We’ve had some impressive swell lately and it was interesting to hear some surfers debate about ‘double-overhead’ and ‘6 foot faces’ while others said it was ‘12 foot’ and the swell forecasts were predicting 4-5 metre waves. There’s a lot going when it comes to describing wave height, not to mention the impressive use of Imperial measurements that was phased out in the early ’70s!

Wave height is simply the vertical distance from the wave crest (highest point of a wave) to the wave trough (lowest point), but measuring wave height accurately is not easy. You can do it by eye, which is subjective and prone to error, or you can measure it directly using instruments, which is accurate, but expensive and logistically difficult. Fortunately, NSW has a network of offshore wave rider buoys, managed by the Manly Hydraulics Laboratory that continuously measure wave height, period (time between the waves) and wave direction. It’s worth looking at the Sydney or Port Kembla wave data, which gives you almost live measurements of wave height.

If you do, you’ll notice they talk about significant wave height (given by Hs), which is the average of the highest one-third of waves measured over a certain time period. Wave height varies from wave to wave and we’re mostly interested in the bigger waves, not just for surfing purposes, but for dealing with coastal hazards and management issues, such as beach erosion. A good way to think of significant wave height is that it’s probably close to the height of most of the waves in a large wave set. Then there’s the maximum wave height (Hmax), which is just that – the largest wave recorded over a certain period. The largest wave ever recorded in NSW was 17.7 m during the famous June 2016 East Coast Low.

One thing to be aware of when it comes to wave height is that many surfers often under-estimate the actual wave height by about half. It’s part of surfing culture and can be traced back to Hawaii where the original surfers used to sit offshore on their boards estimating the height of the back of big waves as they passed them by. However, by doing this, they only really saw half the true wave height, something called the wave amplitude. If they measured it from the front of the face of the wave while staring up at the crest about to break on their heads, it would be a different story.

So when a surfer talks about the surf being ‘3 foot’, which doesn’t sound that big, it’s probably actually closer to ‘6 foot’, or 2 metres, which is big! So be careful – surfers all know what it means, but the average beachgoer probably doesn’t.

Have a question for Dr Rip? Email rbrander@unsw.edu.au