Development and validation of a three-dimensional morphological model

A depth-averaged two-dimensional (2D) numerical model for unsteady flow and nonuniform sediment transport in open channels is established using the finite volume method on a nonstaggered, curvilinear grid. The 2D shallow water equations are solved by the \ISIMPLE(C)\N algorithms with the Rhie and Chow’s momentum interpolation technique. The proposed sediment transport model adopts a nonequilibrium approach for nonuniform total-load sediment transport. The bed load and suspended load are calculated separately or jointly according to sediment transport mode. The sediment transport capacity is determined by four formulas which are capable of accounting for the hiding and exposure effects among different size classes. An empirical formula is proposed to consider the effects of the gravity on the sediment transport capacity and the bed-load movement direction in channels with steep slopes. Flow and sediment transport are simulated in a decoupled manner, but the sediment module adopts a coupling procedure for the computations of sediment transport, bed change, and bed material sorting. The model has been tested against several experimental and field cases, showing good agreement between the simulated results and measured data.

Depth-Averaged Two-Dimensional Numerical Modeling of Unsteady Flow and Nonuniform Sediment Transport in Open Channels

The use of computational models for solving sediment transport and fate problems is relatively recent compared with the use of physical models. Several considerations govern the choice between physical and computational models; namely, the nature of the problem that needs to be solved, the available resources, and the overall cost associated with the problem solution. In some specific problems, a combination of physical and computational models can be used to obtain a better understanding of the processes under investigation de Vries 1973 . Using computational hydrodynamic/sediment transport models, in general, involves the numerical solution of one or more of the governing differential equations of continuity, momentum, and energy of fluid, along with the differential equation for sediment continuity. An advantage of computational models is that they can be adapted to different physical domains more easily than physical models, which are typically constructed to represent site-specific conditions. Another advantage of computational models is that they are not subject to distortion effects of physical models when a solution can be obtained for the same flow conditions identical Reynolds and Froude numbers, same length scale in the three directions, etc. as those present in the field. With the rapid developments in numerical methods for fluid mechanics, computational modeling has become an attractive tool for studying flow/sediment transport and associated pollutant fate processes in such different environments as rivers, lakes, and coastal areas. Representative processes in these environments include bed aggradation and degradation, bank failure, local scour around structures, formation of river bends, fining, coarsening and armoring of streambeds, transport of point source and nonpoint

/pdf/sediment-transport-modeling-review-current-and-future-94gl15yhq8.pdf

Sediment Transport Modeling Review—Current and Future Developments

Numerical modeling of flow and scour below a pipeline in currents: Part II. Scour simulation

A large-eddy simulation has been performed to study the detailed sediment dynamics that occur in channel flow over ripples. The code solves the Navier-Stokes equations and an advection-diffusion equation for the sediment, and has been verified to produce quantitatively accurate hydrodynamic results for the flow domains used in this study. The main features of interest in this flow are seen because of the high resolution of the grid, especially near the ripple boundary. This yields a detailed bottom shear stress distribution, properly identified coherent structures, and resolved sharp gradients in the sediment concentration. Globally, the sediment is carried up into the flow from locations where the shear stress is high—on the upslopes of ripple crests—and is advected downstream by the current and upward by the vertical velocity component. Regions of positive vertical velocity are associated with Gortler vortices, which represent a significant sediment transport mechanism in this flow.

Large-Eddy Simulation of Sediment Transport: Currents over Ripples

The prediction of sediment erosion and deposition magnitudes has challenged many investigators. One- and two-dimensional sediment transport models are commonly used for predicting sediment scour and deposition. They are adequate for many engineering applications; however, for some problems, only a three-dimensional model should be used. A three-dimensional model is necessary for predicting sediment erosion and deposition whenever significant secondary currents exist, such as in river bends, crossings, distributaries, or diversions. In diversions, for example, the sediment load may not split in the same proportions as the flow. A three-dimensional model can be used to estimate the sediment split. This paper presents a description of the three-dimensional numerical model CH3D-SED and the governing equations for the model. The validation of the model and its application to the Deep Draft Navigation Project on the lower Mississippi River also are presented.

Application of 3d mobile bed, hydrodynamic model

A new mathematical formulation is established for two-dimensional (plan view) unsteady water flow, sediment transport, and bed evolution in natural watercourses such as reservoirs, estuaries, coastal water where depth averaging is appropriate. Sediment mixtures in natural watercourses are represented through a suitable, and unlimited, number of discrete size classes, any of which may be subject to either suspended-load or bedload transport (or both) depending on prevailing local hydraulic conditions. Governing equations for water and sediment are transformed (completely) into curvilinear coordinates and then depth-averaged, when appropriate. The system of governing sediment equations at a computational point is solved in a new implicit, finite-difference, fully coupled manner. Furthermore, the sediment equations are coupled with the water flow in an iterative manner in order to account for the feedback between the flow field and changes in both bed elevation and bed-surface size distribution. The mathematical formulation and associated numerical solution, incorporated into the new MOBED2 computational code, are tested through simulation of mobile-bed dynamics in the Coralville Reservoir on the Iowa River, Iowa. Initial proof-of-concept tests confirm the feasibility of the new approach by reproducing the proper qualitative behavior of the overall modeling concept.

2‐D Bed Evolution in Natural Watercourses—New Simulation Approach

: This report describes the theoretical principles of three-dimensional sediment transport and bed evolution processes and numerical solution of the appropriate governing equations. It also includes technical documentation and user's instructions for the sediment-operations program module developed as an integral part of the CH3D-WES code. The generalized CH3D-WES code provides numerical simulation of three-dimensional unsteady water flow, sediment transport, and bed evolution in natural watercourses. Sediment mixtures are represented through a suitable and unlimited number of discrete size classes, any of which may be subject to either suspended load or bed load transport (or both) depending on prevailing local hydrodynamic conditions. The governing dimensionless sediment equations are SIGMA-stretched in the vertical direction and transformed into general (nonorthogonal) curvilinear coordinates in the other two directions. The generalized CH3D-WES code includes the feedback between the flow field and changes in bed elevation, bed-surface size distribution, and the density of the mixture containing water and suspended sediment. This report contains detailed descriptions of the sediment-operations program module, memory and time requirements, and an input-data guide for the module. The new mobile-bed numerical procedures are tested by applying the CH3D- WES code with its new sediment-operations program module to a natural prototype case: the Mississippi River at the Old River Control Structure complex. Alluvial channels, Sedimentation, Bed load transport, Suspended sediment transport, Numerical modeling.

/pdf/three-dimensional-numerical-simulation-of-mobile-bed-3kipukdmuq.pdf

Three-dimensional numerical simulation of mobile-bed hydrodynamics

This work presents the development and calibration of a two dimensional river flow, sediment transport and bed evolution model that couples the active-layer and multiple size-class approach for sediment transport modeling with an enhanced advection algorithm. The governing equations are solved using the fractional step method. This resulted in three successive steps (advection, diffusion and continuity) for the flow equations, and two steps (advection and diffusion) for the sediment transport and bed evolution equations. The focus of the research is the improvement of the advection computation (both in water and sediment computation) by reducing the limitation it imposes on the time step. Overcoming this restriction is enabled by introducing modifications to the characteristic method. Implementing these adjustments allows the characteristic curve to extend throughout multiple computational cells. After evaluating the advection computation for both flow and sediment transport by comparison of the proposed algorithm with various classical methods, the developed model is assessed using field measurements conducted on the Danube River. Analysis of the measured and computed values confirmed the developed model’s reliability.

Miodrag Spasojevic

Papers

Application of 3d mobile bed, hydrodynamic model

2‐D Bed Evolution in Natural Watercourses—New Simulation Approach

Three-dimensional numerical simulation of mobile-bed hydrodynamics

Two dimensional river flow and sediment transport model

Two- and Three-Dimensional Numerical Simulation of Mobile-Bed Hydrodynamics and Sedimentation