Effect of Soil Type and Geometry on the Peak Flow and Depth of Breach Resulting from Overtopping Failure of Embankment Dam

Experiments were conducted in a hydraulic laboratory for eight physical models representing homogeneous embankment dams using two types of soil: silty sand and clayey sand. The models were exposed to overtopping flow, which causes the overtopping failure. The models of the embankment were constructed with a width of 0.6 m, a crest height of 0.3 m, and two different side slopes, 1:2 and 1:1. Two different inflows, 15 L/min, and 30 L/min, were used as a hydraulic variable. Two stages were conducted for the tests; at the first one, silty sand was used to build the embankment consisting of four experiments, and four other experiments were conducted at the same hydraulic and geometric conditions, but with a clayey sand embankment. The necessary calculations were made to calculate the outflow, the depth of the breach, and the breaching process. The analysis and comparison of the results of the eight tests revealed that the breaching process consists of three stages. The first is the stage of breach initiation, while the other two stages are the feature of breach development, during which the two peak discharge phenomenon occurs. The first peak is a result of the beginning of the flow of the stored water upstream, while the second peak occurs as a result of the continuing process of erosion and breach growth. Finally, the clayey sand soil delayed the timing of the first peak discharge, shortened the time interval between the first and second peak discharges, and delayed the breach initiation time by (73.33% and 58.82%) with a slope of 1:2, while when the slope is 1:1, the percentage of delay decreases and becomes (33.33% and 13.33%). In addition, the breach depth (Hf) for clayey sand soil has lower values. Generally, the embankment slope of 1:1 decreased the impact of the variation in soil type, and the embankment slope ratio is considered the primary factor influencing the depth of the breach. The experimental results show that the stability breakdown in clayey sand occurs later than in silty sand, and the dam's geometry has a significant and clear impact on breach depth and failure-resistant stability. Keywords: Embankment dam, Breach depth, Overtopping failure, Soil type, Flooding risk, Peak discharge