So what is wave period and why should us kayakers care about it? If you want to avoid getting swamped by a freak wave offshore or while launching then it is a good to learn enough to keep yourself safe.

A Magic Seaweed forecast showing the swell direction and period.

Waves are generated by wind blowing in a particular direction over a period of time. If you blow across a tray of water you get lots of small ripples travelling in the same direction. Ocean swell is generated in exactly the same way, but on a much larger scale. Wave period is the distance between the crests of adjacent swells travelling past a stationary object, measured in seconds. The more seconds between swells, the higher the wave period and the more powerful the swell.

Wave period increases the further that it travels. If swell has been generated by storms or strong winds a very long way away, then the period could be as high as 20 seconds, although 12 to 16 seconds is still considered a long period. In the UK this tends to occur in the late summer and autumn when hurricanes off the east coast of the US can send swells all the way across the Atlantic until they hit our W and SW facing coastlines.

Shorter period swell is generated by wind blowing right now or from very close by. It causes a choppy, bumpy sea with lots of small peaky waves close together. The waves aren’t particularly powerful but they are numerous and disorganised. Not terribly fun to be out in, but these swells will quickly fade when the wind drops.

If you are out in your kayak on a windless day and there is a long period swell running you will notice long, uniform swells moving quickly in the same direction. Sooner or later these swells will hit a beach or reef and you really don’t want to be there in your kayak when that happens.

No problem you say, we just won’t launch from those sites. Unfortunately, it is a bit more complicated than that due to a couple of characteristics of long period swell. Firstly, they tend to bunch up into sets of waves as they travel across the ocean, with long gaps in-between. This means that the ocean could be almost smooth and flat for twenty or thirty minutes and then a set of five or more large waves could arrive and break, before the ocean again returns to a benign state. You could really get caught out if you arrive at your launch site in one of these gaps and then a set arrives just while you are launching.This is also an issue if you are paddling offshore on an otherwise flat sea and happen to be paddling over a raised reef or pinnacle of rock just as a set of waves arrives that are big enough to break. The combination of long period swell and shallower water cause the so called ‘freak waves’ that you hear about from sailors.  

The second characteristic of longer period swell is that it is able to ‘bend’ into protected bays and stretches of coastline that would otherwise be protected. They do this because they are more affected by changes in the sea floor. Take my local stretch of the North Devon coastline for example. This faces pretty much north and you can see Wales easily on a clear day. With open ocean swell coming from the SW you would expect this coastline to be protected and mostly calm, and luckily it is for most of the year. But when there is a big long period sell running it bends almost 120 degrees around Bull Point and you can get some really big waves breaking on the reefs as far up the coast as Minehead.

I was walking the coast path a couple of years ago and watched a group of three kayakers making their way up the coast towards Lee Bay after launching at Ilfracombe. There was a decent swell  running and as they rounded a small point I realised that they were much too close in and were passing over a finger of shallow reef. I started waving and shouting but was too late and a set of waves arrived hitting all three and washing two of them onto the rocks. They were very lucky and got away without serious injury, but it could have been much worse.  

So what can we do to keep ourselves safe? The main thing is to be aware of swell and how it affects the places we fish. Just because you have launched at a spot ten times during the summer doesn’t mean that it will be safe when the Autumn swells arrive. Surfers are well aware of swell period and surf forecasting sites will always include the direction and period of the swell. Alongside your usual weather checks make sure you check sites like Magic Seaweed or Surf line. These sites receive data from offshore buoys and satellites and are amazingly accurate. If you are out in an unfamiliar area when you know there is a long period swell running then it pays to be extra careful and avoid shallow reefs or any area where you see white foam on an otherwise still sea. This suggests that a wave has broken there recently so you should avoid at all costs.

This reef in North Devon causes waves to break far out to sea when a long period swell hits the pinnacle of reef.

This article was written and donated to the site by Dave Hitchins, an experience ‘yak angler and it’s greatly appreciated!