Planning of sediment bypass tunnel for hydropower dams

Abstract

In Japan sedimentation in hydropower reservoir is one of the most important problems for sustainable power generation. Many J-POWER Electric Company's dams and reservoirs were installed in post war reconstruction period, then for decades reservoirs have stored much sedimentation inside up to sedimentation ratio 10% because of high degree of sediment production and river flow regime. We have been trying to excavate sedimentation out of reservoirs to avoid loss of reservoir capacity, aggradation of upstream riverbed and obstacle for intake and outlet functions. It is time to change sedimentation management of hydropower reservoir, because of the reasons bellow. There is too much sediment to remove, resulting too much cost for excavation, and no disposal site near reservoir, on the other hand, sediment flow is strongly requested in dam downstream river and coastal area. It is not effective to excavate and dispose sediment just considering hydropower generation, appropriate sedimentation management is needed both for power generation and river basin sustainability. Sediment bypassing is one of the most effective methods instead of excavation. In Japan 4 sediment bypasses have been installed. Sediment bypass does also have some difficulties in installation and operation. We study the feasibility of bypass installation on 2 J-POWER reservoirs and in this paper we show the site condition of sedimentation, expected effects, difficulties of bypass and our challenges to them.