Simplifying the Complex: Pool Stage-Frequency Analysis of USACE’s Largest Reservoir

Abstract

Lake Sakakawea in North Dakota, U.S.A., was formed following the closure of Garrison Dam in 1953 and is the largest U.S. Army Corps of Engineers (USACE) reservoir. It is the second of six mainstem reservoirs located in series on the Missouri River operated as a system by USACE. Critical for dam safety risk analysis, a pool stage-frequency curve, extending to water levels above the top of dam, was developed for an Issue Evaluation Study using a combination of tools, including Monte Carlo simulation with the recently released USACE reservoir frequency analysis software, RMCRFA. This paper describes the hydrologic tools leveraged to expedite the pool stage-frequency analysis and discusses the challenges overcome during the study. The methods used to reduce a complex reservoir system to a simplified single reservoir model are presented, and statistical challenges associated with mixed population rain/snow floods, paleofloods, and regulated basin data are discussed. The simplified approach was capable of incorporating a variety of hydrology inputs, while providing a reasonable approximation for the complex reservoir operations. The results were transparent and easily understood by decision makers tasked with making portfolio-wide investment decisions.