From the concern about the acceleration of dam sedimentation due to the global climate change, a numerical model was constructed, using the time series data of heavy rainfall and sedimentation in the Midorikawa Dam Reservoir. The model reproducibility was successfully verified. Next, using this model, the sedimentation rate increase was estimated based on the rainfall increase proposed by MLIT for the end of this century. The results of the numerical simulation showed that the sedimentation rate will become 1.9 to 3.2 times. The reason for such a large amount of sedimentation was considered to be the expansion of the collapsed area by the rainfall intensity increase and to be the activation of coarse gravel movement in the upstream valleys.

In a discharge tunnel of Amagase dam redevelopment project, a stilling basin section is an extremely large cross-sectional tailrace tunnel to still a high pressure and large capacity of discharged water from the main gate. Bench cut method with pilot side and central drifts is applied, because tunnel passes through a fragile fracture zone intervening in solid sandstone and mudstone with small overburden. And observational construction method is applied during tunnel excavation. Soon after construction started, it was found that fracture zone was wider than expected during side drift excavation. Thus, RC pillar support, more effective side wall stabilization effects than initial plan, is developed. This paper reports a design, construction and observational construction on the first Japanese stilling basin constructed in the form of extremely large cross-sectional tunnel.