Hydro-abrasion processes and modelling at hydraulic structures and steep bedrock rivers
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2021
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Doctoral Thesis
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Abstract
Worldwide, reservoirs are subjected to sedimentation due to inflowing sediment. The continuous sediment accumulation negatively affects the sustainable use of single- and multi-purpose reservoirs. Sediment Bypass Tunnels (SBTs) are effective and sustainable countermeasures. They route the incoming sediment-laden flow around the dam to reduce or totally prevent reservoir sedimentation and provide sediment continuity. Due to the high sediment transport rates combined with high flow velocities, severe hydro-abrasion may occur at the SBT inverts. Similar hydro-abrasion mechanisms also occur at other hydraulic structures such as low-level outlets, sediment sluicing tunnels and channels, weirs and stilling basins, and in steep bedrock rivers and mountain torrents. Hydro-abrasion poses serious problems for the sustainable use of hydraulic structures since it puts the structural and operational safety at a risk and causes high maintenance costs.
This study aimed to advance the understanding of the abrasion mechanics and to develop a predictive, mechanistic hydro-abrasion model for both the design of sustainable hydraulic structures, and for better prediction of river & landscape evolution. The investigation is based on systematic laboratory tests and data collected from previous laboratory and recent field studies. To achieve the study goals, five research tasks were established, in which the following was experimentally investigated: (i) mean and turbulent flow characteristics in supercritical narrow open-channel flows (Task A), (ii) single particle motion (Task B), (iii) hydro abrasion depth and pattern in various flow conditions using different bed lining materials and particles (Task C). Tasks A to C were devoted to (iv) the development of a realistic-mechanistic hydro-abrasion model to predict the abrasion both in the field and at laboratory scale (Task D), and (v) recommendations for engineering application of the model (Task E). The experiments were conducted in supercritical narrow open channel flows with Froude numbers in a range of 2 ≤ Fo ≤ 5, and low width-to-depth aspect ratios 1 ≤ b/ho ≤ 4.44. In the Task C experiments, erodible foams and low-strength mortar mixtures were used to obtain measurable abrasion rates within a reasonable time. This study particularly focused on the effect of particle and bed lining material hardness, bed cover and low aspect ratio on hydro-abrasion, which were not studied and interlinked in previous studies.
The Task A results show that four well-developed large scale streamwise vortex pairs are formed in the hydraulically smooth and planar bed conditions, i.e., in absence of any bed forms across the flume. Such secondary currents re-distribute the bed shear stress, thus affecting the cross-sectional distribution of sediment transport and hydro-abrasion patterns.
In the Task B experiments, the particle transport mode, i.e., rolling, sliding and saltation, particle velocities, hop heights, and lengths were determined from the images recorded with a high-speed camera. The results show that the bed roughness is the key parameter affecting the particle saltation trajectories, while particle properties such as diameter and shape have a negligible effect. From the particle saltation trajectory data, non-dimensional equations were developed and used in the abrasion prediction model.
The hydro-abrasion experiments (Task C) reveal that the abrasion pattern depends on the aspect ratio and bed lining material homogeneity. For the foam experiments at b/ho ≤ 2, the abrasion concentrates at the flume center forming a continuous incision channel, whereas two incision channels were formed near the flume sidewalls at b/ho = 4. This outcome shows that abrasion patterns observed in the foam tests match well with the bed shear stress distributions. In mortar experiments, potholes were formed instead of the incision channels due to the non-homogeneity of the cementitious material. The Task C experiments also show that increasing particle hardness causes higher abrasion rates. Furthermore, an exponential cover effect term is the best representative of the present data relating sediment transport to abrasion rates.
In Task D, the experimental results from Task A−C were used to enhance the state-of-the art mechanistic hydro-abrasion model developed by Sklar and Dietrich (2004) by (i) replacing the original particle velocity and hop length formulae with the newly-developed equations, (ii) adding the exponential cover effect term to the model, and (iii) introducing two new terms that account for the particle hardness effect and particle saltation probability. The hydro-abrasion coefficient, kv of the enhanced abrasion model (SAMD) was calibrated by using the present data and the field data from Pfaffensprung, Runcahez, and Solis SBTs, and a constant kv = (5.0 ± 2.4) × 104 is proposed. The enhanced model was validated using independent data. The results show that SAMD predicts the hydro-abrasion rates for various bed lining materials, i.e., natural bedrocks and concretes, within an error range between +92% and −33%, which is a significant improvement over previous abrasion prediction models. The enhanced model performs well and applies for both laboratory and field applications indicating that the laboratory findings can be upscaled to the prototype scale.
Recommendations for engineering application of the present findings are given in Task E.
Overall, the present investigation contributes to the understanding of supercritical narrow open channel flows, particle motion in such high-speed flows, and hydro-abrasion mechanics. Therefore, the present research outputs can be used for the improvement of algorithms in numerical predictive models such as for landscape evolution, 3D flow and sediment transport simulations. Furthermore, they will contribute to the layout and design of sustainable hydraulic structures prone to hydro-abrasion.
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03820 - Boes, Robert / Boes, Robert
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Is original form of: https://doi.org/10.3929/ethz-b-000506311