Geology Reference
In-Depth Information
law of viscous drag can be applied to derive the settling
velocity as:
the fundamentals for the present understanding of
cohesive and mixed sediment transport.
2.2.2.1
Initiation of Motion
Generally speaking, sediment motion is initiated when
the fluid force exceeds the submerged gravitational
force (Fig.
2.1
). In natural environments, initiation of
motion can be very complicated and influenced by
numerous factors including the characteristics of the
flow (laminar or turbulent), sediment size and shape,
sediment sorting, and by presence and characteristics
of bedforms. One of the most commonly used tools
is the Shields parameter (Eq.
2.7
) and the Shields
diagram (Fig.
2.3
). A critical Shields parameter (T
c
),
above which sediment motion is initiated, is defined in
the same form as Eq.
2.7
:
2
1
(
s
1)
D
(2.8)
w
s
18
n
where
n
= kinematic viscosity. For larger grains
that have faster settling velocities, the drag force
is determined based on the quadratic friction law
(e.g., Eq.
2.3
). Soulsby (
1997
) examined a large
amount of existing data and developed an empirical
formula as:
n
§
1
¶
(
)
2
3
w
10.36
1.049
D
10.36
(2.9)
2
¨
·
*
D
©
¸
where the dimensionless grain size, another commonly
used parameter for sediment transport, is:
2
*_
u
t
c
c
q
(2.11)
c
(
rr
)
gD
(
s
1)
gD
s
1
3
§
( )
s
g
¶
(2.10)
D
¨
D
where the bed shear stress in the original Shields
parameter is replaced by the critical bed shear stress
(
t
c
) and u
*_c
= critical bed shear velocity. The original
Shields diagram has shear velocity
u
*
on both the
horizontal and vertical axes and is quite difficult to
use. Soulsby (
1997
) provided a direct relationship
(Fig.
2.4
) between the critical Shields parameter
(
q
c
: Eq.
2.11
) and the dimensionless grain diameter
(
D
*
: Eq.
2.10
):
·
*
2
n
©
¸
Based on the above Eqs.
2.8
and
2.9
, the settling veloc-
ity for coarse silt (5.0 phi or 0.031 mm) to medium
sand (1.0 phi or 0.5 mm) ranges approximately from
0.1 to 8 cm/s.
Sediments which are finer than medium silt (6 phi
or 0.016 mm) are often referred to as cohesive sedi-
ments. They tend to form aggregates which are larger
than the individual grains but with lighter “overall”
density than the mineral grains. The settling of cohe-
sive grains is complicated and comprises a significant
part of the processes of cohesive sediment transport,
and is discussed in the following section on cohesive
sediment transport.
0.24
(
)
0.020
D
q
0.055 1
e
for
D
(2.12)
5
*
cr
*
D
*
0.30
(
)
0.020
D
(2.13)
q
0.055 1
e
for
D
a
5
*
cr
*
1
1.2
D
*
Equations
2.12
and
2.13
yield the critical shear stress
conveniently from sediment grain size. Intuitively, the
larger the grain size, the more fluid power (i.e., a higher
critical shear stress) it needs for the initiation of
motion. However, the relationship is not linear. Soulsby
(
1997
) suggested that the above simple and straight-
forward equations are also valid for wave motion and
combined wave and current.
2.2.2
Transport of Non-cohesive
Sediments in Tidal Environments
Transport of non-cohesive sediment has been studied
extensively and is much better understood than the
transport of cohesive sediment and of mixed sediment.
The following discussion on non-cohesive sediment
transport serves two purposes. Firstly, some tidal
environments or parts of them are composed of
non-cohesive sediments and the subsequent trans-
port relationships are directly applicable. Secondly,
theories on non-cohesive sediment transport provide
2.2.2.2
Bedload Transport
After the sediment motion is initiated, it can be trans-
ported in three modes, i.e., bedload, suspended load,
and washload. Washload has little to no significance in
