2. Characteristics and Motion of Sédiments
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2.2 Sédiment Classification
Natural sédiments can be classified as into coarse or non-cohesive or cohesionless and fine or cohesive.
Silt and clay are the major constituents of cohesive sédiments, the size
of which varies up to a fraction of a micrometer. It is noted that often the
beds and banks of natural and artificial channels consists of cohesive sédiments subjected to érosion, which are highly susceptible to be carried under
suspension and eventually deposit at locations along the flow direction.
The basic différence between or non-cohesive and cohesive sédiments
is that the cohesive sédiments can form agglomérations with size, density,
and strength much different than those of the original particles under the
influence of inter-particle attractive forces. In addition, these properties are
not a constant and are bound to be influenced by the forces acting on them
during its formation, which in turn can vary with time and quality of pore
or ambient water.
Electromagnetic forces in addition to physical drag and lift hydrodynamic forces influence behaviour of cohesive sédiments. The movement of
non-cohesive open coast marine sédiments dépends on motion of the sea
and physical properties of the sédiments. The flow field induced by waves
and tides or that due to the gradient formed by salinity, température and
pressure are the most important driving forces, while extreme events like
underwater earthquakes, tsunamis, storm surges will also contribute to the
sédiment motion. While stating the driving forces, it is mandatory to mention the résistive forces, which are governed mainly by sédiment size and
characteristics.
2.3 Particle Size
It is the most important parameter, as it partially influences the mode of
sédiment transport and its corresponding mechanism. Sieve size distribution
is one of the most commonly adopted method to represent the particle
size by plotting the results as weight of material retained against sieve
single size to produce a cumulative size frequency curve. A frequently used
classification by Coastal engineers for sédiment transport calculations is as
follows.
2.3.1 Soil classification
The classification between coarse and fine sédiments is shown in Table 2.1
and is attributed to the mutual interaction of grains in water environment
21
2.2 Sédiment Classification
Natural sédiments can be classified as into coarse or non-cohesive or cohesionless and fine or cohesive.
Silt and clay are the major constituents of cohesive sédiments, the size
of which varies up to a fraction of a micrometer. It is noted that often the
beds and banks of natural and artificial channels consists of cohesive sédiments subjected to érosion, which are highly susceptible to be carried under
suspension and eventually deposit at locations along the flow direction.
The basic différence between or non-cohesive and cohesive sédiments
is that the cohesive sédiments can form agglomérations with size, density,
and strength much different than those of the original particles under the
influence of inter-particle attractive forces. In addition, these properties are
not a constant and are bound to be influenced by the forces acting on them
during its formation, which in turn can vary with time and quality of pore
or ambient water.
Electromagnetic forces in addition to physical drag and lift hydrodynamic forces influence behaviour of cohesive sédiments. The movement of
non-cohesive open coast marine sédiments dépends on motion of the sea
and physical properties of the sédiments. The flow field induced by waves
and tides or that due to the gradient formed by salinity, température and
pressure are the most important driving forces, while extreme events like
underwater earthquakes, tsunamis, storm surges will also contribute to the
sédiment motion. While stating the driving forces, it is mandatory to mention the résistive forces, which are governed mainly by sédiment size and
characteristics.
2.3 Particle Size
It is the most important parameter, as it partially influences the mode of
sédiment transport and its corresponding mechanism. Sieve size distribution
is one of the most commonly adopted method to represent the particle
size by plotting the results as weight of material retained against sieve
single size to produce a cumulative size frequency curve. A frequently used
classification by Coastal engineers for sédiment transport calculations is as
follows.
2.3.1 Soil classification
The classification between coarse and fine sédiments is shown in Table 2.1
and is attributed to the mutual interaction of grains in water environment