It has been supposed that the oval shape reaches whose width changes from wide to narrow or from narrow to wide in mountain rivers control the sediment produced by landslides and debris flows. The sediment control function was investigated through the experiments with straight flumes and curved flumes. The experiments gave some interesting discoveries. Firstly, channel gradient has the greatest influence on sediment control. The occurrence of hydraulic jump also chages the sediment control at the wide reaches. When hydraulic jump is formed narrower outlet gives more sediment control. When excessive sediment is supplied to un equilibrium condition, sediment discharge does not increase quickly, and control sediment. Secondary flow is generated from the side walls to the center of the flume on the flume bed near the outlet, which score the bed along the walls and control sediment discharge. The sediment control effect at wide reaches in explained with a one-dimentional computer simulation to some extent.
The method to compose a hydrograph of a large basin(Tsukizaki)using the observed discharge of arepresentative unit' basin(Kakinokidai)was already developed in the second report of the study on the Urajiro River Experimental Basin. The purpose of this report is to apply the method to a larger basin. In the large basin, there are two stream gaging stations Futase-bashi and Tsukizaki by name. The former one, with a larger drainage area, is a target station for constituting the hydrograph. The drainage area of the basin is divided into 22 unit basins. The latter one, with a smaller drainage area, is a representative unit basin. It assumed that the hydrograph at the target station can be as a sum of hydrographs 22 unit basins which are proportional to that of the representative unit basin as expressed by Eq. (1). Time lag is introduced to this procedure as described by Eq. (2) and is defined as the ratio of channel length to the flow velocity which is calculated using time lag between observed peak discharge and the peak rainfall intensity. As the result, little difference was found in peak discharge and peak time between the composed and observed hydrograph at the target station. So the procedures have proved to be good method to constitute a hydrograph of a large basin(drainage area:184km2)out of that of small basin(drainage area: 9.0km2). This result also means that the effect of channel storage, the change of flood wave are negligible in this large basin.