D-R-A-F-T
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 hot start the run that simulates the period of
interest.
Boundary conditions: BC cards
BC cards are used to prescribe concentrations at the water boundaries 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
throughout the area of interest, producing erroneous results. The boundaries should
be sufficiently removed from the problem area and an attempt should be made to
adjust boundary concentrations that are seriously different from near-equilibrium
conditions. This process does not apply to cohesive sediments.
For boundaries at which there is always flow out of the model, for example, a
downstream section in a nontidal river, boundary concentrations can be left
unspecified, and the program will calculate the out flowing concentrations.
However, invoking boundary condition buffering for tidal situations is controlled by
the variables IBCFACT and BCFACT on the BC card.
Description of Output:
Output from a model run consists of summaries of input data, computed parameters,
and computed results. Input data summaries include an echo of all card image input
data and a tabulation of options and sediment characteristics that have been chosen.
A number of data set codes are output that are of use primarily in debugging. A
listing of program dimensions is provided and data management system banners
from input files (geometry, hydrodynamics, and hot start data) are printed. These
summaries should be carefully reviewed to ensure that input data were correctly
specified and interpreted by the program.
At selected time-steps, some results and some associated parameters are printed for
selected elements, if requested on the TRE card. Standard results output includes
suspended sediment concentration in kilograms per cubic meter at the nodes, flow
speed in meters per second; water depth in meters; total bed change in meters from
the start of the run; volume of bed change in cubic meters for the elements; and net
bed change (algebraic sum) and gross bed change (sum of absolute values) in cubic
meters over the entire mesh to that point in the run.
A number of specialized output results are available through the trace printout (TR)
cards. Most are detailed listings of the input data or parameters calculated from
input data.
Run Control 61
Users Guide To SED2D-WES
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