Marsh Porosity Corrections
Marsh Porosity parameters are transferred to the SED2D model by way of the
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
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
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.
Conceptual Program Design 11
Users Guide To SED2D-WES