Environmental Engineering Reference
In-Depth Information
CHAPTER 3
3.1 INTRODUCTION
Sediments are fragmented material, primarily formed by the physical and
chemical disintegration of rocks from the earth's crust; they can be divided
into cohesive and non-cohesive sediments. For non-cohesive sediments
there are no physical-chemical interactions between individual particles
and the size and weight of the individual particles are important factors in
their behaviour. In cohesive sediments the physical-chemical interactions
between particles are important factors in the initiation of motion (erosion)
and also in the transportation (flocculation); with cohesive sediments the
size and weight of a particle have less significance. Most of the discussion
on sediment transport in these lecture notes will deal with non-cohesive
sediments.
Some of the major features of the individual particles and the sediments
can be described by:
•
Density and porosity
•
Particle size and size distribution
•
Shape
•
Fall velocity
•
Dimensionless parameters, such as the particle parameter, particle
mobility parameter,
excess shear stress parameter,
dimensionless
particle Reynolds number and transport rate parameter.
3.2 DENSITY AND POROSITY
The density of a sediment particle is the mass per unit of volume and
it primarily depends on the mineral composition. Non-cohesive material
originates generally from the disintegration or decomposition of quartz.
The density (
ρ
s
) of quartz is approximately 2650 kg/m
3
and the density of
clay minerals ranges from 2500-2700 kg/m
3
.
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