Flashfloods are one cause of river accidents. During a flashflood, the water level rises suddenly. A flashflood can result when a small dam, formed by a landslide or debris flow, bursts during a torrent. When such a dam blocks a river, the flood that follows its collapse can cause a disaster downstream. The time until collapse is short. To prevent damage from the natural collapse of small landslide dams, it is important to understand the characteristics of dam shape and the discharge hydrograph. We studied landslide dams that were smaller than the dams examined in previous studies. We experimented with small landslide dam outbursts in a rectangular flume. The temporal variation in the shape of the landslide dams and the erosion rate around the crown of the landslide dam are examined. In addition, the rapid collapse of a landslide dam was examined to determine whether it would cause a flash flood, based on the hydrograph. The results are summarized as follows:1) when there is discharge in the flume, the length of the bottom of the landslide dam is increased;2) for a small landslide dam when the inflow discharge and volume of sand are both large, the upstream side of the landslide dam slope is not eroded; however, when the volume of sand is low, it is eroded and moves downstream;3) the erosion rate at the crown of a landslide dam is essentially constant, and the rate is related to the inflow discharge;4) when a small landslide dam forms, the discharge decreases immediately; and5) when the inflow discharge increases, the peak discharge approaches the inflow discharge.

Key wordsFlandslide dam, experiment, hydrograph, peak discharge