Coasts: Questions and Answers

A river or sea uses four erosional processes. These include hydraulic action, where the force and pressure of the water can dislodge particles. Abrasion where sand, stones and boulders, carried by the water, rub against surfaces and wear them down. Attrition where particles carried by the river also collide into each other becoming smoother and rounder. Solution is where small particles are dissolved by chemical processes in the water. These combine to erode both the bed and banks, of the channel, by vertical and lateral erosion.

Constructive waves add material to the shore. They have a strong swash and weaker backwash and so do not remove much material from the beach. They are small in height and have a low breaking frequency of around 6-8 waves per minute.

Destructive waves are high, steep and powerful waves. They have a weak swash and a strong backwash, which removes material from the beach. They have a high breaking frequency of around 10-15 waves per minute.

A spit is a long, narrow finger of sand or shingle jutted into the sea from the land

Spits form on coasts where there is significant longshore drift.

If the coastline changes orientation and bends sharply, sediment is then deposited out to sea.

As it builds up, it starts to form an extension from the land.

This process continued with the spit gradually growing further out into the sea. Strong winds or tidal currents can cause the end of spits to become curved to form a feature called a recurved end.

In the sheltered water behind the spit, deposits of mud have built up. An extensive salt marsh has formed as vegetation has started to grow in the emerging muddy islands.

Cracks at base of the headland within the inter tidal zone become exposed through hydraulic action which pressurises air, forcing the crack to widen. Cracks are further widened by weathering processes such as salt crystalization and wet and dry weathering that affects chalk. Over time the cracks widen and develop as wave cut notche. Further processes of abrasion and hydraulic action deepen the notch to form caves.

As a result of wave refraction, which distorts the wave direction, destructive waves concentrate their energy on the sides. This deepens the cave. Wave refraction affects all three sides of the headland. If two caves are aligned the waves may cut through to form an arch. Wave cut notches widen the base of the arch. Vertical joints are exposed by tall breakers associated with destructive waves. Joints can also be weathered from above such as through carbonation in limestone. Here blowholes May form.

Over time, the arch becomes unstable and collapses under its own weight (gravity) to form a pillar called a stack e.g. Old Harry in the Dorset coast. The stack is further eroded at its base creating wave cut notches. Sub aerial processes continue to weaken the stack from above. Eventually the exposed stack will collapse to form a stump. This broken material is further eroded through attrition and transported and deposited.

TROUGH: The lowest part of a wave

CREST: The highest part of a wave

WAVE LENGTH: The horizontal distance between two crests or two troughs.

WAVE HEIGHT: The vertical distance between a crest and a trough

Waves are caused by the wind dragging on the surface of the water.

3 things that influence the size of a wave:

1. The strength of the wind
2. The length of time the wind is blowing for.
3. The fetch (the distance over which the wind has blown).

Swash: When a wave breaks, water rushes up the beach.
Backwash: the water then runs back down the beach, back out to the sea, under the force of gravity.