Sediment transport process in mountain streams is not entirely controlled by flow discharge. As a result, the process does not concomitantly follow flow discharge pattern but shows non]consecutive partly independent dynamism. The process has been a subject of extensive studies in theoretical, experimental, as well as observational aspects. Based on direct sampling, experimental bed load equations have been tested to examine the relationship between flow discharge and sediment discharge. The sampling and observation efforts led to the development of more indirect but stable sediment transport monitoring methods (hereafter gindirect method") in recent years. One of the indirect methods, a hydrophone sediment discharge measuring system (hereafter ghydrophone system"), has been intensely studied for the development of practical sediment transport monitoring. Hydrophone systems have been tested quantitatively in relation to bed load equations as well as to directly sampled sediment discharge in 100 and 200 km2]scale river basins.
Quantification efforts have been carried out both as site]specific comparison between direct and indirect methods, and as a year]long basin]scale examination utilizing reservoir sedimentation data. These efforts enabled us to take a longer view than a snap shot view of sediment discharge, which is transported in a collective form seen as sediment waves. Examination of the extent to which erosion and flood control structures such as sabo dams transform sediment discharge of floods has been a major practical concern for a long time. Bed load equations can analyze the effects of structures at a given moment but are not adequate to quantify those exerted upon collective forms of sediment discharge. Therefore, indirect measurement, together with flow discharge observation, was conducted in an upstream segment of a mountain stream in order to study the transformation processes and effects of structures. Since direct sampling is not readily possible in the segment, bed load analytical equations developed in a similar mountain stream was applied to quantify the sediment transport process in the segment. The result indicated that sediment discharge flowed down as a wave at a velocity about one]third of that of average stream flow and that the maximum sediment discharge did not grow as much as the aggregated sediment discharge of the waves in the structurally regulated segment of the stream. Flood cases studied here were minor in their intensities. Further observational efforts and analysis are needed to examine whether the quantification method is robust enough and whether the results obtained here hold to a larger flood. This study is a preliminary attempt to introduce an analytical framework that is applicable to larger flood cases.


Key wordsFbed load, sediment discharge wave, effects of erosion control structures