Page 58 - Sedimentology and Stratigraphy

Nichols/Sedimentology and Stratigraphy 9781405193795_4_004
Final Proof page 45
26.2.2009 8:15pm Compositor Name: ARaju
The Behaviour of Fluids and Particles in Fluids 45
to carry coarse material along the base of the flow and
finer material in suspension. Material may be carried
in water hundreds or thousands of kilometres before
being deposited. The mechanisms by which water
moves this material are considered below.
Air Wind blowing over the land can pick up dust and

sand and carry it large distances. The capacity of the
wind to transport material is limited by the low density
of air. As will be seen in section 4.2.2 the density con-
trast between the fluid medium and the clasts is critical
to the effectiveness of the medium in moving sediment.
Ice Water and air are clearly fluid media but we can
also consider ice as a fluid because over long time per-
iods it moves across the land surface, albeit very slowly.
Iceistherefore aratherhigh viscosity fluidthatiscap-
able of transporting large amounts of clastic debris.
Movement of detritus by ice is significant in and around
Fig. 4.1 Laminar and turbulent flow of fluids through a tube.
polar ice caps and in mountainous areas with glaciers
(7.3.2). The volume of material moved by ice has been
very great at times of extensive glaciation.
geneous fluid almost no mixing occurs during lami-
Dense sediment and water mixtures When there nar flow. In turbulent flows, molecules in the fluid
is a very high concentration of sediment in water the move in all directions but with a net movement in the
mixture forms a debris flow, which can be thought of transport direction: heterogeneous fluids are thor-
as a slurry with a consistency similar to that of wet oughly mixed in turbulent flows. Experiments using
concrete. These dense mixtures behave in a different threads of dye in tubes show that the lines of flow are
way to sediment dispersed in water and move under parallel at low flow rates, but at higher flow velocities
gravity over land or under water as debris flows (4.5.1). the dye thread breaks up as the flow becomes turbu-
More dilute mixtures may also move under gravity lent (Fig. 4.1).
in water as turbidity currents (4.5.2). These gravity- Flows can be assigned a parameter called a
driven flow mechanisms are important as a means of Reynolds number (Re), named after Osborne Rey-
transporting coarse material into the deep oceans. nolds who documented the distinction between lami-
nar and turbulent motion in the late 19th century.
4.2 THE BEHAVIOUR OF FLUIDS AND This is a dimensionless quantity that indicates the
PARTICLES IN FLUIDS extent to which a flow is laminar or turbulent. The
Reynolds number is obtained by relating the following
A brief introduction to some aspects of fluid factors: the velocity of flow (y), the ratio between the
dynamics, the behaviour of moving fluids, is pro- density of the fluid and viscosity of the fluid (n – the
vided in this section to give some physical basis to fluid kinematic viscosity) and a ‘characteristic length’
the discussion of sediment transport and the forma- (l – the diameter of a pipe or depth of flow in an open
tion of sedimentary structures in later sections. More channel). The equation to define the Reynolds num-
comprehensive treatments of sedimentary fluid dyna- ber is:
mics are provided in Allen (1994), Allen (1997) and
Re ¼ y l=n
Leeder (1999).
Fluid flow in pipes and channels is found to be lami-
4.2.1 Laminar and turbulent flow nar when the Reynolds value is low (<500) and
turbulent at higher values (>2000). With increased
There are two types of fluid flow. In laminar velocity the flow is more likely to be turbulent and
flows, all molecules within the fluid move parallel to a transition from laminar to turbulent flow in the
each other in the direction of transport: in a hetero- fluid occurs. Laminar flow occurs in debris flows, in