where α is solved from these reference values above and the general form is obtained
by replacing Qse and ρe with the time-varying values of each.
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.
Lateral Boundary Condition
The lateral boundary condition is applicable to inflow, outflow, or areas of flow
Inflow Boundary Conditions
Outflow Boundary Conditions
Reversal Boundary Conditions
In the case of tidally fluctuating flow across a model boundary the 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
16 Conceptual Program Design
Users Guide To SED2D-WES