4/98
SED2D-WES Version 4.3 Beta
Mass deposition rates are converted to volumetric deposits by the specified density for the type 1 layer, and erosion
rates are converted to a corresponding volume by the actual density of the eroding layer.
Use of the layer type can be used to control whether or not erosion and consolidation are allowed to occur,
and to keep track of sand layers in a mixed bed problem. The layer structure and time - varying consolidation can
be used to specify a subsidence rate for the modeled area.
In the case of tidally fluctuating flow across a model boundary the
e.
Boundary Condition Buffering.
specification of an accurate concentration is not simple. In earlier versions of STUDH and SED2D-WES the
boundary condition was either always specified or always not specified. If a node along the boundary had flow
entering the model the normal convention would be to specify a concentration. However, in older versions
when the tide turned and flow left the model that specification was still applied. This creates artificial
conditions that lead to severe oscillations near the boundary.
In the current version (4.3) this situation has been addressed in two steps. First, the logic has been added
to the code to allow the model to determine whether to apply the concentration specification (Dirichlet BC) or
whether to apply a zero concentration gradient BC (von Neuman). The gradient BC allows the concentration to be
solved from the interior concentration field of the model. This provides some relief; but strong concentration
gradients reaching the boundary can result in abrupt jumps in the concentration as the tide turns to enter the
model and the concentration returns to the Dirichlet specification. This is the result of not accounting for the
concentration history of waters that have crossed the boundary.
In order to provide a form of memory of the concentration history under dynamic tidal conditions a
method termed "boundary condition buffering" was developed. This technique assigns a finite (MBB parameter in
the program include file) number of buffer chambers to each boundary node. The program maintains the specified
nominal boundary concentration Cb in the last chamber. At the beginning of the simulation all buffer chambers
are initialized to Cb. As flow leaves the model the concentration of the exiting water is stored in the first chamber
and all remaining buffer chamber concentrations are shifted to the next higher chamber, keeping the last chamber
at Cb. Then a mixing factor is applied to the chambers to simulate the diffusive processes external to the model.
When the currents turn and begin to enter the model the chamber values are shifted back one chamber per time
step and the mixing process repeated. This procedure results in memory of the history of concentrations crossing
the boundary, delays full specification of the nominal boundary concentration Cb and generally provides more
realistic boundary conditions. Furthermore, the buffering also provides a buffer for the changes that any plan
alternatives to be tested may have on the boundary conditions.
The ability to simulate 1D elements was added to SED2D-WES.
The
f.
One-Dimensional Elements.
formulation assumes a trapezoidal cross section, consistent with the 1D formulation in RMA2.
The
16
WORKING DRAFT
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