Coastal Systems - How Wind Creates Waves

The primary cause of erosion along a coastline is by wave action. Waves are formed by the wind blowing across the surface of the water.

Wave action

The primary cause of erosion along a coastline is by wave action. Marine processes increase with high energy waves. These waves are destructive waves which are high frequency waves (occurring 10-15 times per minute) and they are tall, which means that they crash onto the beach and scour away at the land, removing material out to sea. Their swash up the beach is less powerful than their backwash, causing a net beach loss. They cause erosion, primarily by abrasion, whereby material is thrown at cliff faces and wears it away.

The primary cause of deposition is also to do with the wave action. Low energy waves known as constructive waves are less frequent (6-9 times per minute) and roll onto the beach. They lose energy when rolling up the beach so deposit any material that they are carrying. Their forward swash is more powerful than their backwash leading to net beach gain.

How waves are formed

Waves are formed by the wind blowing across the surface of the water. Initially, the blowing wind will create ripples and these will then turn into waves. As waves reach the coast the lower part of the wave will slow down due to friction. The upper part of the wave then falls forward and breaks onto the beach.

What makes the difference between a destructive and a constructive wave is wind strength, which is affected by local weather systems and the fetch.

The fetch is the distance of the sea, over which the wind has travelled. For example, if the wind blows over an area of sea which is several thousand miles long, destructive waves will form as they have had the time to generate greater energy. Whereas, a shorter fetch will result in smaller, constructive waves.

As well as the fetch, the strength of the wind can affect the type of wave. Even in a narrow section of coast, strong winds associated with storm conditions can lead to destructive waves. These waves can cause wave quarrying where high energy waves smash against the cliff faces and remove chunks of rock into the sea.

The Coriolis force also affects the impact of wind on waves. The Coriolis force is where the earth’s rotation affects the way that winds travel over the earth. In the northern hemisphere, winds are deflected clockwise and counter-clockwise in the southern hemisphere. This has an impact as some coastlines depending on their orientation as they will have water deflected at them in a much more direct head-on way than others as a result of this force.