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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.
Abrasion in a concrete-lined sediment bypass tunnel is estimated using a Japanese state-of-the-art prediction model and validated by measured invert abrasion data at Asahi Reservoir, Japan. The model is described in detail, certain shortcomings are disclosed, and a revised version is proposed. The model consists of a kinetic energy term accounting for the impact by saltating particles, and a friction work term accounting for the grinding stress. It is found that the latter term yields concrete abrasion values being consistently a multiple compared to its kinetic term contradicting other research. Based on that, and a possible particle impact angle inconsistency, it is proposed to omit the friction work term. It is shown that the calculated abrasion is overestimated by 138% on average compared with that measured, if both terms are accounted for. However, promising results are obtained with only 30% overestimation by neglecting the friction work term.
To achieve the sustainable use of dams, the development of methods for sediment management in reservoirs is required. One such method includes the use of Sediment Bypass Tunnels (SBTs) to divert sediment around a dam, thereby preventing sedimentation in the reservoir. However, SBTs are prone to severe invert abrasion caused by the high sediment flux. Therefore, it is necessary to establish a measurement system of the sediment transport rate in these tunnels. One system to measure sediment transport in rivers is the Swiss plate geophone, which can register plate vibrations caused by particle impact. In Japan, the Japanese pipe microphone is used, and sediment transport is measured based on the sound emitted by the particle impact. In this study an attempt was made to optimize the advantages of both systems by fixing a microphone and an acceleration sensor to a steel plate. The results of calibration experiments with this new system are presented and compared with the existing methods. It was found that the acceleration sensor can detect sediment particles larger than 2 mm in diameter. Moreover, a new parameter, referred to as the detection rate, was introduced to describe the correlation between the actual amount of sediment and the registered output. Finally, two parameters - the saturation rate and hit rate - are introduced and exhibit strong correlation with the detection rate.
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.
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.
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.
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.
Due to high bedload sediment transport, many sediment bypass tunnels (SBT) are prone to severe invert abrasion. However, there is little information about the flow characteristics in SBTs after invert abrasion initiated and progresses with time. In the present study, laboratory flume experiments were performed to investigate how the hydraulic conditions change after abrasion patterns developed on the invert. A typical invert abrasion pattern was produced using 3D-printing technique and implemented in the laboratory flume. Flow depths were measured to compare the initial with the abraded state.
Furthermore, turbulence measurements using 2D-laser Doppler anemometry technique were performed to obtain the mean and turbulence flow characteristics. This paper describes results of these measurements focusing on the streamwise and vertical flow velocities, turbulence intensities and Reynolds shear stress.