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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.
Four dams in Japan and Switzerland with Sediment Bypass Tunnels (SBT) as a measure against reservoir sedimentation were monitored to analyse the effects of sediment supply on the downstream environment based on up- to downstream differences in geomorphological and biological characteristics. SBT operation times ranged from 93 years at Pfaffensprung and 17 at Asahi to only three years at Solis and no operation at Koshibu. Sediment grain size distribution was monitored, and microhabitats and invertebrates were analysed in terms of richness and composition. Results showed that grain sizes were coarser down- than upstream at dams with newly established SBTs, while they were similar or finer for dams with long SBT operation. Analysis of biotic data revealed that microhabitat and invertebrate richness was low directly below the dam but increased further downstream the longer the SBT operation. Sedentary species dominated at locations where bed conditions were stable, e.g. directly downstream of the dam at Koshibu. Recovery of downstream environment with increasing SBT operation time was disclosed by the Bray–Curtis similarity index, which evaluated an overlap between up- and downstream reaches for both microhabitat composition and invertebrate communities. With increasing operation time, both indices increased, revealing the positive effects of long-term SBT operation.
Particle dynamics are investigated experimentally in supercritical high-speed open channel flow over a fixed planar bed of low relative roughness height simulating flows in high-gradient non-alluvial mountain streams and hydraulic structures. Non-dimensional equations were developed for transport mode, particle velocity, hop length and hop height accounting for a wide range of literature data encompassing sub- and supercritical flow conditions as well as planar and alluvial bed configurations. Particles were dominantly transported in saltation and particle trajectories on planar beds were rather flat and long compared with alluvial bed data due to (1) increased lift forces by spinning motion, (2) strongly downward directed secondary currents, and (3) a planar flume bed where variation in particle reflection and damping effects were minor. The analysis of particle saltation trajectories revealed that the rising and falling limbs were almost symmetrical contradicting alluvial bed data. Furthermore, no or negligible effect of particle size and shape on particle dynamics were found. Implications of experimental findings for mechanistic saltation-abrasion models are briefly discussed.
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.
The derivation of an abrasion prediction model for concrete hydraulic structures valid in supercritical flows is presented herein. The state of the art saltation-abrasion model from Sklar and Dietrich (2004) is modified using the findings of a recent research pro-ject on the design and layout of sediment bypass tunnels. The model correlates the im-pacting parameters with the invert material properties by an abrasion coefficient kv. The value of this coefficient is verified by a similarity analysis to bedrock abrasion in river systems applying a correlation between the abrasion rate and the bed material strength. A sensitivity analysis reveals that the saltation-abrasion model is highly dependent on an adequate estimation of kv. However, as a first order estimate the proposed model en-ables the practical engineer to estimate abrasion at hydraulic structures prone to super-critical flows.
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.
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.
