Waves are the main factor of shaping the coast. Their erosive capacity is partly due to the effect of impacts of water against the coast and partly due to the action of beach material moved by the waves themselves. Nevertheless, there are several factors that influence the erosive force of waves: Strength of wind, length of time wind blows and the fetch.
Waves with a large fetch and subjected to strong winds are generally known as destructive waves. These waves are steep and tend to break downwards onto the beach. They possess both a strong swash and a strong backwash, the latter being sufficient to erode the beach yet the former being able to build a large berm by adding material to a zone, which the backwash cannot reach. Therefore, destructive waves tend to erode the beach yet leave a constructional feature at the highest point reached by the waves during their period of occurrence (usually a storm).
Destructive waves erode the coastline via a number of processes. The most powerful and active form of wave erosion is hydraulic pressure (Wave Quarrying). This occurs when waves break against the face of a cliff and cause the air in rock cracks to be compressed with pressures reaching up to 50 kg/cm2. The retreat of the wave causes rapid expansion of the compressed air, exerting a huge force, which can be greater than the initial compression. In areas subjected to large amounts of hydraulic pressure, a wave-cut platform is created where the continuous pounding by waves at the base of the cliff causes large amounts of stress and causes the cliff to retreat. (I love my baby)
The net effect of hydraulic action is to pick out weaknesses in the cliffs and prepares the cliff face for further erosion by other processes. Abrasion is where beach material suspended in the water strikes the cliff wearing it away, and is accelerated when hydraulic action opens cracks in the rock supplying a larger surface area for erosion. Similarly attrition describes beach particles striking each other and the cliff gradually reducing in size over time. The combination of erosional processes has the effect of steepening the cliff, particularly if wave action and removal of eroded material from the base of the cliff exceeds the supply from above.
Constructive waves describe those, which are lower, and whose backwash is insufficient to erode the beach. Constructive waves slowly replace the beach material removed into the offshore zone by destructive breakers. However, they still cause weakening in the cliffs in the form of weathering. The growth of chloride salts derived from seawater attacks a wide variety of rocks – particularly porous/permeable rocks in a process called Salt Crystallisation. The resulting effect is that beaches subjected to a larger proportion of constructive waves generally have a gentler gradient.
Indeed the main geomorphological agent on the coast are the waves but sub aerial processes, slope transport, vegetation (e.g. plant collonisation of salt marshes and sand dunes), man and animals also play important roles. Similarly, the beach profile will be determined by the nature of the coast. The type of rock, height of cliffs (amount of material), dip of rocks, aspect (exposure to damaging waves), width of beach, width of wave cut platform and human interference will each leave their own unique trademark on the beach profile.