Wednesday 22 February 2017

Coastal erosion a concern but a natural process


Published 05/12/2012 | 09:14

COASTAL EROSION is an issue of great concern to those who live near low-lying or soft shorelines.

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Yet it is an entirely natural process based on sediment budgets. Our very long coastline can be split into small unit called sediment cells or compartments; small stretches of coast that have a relatively well defined sediment budget.

A broad, sweeping bay enclosed between two prominent headlands serves as a good example of a sediment compartment. The sediment in the bay forms a relatively discrete entity with little inputs from or outputs to the adjoining coastline on either side of the bay.

Sediment is a catch-all term for rock particles ranging from the very big to the extremely tiny. Big particles of sediment have common names like boulders and rocks. Smaller ones include cobbles and stones. Pebbles and gravel are smaller again.

And, of course, each type can subdivided such as 'coarse gravel', 'pea gravel' and 'fine gravel' and a host of other terms when the gravel is used for construction work, surfacing paths and drives or dressing graves. Similarly the tiny particles of rock that we call sand grains are described in many ways. Silt is finer than sand and clay makes up the very tiniest particles.

All of these sediment types may come either mixed together ¯ as in the image above ¯ or sorted by the action of the sea into their separate kinds giving us beaches of stones, gravel or sand and expanses of mudflat composed of silt and clay particles.

If the sediment in the sediment compartment is relatively stable then the budget is in balance in terms of sediment in versus sediment out. However, if sediment is getting moved out of the compartment and taken away offshore then the compartment suffers a sediment deficit.

In that case, to satisfy the deficit and to restore the balance the sea withdraws sediment from the reserves stored up on the land resulting in coastal erosion. If the movement of sediment out of the compartment is an on-going event then coastal erosion becomes a continuous feature with consequent shoreline retreat, loss of land and threats to property.

If coastal defences are put in place to halt the coastal erosion they may stop the sea withdrawing sediment from the land at a particular point in the short term but they fail to stop the longer term movement of sediment out of the compartment resulting in other problems.

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