GeographyISC Class 11 · Ocean Water Movements

Waves and Tsunamis: Parts and Calculations (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Waves are formed by wind friction on the sea surface; water particles move in circles while the wave form travels. A wave has crest, trough, height, amplitude (half the height), wavelength and period; speed = wavelength / period and the wave breaks when steepness exceeds about 1/7. Tsunamis are very long waves caused by undersea earthquakes, landslides or eruptions.

Understand Waves and Tsunamis: Parts and Calculations

Formation. Wind friction transfers energy to water; size depends on wind speed, duration and fetch (distance of open water), so long fetches in the Southern Ocean make very large waves. Water particles move in near-circular orbits; the energy and wave form move forward. Near shore, friction with the bottom slows the wave, shortens wavelength, raises height and the crest topples as a breaker; water rushes up as swash and returns as backwash. Swell is smooth regular undulation travelling far from its storm.

Parts and measures. Crest (highest), trough (lowest); wave height is trough-to-crest distance; amplitude is half the height; wavelength is the distance between successive crests; period is the time for two successive crests to pass a point; frequency = 1 / period; speed = wavelength / period; steepness = height / wavelength, and a wave breaks when steepness exceeds about 1/7.

Tsunamis. Very long waves from undersea earthquakes, landslides or volcanic eruptions, not wind; the Indian Ocean tsunami of 26 December 2004 is the chapter's example.

Link to coasts. Breaking waves erode and deposit; destructive waves have strong backwash, constructive waves strong swash.

Waves and Tsunamis: Parts and Calculations: key facts

Wave speed: $S p ee d = w a v e l e n g t h / p er i o d ∣ F r e q u e n cy = 1/ p er i o d$
Amplitude: $A m pl i t u d e = h e i g h t /2∣ H e i g h t i s t r o ug h t ocr es t$
Steepness: $S t ee p n ess = h e i g h t / w a v e l e n g t h ∣ B r e ak s ab o v e ab o u t 1/7$
Tsunami: Cause: undersea earthquake, landslide, eruption
Very long wave
Example: 26 December 2004

How to answer Waves and Tsunamis: Parts and Calculations questions

Questions are 3 to 4 marks: parts of a wave, formation, or numerical on speed and steepness.

1Draw the wave and label crest, trough, height, wavelength.
2Write the formula, substitute and give the unit.
3Check steepness against 1/7 (about 0.143).
4Distinguish wind waves from tsunamis by cause.

Waves and Tsunamis: Parts and Calculations in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Wave speed

Suppose the distance between crests is 90 m and the period is 10 s.

Find the speed and frequency.

Try it first, then show the solutionSolution

Speed = 90 / 10 = 9 m/s.
Frequency = 1/10 = 0.1 per second.

Answer: 9 m/s; 0.1 per second.

Memory hook: Wavelength over period.

Will it break?

Suppose a wave of height 3 m and wavelength 18 m approaches the beach.

Will it break?

Try it first, then show the solutionSolution

Steepness = 3/18 = 0.167.
1/7 is about 0.143; 0.167 is more than this, so it breaks.

Answer: Yes, it breaks.

Memory hook: Steepness above 1/7 breaks.

Tricks to remember Waves and Tsunamis: Parts and Calculations

Short memory aids. Say each one aloud once, then use it on a question the same day.

Fetch builds waves
Long fetch, strong wind, long time: big waves.
Use it when: Wave size.
Half the height
Amplitude is half the wave height.
Use it when: Amplitude.

Waves and Tsunamis: Parts and Calculations: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Amplitude · 1 mark · MCQBoard level

If the wave height is 3.6 m, the amplitude is:
(a) 7.2 m
(b) 3.6 m
(c) 1.8 m
(d) 0.9 m

Try it first, then show the solutionSolution

Half the height.

Answer: (c) 1.8 m.

Example 2: Tsunami cause · 2 marks · Short answerBoard level

State two causes of tsunamis and give the chapter's example date.

Try it first, then show the solutionSolution

Undersea earthquakes, landslides or volcanic eruptions.
Indian Ocean tsunami of 26 December 2004.

Answer: Undersea earthquake or landslide; 26 December 2004.

Example 3: Speed and frequency · 4 marks · Long answerHarder board level

A wave has wavelength 120 m and period 12 s. Find its speed, frequency and, for a height of 4 m, its amplitude and steepness. Say whether it will break.

Try it first, then show the solutionSolution

Speed = 120 / 12 = 10 m/s.
Frequency = 1/12 = 0.083 per second.
Amplitude = 4/2 = 2 m; steepness = 4/120 = 0.033.
0.033 is less than 1/7 (0.143), so it will not break.

Answer: 10 m/s; 0.083 per second; 2 m; 0.033; does not break.

Example 4: Fetch · 4 marks · Long answerHarder board level

Explain how wind speed, duration and fetch affect wave size.

Try it first, then show the solutionSolution

Stronger wind transfers more energy.
Longer duration builds bigger waves.
Longer fetch lets energy accumulate.
So long fetches in the Southern Ocean give very large waves.

Answer: Wind speed, duration and fetch all raise energy; long fetches give large waves.

Example 5: Compare waves · 5 marks · Case-basedHOTS / competency

Case: A coastal village feels a smooth long swell from a storm far away and, separately, a huge wave from an undersea earthquake. (a) Name each. (b) Compare their causes. (c) State what happens to wavelength and height as a wind wave nears shore.

Try it first, then show the solutionSolution

(a) Swell and tsunami.
(b) Swell comes from wind in a distant storm; tsunami from an undersea earthquake, landslide or eruption.
(c) Wavelength shortens and height increases until the crest topples as a breaker.

Answer: (a) Swell, tsunami (b) wind versus seafloor disturbance (c) wavelength shortens and height rises.

Where students lose marks in Waves and Tsunamis: Parts and Calculations

The slips examiners see most often, and what to write instead.

Mistake: Treating amplitude as the full height
Half rule forgotten.
Wrong working
Amplitude = 3.6 m
Right working
Amplitude = 1.8 m
Do this: Amplitude = height / 2.
Usually costs: 1 mark.
Mistake: Inverting the speed formula
Period over wavelength.
Do this: Speed = wavelength / period.
Usually costs: 2 marks.
Mistake: Saying tsunamis come from wind
Cause confused.
Do this: Tsunamis come from undersea earthquakes, landslides or eruptions.
Usually costs: 1 mark.

Exam tips

Draw and label a wave.
Quote 1/7 about 0.143.
Give units in m/s.
Distinguish swell from tsunami.

Practice questions on Waves and Tsunamis: Parts and Calculations

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The distance between two successive crests is the:
(a) height
(b) amplitude
(c) wavelength
(d) period
Hint
Horizontal distance.
Answer
(c) Wavelength.
Q2 · 3 marks · Short answerHarder board level
A wave of height 2 m and wavelength 20 m reaches shore. Find its steepness and say if it breaks.
Hint
Height over wavelength.
Answer
Steepness = 2/20 = 0.1; 1/7 is about 0.143; 0.1 is less, so it does not break yet.
Q3 · 3 marks · Short answerHarder board level
A wave passes a pole with a period of 8 s and speed 12 m/s. Find the wavelength and frequency.
Hint
Wavelength = speed x period.
Answer
Wavelength = 12 x 8 = 96 m; frequency = 1/8 = 0.125 per second.
Q4 · 4 marks · Long answerHarder board level
Explain why waves break near the shore.
Hint
Friction with the bottom.
Answer
Friction with the shallow bottom slows the base of the wave, shortening wavelength and increasing height; the crest steepens and topples forward as a breaker.
Q5 · 4 marks · Case-basedHOTS / competency
Case: A coast has strong destructive waves in winter and gentle constructive waves in summer. (a) Name the swash and backwash. (b) State the effect on the beach in each season. (c) Name one cause of swell.
Hint
Backwash strength.
Answer
(a) Swash is the rush up the beach; backwash the return flow. (b) Destructive waves with strong backwash remove material; constructive waves with strong swash build the beach. (c) Distant storm winds.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Frequency formula?Show answer
1 / period.
2.Breaking steepness?Show answer
Above about 1/7.
3.Rush up the beach?Show answer
Swash.
4.Tsunami date in chapter?Show answer
26 December 2004.
5.What is fetch?Show answer
Distance of open water over which wind blows.

Waves and Tsunamis: Parts and Calculations: frequently asked questions

Does water travel with a wave?

No, particles move in circles and return nearly to place.

Why do waves break?

Shallow water raises height and shortens wavelength.

What is swell?

Smooth waves that travelled far from their storm.