Geology Reference
In-Depth Information
In addition to particle size, the settling velocity of flocs
is influenced by many factors including salinity and
organic content, temperature, sediment concentration,
water depth, as well as flow velocity. All the factors
that influence flocculation processes, as discussed
above, also influence the settling velocity of flocs.
Results from numerous laboratory studies are summa-
rized in Mehta (
1986b
). Methods and results from
in
situ
field measurements are summarized in Dyer et al.
(
1996
). Van Rijn (
2007b
) proposed a general formula
to estimate the settling velocity of cohesive particles,
or flocs (
w
s_f
):
and Dyer (
2007
) formulas is that they are purely
empirical and the dimensions of the parameters were
neglected.
Unique to cohesive sediment, the settling velocity
of flocs is also influenced by water depth. This is due
to differential settling, because larger flocs settle faster
than smaller flocs and therefore may collide with them
during the settling. The collisions may result in even
larger flocs. Therefore, the size of the flocs increases
with water depth in tranquil water. This results in an
increasing trend of settling velocity of flocs with water
depth. On the other hand, the floc size and correspond-
ing settling velocity are controlled by the shear stress
from fluid motion and settling, which tends to break
the weakly bounded flocs (Hunt
1986
; Winterwerp
2002
). This balance tends to create an equilibrium floc
size under a particular set of conditions.
Cohesive sediments are transported mostly as sus-
pended load and the rate of transport can be estimated
by integrating the product of concentration and veloc-
ity with respect to depth (Eq.
2.16
). Equation
2.16
provides transport by convection only and may not be
applicable for many cases. A numerical model of cohe-
sive sediment transport is typically developed by solv-
ing the convection-diffusion equation. A simpler
depth-averaged convection-diffusion equation neglect-
ing settling can be written as
wwff
(2.22)
s
_
f
s
_
s
floc
hs
where
w
s_s
= the settling velocity of single suspended
particle in clear water,
flo
f
= flocculation factor, and
h
f
= hindered settling factor. For cohesive fine sedi-
ment, the individual particles are small. The
w
s_s
can be
determined based on the Stokes law (Eq.
2.8
). The
flocculation factor (
flo
f
) is determined empirically to
account for the influence of flocculation on the fine
grain settling. Another important consideration for
fine-grain deposition is the hindered settling, which
occurs at a concentration that is greater than 10 kg/m
3
.
Hindered settling is the effect that the settling velocity
of the flocs is reduced due to an upward flow of fluid
displaced by the large amount of flocs. The hindered
settling factor (
h
f
) is less than one and is determined
empirically. Significant hindered settling may occur
near the bed during slack tides (Van Rijn
2007b
).
Based on a relatively large amount of field measure-
ments in several northern European estuaries using an
in-situ
video method (INSSEV) developed by Fennessy
et al. (
1994
), Manning and Dyer (
2007
) developed a
series of empirical formulas predicting the settling
velocity of flocs. Different from most other approaches,
Manning and Dyer (
2007
) separated the flocs into two
populations based on the size, the macrofloc (>160 Pm)
and the microfloc (<160 Pm), arguing that their set-
tling behavior is significantly different. The settling
velocity of macroflocs is a function of suspended par-
ticle matter concentration (in mg/l) and turbulent shear
stress, while the settling velocity of microflocs is only
controlled by the turbulent shear stress. The advantage
of the Manning and Dyer (
2007
) model is that it is
based on a large set of in-situ field measurement under
a variety of conditions. A weakness of the Manning
t t t
c
c
c
1
t
t
c
§
¶
u
v
hk
¨
·
x
t t t t
t
x
y
h
x
t
x
©
¸
(2.23)
1
t
§
t
cS
¶
hk
hy
0
¨
·
y
t
t
y
h
©
¸
where
c
= depth averaged concentration,
u
= depth
average velocity in
x
direction,
v
= depth average
velocity in
y
direction,
k
x
and
k
y
= dispersion coefficient
in
x
,
y
direction, respectively,
h
= water depth, and
S
= source and sink terms. Equation
2.23
can be solved
numerically with boundary conditions describing a
particular tidal environment.
Deposition of cohesive sediment is a complicated
process due to the concentration- and depth-dependent
flocculation. Based on a series of laboratory experi-
ments, Krone (
1962
) found that deposition occurs when
the bed shear stress falls below a critical value for depo-
sition, e.g., during slack tide. Krone (
1962
) further pro-
posed that deposition can be quantified based on a
