4/98
SED2D-WES Version 4.3 Beta
λu
De =
Pe
which provides generally for the diffusion coefficient proportional to the current velocity. However, as prescribed
on the PE card there is a minimum value of De based on a specified minimum velocity (VPEC).
Table 3 lists some previous applications and the effective diffusion coefficients that were used.
Table 3
Example Dispersion Coefficients
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Typical
Current
Typical Element
Coefficient
m 2sec
Location
Speed, mps
Size, km
Medium-size river
1 - 1.5
0.1 - 0.5
100
Open bay
0.5 - 1.0
0.75
100
Tidal river
0.2 - 1.0
0.1 - 0.3
5 - 10
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Initial concentration (IC cards). The nodal concentrations at the first time-step are specified on the IC
k.
cards, or in a file if a hotstart is used.
Depending on the length of a run, the initial concentrations can have a significant effect on the results. If
they are too high, deposition will be high for the first few time - steps. The run should be long enough to overcome
start - up anomalies. If the initial concentrations are too low, the model may artificially erode the bed until an
equilibrium concentration is reached. It is best to use field data to obtain an approximation to the actual initial
concentrations; to make a spin-up run to stabilize the concentrations (see $H card), then hotstart the run that
simulates the period of interest.
Boundary conditions (BC cards). BC cards are used to prescribe concentrations at the water boundaries
l.
of the models. Concentrations need not be specified at land boundaries. Boundary concentrations should be based
on field measurements.
If sand concentrations are too low on an incoming flow boundary, the model will erode material from the
bed (if the specified bed thickness is adequate) to transport a volume of sediment that is equal to the bed material
transport capacity. If sand concentrations are too high, the excess material will deposit, again bringing the
concentration to that needed to satisfy transport capacity. The rows of computational elements near the boundaries
will have erroneous deposition/erosion effects under these conditions.
For example, a too-high boundary
concentration will form a delta at the inflow point. If the model run is long enough, the delta will propagate
28
WORKING DRAFT
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