Study on river bed variation calculation focusing on motion initiation condition of sediment in mountainous river

Satoshi NIWA, Taro UCHIDA and Yoshifumi SATOFUKA


In this study, for the Haruki River basin of the Fuji River system, we investigated motion initiation condition of sediment with high reproducibility using time series data of river bed fluctuation by sand surface gauge and river bed change calculation. Specifically, we confirmed the effects of calculation results by motion initiate evaluation methods and representative particle size for evaluating riverbed erosion, and, discussed the reproducibility of floods accompanied by large-scale earth and sand production. Here, the method (A) is an evaluation for individual transport of gravel particles, Egiazaroff formula corrected by Ashida and Michiue (1972) was used in the evaluation formula. The method (B) is an evaluation for collective transportation of riverbed sediment with optional thickness and uses a formula to compare shear stress and shear strength in the river bed with the depth of representative particle diameter by Takahashi (1977). The conclusions obtained through the study are the following two points. First, in the method (A), which use the average particle size of the river bed, the motion initiation water depth of the mountain river was underestimated, and the erosion depth of the flow path and the amount of sediment discharged to the downstream area are overestimated. Secondly, in mountainous rivers, collective transport initiation due to sliding failure is dominant for riverbed erosion than individual transport initiation of gravel particles, by giving motion initiation condition of sediment assuming sliding of representative grain size (average grain size and d60), reproducibility is improved for floods accompanied with large-scale earth and sand production in mountain rivers.

Key words

mountain rivers, large-scale sediment production, motion initiate evaluation, numerical calculation