Search

This article aims to provide a comprehensive appraisal on the current status of three-dimensional (3D) numerical modeling of local scour around instream hydraulic structures. Many computer models have been developed or are in development to simulate riverbed morphology. Local scour, however, is a special category that requires in general a 3D model based on some averaged/filtered form of the Navier-Stokes (NS) equations and a 3D sediment transport model. Only limited number of 3D scour codes are available for practical usage; there is also a lack of literature on the limitations of the models in capturing the scour-producing flow and the sediment transport solvers embedded within. For 3D local scour modeling, the main intrinsic uncertainty lies primarily in the sediment part. In this article, the basic theory and the state of the art of current 3D local scour models are reviewed, focusing primarily on the sediment solver, and future research needs are discussed. [ABSTRACT FROM AUTHOR]

Experimental sediment transport and river morphologic studies in laboratory flumes can use two sediment-supply methods: an imposed feed at the upstream end, or a recirculation of sediment from the downstream end to the upstream end. These methods generally produce similar equilibrium bed morphology, but temporal evolution can differ. The adjustment of natural rivers may be reproduced by both modes. Nevertheless, computer models of river morphodynamics typically use a sediment-feed boundary condition, which can impact the simulated evolution of transient features such as bedforms. The effect of sediment transport boundary conditions on bedform dynamics was analyzed through numerical experiments using a two-dimensional, depth-averaged sediment transport model. Two different boundary conditions were imposed at the inlet (constant sediment feed and sediment recirculated from the outlet) for two bedform scales (dunes and bars). The type of sediment transport boundary condition greatly influenced dune development. The sediment-recirculating condition produced a more dynamic bed morphology with dunes of higher amplitude. The associated zones of higher shear stress had a direct impact on the hydrodynamics and patterns of sediment transport. In the case of the bar bed morphology, the simulated bars had similar mean length and height for both sediment boundary conditions. However, the sediment-recirculating case produced a more dynamic bed, in which the dominant bar length varied over time. Finally, the simulated bed morphology with bars, agreed much better with that observed when using a calibrated sediment transport equation to match sediment discharges instead of the standard empirical sediment transport equations available in literature. [ABSTRACT FROM AUTHOR]

A numerical model for simulation of embankment dams' gradual breaching because of overtopping, named embankments breach software (EBS), is introduced in this paper. The model solves the equations for the depth-averaged flow (modified for shallow water flow on steep slopes), the turbulence model, the sediment transport, and the bed elevation change in a coupled manner. An upwind cell center finite volume method is applied for developing the discrete form of the differential equations. A linearized scheme with second-order accuracy in space and explicit time integration on unstructured triangular meshes is utilized in this model. To simulate the flow over dry sloping surfaces of the embankment dam configuration, the developed model considers a powerful wetting and drying technique. Furthermore, the abilities of the model enhanced simulation of sudden bank failure of the breach channel because of the base erosion and transport of the failed solid material. [ABSTRACT FROM AUTHOR]

A three-dimensional (3D) numerical model based on unstructured grids has been used to study flow and bed evolution in a laboratory meandering channel bend. The model employs the Reynolds-averaged Navier-Stokes equations and the low-Reynolds-number turbulence model for flow simulation. The model incorporates the suspended-load transport module and the bed-load transport module to simulate sediment transport and employs the sediment continuity equation to calculate channel bed evolution. The numerical simulation is performed in a meandering channel with a movable bed, in which the suspended load is comparable to the bed load. The model captures the main features of the bathymetry at equilibrium conditions. The streamwise velocity profiles and the suspended sediment concentration profiles at equilibrium conditions are also predicted by this simulation, and the agreement with the measured data is generally reasonable. The influence of including the suspended load is revealed by the comparison between the results obtained with total-load model and bed-load model in the meandering channel. [ABSTRACT FROM AUTHOR]

The major obstacles to simulating flood flow in the Yellow River are its high sediment concentration, complex compound cross section, and rapid change in channel planform. This paper presents an improved one-dimensional numerical model that takes into account the effect of sediment concentration and bed change on mass and momentum conservation of flood flow in the Yellow River. The model is calibrated and then validated by simulating three individual flood events. Results show that an increase in sediment concentration leads to a reduction in flood wave celerity and peak discharge. The generalized likelihood uncertainty estimation (GLUE) method is used to evaluate the uncertainty of modeling results. A sensitivity index, analogous to the Nash-Sutcliffe efficiency factor, is adopted to quantify the sensitivity of calibration parameters. The modeling results are sensitive to the choice of Manning's roughness coefficient and the empirical recovery coefficient for suspended sediment transport at reaches of transitional channel planform. [ABSTRACT FROM AUTHOR]

Results from a numerical modeling study are presented to investigate the tidal elevations, tidal current velocity, bed deformation, and suspended sediment concentration in the Qiantang Estuary, China. The Qiantang Estuary is well-known for its macrotide, which generates a hydrodynamically complex environment. This presents challenges for numerical modelers to accurately simulate the flow field and sediment transport in the region. This paper presents a mathematical model using finite volume method with unstructured mesh to simulate the tide-induced water elevation, current velocity, bed deformation, and suspended sediment transport in the Qiantang Estuary. The parameters in the model were determined using the long-term observed field data of the Qiantang Estuary. The simulated tidal elevations and current velocities agree well with the field observations. The numerical prediction of the bed deformation in 5 months is reasonably compared with the field measurements carried out in the same period. However, relatively large deviations exist between the simulated and observed suspended sediment concentrations, which are discussed in this paper. [ABSTRACT FROM AUTHOR]

A stable and well-balanced finite volume method was proposed to solve the shallow water equations coupled with a sediment transport equation. An approximate Riemann solver with a monotone upstream-centered schemes for conservation laws (MUSCL) reconstruction was used for the computation of the flux terms. A special but relatively simple treatment of source terms made it possible to ensure numerically and physically balanced computational results on very steep sloped beds. Various numerical simulations conducted on rigid beds have demonstrated that the proposed numerical scheme could provide stable computational results in the cases of flat and nonflat bottoms. From the comparisons of experiments on erodible beds in one- and two-dimensional channels, good agreement was obtained when proper parameters for turbulent flow and sediment transport were provided. Lastly, the effects of the parameters in relation to hydraulic characteristics included in the flow and the sediment transport models were investigated and briefly discussed. [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]

A numerical model for Type I circular sedimentation tanks (center feed) has been developed to provide useful information for circular tank operation. The unsteady flow process in circular sedimentation tanks is divided by numerous time intervals in which flow and sediment transport are considered to be steady. The other feature for this model is that particle size distribution of raw water is nonuniform. A numerical experiment based on the proposed model is carried out, and the results were compared with results from other models. The comparison shows that this model can provide more information, such as variations of water elevation, overall removal efficiency, bottom sludge thickness, and particle size distribution at outlet. In addition, this model had the advantage of optimizing the tank dimensions based on the sludge raking frequency and preferred removal efficiency. After simplification of the model, this model is practical in determining the overall removal efficiency and dimension of the tank based on different inflow situations for tank design. [ABSTRACT FROM AUTHOR]

Numerical simulations of free-surface flows are important to provide a prediction tool for the optimal management of irrigation canals. Here, we consider an alternative to solving the shallow-water equations. We propose a free-surface model in which the vertical component of the water current is fully resolved. We believe that such a detailed description can be useful to model the flow around gates or in other situations where the vertical structure of the flow will be important such as in the case of sediment transport and deposition. Our approach is based on a two-fluid lattice Boltzmann model. We compare the predictions obtained from numerical simulation and experiments performed on a laboratory microcanal facility. [ABSTRACT FROM AUTHOR]

Incision and the ensuing widening of alluvial stream channels represent important forms of channel adjustment. Two accompanying papers have presented a robust computational model for simulating the long-term evolution of incised and restored or rehabilitated stream corridors. This work reports on applications of the model to two incised streams in northern Mississippi, James Creek, and the Yalobusha River, to assess: (1) its capability to simulate the temporal progression of incised streams through the different stages of channel evolution; and (2) model performance when available input data regarding channel geometry and physical properties of channel boundary materials are limited (in the case of James Creek). Model results show that temporal changes in channel geometry are satisfactorily simulated. The mean absolute deviation (MAD) between observed and simulated changes in thalweg elevations is 0.16 m for the Yalobusha River and 0.57 m for James Creek, which is approximately 8.1 and 23% of the average degradation of the respective streams. The MAD between observed and simulated changes in channel top width is 5.7% of the channel top width along the Yalobusha River and 31% of the channel top width along James Creek. The larger discrepancies for James Creek are mainly due to unknown initial channel geometry along its upper part. The model applications also emphasize the importance of accurate characterization of channel boundary materials and geometry. [ABSTRACT FROM AUTHOR]

The article intends to examine the developmental stages of one-dimensional, two-dimensional, and three-dimensional models and elucidate their key applications, strength, as well as limitations. It also offers perception about future trends and needs concerning hydrodynamic and sediment transport models. It also concentrates on multidimensional computational models which are the two- and three-dimensional models. It also highlights an overview of the one-dimensional models to provide a comparison of the one-dimensional model features with the main features of the two- and three-dimensional models.

A two-dimensional dynamical model is established to simulate current circulation and sediment transport induced by storm surges in a schematic harbor. The model accounts for both settling and resuspension processes of cohesive fine sediments in a salty water environment. The numerical results demonstrate that the current follows wind albeit with a time lag, and so does sediment concentration. The time lag between current and wind is of the order of interval needed for wind to transfer momentum to the whole water body. Whereas, the retarded dynamic balance of sediment entrainment and settling fluxes is responsible for the time lag between sediment concentration and current. When a tropical cyclone, or conventionally so-called typhoon in the Pacific area, is passing through the harbor, the higher sediment concentration inside the harbor is found to the right of the path rather than to the left of it. Bed deformation is found to be substantially influenced by typhoon tracks relative to the harbor. For a harbor facing east, a northward typhoon results in erosion and the southward or westward one causes deposition. Both erosion and deposition occur along the coast with little bedform variations near the harbor center. [ABSTRACT FROM AUTHOR]