SWIM SERIES / SWELL
What makes swell and how does it affect swimming conditions?
When wind blows over the sea, it transfers energy to the water and a swell is born. It starts as a ripple, but a strong wind blowing over a large area for a long time will create a powerful swell capable of travelling across entire oceans; scientists found that a wave breaking off Lands End was made in a storm off the Falkland Islands, 8,000 miles away. The important element to understand here is that it’s the energy that travelled this distance, not the water. If you swim in swell in deep water, you will notice it lift you up and forwards a little, then back down to where you started. What you are experiencing is the energy passing through you, transferred by an orbital wave column that lifted you up and down – it’s a lot to take in, so have a look at the drawing opposite.
When a swell reaches shallow water, the underwater wave column is compressed and the waves get steeper. The next stage is the most exciting; when the depth reduces to 1.3 times the wave height, the lip peels over and the wave breaks, followed by a cacophony of brilliant white. This depth to height ratio creates a natural threshold with two very different swimming zones; in the deeper water you simply get lifted up and down by the swell, but in the shallower side you get tumbled to the beach by the surging waves. Swimming in this surf zone requires a constant lookout for the next wave - when it approaches you have two options; swim around the peeling waves, or under the walls of whitewater. That is unless you want to bodysurf, in which case front crawl towards shore as fast as you can and when you feel the wave pick you up, thrust out your arm like superman and enjoy the ride.
If you’re looking at the forecast before you get to the beach, it’s important to consider both a swell’s height and its period, defined as the time in seconds between waves. As a general rule, longer period swells are more organised and powerful, found on exposed coastlines like Cornwall and the Outer Hebrides. In contrast, shorter period swells are more common in the North Sea and English Channel, with weak, messy waves. To give a sense of the power/period relationship, doubling the period will increase the wave height by 50% when the swell reaches shallow water. This means a 2 metre swell at 20-second period will grow an extra metre taller than the same sized waves with a 10-second period. When you’re in the water this makes a huge difference; the lip punches out with the force of a heavy-weight boxer and the undertow holds you down like a sumo wrestler. While this doesn’t necessarily mean you should avoid these conditions, it’s always good to know who you’re getting into the ring with.
The appearance of a swell on the horizon not only transforms the shallow water into a surf zone; it changes the currents too. When waves break, water surges up the beach and then along it - this is a known as a Longshore Current. However, the water really wants to get back out beyond the breakers, where it was perfectly happy before the wave catapulted it to shore. To get there it uses a rip current, either flowing along an obstruction that faces out to sea or through a deeper channel where the waves don’t break. Obstruction rips are easy to locate because they will be running alongside a wall, cliff or jetty. Deepwater Rips are more difficult to spot, because the topography that creates them is hidden underwater. Look out for darker water [because it is deeper] with traces of sediment washed out from the beach. When there’s a swell, expect multiple rips on a beach and bear in mind that they change with the tide.
“ Obstruction rips are easy to locate because they will be running alongside a wall, cliff or jetty. “
For most people swimming at the beach, the greatest terror is to be taken out to sea in a rip. A common reaction is to swim straight back to shore, but this is the worst thing to do – not even an Olympic gold medalist could outswim a rip. By the time most people realise this they’re exhausted, then they panic. Then they drown. The tragedy is that this can easily be avoided by swimming just a little way across the current, which is rarely more than 20 metres wide. Once there, swim in with the waves that are pushing water up the beach. If there is a lifeguard patrolling the beach you don’t even need to do this; simply lie back and raise your fist to signal for help. A sensible strategy before you get in the water is to find high ground and look out for rips, but they can still catch you out. If there is a lull in the swell there will be no rips. But if a set of powerful 20-second period waves appear, it will suddenly create a ‘Rip Pulse’ that speeds up to 2.5m/s in just a few heartbeats, taking you 10, 20, 30 metres out. The secret is to stay calm, and a perfect way to achieve this is to spend lots of time in the swell so that you are confident swimming in and around waves and rips.
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