Waves and wave energy

Overview

How a wave breaks, swash and backwash and characteristics of a constructive wave.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   How is a wave generated, and what determines how much energy it carries?

   Model answer

   Wind blowing across the water surface transfers energy to it by frictional drag, and the water is set into a circular orbital motion. The energy of the resulting wave depends on three things: the strength of the wind, the length of time it blows for, and the fetch, which is the uninterrupted distance of open water over which it blows. A strong wind blowing for a long period across a very long fetch produces the highest-energy waves, which is why the most powerful waves in the British Isles arrive on coasts exposed to the south-west across the Atlantic.

2. 2

   Describe how a wave breaks, and explain why it breaks at all.

   Model answer

   In deep water the water particles move in closed circular orbits and the wave form advances while the water itself does not travel far. As the wave enters shallow water, roughly where the depth is less than half the wavelength, the base of the orbit is slowed by friction with the sea bed. The orbits become elliptical, the wave slows and shortens in wavelength, and because the same energy is packed into a shorter distance the wave grows in height and steepens. Eventually the crest is moving faster than the base and can no longer be supported, so it topples forward and breaks, releasing its energy as the swash rushes up the beach.

3. 3

   What are swash and backwash?

   Model answer

   Swash is the rush of water up the beach after a wave has broken, driven by the momentum released at the break and running up at the angle at which the wave approached. Backwash is the return of that water down the beach under gravity, which always runs straight back down the steepest gradient, at right angles to the shoreline. The relative strength of the two determines whether a beach gains or loses material.

4. 4

   What are the characteristics of a constructive wave?

   Model answer

   Low in height, typically under a metre, with a long wavelength and a low frequency of about six to eight waves per minute. They are usually generated by distant storms and arrive as a long swell. They break as spilling waves in which the crest gently tips forward, producing a strong swash that carries material a long way up the beach, and because the low frequency gives the water time to soak into the beach before the next wave arrives, the backwash is weak. Since swash exceeds backwash there is net movement of sediment onshore, so these waves build the beach up, building a steeper beach face with berms.

5. 5

   What are the characteristics of a destructive wave?

   Model answer

   High, steep waves with a short wavelength and a high frequency of about ten to fourteen per minute, generated locally by strong winds over a long fetch, typically in storm conditions. They break as plunging waves in which the crest curls over and crashes down almost vertically, so much energy is directed downwards rather than up the beach and the swash is short and weak. Because the waves arrive in quick succession the previous backwash is still running down the beach when the next wave breaks, so backwash exceeds swash and material is dragged offshore. These waves comb material down the beach, lowering and flattening the beach face, leaving a storm ridge at the back and building an offshore bar.

6. 6

   Why is the same beach steeper in summer than in winter on many coasts?

   Model answer

   Because the dominant wave type changes with the season. In calmer summer conditions constructive waves dominate, moving sediment onshore and piling it up the beach face as one or more berms, so the profile is built up and steepened. In winter, frequent storms generate destructive waves that comb material off the beach face and deposit it offshore as a bar, lowering the profile and leaving a flatter, wider beach with only the storm ridge at the back standing steeply. It is important to state this as a tendency of coasts with a marked seasonal storm pattern rather than as a universal rule, since it does not hold everywhere.

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