THE MECHANISM OF MARINE EROSION

The most powerful agents of marine erosion are waves. Their origin is due to the sweeping of winds over the water surface, which sets a series of undulating swells surging forward. These become higher and swifter. A normal wave in an open ocean may measure 6 m (20 feet) high (the vertical height between the crest and the trough) and 120 m (400 feet) long (the wavelength or the horizontal distance between one crest and another). During storms these are greatly increased, depending on the speed and duration of the winds.

On approaching shallow water near the shores, their speed is reduced and the waves are curved or refracted against the alignment of the coast. Shallow water, when it is less than the height of the waves, checks their forward movement, the crests curl over and break into the shores in a mass of foam as breakers.

The water that finally rushes up the beach and hurls rock debris against the land is termed swash.

The water is sucked back and retreats as backwash.

Another element in offshore drift is the undertow, which flows near the bottom away from the shore. This current exerts a pulling effect which can be dangerous to seabathers.

Marine agents of erosion operate in the following ways to transform the coastal landscape.

1) Corrasion: Waves armed with rock debris of all sizes and shapes charge against the base of the cliffs and wear them back by corrasion. Oncoming currents and tides complete the work by sweeping the eroded material into the sea.

 

2) Attrition: The constantly moving waves that transport beach materials such as boulders, pebbles, shingle and fine sand also hurl these fragments against one another, until they are broken down by attrition into very small pieces. The grinding and polishing of such fragmental materials against cliff faces and against each other is largely responsible for fine sand which forms the beaches that are so typical of the seaside resorts.

 

3) Hydraulic action: In their forward surge, the waves which splash against the coast may enter joints and crevices in the rocks. The air imprisoned inside is immediately compressed. When the waves retreat, the compressed air expands with explosive violence. Such action repeated again and again soon enlarges the cracks and rock fragments are prised apart.

 

4) Solvent action: On limestone coasts, the solvent action of sea water on calcium carbonate sets upchemical changes in the rocks and disintegration takes place. This process is limited to limestone coasts.

The rate of marine erosion depends on the nature of the rocks, the amount of rock exposed to the sea, the effects of tides and currents, and human interference in coast protection. Other effects such as volcanicity, glaciation, earth movement and organic accumulations have also to be considered.