Actual condition of sediment discharge and riverbed variation during heavy rain
in a mountain stream to have a large-scale landslide : An example of Harukigawa Fujigawa water system Hayakawa

Satoshi NIWA, Taro UCHIDA, Jun-ichi KANBARA, Takeshi MORIYA, Takeo MITSUNAGA and Yoshifumi SATOFUKA

To clarify sediment dynamics in mountain rivers during the heaviest rainstorms, we observed river water levels and riverbed level at Harukigawa, Yamanashi prefecture. Due the large]scale landslide occurred in more than a hundred years ago, large steep devastated area is located in the upper end of Harukigawa watershed. Here we successfully described detailed sediment dynamics in three large rainstorms due to typhoons in 2011, and, several results were found. About sediment movement of mountain stream with a landslide, we found the following things. At steep mountain river, whether or not to occur riverbed variations is determined by the water level conditions. In addition, when the continuous rainfall exceeds the number 100mm, sediment discharge will be frequent. After that, the high concentration sediment discharge with riverbed rise occurs more than once, and the more the water level is high, a large erosion and deposition occurs. During floods, the duration of the high concentration sediment discharge with riverbed rise was about up to about one hour. Therefore, temporal change of sediment discharge and river bed variation has been found that not determined by only the hydraulic conditions. The timing of the riverbed change to 800m downstream from the confluence of the tributary is affected by the sediment discharge waveform from the tributary. If the scale of the sediment discharge is small, the impact distance will be smaller.