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A new numerical model for computing the formation of sedimentary structures in a fluvial environment is presented. The model has been used to explain the strata-forming processes under antidunes in detail, including water velocities, turbulence, free surface location, erosion, sedimentation and formation of each sediment layer. The Reynolds-Averaged Navier-Stokes equations were solved together with the k-epsilon turbulence model to determine the water flow field and the bed shear stress on a width-averaged two-dimensional grid. The sediment transport was computed by solving convectiondiffusion equations for multiple sediment sizes. An empirical formula for resuspension at the bed was used, enabling the computation of erosion and deposition for each sediment fraction. Five sediment sizes were used, from 0.5 to 2.5 mm. The bed sediment stratification was recorded using a very fine grid, resolving the bedding with up to 1000 cells in the vertical direction. A novel algorithm was used to vertically adapt the bed grid, enabling the computation of very large gradients of the grain size distribution, including thin laminae. The model was tested by computing sediment deposition in a channel with a Froude number of 1.21. The hydraulic parameters and the average sediment size were selected similar to an earlier laboratory flume study where downstream-migrating antidunes were observed. The antidunes started as small bed irregularities and increased in size as they moved in the downstream direction. Recirculation zones were observed behind bedforms with large slopes on the lee side. Here, sediments deposited in slides. Smaller sediment waves moving on top of the bedforms, depositing layers of a fine and a coarse material simultaneously. After the initial phase, stationary antidunes formed in the upstream and middle part of the flume, producing wavy sedimentary structures. Sinusoidal crosslamination formed in deposits under bedforms of different downstream velocities. The antidunes in the downstream part of the flume moved in the streamwise direction. They were affected by occasional upstream-moving surface waves, causing complex sedimentary structures to form. Figures and videos show the strata-forming process in detail, including velocity vectors, turbulence and layers of different particle diameters. [ABSTRACT FROM AUTHOR]

A numerical model is presented that compute the geometrical dimensions and movement of downstream migrating antidunes. The model solves the Navier-Stokes equations together with the k-epsilon turbulence model to find the water flow field over the bedforms. A two-dimensional width-averaged grid is used. The bed elevation changes are computed by solving the convection-diffusion equation for suspended sediments and bedload, together with the Engelund-Hansen sediment transport formula. The free surface is computed with an algorithm based on water continuity in the surface cells. Non-orthogonal adaptive grids were used, moving vertically with the computed location of the bed and the free water surface. The numerical model was tested on data from a physical model study where regular downstream migrating antidunes had been observed. The numerical model started out with a flat bed and the trains of antidunes formed over time. Many of the physical processes observed in earlier studies were replicated by the numerical model. Four dune parameters were computed in the current tests: The antidune wavelength, height and celerity, together with the average water depth. The antidune wavelengths were best predicted with an accuracy of 3 to 8% compared with the measurements. The antidune heights were computed with a deviation of 11 to 25% compared with an empirical formula. The water depths over the antidunes were predicted with an accuracy of 3 to 9% related to the measured values. The average antidune celerity was the parameter with largest deviation: For the coarsest grid it was overpredicted with 37%. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Sediments filling reservoirs is a common problem in the world today, with an estimated 1% of the capacity of hydropower reservoirs being lost annually through sedimentation. One of the most used techniques for reducing this problem is reservoir flushing. During a flood, the water level is drawn down, causing increased velocities, therefore facilitating erosion and sediment transport. During the flushing, water from the reservoir will be lost, resulting in significant economic implications for the reservoir owner. The success of reservoir flushing depends on several parameters, including water discharge, sediment properties and reservoir geometry. This study describes the use of Computational Fluid Dynamics (CFD) as a modern method to predict the reservoir flushing process. A three-dimensional numerical model (SSIIM 2), with an adaptive, non-orthogonal and unstructured grid has been used. Through the application of special modified algorithms (e.g., wetting/drying, free water surface), numerical modelling of sediment movement can be an alternative for planning and optimizing the flushing process for complex reservoir geometries. The numerical model was tested against data from a physical model study of the Kali Gandaki hydropower reservoir in Nepal. The total quantity of flushed out sediments, and the bed deformation in six cross-sections, were compared, highlighting a good correspondence between the results. These include the cross-sectional shape of a 90-degree bend, for which secondary currents influenced the results. The study indicates that numerical models might become a useful tool for reservoir flushing predictions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model was used for calculating the velocity and bed level changes over time in a 90° bended channel. The numerical model solved the Reynolds-averaged Navier-Stokes equations in three dimensions to compute the water flow and used the finite-volume method as the discretization scheme. The k-[variant_greek_epsilon] model predicted the turbulence, and the SIMPLE method computed the pressure. The suspended sediment transport was calculated by solving the convection diffusion equation and the bed load transport quantity was determined with an empirical formula. The model was enhanced with relations for the movement of sediment particles on steep side slopes in river bends. Located on a transversally sloping bed, a sediment particle has a lower critical shear stress than on a flat bed. Also, the direction of its movement deviates from the direction of the shear stress near the bed. These phenomenona are considered to play an important role in the morphodynamic process in sharp channel bends. The calculated velocities as well as the bed changes over time were compared with data from a physical model study and good agreement was found. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model was applied to compute uniform and nonuniform sediment transport and bed deformation in an S-shaped laboratory channel located at the University of Innsbruck, where detailed measurements of the velocity field and bed elevation changes were made. The channel had two bends, a trapezoidal cross section, and a slope of S=0.005. Gravel with a mean diameter of 4.2 mm was used as movable bed material and for sediment feeding. Wu's formula for multiple grain sizes was compared with van Rijn's formula using one grain size. Fairly good agreement was found between the computed and measured bed elevations for both approaches, whereas Wu's formula could further improve the numerical results. Looking at the physics of the erosion pattern, the computed scour areas were located slightly more downstream than what was observed in the physical model. The current study also includes several parameter tests: grid distribution in vertical, lateral, and longitudinal direction; time step; number of inner iterations/time step; active sediment layer thickness; and the Shields coefficient. The variation of those parameters gave some differences in the results, but the overall pattern of bed elevation changes remained the same. [ABSTRACT FROM AUTHOR]

Summary: 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. [Copyright &y& Elsevier]