Modeling reservoir sedimentation associated with an extreme flood and sediment flux in a mountainous granitoid catchment, Japan

Abstract Extreme events, such as large storms and floods, have a major effect on the transport of sediment and contaminants and, hence, on water quality. To be realistic, models designed to predict the short-term transport and fate of sediment and contaminants must include large floods and storms, an idea not incorporated into existing models based on uniform sediment flux. Sediment transport in and to river channels in mountainous terrain is strongly influenced by precipitation, particularly when heavy precipitation triggers a surface landslide in the catchment area. This study provides a quantitative measure of such sediment transport. The Yahagi Dam watershed was modeled using a 50-m grid-based digital elevation model (DEM), and flow directions were determined for each of the slopes present. The study objective was to develop a general algorithm for simulating water flow and sediment movement in a dammed mountain basin using water and sediment dynamics modeling. The nonuniform flow model used for stream channels can incorporate energy loss around dam reservoirs, and the resulting algorithm can estimate all phenomena related to sediment movement. The new method was verified by applying it to the Tokai rainfall disaster and the Yahagi Dam basin, Japan. In this basin, the volume of bedload sediment deposited is 993 × 103 m3/y, suspended-load sediment is 669 × 103 m3/y, and wash load sediment is 900 × 103 m3/y, as determined from sediment production in each tributary of the Yahagi River. Most sediment is probably introduced during extreme flood events owing to lithological factors, and thus extreme rain events that now accompany climate change are very important in sediment supply. During extreme flood events, the river's effective tractive force (τ*e, d90) exceeds the critical tractive force (τ*c), especially upstream of the Yahagi Dam. Rainfall-related sediment disasters can be accurately predicted using water- and sediment-dynamics models that are founded on realistic geomorphological characteristics and sediment flux of rivers.

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