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SED2D-WES Version 4.3 Beta
selected, it will print only at the locations specified on the TRE, TRT and TRN cards if they are present (Note: the
TRN and TRT cards are not fully operational in this version of SED2D-WES).
The EF card specifies two flags that affect how the computations proceed. The flag denoted as IHYDOPT
permits the user to allow the flow field to be "adjusted" during the run. That is, as the bed moves up and down
because of erosion or deposition, the depth of flow is adjusted at each node to maintain the same water surface
elevation and the velocities are adjusted to maintain the same unit flow at each node. This option is included to be
consistent with earlier versions of STUDH. Whether this adjustment is appropriate is a matter of significant
differences in opinion, even among the authors of this documentation.
One side of the issue maintains that this artificial adjustment of the flow field can lead to numerical
inaccuracies, and is physically unsupportable. That position highly recommends that the user choose the default
option, IHYDOPT = 0. However, by choosing IHYDOPT = 0 the user must realize that he or she is accepting the
assumption that the changes in the bed calculated by SED2D-WES are small, and they do not have a significant
impact on the hydrodynamic solution. When the bed changes become large enough that this assumption is no
longer valid, the user should stop the SED2D-WES run and generate a new hydrodynamic flow field using the new
geometry file generated by SED2D-WES (which contains the new bed elevations). The user may then "hotstart"
the sediment run using the new flow field and the initial conditions saved from the previous sediment run. This
process is repeated until the whole period of simulation has been covered.
The counter position to the above argument is that by not adjusting the flow field the model may produce
excessive erosion or deposition. As the sediment deposits, if the flow field is adjusted the velocities will increase,
increasing the shear stress and potentially reaching a level that will inhibit further deposition. If the sediment is
being eroded, then adjusting the flow field will result in lower velocity, lower shear stresses and potentially reach a
level where erosion will be inhibited.
The adjustment can provide some measure of control on excessive bed
change. Both positions on the issue agree that the user must ultimately be responsible for interpreting the model
results and insuring that when the bed changes reach a level where the hydrodynamics will be dramatically altered
that the hydrodynamics are reassessed.
The second variable on the EF card is the DEPLIMIT variable. This variable causes the sediment run to
abort when a "significant" bed change occurs at one or more nodes. That is, the user may specify a percentage of
the total water column as the characteristic length beyond which the assumption of a pre-calculated flow field is no
longer valid. The default is DEPLIMIT = 0.25, or when the bed change at a node is greater than 25% of the water
depth at that node then the program will abort. The hotstart files requested on the $L cards will be saved, so that
the user may generate a new hydrodynamic flow field and then "hotstart" the sediment simulation from the point
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WORKING DRAFT
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