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Sediment bypass tunnels are an effective countermeasure against reservoir sedimenta-tion. They are operated at supercritical sediment-laden open channel flow conditions. The major drawback of these tunnels, besides high construction costs, is the severe invert abrasion caused by these flows provoking high annual maintenance costs. The project goal was to analyze the fundamental physical processes and to develop design criteria to decrease these negative effects. A laboratory study was performed in a scaled hydraulic model flume. The project was divided in three main test phases giving new insights into the dynamics of turbulence structures and particle motions, resulting bed abrasion and their interactions in a supercritical open channel flow, respectively. In phase A the mean and turbulent flow characteristics were investigated. In phase B single sediment particle motion was analyzed. In phase C the invert abrasion development in time and space was examined. Phase A revealed that secondary current cells affect the turbulent flow pattern leading to high bed shear stress at the wall vicinity. In phase B it was found, that particles were dominantly transported in saltation. Relationships between the saltation probability, and particle hop lengths and heights to the flow Shields parameter were found. The specific impact energy was determined by the impact velocity, number of impacts and the amount of particles transported in time. In phase C the results show that bed abrasion progresses with time both in the lateral and vertical direction. Two lateral incision chan-nels developed along the flume side walls at narrow flow conditions occurring at low flume-width to flow-depth aspect ratios b/h < 4-5, whereas randomly distributed pot-holes were found at wide channels where b/h > 4-5. The observed abrasion patterns match well with the spanwise bed shear stress distributions found in phase A. Further-more it was found that the abraded mass linearly increases with the transported sedi-ment mass allowing for a linear fit. Further results showed that abrasion increased with flow intensity and sediment transport rate, with highest values for the mean particle diameter category, whereas abrasion decreased with increasing material strength. Finally, a new formulation was developed based on Sklar’s saltation abrasion model. A new abrasion coefficient CA is introduced correlating the impact energy and material properties with the gravimetric abrasion rate.

Without adequate measures, reservoirs are not sustainable, neither the reservoir itself due to continuous sedimentation, nor the downstream ecosystem due to altered sediment continuity. Appropriate actions are inevitable and require a systematic sedimentation management. Sediment bypassing constitutes one effective strategy that routes sediment load around reservoirs during floods. A sediment bypass system has the advantage that only newly entrained sediment is diverted from the upstream to the downstream reach thereby re-establishing sediment connectivity. Hence, such a system contributes to a sustainable water resources management while taking the downstream environment into consideration. This paper gives a state-of-the-art overview encompassing design, bypass efficiency, hydraulics, challenges due to abrasion, positive effects on both downstream morphology and ecology, and makes design recommendations.

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.

In order to prevent reservoir sedimentation, sediment bypass tunnels can be an efficient countermeasure restoring sediment continuity of impounded rivers. Although supercritical open channel flow conditions in these tunnelsprevent tun-nel blockage, in combination with the high bypassed sediment volume it may lead tosevere abrasion damages on inverts. Consequently,wear termed hydroabra-sionoccurs. Based on laboratoryexperiments, a theoretical model was devel-oped to predict abrasion rates and service life timeof sediment bypass tunnels. Insituexperiments are further conducted for model calibration to provide an abrasion prediction approach for field applications.Finally,recommendations concerning the hydraulic design of the tunnel as well as the structural design ofthe invertareprovided.

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.