Geoscience Reference
In-Depth Information
11.4 SIMULATION OF TRANSPORT OF COHESIVE AND
NON-COHESIVE SEDIMENT MIXTURES
When the fraction of clay and fine silt is larger than about 10%, a sediment mixture
composed of cohesive and non-cohesive particles may exhibit cohesive properties. The
flocculation and consolidation of the cohesive particles may affect the erosion, deposi-
tion, and transport of the non-cohesive particles significantly. In particular, when the
fraction of cohesive particles is appreciable and the sediment concentration is high, the
floc structures formed by the cohesive particles will involve the non-cohesive particles.
Interactions between cohesive and non-cohesive particles should be taken into
account in the simulation of the mixed cohesive and non-cohesive sediment trans-
port, but this is difficult because the interaction mechanisms are little understood. In
the case of low sediment concentration, such interactions may be ignored and, thus,
the following modeling framework is often used (Ziegler and Nisbet, 1995; Ziegler
et al
., 2000; Wu and Vieira, 2002).
The entire sediment mixture is divided into a suitable number of size classes. The
depth-averaged 2-D transport equation of the
k
th size class of sediment is
E
s
,
x
h
∂
E
s
,
y
h
∂
+
∂(
hU
y
C
k
)
∂
∂(
hC
k
)
+
∂(
hU
x
C
k
)
=
∂
∂
C
k
∂
+
∂
∂
C
k
∂
∂
∂
t
x
y
x
x
y
y
+
E
bk
−
D
bk
(11.43)
where
C
k
is the depth-averaged concentration of the
k
th size class of sediment. For
brevity, the 1-D, width-averaged 2-D, and 3-D transport equations are omitted here.
The deposition rate
D
bk
is determined by
D
bk
=
α
k
ω
sf
,
k
C
k
(11.44)
where the coefficient
α
k
is determined using Eq. (11.15) for the cohesive size classes
and the methods introduced in Section 2.5 for the non-cohesive size classes; and
ω
sf
,
k
is the settling velocity of size class
k
, determined using one of Eqs. (11.1), (11.2),
(11.4), (11.6), (11.7), and (11.9) for the cohesive size classes and one of the formulas
introduced in Section 3.1 for the non-cohesive size classes.
The erosion rate
E
bk
is determined by
p
bk
E
(
k
)
b
E
bk
=
(11.45)
where
p
bk
is the fraction of size class
k
in the surface layer of bed material, and
E
(
k
)
b
is
the potential erosion rate of size class
k
.
If the cohesive portion is dominant in the bed (surface layer), all cohesive and
non-cohesive particles are usually eroded simultaneously in flocs (even blocks) and
have the same potential erosion rate; thus,
E
(
k
)
b
is actually the total erosion rate
and can be determined using an erosion model of cohesive sediments, such as
Eqs. (11.19)-(11.21). If the cohesive portion is not dominant in the bed, the non-
cohesive particles are eroded in dispersed form and the cohesive particles may be
