SEDIMENT RUNOFF CONTROL AND SORTING BY FOREST ZONE DURING FLOOD AND DEBRIS FLOW EVENTS

One of the roles of forest trees on alluvial fan is to retard the debris and flood flows and protect houses on the alluvial fan. The purpose of the present study is to investigate the flow behavior within the forest-tree area during the flood and debris flows.Two kinds of experiments were made; one is to examine sediment transport rate within the model-tree area and the other to examine the effect of model trees on sediment runoff control and sorting. From the former experiments it is found that the sediment transport rate can be expressed by effective shear stress. From the latter experiments it is found that the runoff of sediment volume from the model-tree area decreases as the model-tree area and the size of the bed materials becomes larger. Further it is found that sorting of sediments depositing from the flows and outflowing from the model-tree area becomes more significant than that of sediments flowing into the model-tree area.