| The sediment runoff, which takes place in relation to a rainfall runoff in the catchment, depends not only on history of sediment supply into stream channels, but also on the stored sediment volume and sediment size distribution along the stream channels. The volume of sediment runoff is not so large in the river section where the armoring processes develops, while it is large where the sediment production just occurs. In particularly when extreme rainfall event occurs, large volume as well as wide range size of sediments are supplied to the stream channels though debris flows or slope failures. In such situation, a characteristics of sediment runoff dramatically changes. Once this kind occurs, it is important for a river administrator to be able to estimate the changes regarding the characteristics of the sediment runoff. The authors proposed a sediment runoff model, which was combined with the rainfall runoff model and the sediment transport model. The model deals with the rainfall runoff, river flow, and sediment runoff. Surface runoff and subsurface flow are computed by two dimensional kinematic wave model and by Darcy law, respectively. Stream channels flows are evaluated with one dimensional kinematic-wave model. The model assumes that sediment is produced from slope as the debris flow or land slide, and transported and deposited in the channel whose bed slope is about 5 degree. Thereafter, sediment is transported by the water flow in channel evaluated using the bed shear stress estimated from one-dimensional kinematic-wave approximation in the river channel. Sediment transport model is capable enough of dealing with mixed sediment sizes, including the bedload, suspended sediment, and wash load. Based on the numerical model, the armoring process is simulated, which is that the sediment runoff decrease as the bed material coarsen in steady water-flow condition. The numerical simulation is also conducted for an actual catchment in Japan, and the results are verified with the observed sedimentation in a dam. |
