The Stochastic Event Flood Model Applied to Minidoka Dam on the Snake River, Idaho

A stochastic event-based, rainfall-runoff model was developed to compute magnitude-frequency estimates for flood peak discharge, runoff-volume and maximum reservoir level at Minidoka Dam. This model was supplemented with probabilistic analysis of historic and potential flood outflows from American Falls Dam. The basic concept of the stochastic model is to employ a deterministic flood computation model and treat the input parameters as variables instead of fixed values. Monte Carlo sampling procedures are used to allow the climatic and hydrologic input parameters to vary in accordance with that observed in nature. Flood peaks rather than flood volume were found to be the primary concern because of the limited flood storage in Lake Walcott behind Minidoka Dam. The magnitude frequency curve developed for peak flows revealed that inflow peak flows in excess of 109,000 ft³/s, the approximate maximum non-overtopping discharge available at Minidoka Dam, would have an annual return period in excess of 10,000-years. The magnitude-frequency curve developed for maximum reservoir elevations confirmed the overtopping probability to be in excess of 1 in 10,000-years.