Geoscience Reference
In-Depth Information
The correction factor
K
t
is determined by
1
n
t
1
+
k
t
1
(τ
b
/τ
p
)
0
<τ
b
≤
τ
p
K
t
=
(11.13)
)
−
n
t
2
(
1
+
k
t
1
)(τ
b
/τ
τ
b
>τ
p
p
where
k
t
1
is an empirical coefficient,
n
t
1
and
n
t
2
are empirical exponents,
τ
b
is the bed
m
−
2
, and
shear stress in N
·
τ
p
is the threshold bed shear stress at which
K
t
attains the
maximum value.
The first expression of (11.13) gives
K
t
a value of 1 in quiescent conditions. Accord-
ing to the experiments of Haralampides
et al
. (2003),
m
−
2
at
τ
p
is about 0.17 N
·
the maximum floc
d
50
; however, like many other parameters,
p
may depend on
the properties of sediment. In accordance with the modified Peng formula (11.7),
n
t
1
has a value of about 0.165. However,
n
t
2
has not been investigated well. From
McConnachie's (1991) experiments,
n
t
2
and
n
t
1
might have values close to each other.
Further investigation is needed to quantify
k
t
1
,
n
t
1
, and
n
t
2
.
τ
11.1.4 Deposition of cohesive sediments
Cohesive sediments deposit as flocs as long as the flocs are strong enough to settle
through the bottom region of high shear (Mehta and Partheniades, 1975). Some flocs
may break up as they approach the bed where the shear is stronger and return to the
flow.
Krone (1962) and Mehta and Partheniades (1975) investigated the deposition pro-
cess of fine sediments and proposed formulas to determine the deposition rate. Their
formulas can be written as
D
b
=
αω
sf
C
(11.14)
where
is the deposition probability coefficient between 0 and 1, which is related to
the bed shear stress
α
τ
b
and approximated as (Mehta and Partheniades, 1975)
⎧
⎨
⎩
1
τ
b
<τ
bd
,min
α
=
1
−
(τ
−
τ
)/(τ
−
τ
)τ
≤
τ
≤
τ
b
bd
,min
bd
,max
bd
,min
bd
,min
b
bd
,max
0
τ
b
>τ
bd
,max
(11.15)
where
τ
bd
,min
is the critical bed shear stress below which all sediment particles have a
full probability to deposit on the bed, and
τ
bd
,max
is the critical bed shear stress above
which all sediment particles remain in suspension yielding a zero deposition rate.
The parameters
τ
bd
,min
and
τ
bd
,max
are related to sediment properties. According to
Krone (1962),
τ
bd
,min
=
0, whereas Mehta and Partheniades (1975) found that
τ
bd
,min
m
−
2
in their experiments). For the sediment having
uniform properties sampled in the San Francisco Bay, Krone found
might be larger than zero (0.2 N
·
τ
=
0.078
bd
,max
m
−
3
. For
the sediment with a broad size distribution, Mehta and Partheniades found that
m
−
2
when the initial sediment concentration ranged from 0.3 to 10 kg
N
·
·
τ
bd
,max
m
−
2
.
might vary from 0.18 to 1.1 N
·
