Deposition Mechanism of Sediment Mixture in a Dam Reservoir
2Associate Professor, Department of Civil and Environmental Engineering, Hiroshima University, Graduate school of Engineering, Japan
3Professor, Department of Civil and Environmental Engineering, Hiroshima University, Graduate school of Engineering, Japan
The decrease in the amount of sediment supply to downstream rivers, which is caused by the deposition in the dam reservoir, have induced problems of river environment such as riverbed degradation, static armor with coarse grain, and coastal erosion. In order to perform countermeasures against them, it is necessary to develop a calculation method of riverbed change of the mixed grain size sediment, which can properly evaluate deposition and sediment flushing mechanism in reservoirs, and the effect of sand supply on the downstream river. However, the conventional method based on the active layer concept doesn’t have the ability to evaluate the change in the sediment porosity. The objective of this study is to clarify the change in sediment volume and the porosity of river sediment through the sediment classification effect due to the flow action in the reservoir.
Based on the binary packing formula, we have developed the Eulerian packing model with various sediment porosity with wide grain size distribution. We applied it to the natural sediment deposited in a dam reservoir. We measured the porosity of deposited sediment at 6 places with various grain size distributions in the reservoir and to develop a model to calculate the porosity of the sediment mixtures. The model is compared with the packing experiment using the material of the dam reservoir. The porosity of the natural sediment is 1.5 times as big as that calculated by the model using the grain size distribution, while it provides good agreement with the closest packing measurement results of the porosity with the deposited sediment in dam reservoir. We modified the Eulerian packing model for natural sediment depositing process by introducing the coefficients in the model.
The experiments with a straight channel that has the transparent side wall of an acrylic plate are conducted to investigate the change in the sediment volume due to water flow action. In the experiment, we make a mountain of the sediment with wide grain size distribution, and the weir at the downstream was gradually lowered. The volumetric change and the deposition shape are measured. The volume of sediment increased by the classification effect in water flow, and the porosity was generally consistent with the modified model for natural sediment.
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