Essay on Waves as agents of Marine erosion

Of all the factors that are capable of shaping a coastline and affecting the coastal geomorphology, waves are the most dominant. At this point it may be pointed out that a detailed description of waves may be found in the third section of the book which deals with ‘Oceanography’.

However, it would not be out of place to refer to the two types of wave’s i.e. destructive waves and constructive waves. The storm waves fall into the category of destructive waves keeping in view the results produced by them.

In these waves, because of their higher frequency and the vertical plunge of water as they break, the backwash is more powerful than the swash. Such waves tend to move eroded material towards the sea.

The constructive waves, on the contrary, have a more powerful forward push of the swash, and because of frictional retardation, a less powerful backwash.

Such waves instead of moving the material seaward bring it towards the coast. They move water-worn pebbles and cobbles which are coarser than gravels up the sea beach.

It is worthwhile to remember that the average pressure exerted by waves on the coast is about a ton per square foot. During a heavy storm the amount of pressure of the waves may increase threefold.

The storm waves at high tide exert greater effect on the coastline, because their impact is felt higher up the beach or on the seaward face of the cliff.

According to Arthur Holmes, the pressure exerted by Atlantic waves averages over 9700 kg per m² during winter, while the pressure may still be greater in great storm exceeding even 30,000 kg per m².

Wave-erosion of the coastal zone is performed in the following ways:

(i) by hydraulic action,

(ii) by corrasive action or abrasion,

(iii) by attrition, and

(iv) by solvent action or solution or corrosion.

The hydraulic action of the wave refers to the force exerted by moving waves on rocks without using its load. Wave erosion of the coastal zone is carried on by the hydraulic action from the sheer physical force of the pounding water and the explosive effect of the air compressed between breaking waves and cliff faces.

Air present in the cracks and crevices in the cliff face is compressed until its pressure is equal to the pressure exerted by the breaking wave. When the wave retreats, the resultant expansion has an explosive effect. This process is repeated with the result that in case of jointed or faulted rock, the fissures get enlarged.

Corrasive action, also called abrasion, plays a very effective role in wave erosion. The corrasive action of sand, gravel and pebbles carried by waves against the shore is the most effective process in marine erosion.

This process is also called ‘artillery action’ of tools moved against solid rock. The mass of fragments of varying sizes are pounded by the waves against the lower part of the cliff. This undercutting ultimately produces an overhanging cliff. The most striking example of undercutting is provided by caves.

Attrition is another process of wave erosion. When rock fragments are moved by the waves against the cliff, they not only strike against the cliff face, but also dash against each other.

Thus, the fragments during the course of their forward and backward movement by the swash and the backwash are themselves worm down by what is called attrition. In fact, attrition is the mechanism by which the particle size of any material is reduced by friction during transport-in this case by ocean waves.

The sea wave operates as an agent of erosion in another way i.e. by corrosion or the solvent and chemical action. This process is more effective on rocks with calcareous cement or on limestone. Otherwise this process is of limited importance in the case of sea water.