Wave types and characteristics

One beach, two kinds of wave

switch the view, then tap a part

Low and long, with a wide gap between crests. The swash runs a long way up the beach and the water has time to soak in, so the backwash that returns is weak — and over a calm spell more sand and shingle is left near the top of the beach than is taken off it.

The same stretch of sand, drawn twice: once under a constructive wave and once under a destructive one. The grey arrows mark the two movements that happen on every wave; the bold arrow shows which one is winning.

Read the beach

Commit to an answer before you reveal it — deciding first is what makes the reveal teach you something.

A photographer shoots the same stretch of beach in March and again in May. In the May picture the sand reaches further out towards the sea, and a low ridge of shingle has appeared near the top of the beach. Which waves have been working on it, and how can you tell?

What you need to know

• Waves are made by wind blowing across the surface of the sea. The stronger the wind, the longer it blows for, and the greater the fetch — the distance of open water it crosses — the bigger the waves it builds.
• When a wave breaks, the water that rushes up the beach is the swash and the water that drains back down towards the sea is the backwash. Every breaking wave produces both.
• Constructive waves are low and long, with a wide gap between crests. Their swash is stronger than their backwash, so they tend to leave material on the beach.
• Destructive waves are tall and steep, with crests close together. Their backwash is stronger than their swash, so they tend to drag material off the beach and into the sea.
• Wave type belongs to the conditions at sea, not to the place. The same beach receives constructive waves through a calm spell and destructive waves in a storm.

The big picture

Waves arrive at the shore as one of two types, and the balance between the water running up the beach and the water running back down it decides whether the beach is built up or worn away.

⚠ Watch out

Writing about a "destructive coastline", as though a coast only ever receives one kind of wave. Wave type comes from the weather out at sea, so one beach can be built up through a calm fortnight and cut back in a single storm. Name the waves, not the place.

Memory hook

Constructive constructs the beach, destructive dismantles it — and the giveaway is which way the water is winning: swash up, or backwash down.

Check yourself

Sketch a beach twice — once under a constructive wave, once under a destructive one — with the swash and backwash arrows sized correctly. Say in one sentence what happens to the sand in each.