When waves reflect back off a sea wall, they meet more incoming waves and this can set up a totally new wave pattern called ‘clapotis’ – from the French word for ‘lapping’.
We can see clapotis in the video above. Notice how it is hard to tell if the waves are coming towards us or moving away. That is because they are doing both – and neither. Welcome to the weird world of clapotis!
Waves are well-behaved things, they carry energy from one place to another in a totally predictable way. The direction of travel only changes when waves reflect or diffract – ie. when they bounce off something or squeeze through a gap or past the edge of a barrier.
When waves hit a sea wall at an angle, the waves reflect and continue heading out in the same way that light reflects off a mirror. This creates a cross-hatch pattern in the water, known as ‘clapotis gaufre’ or ‘waffled clapotis’.
Occasionally waves hit a wall head-on and reflect straight back out in the exact opposite direction that they arrived. The outgoing waves then interfere with the incoming waves and this sets up a new pattern where the waves don’t appear to be travelling in our out, they are just moving up and down. This new pattern is called clapotis – see the video at the top.
In the video below we can see a complex pattern of clapotis and waffled clapotis waves in Reykjavik harbour. But see how the waves appear to be going up and down but not travelling predictably in a single direction. Also notice how the waves nearest us are bigger than those nearer the boats.
The waves in clapotis will be twice the height of the original waves – ie. the waves as they were before they hit the wall. And this is why it can be a potentially dangerous phenomenon if you swimming or in small craft.
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