4/98
SED2D-WES Version 4.3 Beta
d.
Even when sand particles become mobile, there may be no net change in the surface elevation of
the bed. A net change would result only if the rate of erosion was different from the rate of
deposition - two processes which go on continuously and independently.
e.
Cohesive sediments in transport will remain in suspension as long as the bed shear stress exceeds
the critical value for deposition. In general, simultaneous deposition and erosion of cohesive
sediments do not occur.
f.
The structure of cohesive sediment beds changes with time and overburden.
g.
The major portion of sediment in transport can be characterized as being transported in
suspension, even that part of the total load that is transported close to the bed.
2.3
Theoretical Basis
The derivation of the basic finite element formulation is presented in Ariathurai (1974) and Ariathurai ,
MacArthur, and Krone (1977) and summarized below. There are four major computations.
a.
Suspended sediment concentration using convection - diffusion equation with a bed source term
b.
Bed shear stress
c.
Bed source quantity
d.
Bed model
Convection-diffusion equation. The basic convection - diffusion equation is presented in Ariathurai,
a.
MacArthur, and Krone (l977),
C
C
C
C
C
+u
+v
=
Dx +
Dy + α1 C + α2
(1)
x
x
y
y
t
x
y
where
concentration, kg/m3
C=
t=
time, sec
u=
flow velocity in x - direction, m/sec
7
WORKING DRAFT
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