Abstract:Recently, the contributions of slope failures have been difficult to quantify over the time scales of the sediment concentrations in Sangun catchment area, Fukuoka Prefecture. Therefore, to clarify the sources of the sediment mixing in the catchment is important and becomes considering in the future that related the slope failures occurrence. This paper describes how cosmogenic nuclide methods have provided information about the geomorphic process by utilizing 10 Be (half-life = 1.5 ma), measuring the 10 Be/ 9 Be ratio, and quantifying
10Be concentration. The sampled sediments were derived from throughout a drainage basin. To measure the 10 Be/ 9 Be ratios in the sediments, quartz minerals were collected from the sediment and then Be was extracted from the quartz. The 10 Be/ 9 Be ratios were measured by AMS. We assume that the cosmogenic nuclide concentrations stored in the sediment of a river can increase or decrease, depending on the amount of mixing of sediments during transport through the fluvial system. As results, the 10 Be concentrations have been calculated to determine the short term sediment mixing rate in the catchment area. Therefore, the relationship between the morphometrics of the drainage basin, the sediment concentration by 10 Be analysis and the contribution of landslides to landscape changes over timescales of several years have been investigated in Sangun catchment area.
In Japan, the management of sediment in dam reservoirs has become challenging owing to heavy rain. Unexpected increase of sediment volume in dam reservoirs can increase flood risk owing to a rise in the level of the riverbed. It is therefore necessary to conduct sediment management to clarify the area of sediment production to control the amount of sediment transported and the timing of its movement. Sediment discharge from tributaries is often calculated based on catchment area and the deposited sediment volume already present in the dam reservoir. However, our preliminary surveys have implied that it is also necessary to consider regional geology in the evaluation of sediment transport. Moreover, variations in sediment deposition should be evaluated for each site where the geology changes.
In this study, we observe in detail changes in the riverbed in two tributaries with different regional geologies. Changes in the shape of the riverbed were analyzed from unmanned aerial vehicle observation data, following which the effects of erosion and sedimentation on the riverbed were quantitatively evaluated. Finally, sediment discharge was calculated by a one-dimensional numerical model of sediment transport. These results indicate that the tributary that is mostly composed of mudstone produced considerably more sediment than the tributary composed mainly of sandstone. This suggests that an understanding of regional geology is necessary in the evaluation of sediment dynamics and should be considered in addition to the catchment area.
The heavy rain disaster occurred in July 2017, in northern Kyushu, Japan. The river environment greatly changed due to sediment moving accompanied by erosion. It is important regarding disaster prevention to localize watersheds where sediment transportation is active. In this study, the sediment dynamics were discussed on the basis of our investigation about the stored sediment, water-flow, and silica fluxes from sub-basins. As a result, the sediment survey revealed that many sediments consist of sand or gravel and were moved secondarily by water-flow. By the hydrological survey, it was confirmed that the trend of the dissolved silica concentrations varied between the right and the left bank tributaries. It was suggested that the left bank tributaries have a various process of water-flow. Considering the distribution of collapsed slopes, the right bank tributaries have more collapsed slopes than the left bank tributaries. As suggested by the results, the range of the silica flux is wider at the left bank tributaries because the sedimentation shows various distributions. On the other hand, the right bank tributaries were estimated topographically stable since the value of the silica flux is about the same.
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