Auel, Christian
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The Solis reservoir is located in the Alps in Grisons, Switzerland and is operated by the electric power company of Zurich (ewz). Since its construction in 1986, high sediment input during flood events has led to major aggradations in the reservoir. Up to date, nearly half of the original reservoir volume has been filled with sediments from upstream mountain torrents. The deltaic deposition starts extending into the active water volume. Therefore, ewz plans a sediment bypass tunnel to flush the incoming bedload around the dam to the downstream reach. In a first step the reservoir level during flood events is lowered to the minimum operation level. The delta is subjected to free surface flow and the bedload is transported over the delta and deposited further downstream. This sediment relocation decreases the delta volume within the active storage. During further flood events, the incoming sediment is led to the bypass tunnel intake using a guiding structure and flushed through the tunnel. If the flood exceeds the capacity of the bypass tunnel, the surplus flow passes the tunnel intake towards the bottom outlets with the bedload still being flushed through the tunnel. A skimming wall located upstream from the tunnel intake prevents driftwood blocking by leading it to the reservoir front where it can be safely removed. Both the sediment relocation due to water level drawdown and the flushing through the bypass tunnel are investigated and optimized in a hydraulic model at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of ETH Zurich. Additionally, the sediment relocation process in the model is compared with a relocation test in the prototype.
Reservoir sedimentation, a serious problem affecting the majority of reservoirs worldwide, was not systematically accounted for in the past. After 50 years of operation, a constantly decreasing reservoir volume becomes currently a serious challenge for reservoir owners, against which countermeasures have to be developed. This research focuses on sediment routing using a bypass tunnel to convey sediments past a dam.
By transporting sediments into the tailwater past a dam, their accumulation in the reservoir is reduced significantly. However, the global number of sediment bypass tunnels is limited primarily due to high investment and maintenance cost. The main problem of all bypass tunnels is the massive invert abrasion due to high flow velocities combined with high sediment transport rates. Therefore, VAW started two research projects to counter this problem. The main goal of the first project Layout and design of sediment bypass tunnels is to investigate the invert abrasion process by conducting hydraulic laboratory tests and to establish general design criteria for optimal flow conditions in which both sediment depositions in the tunnel are avoided and the resulting abrasion damages are kept at a minimum. The second project Optimizing hydroabrasive-resistant materials at sediment bypass tunnels and hydraulic structures investigates the hydraulic resistance of different tunnel invert materials, such as high performance concrete or cast basalt plates in prototype tests at the Solis bypass tunnel. The sediment transport measurement technique used in this project was optimized during preliminary model tests.
Reservoir sedimentation is increasingly affecting the majority of reservoirs all over the world. As many dams are more than 50 years of age, this problem is becoming more and more seriou403s nowadays. Reservoir sedimentation leads to various severe problems such as a decisive decrease of the active reservoir volume leading to both loss of energy production and water available for water supply and irrigation. These problems will intensify in the very next future, because sediment supply tends to increase due to climate change. Therefore coun-termeasures have to be developed. They can be divided into the three main categories sediment yield reduction, sediment routing and sediment removal. This paper focuses on sediment routing by means of sediment bypass tunnels. Sediment bypass tunnels are an effective measure to stop or at least decrease the reservoir sedimentation process. By routing the sediments around the reservoir into the tailwater in case of flood events sediment accumulation of both bed load and suspended load is reduced significantly. However, the number of sediment bypass tunnels in the world is limited primarily due to high investment and above all maintenance costs. The state-of-the-art design criteria of constructing bypass tunnels are summarized herein; major problems such as tunnel invert abrasion are discussed. The need for further research regarding sediment transport in bypass tunnels and invert abrasion is highlighted.
Reservoir sedimentation is an increasing problem affecting the majority of reservoirs both in Switzerland and worldwide. As many dams are more than 50 years of age, this problem is becoming more and more serious nowadays. Res-ervoir sedimentation leads to various severe problems such as a decisive de-crease of the active reservoir volume leading to both loss of energy production and water available for water supply and irrigation. These problems will intensify in the near future, because sediment supply tends to increase due to climate change. Therefore, countermeasures have to be developed. They can be divided into the three main categories sediment yield reduction, sediment routing and sediment removal. This paper focuses on the sediment routing using sediment bypass tunnels. Sediment bypass tunnels are an effective means to decrease the reservoir sedimentation process. By routing the sediments around the reservoir into the tailwater, sediment accumulation is reduced significantly. However, the number of sediment bypass tunnels in the world is limited primarily due to high investment and maintenance costs. The main problem of all bypass tunnels is the invert abrasion due to high velocities in combination with high sediment transport. Three Swiss bypass tunnel examples suffering invert abrasion are presented in this paper. Furthermore, VAW started a research project to investigate the invert abrasion process by conducting hydraulic scale tests in the laboratory. The goal of this research project is to establish general design criteria for optimal flow con-ditions where both sediment depositions in the tunnel are avoided and the result-ing abrasion damages are kept at a minimum.
This paper deals with an experimental investigation of the mean and turbulence characteristics of supercritical quasi-uniform and gradually varied open-channel flows over a transitional rough bed. These conditions are typical for sediment bypass tunnels. The results show that the log-law holds well in the inner region across the channel. The roughness shifts the velocity profiles downward by an amount of ΔU+. The velocity-dip phenomenon and strong secondary currents exist in the channel for narrow open-channel flow. These currents cause the Reynolds shear stress distributions to deviate from the linear distribution and an undulation on the transversal distribution of the bed shear stress, which matches with the bed abrasion pattern. The streamwise turbulence intensity profiles deviate from the semi-empirical universal function whereas the vertical turbulence intensity profiles fit well with it only at the centerline of the channel. A strong wall effect exists on the turbulence intensities in the outer region.
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.
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.
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.