Positive effects of reservoir sedimentation management on reservoir life: Examples from Japan
(2016)
The effectiveness of different strategies against reservoir sedimentation is demonstrated herein using data sets of Asahi, Nunobiki and Dashidaira reservoirs in Japan. The applied strategies encompass
sediment routing with a bypass tunnel, drawdown flushing during floods and sabo dam construction in the catchment. It is shown that bypassing and flushing are very efficient strategies enlarging reservoir life by 3 to 21 times up to many hundreds of years. Furthermore, it is revealed that also efforts in the catchment, e.g. sabo dam construction, is effective enlarging reservoir life by 2.4 times.
Sediment, which deposits and damages the function of reservoirs, is an essential element of aquatic habitats in downstream ecosystems. We reviewed ecosystem features of degraded channels associated with sediment deficiency below dams and ecosystem responses to changes in sediment conditions after management practices in Japan. Sediment bypass tunnel (SBT) is an effective way to transport sufficient amount of sediment to downstream ecosystems. Based on a concept of suitable mass and size of sediment for ecosystem, some effects and limitations of SBT on downstream ecosystems were discussed.
Sediment bypass tunnels are an effective and sustainable strategy against reservoir sedimentation. Sediments are diverted into the down-stream during floods without deposition in the reservoir, hence mor-phological and ecological variability increases. One major drawback of these tunnels is the severe invert abrasion due to a combination of high flow velocities and bedload sediment transport. The abrasion phenom-ena is briefly described, different abrasion prediction models are pre-sented and their applicability for the estimation of concrete abrasion is discussed.
For long term use of dams, it is required to develop methods of sediment management in reservoirs.
As one method, Sediment BypassTunnels (SBT) are operated in Japan and Switzerland to prevent reservoir sedimentation. SBT reduces sedimentation in reservoirs by routing the incoming sediments around the dam. SBT, however, is prone to severe invert abrasion caused by high sediment flux. Therefore, it is necessary to establish a measurement system of sediment transport rates in the SBT. A geophone was experimentally investigated in a laboratory flume at ETH Zurich. The sediment transport rate is calculated based on the plate vibration caused by hitting of gravels. In this paper, in order to alleviate disadvantages of a geophone, two newly developed sensor systems, a plate microphone and plate vibration sensor, are suggested and the results of their calibration experiments are shown. Finally, they are compared with the existing methods.
Sediment Bypass Tunnels are operated to divert sediment around reservoirs reducing reservoir sedimentation. A major drawback of these tunnels is severe invert abrasion due to high velocity and sediment flows. There is an urgent need to establish innovative measurement systems of sediment transport rates in SBTs. In this paper, three bedload measuring systems, namely hydrophones, geophones, and newly developed plate microphones are introduced and compared. The Koshibu SBT is planned to operate from 2016. Plate microphones combined with geophones and other planned systems are installed in the tunnel. Results of preliminary tests and installation plans of bedload
measurement are presented.
To understand the effects of sediment bypass on environmental recovery of the degraded channels below dams, bed topography and bed materials above and below dam reaches were surveyed by ground-based measurements and aerial photos using quadrocopter. Coarse bed materials such as boulders were more represented below than above the Koshibu dam, where the bypass tunnel had not been in operation yet. The coarse materials formed steps and protruded in the water column within riffles and runs, both of which can increase slow-flow areas, below the dam. On the other hand, sand, gravel, and cobbles were abundant below as much as above the Asahi dam, where the bypass tunnel had been operated for >17 years. The downstream environment in terms of bed topography and grain size seems to have almost been recovered for the Asahi dam. However, less representation of large cobbles and boulders below the dam suggested a possibility of a selective deposition of coarse materials at the upstream of the bypass tunnel inlet.