earticle

영어

A massive sediment export during floods and relatively episodic debris flows, which cause occasionally sediment-related disasters, have different transport forms (i.e., fluvial vs. non-fluvial) but can be categorized hydraulically as a mass movement of sediment and/or boulders by running water. Mass movement by running water is brought about by the tractive force. This force gets stronger in proportion to the flow depth and/or the gradient of stream bed; that is, the tractive force reduces in sites, where stream beds show widen or transformation of gradient. In these sites, sediment deposition, therefore, becomes to be accelerated and dominant. On the contrary, local scouring takes place intensively in sites, where running water has concentrated by the morphological feature of stream bed. Sediment sailing during floods is different from running water in having the discontinuous movement with the repeated cycle of scouring and deposition. Setting up the Sabo facilities such as check dam and groundsill does not intend to stop or to weaken the sediment transport directly. They are constructed to reduce the gradient of stream bed, to broaden the flow width and to maintain the temporal base level against vertical deformation. In other words, human can do only modification of land surface to contribute the preventive purpose. The important issue for watershed disaster prevention can be pointed out the examination and decision of the sites, where sediment can be separated from running water through acceleration of deposition and also local scouring can be restrained through setting up the Sabo facilities. Such works should be given priority in the site near preventive objects than upstream, based on the consideration of the discontinuous property of sediment transport.