Geoscience Reference
In-Depth Information
deposit (Fig. 1.4c) and can be calculated by
measuring the dispersion of grain size around
the mean. This is again a useful parameter as it
can be used, along with grain size data, to infer
information about the environments of sediment
deposition and the history of sediment rework-
ing (e.g. McManus 1988).
)
8 6 4 2 0
−
2
Grain size
(
φ
−
4
−
6
100.0
Clay Silt Sand Pebble
Gravel
Erosion
10.0
1.0
Transportation
Deposition
1.3
MECHANISMS OF SEDIMENT TRANSPORT AND
ACCUMULATION
0.1
Fall
velocity
The transport and deposition of sediment within
and through different sedimentary environments
may occur within a variety of mediums (water,
wind or ice), and the thresholds for sediment
entrainment and transport represent a funda-
mental control on both the character and develop-
ment of specific sedimentary deposits, as well as
their response to fluctuating energy regimes.
The classic work of Hjulström (1935) demon-
strated the relationship that exists between the
velocity of fluid flow and the size (diameter) of
sediment that can be moved within a fluid. At its
most simplistic this demonstrates that sediment
will be deposited when flow rates drop below
the fall velocity for a particle of a given size.
However, the relationship between these two
parameters is non-linear so that, for example,
much higher flow velocities are required to
entrain highly cohesive fine clay and silt-rich
sediments (Fig. 1.5). Although in reality these
entrainment /transport thresholds vary from
this model depending upon sediment substrate
and individual grain characteristics (e.g. shape,
structure, density) as well as the flow charac-
teristics of the fluid medium, a basic grain-size-
flow-velocity relationship is demonstrated that
can be broadly applied within both fluvial and
marine settings. This section outlines some of the
key physical parameters that control sediment
entrainment, transport and settling (deposition),
and highlights the main sedimentary processes
that operate within the different sedimentary
environments discussed in this topic. For further
details about the physics of sediment transport
and deposition reference should be made to texts
such as Allen (1985) or Leeder (1999).
0.01 0.1 1.0 10 100
Log grain size
(mm)
Fig. 1.5
Hjulström's (1935) graph showing the relationship
between flow velocities and sediment grain size and the
corresponding fields in which erosion, transport and
deposition occur.
1.3.1 Sediment entrainment
The entrainment of sediment by a fluid (most
commonly water or wind), and thus the poten-
tial for sediment transport, is determined by the
relationship between (i) fluid density (which is
the weight per unit volume of a fluid - usually
expressed as 'specific gravity'), (ii) fluid viscosity
(the resistance of a fluid to deformation or flow
- this is measured as a ratio between the shear
stress and the rate of deformation) and (iii) the
velocity of fluid flow. These parameters exert an
influence on the nature of the flow regime within
a fluid medium and, in particular, determine
whether flow that occurs immediately above the
sediment substrate (within the boundary layer)
is laminar or turbulent. In situations charac-
terized by laminar flow, flow streamlines run
parallel to the substrate, flow velocity is low
and viscosity is high (Fig. 1.6a). Under turbulent
flow, the streamlines move in a series of random
eddies, flow velocity is high and viscosity is low
(Fig. 1.6a). The threshold between these two
flow states is expressed by the Reynolds number
(
R
), which describes the ratio between mean
velocity over a defined distance or depth, and
