Soil slides can occur when the water level in a lake or a reservoir is lowered. This may take place in situations when a reservoir is flushed to remove sediments. The current study describes a ...three-dimensional numerical model used for the simulation of reservoir flushing that includes the slide movements. The geotechnical failure algorithms start with modelling the groundwater levels at the banks of the reservoir. A limit equilibrium approach is further used to find the location of the slides. The actual movement of the sediments is computed by assuming the soil to be a viscous liquid and by solving the Navier–Stokes equations. The resulting bed elevation changes from the slides are computed in adaptive grids that change as a function of water level, bed erosion and slide movements. The numerical model is tested on the Bodendorf reservoir in Austria, where field measurements are available of the bank elevations before and after a flushing operation. The results from the numerical simulations are compared with these observations. A parameter test shows that the results are very sensitive to the cohesion and less sensitive to the
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Purpose
The purpose of the current study was to present a 3D computational fluid dynamics (CFD) model that can be used to predict long-term (11 years) bed changes in a reservoir due to sedimentation ...and dredging and that can be done with a reasonable computational time (18 h) on a desktop computer.
Materials and methods
The numerical model solved the Navier-Stokes equations on a 3D non-orthogonal unstructured grid to find the water velocities and turbulence. The convection-diffusion equation for suspended sediment transport was solved to find the sediment deposition pattern. Bed changes were computed and used to adjust the grid over time. Thereby, bed elevations over time were computed. The effect of dredging was also included in the model, and how this affected the bed elevation changes. The main feature of the numerical model enabling a reasonable computational time was implicit numerical methods giving the possibility to use long time steps.
Results and discussion
The results were compared with annually measured bed elevation changes in the reservoir over 11 years. This gave 11 figures of bed elevation changes, due to the combined effect of sedimentation and dredging. Comparing the annually computed and measured bed changes, there was a fair agreement for most of the years. The main deposition patterns were reproduced. The amount of sediments removed in three dredging campaigns were also computed numerically and compared with the measured values. Parameter tests were done for the grid size, fall velocity of the sediments, cohesion, and sediment transport formula. The deviation between computed and measured dredged sediment volumes was less than 16% for all these four parameters/formulas.
Conclusions
The 3D CFD numerical model was able to compute water flow, sediment transport, and bed elevation changes in a hydropower reservoir over a time period of 11 years. Field measurements showed reasonable agreement with the computed bed elevation changes. The results were most sensitive to the sediment particle fall velocity and cohesion of the bed material.
A numerical model was used to compute the formation of a braided channel system. The model calculated the water flow field from the fully 3D Navier-Stokes equations on a non-orthogonal unstructured ...adaptive grid. The sediment transport was computed from the Engelund-Hansen formula. A free surface algorithm based on local pressure gradients was used. The model was applied to an idealized geometry of an initially straight alluvial channel, where the evolution of the braided channel system over time was computed. The complex processes and geometry for this case made it very well suited for testing the numerical model. The purpose of the study was also to explain avulsion processes of a braided river in more detail. Figures are presented with water depth, velocity, water level and secondary currents during an avulsion. The effect of the water level changes and the secondary currents are shown. The geometry, sediment size and water discharge used in the numerical model was identical to a laboratory study. Reasonable agreement was found when comparing the active braiding intensity (BIA) computed by the numerical model with measurements from the flume experiment. Parameter tests include sediment transport formula, grid size, secondary current damping and grid parameters related to wetting/drying. The results using the Engelund-Hansen formula show a higher degree of braiding than the van Rijn or Mayer-Peter Müller formula. The secondary current strength is also shown to be very important for the braiding process and the BIA values.
•3D CFD model computes formation of a braided channel system from an initially straight channel.•Results and active braiding intensity compares well with results from a physical model study.•Avulsion process explained in details with images that include secondary currents•Engelund-Hansen formula gives more braiding than van Rijns formula or Mayer-Peter Müllers formula.
► A 3D RANS model was used to compute the suspended sediment distribution in a reservoir. ► The study includes 3D measurements of the sediment distribution using a LISST-SL device. ► The measurements ...were used to validate the numerical model. ► The morphological bed level changes during an entire operation year were simulated. ► The simulations showed good agreement with the measured deposition pattern.
The three-dimensional numerical model SSIIM was used to compute suspended sediment distribution and deposition pattern in a hydropower reservoir. The study also included three-dimensional measurements of suspended sediments in the reservoir using the LISST-SL instrument. The measurement device is based on a laser-diffraction method and measures concentrations and grain size distributions instantly. It was applied to 25 locations in the reservoir where vertical profiles were taken. The measurements and computed results were compared and reasonable agreement was found. In addition, computed bed elevation changes were compared with measured values in the conducted study. The results of the numerical model agree well with the bed levels taken by echo sounding.
The numerical model SSIIM solves the Reynolds-averaged Navier–Stokes equations in three dimensions and uses an adaptive grid which moves in accordance to changes in the water and bed levels. The suspended sediment transport is calculated by solving the convection–diffusion equation and the bed load transport by an empirical formula. The used implicit free-water surface algorithm provides the possibility of using large time step sizes, which makes a simulation of an operation year on a desktop PC possible.
The dependency of the friction factor on the flow direction was investigated experimentally in a milled scale model of an unlined rock blasted tunnel under pressurized flow conditions by reversing ...the flow direction. The experimental data were used to highlight the significance of anisotropic roughness structures and variations in the cross-sectional area on the flow resistance. It is hypothesized that local sudden expansions of the cross-sectional area, which are followed by gradual contractions, contribute significantly to the directional dependency of the friction factor due to potential flow separation. For the reversed case, i.e. when sudden large-scale contractions were followed by gradual expansions, 15% lower friction factors were observed. The results were also used to highlight the scale dependency of these topographical features, the dependency of the friction factor from the tunnel driving direction, and to show the need for the development of methods that can be used to parameterize the directional dependency of hydraulic roughness and friction factors.
A three-dimensional computational fluid dynamics model is applied to predict local scour around an abutment in a rectangular laboratory flume. When modeling local scour, steep bed slopes up to the ...angle of repose occur. To predict the depth and the shape of the local scour correctly, the reduction of the critical shear stress due to the sloping bed must be taken into account. The focus of this study is to investigate different formulas for the threshold of noncohesive sediment motion on sloping beds. Some formulas only take the transversal angle (perpendicular to the flow direction) into account, but others also consider the longitudinal angle (streamwise direction). The numerical model solves the transient Reynolds-averaged Navier-Stokes equations in all three dimensions to compute the water flow. Sediment continuity in combination with an empirical formula is used to capture the bed load transport and the resulting bed changes. When the sloping bed exceeds the angle of repose, the bed slope is corrected with a sand-slide algorithm. The results from the numerical simulations are compared with data from physical experiments. The reduction of the bed shear stress on the sloping bed improves the results of the numerical simulation distinctly. The best results are obtained with the formulas that use both the transversal and the longitudinal angle for the reduction of the critical bed shear stress.
Celotno besedilo
Dostopno za:
DOBA, FGGLJ, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
► Spoiler baffles in culverts are the optimum solution for fish migration. ► Not only reducing the flow velocity but also providing resting areas is essential. ► CFD is a reliable tool for the design ...of new and restoration of existing culverts. ► Spoiler size depends more on the type of migrating species than on culvert size.
Upstream migration of fish through circular culverts is often prevented by velocities in the barrel being higher than that of the natural channel. In this investigation a computational fluid dynamic (CFD) model has been used to test the effects of various spoiler baffle geometries in culverts of varying size to reduce water velocity and increase water depth and thus increase the upstream passage of small fish species. Results indicated that standard baffles designed for specific fish species or groups could be successfully retrofitted to culverts of varying dimensions. Subsequent field tests have confirmed the effectiveness of the design.
The bed changes in a section of the river Danube were computed using a 3‐D computational fluid dynamics model. A time series of discharges during the flood in 2002 was used. The results compared ...reasonably well with regular bed level surveys before and after the flood. The Danube River section was 6 km long and located between Vienna and the Austrian‐Slovakian border. The fully three‐dimensional numerical model solved the Navier‐Stokes equations using the k‐epsilon turbulence closure. Nonuniform sediment transport was computed using the formulas of Wu et al. (2000b), considering hiding‐exposure algorithms. Both bed deformation and sorting processes were calculated. A number of parameter sensitivity tests were carried out on roughness values, parameters in the sediment transport capacity formula, parameters in the hiding‐exposure formulas, critical Shields number, and variations in the formulas for the effect of a sloping bed. Additionally, the sediment inflow to the section was varied together with investigations of physical parameters such as vegetation and groyne structures. The results were most sensitive to the Shields number for critical movement of the sediments. An error analysis was performed to give a quantitative assessment of the parameter sensitivity tests.
A fully three-dimensional numerical model for reservoir flushing has been tested against field measurements for the Angostura reservoir in Costa Rica. The numerical program solves the ...Reynolds-averaged Navier–Stokes (RANS) equations in three-dimensions and uses for discretization the finite-volume method together with a second-order upwind scheme. The used grid is unstructured and non-orthogonal, made of a mixture of hexahedral and tetrahedral cells. In addition to the bathymetry data of the prototype, the model uses grain size distributions of the bed, discharge rates and water levels during the flushing. Simulated bed level changes during the flushing are presented in this study as well as the computed amount of eroded sediments. Where the amount of flushed out sediments show reasonable agreement, differences in the developed flushing channel simulated by the model and compared to the prototype were observed. However, the presented study shows that due to the increasing development of three-dimensional RANS models, the simulation of a reservoir flushing in a prototype becomes feasible.
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