| In rainfall runoff model, flow resistance is assumed as constant. However, according to previous studies (e.g.: Engelund et al. 1967), the resistance changes due to development or vanishment of dune during flood. Almost previous studies have been constructed in bedload dominant condition. In addition to that, in extreme rainfall event, high concentration suspended sediment (SS) is taken into trough of dune and dune height decreases. Consequently, the flow resistance, in especially form roughness, decreases. In this study, authors conducted flume experiments and analysis the flow resistance change when SS is taken into the flow on the dune.The flume has rectangular channel (width/water-depth = 0.4 m/0.08 m). There are 10 fixed dunes on the channel in succession. Sand with 0.8 mm diameter is fixed on surface of the dunes. SS, which is d60=0.13 mm diameter and 10 g/L concentration, is provided into the flow in 2 minutes.Three cases are conducted. In Case 1, dune phase is Dune I and non-dimensional shear stress τ*=0.15. In Case 2, the phase is Dune II and τ*=0.24. In case 3, the phase is also Dune II and τ*=0.40. These τ* are calculated using sand diameter d=0.8 mm.As the result, without suspended sediment, Manning’s roughness coefficients are calculated as 0.020, 0.024 and 0.027 in Case 1, Case 2 and Case 3, respectively.The elapsed time from the beginning to input SS is called t0. In t0=60-90 sec, when during providing SS, the roughness decreases by 5-13% and τ* increases 5-6 times as case without SS. In t0=440-470 sec, after finishing to provide SS, the roughness decreases by 26-30% and τ* increase 5 times as case without SS. Additionally, in case of using the relation proposed by Kishi et al. (1973), the roughness decreases by 50% as case without SS. |
