A case study for topographic characteristics of sediment deposited area
and deposition starting point in recent debris flows

Jun’ichi KANBARA, Taro UCHIDA, Yasutaka TANAKA, Tomo’o MATSUBARA, Kotaro IKEDA and Hikaru TODATE

To understand effects of topography, such as longitudinal gradient, on erosion and deposition patterns of debris flow, we quantified topographic changes due to debris flow using aerial LiDAR datasets for recent debris flow in Japan. In larger catchments, the debris flow fan already formed and the top of debris flow fan located upper stream from the exit of the catchment. While, the top of debris flow fan was the downstream from the exit of the catchment in small catchments. We confirmed that the longitudinal gradient of the top of debris flow fan was around 10 degree, regardless of catchment size. This difference gave an impact on the deposition pattern of debris flow. The distance from the point where longitudinal slope angle was 10 degree to the lower end of deposited area was linearly increased with increase of the stream length. While, the distance from the exit of the catchment to the lower end of deposited area was not related to the stream length where catchment scale was large. Further, we found that the gradients at the position changed from erosion to deposition were highly varied, although studied catchments were located in the same region and debris flows were triggered by the same rainfall event.

Key wordsFdebris flow, LiDAR data, topography, erosion and deposition, landform evolution