Earth and Environmental Science 11–12 · Year 12
Tsunami speed and water depth in a tray, and the shallow-water wave law
Module 6: Hazards
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The idea
A tsunami is a shallow-water wave whose speed depends on depth alone, so it crosses the deep ocean at aircraft speed and slows, steepens and grows as it reaches the coast.
What you need
- a tray at least 1.0 m long (a length of guttering with capped ends works), water, ruler
- a 50 mm block to raise one end and drop it to start the wave (Earthlearningidea), or a paddle to make one pulse, stopwatch, phone camera for slow-motion timing
- a wedge of clay or a sloping board at the far end as the coast
How to do it
- Fill to a depth of 2 cm; make one pulse and time it over a marked 1.00 m; repeat five times and average.
- Repeat at 5 cm and 10 cm.
- Calculate speed and compare with √(g d).
- Add the sloping coast and film the pulse running up it; note the change in height and speed.
- Scale the law up: compute the speed for a 4000 m deep ocean and compare with the NOAA statement that a deep-ocean tsunami moves at over 500 mph.
What you should see
√(g d) predicts 0.44 m/s at 2 cm (2.26 s per metre), 0.70 m/s at 5 cm (1.43 s per metre) and 0.99 m/s at 10 cm; the measured speeds are compared with these, and the law applies when the pulse is long compared with the depth. At 4000 m the same law gives 198 m/s, 713 km/h; NOAA’s ‘over 500 mph’ corresponds to depths above about 5100 m, and the law reproduces NOAA’s own worked figure of about 475 mph in 15 000 feet of water. On the model coast the pulse slows and rises before it breaks.
What changes
- What you change
- water depth
- What you measure
- wave speed
- What you keep the same
- same pulse size
- same timed distance
- same tray
- five trials per depth
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A tsunami is a tall wave at sea (NOAA notes it is smaller in height in the deep ocean and grows as it reaches shallow water).
- Tsunamis are caused by tides.
Safety card
Hazards
- water on the floor
Controls
- towels and a tray
Note
No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES12-13EES11/12-1EES11/12-2EES11/12-3EES11/12-4
- Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-12-02
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.