D-R-A-F-T
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no longer valid.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
hot start files requested on the $L cards will be saved, so that the user may generate a
new hydrodynamic flow field and then "hot start" the sediment simulation from the
point when the depth tolerance was exceeded. For greatest accuracy, make
DEPLIMIT as small as possible and generate new flow fields often.
Geometry cards: GE, GNN, GT, GS, GC, DM
The GE card permits the user to put in an element connection table. The GNN card
permits the user to assign the (x,y) coordinate, and z-elevation of a node. The GT
permits the user to assign or modify an element's material type number. The GS card
allows the user to scale the coordinates provided in the GFGEN binary file by a
constant value. The GC card allows the user to specify lists of nodes that define
channel cross-sections. The GC cards are useful for specifying boundary conditions
along cross-sections that lie on the mesh boundary. The DM cards are used to
specify the marsh porosity parameters, using the same format as RMA2. However,
the user must remember to convert the first three variables if the units are changes
between RMA2 and SED2D WES.
Timing and run length control: TZ card
The TZ card specifies the computational interval and number of time - steps to be
run. Choice of a computational interval is dependent on the size of mesh cells used,
speed of the flow, effective settling velocity of the sediment, and how well the
modeler wishes to resolve small - scale bed features. It is recommended that the
time interval for SED2D WES be identical to the RMA2-WES time interval. To
obtain the number of time-steps needed to reach a given length of run, use the
equation
RunLength
# TimeSteps =
+1
ComputationalInterval
Implicitness factor: TT card
The program uses the Crank Nicholson time - stepping scheme that employs an
implicitness factor. A value of 0.66 is recommended, but variations from 0.5 (equal
weighting of this time - step and the previous time - step) to l.0 (no influence from
the previous time - step) are permitted. A higher value of Theta produces results that
are more stable but numerical (artificial) dispersion of sediment is increased.
56 Run Control
Users Guide To SED2D-WES
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