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

Abstract

Type of problem 2-D (depthaveraged) 3-D required ? Hydrostatic assumption in 3-D? Unsteady flow capability required? Sediment mixture capability required? Distinct treatment of bedload/ suspended load processes? Grid requirements Turbulence model requirements Bed layering capability required? Required Generally not acceptable, since vertical Generally not necessary Required unless sediments are Required in most alluvial rivers Unstructured grid usually High level turbulence modeling Generally not necessary unless Spaso (2001 and M 12 deposition patterns, as input to the design process. Boundary effects are ubiquitous, and are naturally accommodated by three-dimensional modeling. Unless there are strong vertical accelerations near the inlet or the outlet, the hydrostatic pressure assumption may be adequate. Unsteady flow dynamics are generally not relevant for continuous-flow sedimentation basins, so steady-flow models to determine sedimentation rates may be quite appropriate. Unless the inflowing sediment is truly of uniform size, it is generally necessary that the modeling accommodate differential particle sizes, especially since this can have a direct bearing on the longitudinal deposition patterns in the sedimentation basin. To the extent that re-entrainment of deposited sediments in the basin is not an issue, it may not be necessary for the model to accommodate bedload processes and their exchanges with the water column. However if possible re-entrainment near the outlet is under study, it may be necessary to include a full representation of bedload dynamics and exchange with the water column. Since settling basins tend to have regular geometric shapes, a simple Cartesian structured grid may be sufficient. Since the diffusive transport of suspended sediments entering the basin can be an important factor in its design, it is important for the model to include at least a one-equation model for turbulence in the horizontal plane. Bed layering is of importance only if sediment reentrainment in flushing operations is anticipated, and then only if significant stratification of sediment sizes is expected. An example of a model study of sedimentation basins is that of Olsen and Skoglund (1994). 15.2.4 Riverbend dynamics and training works Three-dimensional modeling must be used for the study of mobile-bed processes in riverbends and around their associated training works (bendway weirs, spur dikes, etc.). One-dimensional models simply cannot resolve the detailed interaction between flow and sediment within the cross section. Two-dimensional depth-averaged models cannot normally resolve the secondary currents that are an essential part of this process.

A Model Study of the Estuarine Turbidity Maximum along the Main Channel of the Upper Chesapeake Bay

“…The mass-balance equations are identical to the salt-balance equation in the CH3D-WES model, except for the terms representing settling in the water column and erosion/deposition at the water-bed interface. The mass-balance equation for any size class of suspended sediment in a generalized curvilinear coordinate can be found in Spasojevic and Holly (1994).…”

“…With various processes affecting the ETM, a numerical model is a useful tool to study the characteristics of the ETM. Models ranging from relatively simple tidal average box models (Officer and Nichols 1980) to highly sophisticated, three-dimensional, intratidal models (Spasojevic and Holly 1994) have been employed to study the ETM.…”

CFD modeling of turbidity current deposition

“…SSIIM is one of several CFD tools with the capability to model sediment transport in three dimensions, among them: ECOMSED (Blumberg and Mellor, 1987), EFDC3D (Hamrick, 1992), and CH3D-SED (Spasojevic and Holly, 1994). Readers are referred to Papanicolau (2008) for a complete review of available sediment transport modeling software.…”

Modelling sediment transport over partially non‐erodible bottoms

“…Nevertheless, in contrast with the shallow water equations model well established for the water flow, no fully satisfactory theory has arisen so far to describe the evolution of the bed profile [3]. Indeed, the differences in shapes and sizes of sediments in a specific river, give rise to complex stochastic sediment motion, which remains complicated to describe by a simple model.…”