Sediment transport

Sedimentology

1. Types of Sediment Transport
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Sediment transport occurs in natural systems where the particles are clastic rocks
(sand, gravel, boulders, etc.), mud, or clay; the fluid is air, water, or ice; and the force of gravity
acts to move the particles along the sloping surface on which they are resting.

A. Saltation:
in geology, saltation (from Latin saltus, "leap") is a specific type of particle transport
by fluids such as wind or water. It occurs when loose material is removed from a bed and
carried by the fluid, before being transported back to the surface.
In geology, saltation (from Latin saltus, "leap") is a specific type of particle transport
by fluids such as wind or water. It occurs when loose material is removed from a bed and
carried by the fluid, before being transported back to the surface. Examples
include pebble transport by rivers, sand drift over desert surfaces, soil blowing over fields,
and snow drift over smooth surfaces such as those in the Arctic or Canadian Prairies.
Saltation process:
At low fluid velocities, loose material rolls downstream, staying in contact with the
surface. This is called creep or reptation. Here the forces exerted by the fluid on the particle
are only enough to roll the particle around the point of contact with the surface.
Once the wind speed reaches a certain critical value, termed the impact or fluid threshold,the
drag and lift forces exerted by the fluid are sufficient to lift some particles from the surface.
These particles are accelerated by the fluid, and pulled downward by gravity, causing them to
travel in roughly ballistic trajectories. If a particle has obtained sufficient speed from the
acceleration by the fluid, it can eject, or splash, other particles in saltation, which propagates
the process. Depending on the surface, the particle could also disintegrate on impact, or eject
much finer sediment from the surface. In air, this process of saltation bombardment creates
most of the dust in dust storms. In rivers, this process repeats continually, gradually eroding
away the river bed, but also transporting-in fresh material from upstream.
Suspension generally affects small particles ('small' means ~70 micrometres or less for
particles in air). For these particles, vertical drag forces due to turbulent fluctuations in the fluid
are similar in magnitude to the weight of the particle. These smaller particles are carried by the
fluid in suspension, and advected downstream. The smaller the particle, the less important the
downward pull of gravity, and the longer the particle is likely to stay in suspension.
A recent study finds that Saltating sand particles induces a static electric field by friction.
Saltating sand acquires a negative charge relative to the ground which in turn loosens more
sand particles which then begin saltating. This process has been found to double the number of
particles predicted by previous theory. This is significant in meteorology because it is primarily
the saltation of sand particles which dislodges smaller dust particles into the atmosphere. Dust
particles and other aerosols such as soot affect the amount of sunlight received by the
atmosphere and earth, and are nuclei for condensation of the water vapour.

2. Types of Sediment Transport
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B. Traction:
Traction is the geologic process whereby a current transports sand grains and larger
clasts by rolling or sliding along the bottom. Thus, the grains and clasts interact with the
substratum during transport.
Traction is the geologic process whereby a current transports sand grains and larger
clasts by rolling or sliding along the bottom. Thus, the grains and clasts interact with the
substratum during transport. By contrast, saltation, a related sediment transport process, moves
grains across the bottom by bouncing or hopping. The actual current carries the sediment load
in traction and saltation flows, whereas downslope movement under the force of gravity carries
the sediment in gravity flows. These processes contrast withsuspension settling, in which there
is no current.
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Traction is where large stones or boulders in the river's load are rolled along
by the for ce of the river


C. Suspension:
Suspension - fine light material is carried along in the water. Saltation - small pebbles
and stones are bounced along the river bed. Traction - large boulders and rocks are rolled along
the river bed.

3. Types of Sediment Transport
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Suspension: Transport of sediment by wind or water currents that are strong enough to
keep the sediment particles continuously above the stream bottom or ground. (See traction and
saltation for comparison.)

D. Solution:
Solution - minerals are dissolved in the water and carried along in solution. Suspension
- fine light material is carried along in the water. Saltation - small pebbles and stones are
bounced along the river bed. Traction - large boulders and rocks are rolled along the river bed.