Marsh Porosity Corrections

SED2D_Users_Guide_Draft_12-09-2004
Page Navigation
7

GFGEN geometry and RMA2 hydrodynamic input files.

The Bed Source/Sink Term

The form of the bed source term, S = α1C + α2, as given in Equation 1 is the same for

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.

Sand Transport

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

Equation 11

-C

where

source term

Ceq

equilibrium concentration (transport potential)

sediment concentration in the water column

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. The Ackers-White formula computes the total load,

including suspended load and bed load, and was developed originally for fine sand.

The formulation was later updated to include coarser sands and these revised

coefficients are included in the current model formulation. However, the

appropriateness of the use of SED-2D with the Ackers-White formula diminishes

with coarsening of the sediment.

The characteristic time, tc, is somewhat subjective. It should be the amount of time

required for the concentration in the flow field to change from C to Ceq. In the case

of deposition, tc is related to fall velocity. The following expression was adopted.

Equation 12

Conceptual Program Design 11

Users Guide To SED2D-WES

Index