Abstract
It is expected that the greenbelts will play the rule as a buffer zone against sediment disasters in pediments. This paper describes how the greenbelts should be appropriately used in terms of debris flow control. These conclusions are based upon the results of numerical analyses of sediment runoff process observed in actual basins. The Chizuka and Nozakinakagawa Rivers, which drain from the Ikoma mountain system, were chosen for discussions. The numerical analyses were conducted in order to investigate the changes in debris flow characteristics through the reaches of the rivers having check dams. The results of the numerical analyses indicate that the river-bed gradient tends to have a considerable affection the debris flow behavior. The discharge rate of debris flow abruptly decreases as the sediment deposition increases by a decrease in the river-bed gradient. The sediment transport mode then changes from a debris flow into the hyperconcentrated flow or general contact loads. According to these results, in order to withstand debris flow, the greenbelt needs to have a gentle and sufficiently wide slope. If it is actually not possible to have such a greenbelt, debris flow should be controlled using structures such as check dams or embanked channels along with a certain amount of greenbelt.
