D-R-A-F-T
END card signals the end of the card list pertaining to a specific time step. One
END card must be supplied for every time step. The STOP card signals the end of
the card list pertaining to the entire simulation period.
The $H card is used to control HOTSTART runs of the program. In a cold start, a
model run begins fresh, not using the results of any previous run as a starting point.
In a hot start, some of the variables, such as concentration or bed thickness, begin
with values that were computed in a previous run. The files needed for hot starting a
model run are written if so requested on the $L2 card. In order to make a hot start
run, first submit a cold start model run in which bed structure and/or concentrations
and bed elevation changes are saved (see $L2 card). Then submit a run with the
appropriate hot start switches on the $H card, and specify the appropriate hot start
unit numbers on the $L1 card.
In a cold start run, all of the important processes must spin - up from an artificial
condition such as a uniform sediment concentration field. The spin - up time is the
length of time a simulation must run before the solution has recovered from the
artificial initial condition. For example, in a sand bed problem, if the initial sediment
concentrations are too low in one area the bed may erode during spin - up until
equilibrium concentration is reached. This may happen even if the prototype bed is
stable in that area. To overcome this problem, a hot start run would be made in
which concentrations are hot started from the previous run, but bed structure and bed
elevation changes are cold started.
The TO card specifies the frequency for printing the binary solution. The TR card
specifies the frequency for printing to the ASCII output file, and provides the user
with options for the volume of information to be printed. The primary purpose of the
trace printout controls is to assist in diagnosing problems with a run, but they also
provide for printing of some parameters that may be useful in interpreting model
results. If a trace printout is 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 "hot start" 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
Run Control 55
Users Guide To SED2D-WES
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