Bed-load particle motion in supercritical open channel flow.

Christian Auel, ISMAIL Albayrak, Robert M. Boes

Thursday 2 july 2015

9:15 - 9:30h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 10A. Sediment - Transport

Supercritical sediment-laden open-channel flows occur in spillways, dam outlets, weirs, and sediment bypass tunnels. The latter are an effective means to reduce reservoir sedimentation by bypassing the sediment into the dam tailwater during floods. Due to high flow velocities and large sediment transport severe problems as cavitation and abrasion damages are expected causing significant annual maintenance cost. To counter hydro-abrasion and thus contribute to a sustainable use of bypass tunnels, a research project was initiated at VAW. Its main goals are to study and analyze: (1) mean and turbulence characteristics of supercritical open-channel flows, (2) sediment transport modes, i.e. rolling, saltation, or suspension under different flow conditions, and (3) relationship between the transport modes and rates, and the invert abrasion depth. The results of selected experiments of (2) are presented in this paper. Experiments were conducted in a 13.50 m long glass- and PVC-sided concrete-lined tilting flume. In total 264 tests were performed at various discharges, flow depths, bottom slopes, and particle diameters. Sediment motion was recorded using a High Speed Camera system with a frame rate of 240 fps. The image resolution was 2,560 x 400 pixels corresponding to a recorded area of 1,100 x 200 mm. Image processing was done by a self-developed Matlab Code. All three particle motion regimes were observed and quantified depending on the hydraulic flow conditions. In addition to particle mean velocities, hop heights and lengths, the particle impact velocities at bed collision as a main result were determined leading to corresponding impact energies. These findings will be used to define a correlation with the measured abrasion pattern obtained from test series (3).