Coastal processes

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Suspended load transport.

It is the part of the total load that is moving in suspension without continuous contact with the bed.

The determination of the rate of suspended load transport is straightforward by comparison with measurement of the rate of bedload transport.

Wash load transport

It consists of very fine particles transported in water.

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Figure 2

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Longshore drift

It is the movement of sediments most often sand, along a coast parallel to its shoreline - a zig-zag movement of sediment along the beach .

The largest beach sediment is found where the process begins, updrift, and the smallest, most easily moved, downdrift.

Where waves are strong, the coast will be eroded and sediment carried away and where they are weak sediment will be deposited.

Longshore drift can have undesirable effects for humans, such as beach erosion. To prevent this, sea defences such as groynes or gabions may be employed.

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Where beaches have become severely eroded by longshore drift and little material is replaced by natural processes, then the material may be artificially replaced by beach nourishment.

Net rate:

Gross rate:

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Model theories for the calculation of longshore drift are

Energy flux model

Energetic model

Suspended transport model

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Energy flux model

It is the simplest model used for the calculation of total amount of material moved along the shoreline.

It is based on the amount of energy available in the waves arriving at the shoreline.

The flux energy in the wave direction is determined to be as Ϝ dl,

where

Ϝ -the energy flux of the waves per unit crest width(Ϝ=ECg),

E - the energy per unit surface area= ξgH2/8

ρ –the water density

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g-acceleration due to gravity

Cg-Group velocity

To determine the amount of this energy flux per unit length of coastline consider the figure,

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dx=dl/cosθ,

where θis the angle the wave ray makes with the onshore(y)direction.

Now it is supposed that the energy flux in the alongshore direction is responsible for the longshore sediment transport; therefore we multiply the energy flux per unit legth(dx=unity) of beach by sinθ to obtain

Ϝcosθsinθ= Pl=ECgsinθcosθ= (ρ gCgsin2θ)/16