CS17A - Concurrent Session 17A: Evaluating Dam Breach Inundation for Largely Sedimented Reservoirs

To satisfy regulatory requirements emergency action plans must be prepared, which includes the development of dam breach inundation maps. This paper reports on a study for dam in which 90 percent of reservoir is now filled with sediment that has accumulated over its more than 100 year life. In this case, an assumption (originally suggested by the regulatory) that the reservoir is entirely water, drastically overestimates the potential area of inundation. Specifically, assuming the reservoir is full of water, rather than sediment results in predictions that populated areas would be flooded. Due to the location of the dam relative to populated areas, analysis of the inundation accounting for the sediment in the reservoir determined that inundation remained upstream of populated areas in the upper reaches of the downstream channel. To address this problem, three very different methods were used to model the outflow from a breach and the flow of water and sediment from the reservoir. The methods varied from simplified mechanics analysis to sophisticated particle analysis. The methods were 1) geotechnical kinematic analysis, 2) hydraulic analysis using FlO-2D, and 3) smoothed particle hydrodynamic (SPH) method. The use of three alternative methods addresses model uncertainty concerns. The SPH method is a computational method for simulating fluid flows. Originally developed independently by Gingold and Monaghan (1977) and Lucy (1977) for astrophysical problems, it has become a popular choice amongst modelers for simulating debris and granular flows phenomena. McDougall and Hungr (2004) were the first to use the SPH method for simulating the propagation of flow-like landslides. They proposed a framework capable of incorporating more sophisticated rheological models, as well as the influence of topography and material entrainment along the runout path. Three analysis teams worked independently implementing the different methods. The teams collaborated on the mechanical properties of the sediment. The results of the different sediment runout analyses were quite similar and showed conclusively that following a breach of the dam, the sediments travel a relatively short distance downstream; less than 7,000 feet. At the beginning of the breach the free water in the reservoir is initially, followed by sediment laden water, and the saturated sediment. Calculations for the release of the water and sediment laden water show these materials will move downstream much faster than the released sediments (which represents the largest volume of material that will be released).