Abstract
Dam related issues have become preoccupying, with raising concerns about detrimental environmental impacts and sedimentation in reservoirs. Reduction of the Ayu-fish production is due to increasing turbidity and decreasing coarse sediment supply by dams in the Tenryu River, Japan. The paper summarizes various case studies on excavated sediment supply in Japan and a case of numerical study to evaluate the effect of the supplied sediment on the spawning redds of Ayu-fish in the Tenryu River. Results of numerical simulations show that under the present management scheme of non-supplied sediment from the upstream dams, suitable spawning redds are formed on sandbars at the confluence instead of riffles. However, field surveys indicate that riffles with newly deposited sediment are more suitable for fish spawning.
Highlights
Dam related issues have become preoccupying, with raising concerns about detrimental environmental impacts and sedimentation in reservoirs
The results showed that the riffles of lower reaches at point 304 had the softest riverbed (Fig. 4(c))
We identified areas with gravel riverbeds based on the grain diameter, areas with a high riverbed softness based on the amount of riverbed changes, and areas of riffles based on the flow velocity
Summary
In Japan, there are more than 2700 operating dams, more than 15 m in height with median age of 61 years. Japan is a world leader in the number of implemented sediment management techniques by flushing, sluicing, dredging/excavating, and bypassing. Supplying the excavated sediment is the most dominant technique to recover reservoir function and restore the river ecosystem [5]. The technique consist in excavating or dredging sediments and placing them just below the dam or distributing them at various locations along the river banks. This method is win-win for recovering original functions of reservoir and restoring original natural riverbed state. In Japan, every year, limited sediment volumes are excavated and supplied below more than 27 dams (Figure 2). The following subsections introduce historical analysis for some of the ongoing sediment supply projects in Japanese rivers and reservoirs. Two dimensional numerical modelling in Ternyu River is presented with comparison with the field investigations
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