With many earthen embankment dams reaching the end of their planned service life, the increased possibility of one of these structures failing is generating concern within the dam safety community. Over time, sediment pools fill, and land use and topography change due to increasing development. These transformations in the environment can trigger embankment failure through flood events that cause overtopping and/or piping in the embankments. Understanding how a breach develops over time can improve prediction of flood magnitudes and timing that can be used to develop warning systems and emergency action plans. Research using large-scale physical models of earthen embankments is on-going at the USDA-ARS Hydraulic Engineering Research Unit outdoor laboratory. Soil properties including water content at compaction and compaction energy have been shown to significantly influence how a soil erodes and the rate at which it erodes. These properties directly affect breach timing, formation, and geometry. Large-scale embankment failure tests were conducted to examine the mechanics of breach widening during the reservoir draw down stage of a breach. Data from these laboratory experiments provide a basis for developing numerical models for describing the breach geometry as a breach widens over time. The objective of this paper is to evaluate the time rate of breach widening for four large-scale earthen embankment tests and compare the results to a simplified numerical model, SIMBA, developed to evaluate embankment failure processes. The homogeneous embankments tested were constructed of materials ranging from silty sand to clay loam, 1.3 m in height with a 0.30 m wide notch through the center of the entire height of the embankment. Soil properties that show promise for characterizing breach embankment failure processes and used for performance prediction during the breach widening stage are discussed.
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