Page 59 - Sedimentology and Stratigraphy

Processes of Transport and Sedimentary Structures
46 Nichols/Sedimentology and Stratigraphy 9781405193795_4_004 Final Proof page 46 26.2.2009 8:16pm Compositor Name: ARaju

Fig. 4.2 Particles move in a flow by
rolling and saltating (bedload) and
in suspension (suspended load).

moving ice and in lava flows, all of which have high clay) and low density particles are kept in suspension
kinematic viscosities. Fluids with low kinematic vis- while sand-size particles move by rolling and some
cosity, such as air, are turbulent at low velocities so saltation. At higher flow rates all silt and some sand
all natural flows in air that can transport particles are may be kept in suspension with granules and fine
turbulent. Water flows are only laminar at very low pebbles saltating and coarser material rolling. These
velocities or very shallow water depths, so turbulent processes are essentially the same in air and water but
flows are much more common in aqueous sediment in air higher velocities are required to move particles
transport and deposition processes. Most flows in of a given size because of the lower density and vis-
water and air that are likely to carry significant cosity of air compared with water.
volumes of sediment are turbulent.

4.2.3 Entraining particles in a flow
4.2.2 Transport of particles in a fluid
Rolling grains are moved as a result of frictional
Particles of any size may be moved in a fluid by one of drag between the flow and the clasts. However, to
three mechanisms (Fig. 4.2). Rolling: the clasts move make grains saltate and therefore temporarily move
by rolling along at the bottom of the air or water upwards from the base of the flow a further force is
flow without losing contact with the bed surface. required. This force is provided by the Bernoulli
Saltation: the particles move in a series of jumps, effect, which is the phenomenon that allows birds
periodically leaving the bed surface, and carried and aircraft to fly and yachts to sail ‘close to the
short distances within the body of the fluid before wind’. The Bernoulli effect can best be explained by
returning to the bed again. Suspension: turbulence considering flow of a fluid (air, water or any fluid
within the flow produces sufficient upward motion to medium) in a tube that is narrower at one end than
keep particles in the moving fluid more-or-less con- the other (Fig. 4.3). The cross-sectional area of the
tinually. Particles being carried by rolling and salta- tube is less at one end than the other, but in order to
tion are referred to as bedload, and the material in maintain a constant transport of the fluid along the
suspension is called the suspended load. At low cur- tube the same amount must go in one end and out the
rent velocities in water only fine particles (fine silt and other in a given time period. In order to get the same