Monday, August 20, 2007

Coasts - Energy at the Coast

The coast represents the metting point between the land and sea. Coasts are very dynamic areas and they are constantly change. This change is due to 3 main processes which operate at the coast, 1. Erosion; 2. Transport and 3. Deposition. These 3 processes are all driven by the amount of energy that is available at the coast. The main agents of change at the coasts are waves. Waves are movements of energy throughout the water, but where do waves get their energy from? The answer to this is wind.

As wind blows over the surface of the sea, it creates friction. This frictional drag causes water particles to begin to rotate and energy is transferred forward in the form of a wave. Whilst the water moves forward, the water particles return to their original position. As a wave reaches shallow water, friction between the sea bed and the base of the wave causes the wave to begin to slow down and its shape becomes more eliptical. The top of the wave however, unaffected by the friction, becomes steeper until it eventually breaks. When the wave breaks, water washes up the beach, this is called the swash. The movement of water backdown the beach is called the backwash.

It is the rate at which waves reach the coast which determine whether the main process acting on the coastline is erosion or deposition. There are two main types of waves:

(i) CONSTRUCTIVE WAVES - tend to arrive at the coast at a rate of less than 8 waves per minute, they are low energy waves and are small in height. They have a strong swash and a weak backwash. This means that constructive waves tend to deposit material and build up a beach.

(ii) DESTRUCTIVE WAVES , have much higher energy and tend to arrive at the coast at a rate of more than 8 per minute. They are much larger in height often having been caused by strong winds and a large fetch. These high energy waves have a weak swash but a strong backwash, which erode the beach but pulling sand and shingle down the beach as water returns to the sea.

There are 3 main factors which will affect the strength of a wave and therefore whether it is more likely to erode or build up the coastline:
(i) the strength and speed of the wind - the faster the wind, the more energy is transferred and therefore the bigger the wave that is produced.
(ii) the duration of the wind - this is the length of time for which the wind has blown - the longer the wind blows, the more energy is transferred to the wave
(iii) the fetch - this is the distance over which the wind has blown and therefore how far the wave has travelled. The longer the fetch, the larger the wave is likely to be.

Follow up links:
Excellent Animation showing a wave forming and breaking (Wycombe High School)
Wave Machine Simulator - create your own ocean wave
Ocean Surface Wave - Wikipedia
Waves - includes animation of swash and backwash (BBC Bitesize)
Constructive and Destructive Waves Animation (Wycombe High School)

Key Term Check:
Swash - the movement of water and material up the beach (in direction of prevailing wind)
Backwash - the movement of water and material back down the beach (straight back down due to gravity
Constructive wave - low energy wave with greater swash than backwash - tends to build up the beach
Destructive wave - high energy wave with greater backwash than swash - tends to erode beach

Podcast: Energy at the Coast - Wave Formation
You can listen to a podcast of this post below - to download a copy to listen to on your .mp3 player click here.

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