Shallow Water Turns a Tsunami into a Wall

Lab Brief #5

🎯 Your mission

Set it up. Test it. Be amazed.

⚡ The twist

The slowest changes are the biggest ones.

What you'll learn

In deep ocean a tsunami is barely a metre high and can be hundreds of kilometres long, crossing the Pacific at jet-aircraft speed because wave speed depends on water depth. As the water shallows, the front of the wave slows while the back is still travelling fast, so the wave shortens dramatically and the energy it carries is compressed into height. Nothing is added — the same energy in a fraction of the length is what turns an unnoticed swell into a wall at the shore.

Key words

tsunami
A very long ocean wave, usually generated by a sea-floor earthquake, whose speed depends on water depth.
wave shoaling
The slowing and steepening of a wave as it moves into shallow water, raising its height.

Make a hypothesis first

Predict: which will erode faster?

Say your guess out loud — then find out if you were right.

Get ready

🧰 What you need

shallow tray
water
sloping board
ruler
phone camera

Get ready

⚠️ Safety first

Mop up spills straight away so nobody slips, and keep the tray away from sockets.

Step 1 of 4 · Read

Think about what you just learned: say what "tsunami" and "wave shoaling" mean in your own words before you start.

Step 2 of 4 · Do

Use a shallow tray of water with a sloping base to model shoaling: generate a long low pulse at the deep end and film the wave as it climbs the slope. Measure its height at three depths and describe how height and length changed together.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 3

A tsunami's speed depends mainly on:

How sure are you?😐 maybe

Question 2 of 3

A tsunami grows tall near shore because the wave:

How sure are you?😐 maybe

Question 3 of 3

In deep ocean a tsunami is typically:

How sure are you?😐 maybe

Mind = blown

🪨 Every grain of sand on every beach used to be a mountain.

Nice work!

You got 0 of 3 right.

Next: The Geodynamo and Magnetic Reversals All lessons

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✨ More to explore

🌍 Where you see this in real life: Today's weather, tomorrow's beach, next century's coastline — all this.

🏆 Stretch challenge (try it this week): Look at the sky for 5 minutes and try to predict tomorrow's weather.

👨‍👩‍👧 For the dinner table: “What's the slowest change you can think of?”

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Shallow Water Turns a Tsunami into a Wall

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