Saturday, January 21, 2012

COAST: Wave action

The size of a wave depends on its fetch. The fetch is the distance a wave travels. The greater the fetch, the larger the wave. Wind also has a significant effect on the size of waves. The stronger the wind the larger the wave. As a wave approaches a beach it slows. This is the result of friction between the water and the beach. This causes a wave to break.

There are two main types of wave. These are constructive and destructive waves. :

Constructive waves build beaches. Each wave is low. As the wave breaks it carries material up the beach in its swash. The beach material will then be deposited as the backwash soaks into the sand or slowly drains away. These waves are most common in summer.

Constructive Waves

Destructive waves destroy beaches. The waves are usually very high and very frequent. The back wash has less time to soak into the sand. As waves continue to hit the beach there is more running water to transport the material out to sea. these waves are most common in winter.

Erosional Landforms

Bays and headlands
Headlands form along coastlines in which bands of soft and hard rock outcrop at right angles to the coastline (see image below). Due to the different nature of the rock erosion occurs at different rates. Less resistant rock (e.g. boulder clay) erodes more rapidly than less resistant rock (e.g. chalk).

Bays and Headlands before Erosion

The image below shows the landforms that result from the different rates of erosion.

Bays and Headlands after erosion

Bays and Headlands

Erosion of a headland
A headland is an area of hard rock which sticks out into the sea. Headlands form in areas of alternating hard and soft rock. Where the soft rock erodes bays are formed either side of the headland. As the headland becomes more exposed to the wind and waves the rate of its erosion increases. When headlands erode they create distinct features such as caves, arches, stacks and stumps.

The diagram below shows the sequence in the erosion of a headland.

Stage 1
Waves attack a weakness in the headland.
Stage 2
A cave is formed.
Stage 3
Eventually the cave erodes through the headland to form an arch.
Stage 4
The roof of the arch collapses leaving a column of rock called a stack.
Stage 5
The stack collapses leaving a stump.

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