Numerical simulation of the Debris Flow Occurred at Izumi City on July 10, 1997.

1997年7月10日，鹿儿岛县泉市发生泥石流，造成21人死亡。泥石流是由26°山体边坡破坏引起的。滑坡发生后，泥石流沿6.8°的坡度向下移动，堆积在坡度为3.7° ~ 2.4°的冲积扇上。因此，编制该地区泥石流危险度图，对防治泥石流具有重要意义。本研究的主要内容如下：（1）为研究泥石流的终止过程，进行了坡度从4°或6°迅速减小到2°的明渠泥石流试验。试验床料为0.17mm的细砂和0.84mm的粗砂，粗砂在河床变坡处有较大的淤积。同时沉积区域向下游和上游方向延伸。在细砂情况下，在河道坡度变化点的更下游区域发生大量沉积。同时沉积区域向下游方向延伸。(2)对这些实验进行了数值模拟，基本方程是动量方程和两个连续性方程的固体和流体相。泥沙输移方程是求解基本方程组所必需的。两种不同类型的输沙方程，一个是推移质输沙方程和其他的总负荷输沙方程。在粗砂情况下，用推移质输沙方程计算的结果与试验结果吻合较好。但无论是粗砂还是细砂，用总荷载输运方程计算的结果都不能解释试验结果。

Debris flow occurred at Izumi city , Kagoshima Prefecture on July 10, 1997, and so 21 people died because of this event. The debris flow was caused by the failure of mountain slope of the angle of 26°. After the slope failure the debris moved down slope of 6.8° and deposited on the alluvial fan of lope angle of 3.7° 〜2.4°. It is important to make debris-flow hazard map of this area as its counter measures. The purpose of this study is to accumulate the knowledge for numerical simulation method of the debris flow.The results obtained in this study are as follows :(1) In order to examine the process of debris-flow termination, experiments of debris flow were made in an open channel with rapid decrease in slope from 4° or 6° to 2°. Bed materials used in the experiments are fine sand with the diameter of 0.17 mm and coarse sand of 0.84 mm. In the case of the coarse sand considerable deposition occurs at the changing point of channel slope. At the same time deposition area extends to the downstream and upstream directions. In the case of the fine sand considerable deposition occurs in the more downstream area from the changing point of channel slope. At the same time deposition area extends to the downstream direction.(2) Numerical simulation of these experiments was made ; the basic equations are the momentum equation and two continuity equations for solid and fluid phases. Sediment transport equation is necessary to solve the basic equations. Two different types of sediment transport equations are used ; one is a bed-load transport equation and the other a total-load transport equation. The result of computation with the bed-load transport equation agrees with the experiments in the case of coarse sand. However the result of computation with the total-load transport equation cannot explain the experimental results in the either case of coarse sand or fine sand.