D-R-A-F-T
τ
⎛
⎞
⎟,τ ≤ τ d
P = ⎜1 -
⎜ τ
⎟
⎝
⎠
d
⎛ - 0.2 ⎞
⎜ τ - τ ⎟,τ > τ d
P = exp⎜
⎟
⎝
d⎠
and
P' = MAX ( P, PMIN )
where the default for PMIN is 0.1 . Deposition is computed as:
Equation 19
Deposition = P' V s C
Erosion rate is calculated much as for Option 1 but with an exponent on the excess
shear stress term:
Equation 20
esse
M ⎛τ
⎞
,τ > τ e
⎜ - 1⎟
Erosion =
D ⎜τ e
⎟
⎝
⎠
The Bed Strata Discretization
The sink - source term in Equation 1 becomes a source - sink term for the bed model,
which keeps track of the elevation, composition, and character of the bed. Bed
change computations utilize the Crank - Nicholson weighting of the time-step
contributions.
Sand Beds
Sand beds are considered to consist of a sediment reservoir of finite thickness, below
which is a nonerodible surface. Sediment is added to or removed from the bed at rate
determined by the value of the sink/source term at the previous and present time -
steps. The mass rate of exchange with the bed is converted to a volumetric rate of
change by the bed porosity parameter.
Clay Beds
Clay beds are treated as a sequence of layers. Each layer has its own characteristics
as follows:
Thickness.
Density.
Age.
Bulk shear strength.
Type.
In addition, the layer type specifies a second list of characteristics.
Critical shear stress for erosion.
14 Conceptual Program Design
Users Guide To SED2D-WES
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