Bed structure: SB and CL cards
The initial thickness of the sediment bed at the beginning of a run is 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.
Effective diffusion: DF, PE and DD cards
Diffusion of suspended sediment occurs because of 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
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*
and for the transverse (perpendicular to the flow direction) diffusion coefficient as
Dt = 0.23 Du*
D = water depth, and
u* = shear velocity as given by equation 8.
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.
Allen Teeter of the WES Coastal and Hydraulics Laboratory has suggested that an
equation of the form
De = Κ 1 Κ 2 D u + 10 -5 λ
the element size
K1 and K2
Run Control 59
Users Guide To SED2D-WES