Geology Reference
In-Depth Information
2.2.3
Transport of Cohesive Sediments
in Tidal Environments
Once the current and suspended sediment concen-
tration profiles are determined (Eqs.
2.1
,
2.18
, and
2.19
), the suspended-load transport rate can be calcu-
lated (Eq.
2.16
). However, accurately determining cur-
rent and suspended sediment concentration profiles is
difficult, especially for complicated flow regime.
Various simplified formulas have been developed to
estimate a total rate of suspended load transport (
q
s
). A
commonly used formula was developed by Van Rijn
(
1984b
):
When sediment grain size is very small, from fine silt
to clay, the electrostatic forces between individual par-
ticles become comparable to the gravitational forces.
The sediments do not behave as individual particles
but tend to cohere together forming aggregates, or flocs
(Mehta and Patheniades
1975
). Sedimentologically,
these aggregates behave differently than the individual
small particles and non-cohesive particles of similar
size due to their lower density and weak strength
(Krone
1986
). The entrainment and settling of cohe-
sive particle aggregates are complicated and controlled
not only by physical properties but also by chemical
and biological conditions. Present understanding of
cohesive sediment transport is limited and largely
based on laboratory experiments. A limitation of the
laboratory studies is that natural chemical and biologi-
cal conditions are difficult to simulate (Mehta
1986a
).
In addition to the poor compatibility of laboratory and
field measurements, compatibility among field mea-
surements is also influenced by data collection meth-
ods (Dye et al.
1996
; Eisma et al.
1996
). Basic processes
of flocculation, settling, erosion, and transport are dis-
cussed below. As emphasized by all the studies, cali-
bration and verification using
in-situ
field data are
crucial for quantifying cohesive sediment transport.
In salt water, the positively charged sodium ions
tend to form a cloud of cations around the negatively
charged clay particles promoting the formation of flocs
via the process of flocculation. Flocculation is caused
by particle collisions due to Brownian motion, turbu-
lent mixing, and differential settling, with turbulent
mixing identified as the dominant process for most
natural systems. Flocculation is influenced by many
factors including particle size, sediment concentration,
salinity, temperature, and organic content. The size of
flocs typically ranges from 0.01 mm to over 1.0 mm.
However, the density of flocs is much lower than that
of the clay minerals, or that of a quartz particle of simi-
lar size. In addition, the floc density decreases with
increasing size. When the fluid shearing forces exceed
the strength of the flocs, they will break into smaller
flocs or particles (Winterwerp
2002
).
Settling of fine-grain particles comprises a substan-
tial part in the understanding of cohesive sediment
transport, in which flocculation plays an essential role.
2.4
0.6
¤
³
q
u
u
d
¤³
¤³
1
s
0.012
cr
50
(2.20)
¥
´ ¥
¥´
0.5
¦µ
uh
((
s
1)
d
)
h
¦µ
D
¦
µ
50
*
where
u
and
c
u
= depth-averaged velocity and critical
velocity, respectively. According to Van Rijn (
1984b
),
Eq.
2.20
is valid for water depth from 1 to 20 m, veloc-
ity from 0.5 to 2.5 m/s, and grain size from 0.1 to
2.0 mm, which is applicable to many tidal environ-
ments. Equation
2.20
suggests that suspended load
transport is proportional to velocity to the power of
3.4. Therefore, similar to the bedload transport, the
time-velocity asymmetry will also induce a net
suspended sediment transport (Fig.
2.5
).
Based on a series of experiments in the Large-scale
Sediment Transport Facility at the US Army Engineer
Research and Development Center, Wang et al. (
2002a,
b, 2003
) combined a suspended sediment concentra-
tion model of Nielsen (
1984
, 1986) for non-breaking
waves and that of Kraus and Larson (
2001
) for breaking
waves and proposed a model predicting the sediment
concentration profile under waves as:
¤
³
¤
³
¥
´
¥
´
¥
´
¥
w
´
11
cz
()
c
exp
z
s
(2.21)
¥
´
¥
´
a
1
3
hL
¥
´
¥
´
¤ ³
D
s
w
¥
k
´
¥
´
¥
¦ µ
d
r
¦
µ
¦
µ
where
L
s
= turbulent mixing length (Nielsen
1984,
1986
),
k
d
= empirical coefficient,
D
w
= wave-energy
dissipation due to breaking. Due to their oscillatory
nature of motion, waves may play a significant role in
suspending sediment. The direction of the net sus-
pended-load transport is controlled by tidal flow (the
u
term in Eq.
2.16
); wave forcing may contribute signifi-
cantly to the
c
term, and therefore to the magnitude of
the transport.
