4/98
SED2D-WES Version 4.3 Beta
1 2 1
2
u* =
fcu + fwuom
(10)
2
4
For further information on the shear stress computation equations, see McAnally and Thomas (1980).
When RMA-2 has used the marsh porosity option (DM cards) shear stresses should be adjusted in the SED2D-
WES simulation for more accurate estimates of the bed exchange. Therefore, the marsh porosity information must
be provided (in appropriate units) and the program will compute the needed adjustments. The adjustment is made
by computing a conveyance distribution within the marsh porosity depth distribution based on Manning's equation.
This is then extended to a shear stress distribution that is averaged and a correction factor developed for the
conventionally derived shear stress form one of the options above.
The bed source. The form of the bed source term, S = α 1C + α 2 , as given in Equation 1 is the same for
c.
deposition and erosion of both sands and clays. Methods of computing the alpha coefficients depend on the
sediment type and whether erosion or deposition is occurring.
For sand transport, the supply of sediment from the bed (i.e., the sediment reservoir) is controlled by the transport
potential of the flow and availability of material in the bed. The bed source term is
Ceq - C
S=
(11)
tc
where
S = source term
C = sediment concentration in the water column
tc = characteristic time for effecting the transition
There are many transport relations for calculating Ceq for sand size material. The Ackers-White (1973)
formula was adopted for this model because it performed satisfactorily in tests by WES and others (White, Milli,
and Crabbe 1975; Swart 1976), because it seems to be complete, and because it is reasonably simple. The transport
potential is related to sediment and flow parameters by the expressions in the following paragraphs.
11
WORKING DRAFT
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