So you want to surf a tsunami? In this article, you’ll learn why that’s not possible – and how dangerous it would be to try. Tsunamis have much more energy than a surf wave, so they last for longer and are much faster. They also look very different to a surf wave. However, tsunamis and surf waves do share some basic characteristics, and both arise as a result of shoaling.
Tsunamis and surf share some important features
Tsunamis and surf waves are both water waves. Like all waves, they are a mode of energy transfer and can be described by their wavelength, speed, frequency, period, amplitude (height) and energy. As waves, they also behave in particular ways – they can be reflected, refracted and diffracted, and two waves can interfere with each other.
Shoaling is important for both tsunamis and water waves. Shoaling is an increase in wave crest height above sea level. It happens when waves enter shallow water (such as coastal areas). Shoaling gives tsunamis their height and their devastating impact. (In the deep ocean, tsunami crests are usually much less than a metre above the sea level.) Shoaling also causes ocean swells to grow in height near the shore, forming surf waves.
Tsunamis and surf have different origins
A tsunami is caused by a massive disturbance to a large body of water. The disturbance has to be under the water or in the water. This means that sea-floor tectonic fault ruptures, underwater volcanic activity (such as eruptions of volcanoes on the sea floor), landslides or earthquakes, explosions in the water or a large object (such as a meteorite) hitting the water can all cause tsunamis.
Unlike tsunamis, surf waves are formed by wind. As it blows across the ocean, the wind imparts energy to the water surface, forming waves (ocean swells). As the wind continues blowing, small swells are whipped up into much larger waves. These waves or swells then interact with land (the seabed) as they near the coast and form the surf waves that we see at the beach.
Tsunamis: much faster, much longer
Tsunamis travel very fast. In deep water, they can move at several hundred kilometres per hour – about the speed of a passenger jet. They are about 10 times faster than a wind-generated ocean swell, which usually travels about 50 km/h in deep water. Both tsunamis and ocean swells slow down in shallow coastal water, but a tsunami will still hit land at 30–50 km/h (too fast to run away from).
Tsunamis have very long wavelengths (over 100 km). Ocean swells are about a thousand times shorter – they have wavelengths about 150 m at the most, decreasing to 30–50 m at the coast when they become surf waves. Long waves involve large volumes of water, so when a tsunami hits land, the water keeps on coming as a torrent for several minutes – the period of the wave at the coast is 10–60 minutes. Compare this with surf waves, which dump a small amount of water on the beach over a few seconds and then recede (the period of the surf is usually 5–15 seconds).
Only tsunamis touch the bottom of the sea
Tsunami waves travel through the whole water column down to the sea floor, even to depths of thousands of metres. This is important for predicting the path of a tsunami because the topography of the bottom of the sea guides where the wave is going to travel. Ocean swells are surface waves and do not ‘feel’ the sea floor, except in shallow water.
Breaking versus surging
Surf waves, by definition, are breaking waves (‘breakers’) – they form a whitecap and often have an unbroken front face of water (ideal for surfing) before breaking. A tsunami, though, usually doesn’t break but instead surges onto land. Even if it does break, it doesn’t look like a surf wave. Instead, it forms a turbulent bore, which is a surge of white water without a clear face.
Why can’t you surf a tsunami?
A tsunami usually isn’t a breaking wave and doesn’t have clear water to surf on. It’s also far too fast to surf safely, and it doesn’t end at the beach – instead, it travels long distances inland over a long period before receding back to the coast. Even if you did manage to surf a tsunami, the water would be full of debris from the sea floor, trees, cars and pieces off buildings, and colliding with this could leave you badly injured.
Use an interactive or paper-based Venn diagram to illustrate the key similarities and differences between tsunami waves and surf waves.
Use a Mexican wave to demonstrate how waves transfer energy and to help your students visualise the wave behaviours of reflection, constructive interference and shoaling.
A shallow tray of water is used to demonstrate wave generation and behaviour.
Some clear advice for surfers – why they shouldn’t attempt to surf a tsunami.
Article from The Conversation on how to prepare for a tsumnai, this includes videos.
Real-time water-level data from the Aotearoa New Zealand tsunami gauge network.
This video from NOAA explains how to prepare for and respond to a tsunami.
Find out more aboout lake tsunamis in Aotearoa New Zealand in this Newshub article – looking at the risks and this Geonet article – covering causes, where they have occurred, and how we can be prepared for them.