Author(s): Ikuko Fukuda

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Keywords: Sediment runoff; Runoff analysis; Mountainous area; Bed load;

Abstract: The management of the forest in Japan is changing because of declining of the forestry. As a result, forests are not able to get much sunshine, decrease undergrowth, and become the bare land. Then, the basins which have deteriorated forest are in danger of slope failures when it rains heavily. Thus the state of sediment runoff is influenced by the condition of the forest. The purpose of this study is to clarify the relationship among rainfall, surface water and sediment volume. The studied basin is the Nishitani River, a branch of the Tonda River, in Wakayama Prefecture, Japan, where is covered by coniferous forest. We conducted both observational and computational studies there. About the observation, we have measured precipitation and water level, and sediment volume at the end of the basin. About the calculation, the surface water is estimated by a two-stage tank model and bed load volume is estimated by bed load formula. The simulation models were applied to several rainfall events. Predicted hydrographs reproduce the events appropriately within the error values of 3.9%. And the orders of the sediment volume due to typhoons 1411 Halong matched between the observed and the estimated values, although the estimated value itself was 4 times larger than the observed one. In addition, the same parameters of the tank model were applied to the other rainfall events. However, the repeatability was not so good. The reason can be attributed to flood caused by typhoon 1511 Nangka, bringing the change of some parameters.

DOI: https://doi.org/10.3850/38WC092019-1482

Year: 2019