Abstract: Once encountering extreme rainfall, a check dam is highly prone to flooding and collapse, posing a grave threat to downstream communities; a quantitative risk assessment of dam failure caused by its breaching is urgent. Previous studies overlooked the impact of sedimentation on dam failure. In this paper, we examine the influence of rainfall-induced sedimentation on the failure and reveal a mechanism of the dam behavior under the interaction of rainfall, flooding, and sedimentation. We then develop a mathematical model for quantitative assessment of dam failure risks, extending traditional "rainfall-flooding" modeling to "rainfall-flooding-sedimentation" so as to achieve more accurate assessment of dam failure and its risks. We have applied this model in two failure cases of the Wangmaogou watershed and assessed their risks under different return periods of rainfall. Results indicate that the impact of sedimentation is a significant factor. For the Guandigou 3rd dam, Beitagou dam, and Kanghegou 3rd dam, the judgement and calculation results agree with all the three real failure cases when using the simulation considering the sedimentation factor, while they deviate from the reality of the Beitagou dam failure when neglecting this factor. Detailed comparison indicates that under the rainfall on July 26, 2017, neglecting the sedimentation role advances the failure initiation time of the three dams as listed above by 11 minutes, 13 minutes, and 7 minutes respectively. This facilitates timely issuing of the warnings of emergency and quick evacuation of the people living around the dam, and enhances the accuracy of check dam warnings. For the Guandigou 3rd dam under the rainfalls of different return periods, its failure discharge is 4.82 m3/s and 5.97 m3/s under 10-year and 200-year periods respectively. The other two dams can withstand a 200-year return period rainfall under current siltation conditions.

Key words: check dam, rainfall-flooding-sedimentation, dam breach hazard, risk assessment, disaster warning