An experimental investigation of supercritical uniform and gradually varied open channel flows is presented for a wide range of Froude numbers and flume width-to-flow depth aspect ratios. The instantaneous streamwise and vertical flow velocities were measured in a laboratory flume over the entire width using a two dimensional–laser Doppler anemometry (2D-LDA) system to determine turbulence intensities, and bed and Reynolds shear stresses. The mean velocity patterns show undulation across the flume, indicating the presence of counterrotating secondary current cells. These currents redistribute turbulence intensities and bed and Reynolds shear stresses across the flume. For aspect ratios ≤ 4−5, i.e., narrow open channel flow, the velocity-dip phenomenon is identified both in the streamwise velocity and the Reynolds shear stress distributions. For high aspect ratios, i.e., wide open channel flow, the strength of secondary currents diminish toward the flume center, resulting in a 2D flow farther away from the walls and no velocity-dip phenomenon. Froude number effects on the flow characteristics are less pronounced compared to the aspect ratio effects. At high Froude numbers, the results for narrow and wide open channel flows agree well with literature data. The log-law holds in the inner region across the entire flume width for all investigated Froude numbers and aspect ratios. The Reynolds shear stress distribution agrees well with the computed spanwise bed shear stress distribution. At the flume side walls, the bed shear stresses are 20–50 % higher than the mean values. These results are verified with an engineering example in which high sediment transport and corresponding deep abrasion patterns at the side walls were observed.
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.
Worldwide, a large number of reservoirs impounded by dams are rapidly filling up with sediments. As on a global level the loss of reservoir volume due to sedimentation increases faster than the creation of new storage volume, the sustainability of reservoirs may be questioned if no countermeasures are taken. This paper gives an overview of the amount and the processes of reservoir sedimentation and its impact on dams and reservoirs. Furthermore, sediment bypass tunnels as a countermeasure for small to medium sized reservoirs are discussed with their pros and cons. The issue of hydroabrasion is highlighted, and the main design features to be applied for sediment bypass tunnels are given.
Single glass sphere motion recordings were conducted in a transitional-rough bed open channel at steady and highly supercritical flow similar to hydraulic conditions in sediment bypass tunnels. A high speed camera with a maximum resolution of 2,560 × 2,160 pixels was used to record the movement of bedload particles with diameters of D = 5.3, 10.3 and 17.5 mm. An in-house developed Particle Tracking Velocimetry (PTV) program was used to determine the transport mode and velocities of each particle for a wide range of Froude numbers up to Fo = 6. The relative roughness defined as the ratio of the bed roughness height ks to the water depth h varied from ks/h = 0.02–0.03. Particles were observed to move in rolling and saltation modes depending on the Shields number. The particle velocity shows a linearly increasing relationship with both friction velocity and Froude number nearly independent on the particle diameter. A linear relationship was also found between rolling and saltating particle velocities indicating that particle velocity does not depend on the transport mode in the range of the investigated hydraulic conditions. Scaling of particle velocity with the wave celerity plotted as a function of the Froude number adequately merged external data sets with the present data. As a consequence, a linear fit for a large Froude number range was obtained.