Geoscience Reference
In-Depth Information
(a)
(b)
Flow direction
F
L
c
b
a
F
D
a
b
c
= increasing flow velocity where grains are of uniform size
or
decreasing grain size under conditions of uniform flow
F
W
F
C
(c)
F
L
Fluid lift
F
D
Fluid drag
F
C
Cohesion/friction
F
W
Immersed weight
Fig. 1.7
(a) Schematic diagram illustrating the main forces acting on a sediment particle within a moving fluid medium.
(Adapted from Allen 1985.) (b) The processes of sediment movement within flowing water;
a
, rolling;
b
, saltation;
c
, suspension.
(c) Grain transport due to saltation under conditions of aeolian transport. Grains typically exhibit steeper and longer trajectories
during aeolian transport.
occur at a critical shear velocity, the value of
which varies with sediment grain size (although
this is complicated by substrate specific vari-
ations in grain size, particle density, sediment
packing and grain imbrication). The interrela-
tionship between these variables is highlighted
in the Hjulström graph (Fig. 1.5), which illus-
trates a general (and fairly intuitive) rule whereby
increasing flow velocities are required to entrain
increasingly larger sized particles. This rule, how-
ever, breaks down where the substrate is domin-
ated by very fine sands, silts and clays because
of the cohesive nature of such material. In such
cases, much higher flow velocities are required
to entrain particles and this helps to explain why
fine silts and clays can accumulate within tidally
influenced estuarine and deltaic environments
(see Chapter 7). Velocity-entrainment relation-
ships are also complicated as fluid moves across
a sediment substrate because grains protruding
from the substrate cause flow to be constricted.
This causes the streamlines above the grain to
accelerate and thus to exert a fluid lift (Fig. 1.7a).
Within aeolian environments, wind velocities
that exceed the critical shear stress for specific
sediment grain sizes are also required for entrain-
ment, although they occur at higher velocities
than in water.
1.3.2 Sediment transport
Once entrained, sediment movement occurs in
three ways:
1
as bedload material that is too heavy to be
lifted up into the water and moves by
rolling
along the substrate (Fig. 1.7b);
2
via the process of
saltation
whereby lighter
grains are temporarily lifted into the fluid and
then settle out (in water, saltating grains typic-
ally exhibit short, flat trajectories due to the
cushioning effects of fluid viscosity, whereas
in air the trajectories tend to be steeper and
longer; Fig. 1.7b);
3
in
suspension
where the lightest particles are
held within the fluid and moved, often in an
erratic path (Fig. 1.7b).
For sediments of a given grain size these transport
mechanisms occur along a gradient of increas-
ing flow velocity. Consequently, for a sediment
deposit comprising a mix of grain sizes it follows
that the occurrence of different sediment size
fractions can be attributed to different transport
