Waves and tides at the coast

Geography · Coasts

Waves or tides?

Pick a clue, then choose where it belongs: waves or tides?

7 of 7 still to sort.

Both move the sea at the shore, so they get mixed up all the time. Make a guess for each clue first. The reason pops up straight after.

Predict, then check

Same wind, two different coasts. Which sea gets the bigger waves?

The same wind blows towards two coasts. Coast A has only a short stretch of sea for the wind to cross. Coast B has a long stretch. Which coast gets the bigger waves?

Geography · The shore

tap a part, or peel a layer

At low tide the sea level is at its lowest and beaches and estuaries are revealed.

Tap a part to read what it is. Switch layers on and off to see what sits where.

What you need to know

• Waves are energy moving across water, and wind blowing across the sea is what gives the water that energy.
• Tides are the regular rise and fall of sea level, caused by the Moon's gravitational pull on the Earth's oceans.
• Tides happen whether or not there is wind, so wind is not the cause of tides, and tides are not the cause of waves.
• Have a goSam says: 'It's really windy today, so the tide will be doing something special.' Sam is very sure. What's wrong with Sam's reason?

  Wind isn't what causes the tide, so it can't be the reason.

  Tides happen whether or not there is wind, so wind is not the cause of tides.

• High tide is when the sea level is at its greatest and the water covers the shore.
• Low tide is when the sea level is at its lowest and beaches and estuaries are revealed.
• Have a goYou arrive and the sea has dropped right back. Sand is showing and so is an estuary. High tide or low tide?

  Low tide.

  At low tide the sea level is at its lowest and beaches and estuaries are revealed; at high tide the water covers the shore.

• Fetch is the distance over water that the wind blows. The greater the fetch, the bigger the waves.
• When a wave breaks on a beach, the water pushed up the beach is the swash.
• The water that runs back down the beach is the backwash.
• Have a goA wave breaks and the water slides back down the sand towards the sea. Is that the swash or the backwash?

  The backwash.

  Swash is the water pushed up the beach; backwash is the water that runs back down it.

• Constructive waves have a powerful swash and a weaker backwash, so they help to build up beaches through deposition.
• Destructive waves have a powerful backwash and a weaker swash, so they can reduce beaches because of the erosion they cause.

The big picture

Waves and tides both move the sea at the shore, but they have different causes. Wind blowing across the sea gives waves their energy, while the Moon's gravitational pull causes the regular rise and fall of sea level called the tide. You will also meet fetch, swash and backwash, and see how constructive and destructive waves treat a beach differently.

⚠ Watch out

Thinking a tide is just a very big wave, or that the wind pushes the tide in. A tide is the regular rise and fall of sea level caused by the Moon's pull, and it happens whether or not it is windy.

Memory hook

Wind makes waves. The Moon moves the tide. And swash is the water that surges up the sand, so backwash is the water that comes back.

Check yourself

Cover the page. A wave breaks on a beach. Say out loud which water is the swash and which is the backwash. Then say which type of wave has the powerful backwash.