Geopix UK - Other Titles

Coastal Systems

Coasts are never stable for long. They are - naturally - constantly subject either to erosion or to deposition, although there is usually an overall balance between the two, but not necessarily in the right location for our human purposes.

Taken separately, erosion is usually more of a problem, especially if it is your house which is about to disappear into the sea! We want to protect our precious coastline, but this is a very expensive option. Is it better in the long term for natural forces to take their course and allow the coast's natural defence - the deposited beach material - to do the job for us, allowing some loss of land?

Unfortunately, we sometimes interfere with these defences by dredging this ready supply of deposited sand. Hallsands, Devon, and Folkstone Warren, Kent, are two examples which have suffered damage as a result of the extraction of sand from offshore bars for use in the construction of nearby harbours.

In residential and industrial coastal areas, the coast may be protected by sea walls, which sometimes actually increase scouring of the beach as the water bounces back off the wall. What is a better way of protecting the coastline?
The photograph shows an open arrangement of concrete blocks which absorbs the energy of the waves. (In the background is the Cape Town stadium for the 2010 football World Cup.)

Many seaside resorts try to prevent the movement of material away from their beach by the process of longshore drift. The beach attracts visitors as well as keeping the sea in its place. The usual method is to construct what are often called breakwaters - but more correctly referred to as groynes - across the beach. The effectiveness of this sediment trap can be gauged by measuring the difference in height between the sand on the 'up-drift' side and the 'down-drift' side of the groyne.

Groynes may be an effective method of retaining the beach in resort A, but resort B will be deprived of its rightful supply of sand and shingle, and suffer increased erosion as a result.
A similar problem may be caused when jetties and harbour walls are built out from the shore. In other words, solving one problem may well cause another. Before interferring with nature, we ought to make an attempt to understand how it works.
The photograph shows a cliff which is being eroded on one side of the groyne, but not on the other side where it is protected by shingle banked up against the groyne.