershed. This is accomplished by utilizing a deterministic flood computational model (HEC-1) (USACE, 1990) and treating the hydrometeorological input parameters as variables instead of fixed values. Monte Carlo sampling procedures are used to allow the hydrologic and meteorological input parameters to vary in accordance with values observed in nature. Several thousand years of extreme storm and flood annual maximum values are generated by computer simulation. These maxima are used in assembling annual maxima series representing multi-thousand years of flood events. A plotting position can then be used to compute the annual exceedance probabilities for flood peak discharge, flood runoff volume, and maximum reservoir elevation.