Forecasting Tutorial: Wave Period Explained
Understanding The Magic Number
At Coastalwatch we often receive emails with questions about our surf forecasts. Here, Chief Swell Forecaster Ben Macartney explains how Swell Period affects surf quality and answers a reader question: Which is better, long period or short period swells?
When it comes reading forecast graphs, swell period is definitely the magic number. Indeed, for a forecaster, explaining the implications of swell period is a bit like the birds and the bees – of surfing. The basic definition is as follows: Swell period is a measure of the time, in seconds, between successive wave crests (or troughs) passing through a stationary point.
A good way to get a handle on it is to manually count seconds or time set waves reaching a stationary point. For example, if you were watching a fishing boat anchored beyond the breaking zone, you could count the seconds between one wave going under the boat and the next . Or you can measure time between one wave hitting a rock ledge and the next - and there you would have an approximate wave period.
However, wave period gives you a lot more information about a swell than just how long you’ll have to wait before the next wave lands on your head. Primarily, it provides a deep insight into the underlying energy contained in a swell. The shorter the period, the weaker and slower the swell, and the closer to the surface it travels. The longer the period, the more wave energy is contained within the swell and the faster and deeper beneath the sea surface it travels.
Indeed, it’s the mathematical relationship between wave period and wave speed and how far the wave energy penetrates beneath the sea surface that allows us to calculate, with great accuracy, things like swell arrival times as well as breaking wave heights. Big interval groundswells of 15 to 20 seconds carry much more energy, penetrating over 1000 feet below the sea surface (for 18 second intervals). This means groundswells feel the sea floor in much deeper water than windswells (a windswell is loosely any wave exhibiting periods of 6 seconds to 12 seconds) and hence will refract and react the sea floor further offshore.
So, getting back to the question; when asking what's better; long periods or shorter ones, it depends on what kind of waves you are looking for. More often than not, big groundswells react to a sand bank in deeper water, often beyond the rips and gutters that shape good beach break surf – and hence tend to close out. Conversely, windswells are often more broken up and only begin to ‘feel’ sand banks in much shallower water, and are therefore more likely to produce good surf as they peel off along gutters and rips.
If you're looking for reefs and point breaks, then longer period groundswells generally produce better surf - but this of course is heavily dependent on the characteristics of an individual break – or more specifically the bathymetry; the shape and depth of the seafloor that the approaching swell interacts with. Most world class point breaks love a long interval groundswells that wrap in from oblique angles, producing long, high quality waves.
However, really big periods of 18 to 20 seconds don’t always equate to good surf. Such high energy swells can be too much for some breaks, with all that deep water energy drawing too much water off the reef before the wave breaks.
Anyway, I hope that helps. For a more technical overview on Wave Period click here.
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