Tamotsu Takahashi

PREDICTION OF BRAIDED CHANNELS PATTERN AND SEDIMENT TRANSPORTATION

In this study, we propose a simulating method to predict channel pattern in braided reach. Stream length which is defined as the distance from a channel bifurcating point to next bifurcating point, can be predicted by the channel width and the Froude number of the stream channel. We calculate the channel pattern using the channel length, separating rates of flow discharge and angles of channel bending at bifurcating paints.The channel pattern calculated by the simulation model agree well with the experimental data. And also, the temporal variation of sediment discharge through braided channels can be estimated by this model.

Bed Variation in a Gradually Widening Channel

In braided channels, channels vary spatially and temporally, which results in the big variation of the sediment dischrge. It is important to know the channels pattern and process of stream channels variation. Channel patterns can be predict by the length and width of each stream channel. In this paper, two dimensional simulation model are employed to clarify the process of sand bar formation and channel diverging phenomenon in a gradually widening channel, in order to investigate the length of a stream channel. The results of the calculation are compared with the data obtained from flume tests to verify the numerical model.

A Numerical Simulation Method of the Mud Flow Deposition

This paper discribes a numerical simulation method for the deposition of the debris flow with high content of fine and cohesionless sediment. The model assumes that the excess quanty of sediment than the equilibrium deposits with the settling velocity. The validity of the simulation method is proved by the experiment in which the steep channel abruptly debouches to flat area.

Fan Formation by the Sediment Runoff Due to River Bed Erosion

We had a localized torrential downpour near the Fukui-Gifu prefectural border in September, 1965 (we call this Okuetsu Rainstorm). Especially, Nishitani village in Fukui prefecture (after this disaster, residents in this village moved to another place and now this district is included in Ohno city) suffered crushing damage from this storm. Namely, in Nakajima district which was the main part of the Nishitani village, before the disaster the number of households was 154, but after the disaster, 58 households were swept away and 86 were buried. A distinguishing feature of this disaster was that the bulk of the houses in the main part of the Nakajima district situated on the debris fan which had been formed by the sediment runoff from the Kamata river was buried second floor deep in sediment but not swept away. Judging from the fact-finding survey and data analysis, these houses were buried by the sediment which outflowed and deposited from the Kamata river taking the type of the sediment flow rather than the debris flow.In this paper, in order to reproduce the burying process of the Nakajima district by the sediment flow, sediment hydrograph of the Kamata river is evaluated by the calculation of the bed level variation at first, then by using this sediment hydrograph as a input boundary condition, simulation of the horizontally twodimensional bed level variation in Nakajima district is carried out, and it is found that the calculated results are comparatively in good agreement with actual phenomena.