Landslide dams occasionally induce catastrophic outburst floods or debris flows mostly by overtopping. However, adequate information about changes of landslide dam features and grain size distribution due to overtopping are still lacking. The 2008 Iwate and Miyagi inland earthquake triggered many landslide dams. Here we report the results of prompt survey for landslide dam features and grain size distribution just after the overtopping at one of the landslide dams triggered by the Iwate and Miyagi inland earthquake. Erosion widths ranged from 19 to 32 m and longitudinal riverbed gradient after the overtopping ranged from 2.4 to 7.5 degree. Also, we found that amour coating occurred at the stream bed obviously. Based on the observed width and peak discharge, it can be estimated that the value of alpha in the regime theory was smaller than 1.9.

Key wordsFlandslide dam, the 2008 Iwate and Miyagi inland earthquake, overtopping