Abstract: Flume experiment with varied bed slope gradients (7°, 8°, 9°, 10°, 11°, 12°, 13°) was conducted to explore the downcutting erosion of landslide dam breach. Test results demonstrated that despite different slope gradients, the downcutting erosion of landslide dam breach under overtopping failure mode can be divided into three stages: breach penetration stage, accelerated erosion stage, and decelerated erosion stage. The slope gradient of trench bed affects the accelerated erosion stage. When the bed slope gradient is greater than or equal to 9°, the dominance of downcutting erosion supplemented by retrogressive erosion is weakened. Moreover, the duration of breaching is in a non-monotonic relation with slope gradient, shortens at first but then elongates along with the expansion of slope gradient. Average downcutting erosion rate and retrogressive erosion rate are also in non-monotonic relation with slope gradient, rises at first but then declines with the increase of slope gradient. When the slope gradient is smaller than 11°, the sensitivity of duration to slope gradient is higher than that to reservoir capacity in front of the dam; and vice versa.

Key words: landslide dam, trench bed slope, overtopping process, longitudinal breaching, retrogressive erosion