Supercritical open-channel flows occur in many hydraulic structures including spillways, dam outlets, weirs, and bypass tunnels. Due to high flow velocities and high sediment flux severe problems such as cavitation and abrasion damages are expected, particularly in sediment bypass tunnels. Given the fact that sedimentation affects the majority of worldwide reservoirs, sediment bypass tunnels as a countermeasure become more and more important. These are an effective means to decrease reservoir sedimentation by bypassing the sediments into the dam tailwater during floods. However, hydro-abrasion on tunnel inverts causes significant annual maintenance cost. To enhance the use of sediment bypass tunnels as an effective countermeasure against reservoir sedimentation, the presented research project was initiated at VAW in early 2011. The main goal is to establish general design criteria for optimal hydraulic conditions to avoid sediment depositions in the tunnel and to keep the invert abrasion damages occurring in most existing tunnels at a minimum. The project is divided in three single test series investigating the (1) mean and turbulent flow characteristics, (2) particle motion, and (3) invert abrasion caused by sediment transport. Besides new insights in the three listed topics, paramount interest is given to their interrelations. Knowledge of the flow characteristics is indispensable to analyze the particle motion and to amplify the invert abrasion phenomena in super critical open-channel flows.
