The waves are one of the most powerful forces involved in the change of coastal areas. These waves are brought about by winds blowing over the surface of the water. Waves are produced by the friction created by the wind. This friction produces a swell in the water. The wind energy makes the particles of water to rotate inside that swell and this takes the wave forward.
Water particles only move forward in small circles as waves pass. When waves hit the coasts it releases the energy on shorelines. The movement of the surface water very rarely affects the still deep bottom water of the oceans. As a wave come near the beach, it slows down. This is due to the friction between the moving water and the sea floor. The wave breaks when the depth of water is even less than half of the wavelength of the wave.
The size and energy of a wave are affected by :
- The period for which wind blowing over the sea surface.
- The power of the wind
- Fetch – the distance travelled by a wave.
When the wave hits the beach, it is called swash and when the same wave returns into the sea, it is called backwash.
There are two types of waves that affect the coastal regions and bring about change.
1. Constructive waves
Features of constructive waves
- Swash is stronger than backwash.
- It is flat and low in height
- Long wavelength
- The strong swash takes the sediments to the beach and form berm.
- These are formed in calm water and are less strong than destructive waves.
- These waves bring sediment with them and deposit on the beach and further build them.
2. Destructive Waves
- The backwash is stronger than the swash.
- Storms help in the formation of destructive waves.
- The strong winds blowing over a long period supports destructive waves.
- A long fetch and high wave energy is one of the main features of destructive waves.
- They induce the erosional process along the coast.
- The waves are high and steep and have a shorter wavelength.
The destructive and constructive waves give rise to two kinds of processes in the coastal areas:
- Coastal Erosion
- Coastal Deposition
The wearing down and breaking of rocks and other material along the coastline is called coastal erosion. This erosion is the result of destructive waves that wear down the coastline in a number of ways :
- Hydraulic action – Air trapped in the cracks and joints of the cliff face get compressed when a wave hits the cliff which causes erosion.
- Abrasion – Waves bring bits of rock and sand with them which grind the surface of the cliff and causes erosion.
- Attrition – The rocks and pebbles on the shore grind against each other when waves hit them resulting in the breakdown of these rocks and pebbles.
- Solution – Some rocks like chalk or limestone are easily dissolved by acids contained in the sea water.
Waves hit the shore at some angle and go back at 90 degrees.
There is a variety of material which is transported from one place to another through waves. This movement is called transportation. This material is generally:
- Eroded from the cliff.
- Transported by longshore drift.
- Brought by constructive waves.
- Carried to the coastline by rivers.
The continuous backwash and swash transport the sediments and other material sideways along the coast. This type of movement of material is called longshore drift. It happens in a zigzag manner.
Waves and tides transport sediments in four ways. Further, these contribute to the movement of sediment by longshore drift.
Sediments settle down when the sea loses its energy. This is called deposition. Stronger swash and a weak backwash with constructive waves contribute to coastal deposition.
Deposition occurs when:
- Waves enter an area of lesser depth.
- Waves hit a sheltered area like a bay.
- A weak wind
- The amount of material to be transported is in good quantity.