4/98
SED2D-WES Version 4.3 Beta
1.4
Limitations
Both clay and sand may be analyzed, but the model considers a single, effective grain size during each
run. Therefore, a separate model run is required for each effective grain size. Fall velocity must be prescribed
along with the water surface elevations, x-velocity, y-velocity, diffusion coefficients bed density, critical shear
stresses for erosion, erosion rate constants, and critical shear stress for deposition.
The program does not compute water surface elevations or velocities; these data must be provided from an
external calculation of the flow field. For most problems, a numerical model for hydrodynamic computations,
RMA2-WES, is used to generate the water surface elevations and velocities. An implicit assumption of the
SED2D-WES model is that the changes in the bed elevation due to erosion and/or deposition do not significantly
affect the flow field. When the bed change calculated by the model does become significant and the externally
calculated flow field supplied by the user is no longer valid, then the SED2D-WES run should be stopped, a new
flow field calculation should be made using the new channel bathymetry generated by SED2D-WES, and the
SED2D-WES run should be restarted with the new flow field as input.
2.
PROGRAM DESIGN
2.1
Capabilities of the Program
Either steady-state or transient flow problems can be analyzed. The exchange of material with the bed can
be calculated or suppressed. Default values may be used for many sediment characteristics or these values may be
prescribed by input data. Either the smooth wall velocity profile or the Mannings equation may be used to
calculate bed shear stress due to currents. Shear stresses for combined currents and wind waves may be calculated.
2.2
Conceptual Basis
The program is based on the following conceptual model:
a.
Basic processes in sedimentation can be grouped into erosion, entrainment, transportation, and
deposition.
b.
Flowing water has the potential to erode, entrain, and transport sediment whether or not
sediment particles are present.
c.
Sediment on the streambed will remain immobile only as long as the energy forces in the flow
field remain less than the critical shear stress threshold for erosion.
6
WORKING DRAFT
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