4/98
SED2D-WES Version 4.3 Beta
larger number of layers, each having a small thickness. When a thin layer is filled up it will move down into the
bed and be given a higher layer type, and the effect of the overburden will be calculated more accurately than if a
small number of thicker layers are used.
Bed structure (SB and CL cards). The initial thickness of the sediment bed at the beginning of a run is
i.
specified on the SB (noncohesive) and CL (cohesive) cards. If that thickness is eroded, it is assumed that
nonerodible rock has been reached. The CL cards specify which layer types (CI cards) are present, the thickness of
each layer, and the age of each layer. In hotstart runs ($H card), the bed structure from a previous run is used and
information on the CL and SB cards is disregarded.
Diffusion of suspended sediment occurs because of
j.
Effective diffusion (ED, PE and DD cards).
turbulence in the flow field. When the transport equation is simplified by averaging over depth, as in SED2D-
WES, dispersion is introduced because of vertical variations in the flow field and settling of the sediment through
the water column. In practice, this effect is lumped together with turbulent diffusion and the effect of averaging in
time and the combined effect is called dispersion or effective diffusion.. In this program, these various effects are
combined in a pair of effective diffusion coefficients given on the ED card.
Selection of appropriate values for the dispersive coefficients is not a straightforward task. Elder (1959)
gave approximate expressions for longitudinal (direction of flow) turbulent diffusion coefficients as
De = 5.93 Du*
(20)
and for the transverse (perpendicular to the flow direction) diffusion coefficient as
Dt = 0.23 Du*
(21)
where
D = water depth, and
u* = shear velocity as given by equation 8.
Experimentally derived values of the constants in Equations 20 and 21 are often orders of magnitude greater than
those given. This is attributed to nonuniformity of the flow, wind effects, wave effects, and so on.
In choosing an effective diffusion coefficient to use in numerical modeling, consideration must also be
given to the mesh cell size. Exact relations are not available, but generally, larger element sizes require larger
diffusion coefficients.
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WORKING DRAFT
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