Coastal Processes » Waves: Constructive, Destructive and Refraction

How waves form

Waves are made by wind. As wind blows across the sea, friction drags on the surface of the water and pushes it into ripples. The ripples grow into waves. Three things decide how big the waves become: how strong the wind is, how long it blows for, and the fetch. Remember, fetch is the distance of open water the wind has blown across. A long fetch gives big waves with a lot of energy.

In open water the wave moves forward but the water does not. Each bit of water just moves in a small circle, like a cork bobbing on the sea. Near the coast the sea gets shallower. The bottom of the wave is slowed by friction with the sea bed, but the top keeps moving. The wave gets steeper, tips forward and breaks on the beach.

Key terms:

• Wave height: the distance from the bottom of a wave (the trough) to its top (the crest).
• Wavelength: the distance from one wave crest to the next.
• Constructive wave: a low wave with a long wavelength that builds up a beach.
• Destructive wave: a high, steep wave that wears a beach away.
• Wave refraction: the bending of waves as they approach an irregular coastline.

Swash and backwash

When a wave breaks, water rushes up the beach. This is the swash. The water then drains back down the slope under gravity. This is the backwash. These two movements decide whether a beach grows or shrinks:

Constructive and destructive waves

Not all waves behave in the same way. Geographers sort them into two types, depending on the shape of the wave and which of swash or backwash is stronger.

A constructive wave is low and gentle. It has a long wavelength and rolls up the beach slowly, spilling forward. Its swash is strong and its backwash is weak. The water also has time to sink into the beach before the next wave arrives. So material is left behind and the beach builds up.

A destructive wave is high and steep. It has a short wavelength and crashes down onto the beach, so the swash is weak. Most of the water runs straight back down the beach, so the backwash is strong. Another wave arrives before the last one has drained away. Material is pulled out to sea and the beach is removed. Destructive waves also attack the cliff behind the beach, so they erode.

Comparing the two types

Wave refraction

Most coastlines are not straight. Some parts stick out into the sea as headlands. Other parts curve inwards as bays. Where the coastline is uneven like this, waves change direction as they arrive. This is wave refraction. Describe it as steps, in order:

1. Waves travel towards the coast in deep water, roughly parallel to each other.
2. The sea is shallower around a headland, so the part of each wave nearest the headland touches the sea bed first and slows down.
3. The parts of the wave in the deeper water of the bay keep moving faster, so the wave bends and wraps around the headland.
4. The waves converge on the headland, so their energy is concentrated there. The headland is eroded quickly.
5. In the bay the waves spread out (diverge). Their energy is spread over a wide area, so it is lower. The waves are weaker, and sand and shingle can be deposited.

Refraction helps explain why headlands are eroded strongly and have cliffs, while the bays beside them often have beaches. How headlands and bays form is explained in the lesson on headlands, bays, caves, arches and stacks.

Exam-style question

Fig. 1.1 shows made-up data about waves measured at two beaches on the same day.

(a) Identify which beach has destructive waves. [1 mark]

(b) Describe two differences between the waves at the two beaches. [2 marks]

(c) Explain why Beach B is likely to be losing sand. [3 marks]

(d) Explain how wave refraction affects the energy of the waves at a headland and in a bay. [6 marks]