Laboratory experiments on failure characteristics of non-cohesive sediment natural dam in progressive failure mode

Natural dam failure may cause disastrous floods in the downstream, especially for dam outbursts in progressive failure mode. Until now researches about the characteristics of progressive failure are still completely unclear. In this paper, a series of flume tests were conducted to investigate the formation mechanism of progressive failure of natural dam, the breaching characteristics and the effect of outburst flood on the channel bed. According to the results, three typical features in the progressive failure process of natural dams were observed: seepage on downstream slope, slide of the downstream slope and overtopping. With phreatic line rising continuously, seepage occurred on the downstream slope, which caused slide of downstream slope of the dam. The breaching discharge hydrographs were asymmetric on both sides of peak discharge, which was similar to the breaching discharge of overtopping. The relationship between breach width and depth was like a shape of "S", which followed a logistic function. In the breaching process, sediment in the channel bed near the dam was first deposited and then scoured, where the flow velocity was large with strong turbulence; while flow on channel bed far away from the dam showed lower velocity and the flow regime transferred from supercritical flow to subcritical flow. Sediment deposited at first and then scoured gradually on channel bed, but only deposited at a large distance from the dam. The outburst flood eroded the channel bed continuously, the erosion mechanism was from tractive erosion to backward erosion, and returned to tractive erosion finally. In addition, the further away from the dam, the weaker the erosion intensity was. The results of this paper enriched the understanding of the failure mechanism of progressively outburst natural dam, which is helpful to predict breaching parameters in progressive failure mode and to provide some reference about flood prevention.