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.

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

DF 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*

and for the transverse (perpendicular to the flow direction) diffusion coefficient as

Dt = 0.23 Du*

where

D = water depth, and

u* = shear velocity as given by equation 8.

Experimentally derived values of the constants in Equation 62 and Equation 63 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.

Allen Teeter of the WES Coastal and Hydraulics Laboratory has suggested that an

equation of the form

(

)

*

2

where

λ

=

the element size

K1 and K2

=

constants

Integrated Publishing, Inc. |