This formulation is provided through the DD card, where the variables K1, K2 and K3

( 10 -5 above) are specified.

parallel and transverse to the direction of flow. Since the coefficients in the present

version of SED2D WES apply in the x - and y - directions, not necessarily in the

flow directions, these equations can be used only as a guide.

Fortunately, in most applications, effective diffusion is smaller than convection by

the calculated flow velocities, so a wrong choice does not affect the results very

much unless the chosen coefficient is far too large. The best approach then is to use

a moderately high value (say 50 m2/sec) during the first few runs, and then reduce

the coefficients until the run becomes numerically unstable. This will allow the user

to determine a range of values for which the model gives a converged solution. The

user can then perform sensitivity analyses to determine how the solution changes as

the effective diffusion is varied over this range. If the solution does not vary greatly

then the model is "insensitive" to this coefficient, and no further testing is needed. If

the solution varies widely as this coefficient is varied then the user must rely upon

validation of the model against field measurements in order to determine the

appropriate values. If no field data is available for comparison, the user should use

as small a value as possible, effectively de-emphasizing the importance of these

terms in the overall solution of the system of equations.

The PE card provides a method of specifying the effective diffusion in an automatic

fashion based on the Peclet number:

λu

If the user specifies the Peclet number, then the effective diffusion is

λu

which provides generally for the diffusion coefficient proportional to the current

velocity. However, as prescribed on the PE card there is a minimum value of De

based on a specified minimum velocity (VPEC).

The table below lists some previous applications and the effective diffusion

coefficients that were used.

Table Example Dispersion Coefficients

Current

Typical Element

Coefficient

m 2sec

Typical Location

Speed, mps

Size, km

Medium-size river

1 - 1.5

0.1 - 0.5

100

Open bay

0.5 - 1.0

0.75

100

Tidal river

0.2 - 1.0

0.1 - 0.3

5 - 10

The nodal concentrations at the first time-step are specified on the IC cards, or in a

file if a hot start is used.

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