Looking Beyond the Deterministic to a More Robust Dam Breach Analysis

An important component of any dam safety program is the dam breach analysis and resulting flood inundation maps which are critical for assisting emergency management agencies in minimizing loss of life. Although a dam breach analysis follows a prescribed deterministic methodology, it includes a significant degree of uncertainty, and depending upon the assumptions made can lead to either underestimating the breach consequences or to overly conservative breach results and does not provide a true understanding of the critical magnitude and areas of risk. Due to the high degree of uncertainty surrounding the size, shape, and formation time of a hypothetical breach of Camanche Main Dam and Dikes and potential magnitudes of these breaches, East Bay Municipal Utility District (EBMUD) decided to perform a probabilistic dam breach analysis in addition to traditional deterministic methods. Camanche Reservoir is owned and operated by EBMUD and located on the Mokelumne River in the Sierra Nevada foothills in Northern California. The reservoir, formed by the main dam on the Mokelumne River and six auxiliary dikes along the northern and southern ends of the reservoir, is operated to provide flood control, meet downstream in-stream flow and water rights obligations, provide recreational services, and generate hydroelectric power. The main dam and dikes consist of zoned compacted earthfill structures, rising to heights ranging from 40 to 170 feet with impounded reservoir volumes ranging from 40,000 to 430,000 acre-feet at full pool. This paper discusses the analysis performed for one of the southern dikes, Dike 2, including the methods used for setting up and forming the statistical distributions for the input breach parameters, developing and calibrating the HEC-RAS 2D model, and the process of running the Monte Carlo simulation to determine the distribution and probabilities of peak breach flow rates. Results from the probabilistic analysis of Dike 2 helped provide great insight on several key factors that were overlooked during the deterministic analysis of the dikes. The findings of this robust approach for dam breach analysis highlights a powerful application of the probabilistic approach in being able to inform the results from a deterministic study.