William Thomson, author of World of Tides, explores the what’s, why’s and how’s when it comes to tides, so you can take one glance at a tide app and quickly predict what type of seas to expect
If you go sea swimming at the same time every day, on the same beach, you may notice that high and low tides alternate every week. If it is high tide in the early morning today, it’ll be low tide at the same time next week, then high tide the following week. The daily transition is subtle, almost unrecognisable, but the weekly transformation is dramatic; the beach literally changes shape, waves behave differently and currents flow the opposite direction. These changes have a huge impact on our swims, which is why understanding the tide is crucial for swimmers.
Up and down the Channel
We all know that the tide is affected by the moon. But few people understand exactly where the water comes and goes as the tide ebbs and flows. Where I live in Deal, Kent, many people ask if the tide is rising in Calais [across the channel] when it is falling in Deal. Their thinking is that water sloshes back and forth across the channel, and they are almost right. In fact, it pours up and down the English Channel. So when it is low tide in Deal, it is low tide in Calais – and high tide in Lands End. Imagine a huge wave, a tide wave, with Deal at its trough and Lands End at its peak. Then, over the course of six hours, the peak travels up the channel towards Deal, bringing high tide. This is one of three tide waves around Britain; another flows up the west coast and a third travels down the east coast, where its peak meets the English Channel wave and they travel up the coast of Europe together.
Spring and neap tides
These tide waves are shaped by the gravitational pull from the moon and set in motion by the spinning of the earth. The time that peaks pass is directly connected to the moon phase, and the reason high tide is [on average] 50 minutes later every day is because for every 24-hour rotation of the earth, the moon has made 12 degrees of its orbit, and it takes 50 minutes for the earth to realign with its new position. The lunar orbit also affects the height of tide; when the moon and sun [which also exerts a gravitational pull on our seas] are in line with the earth at the Full Moon and New Moon, their gravitational pulls work together and we get stronger tides with higher highs and lower lows. These are called Springs, not because of the season but because they ‘spring’ up and down with more energy. In contrast, when half the moon is illuminated during the First and Third Quarter phases, it is at right angles to the sun and earth and the combined gravitational pull is reduced, creating weaker tides called Neaps.
As the peaks and troughs of tide waves pass, the most obvious change is the depth. The seabed is rarely a consistent slope and on most beaches [Brighton is a perfect example] there is a gently sloping seabed that gets steeper close to shore. This means that at high tide, when the water covers the steep beach, you are quickly out of your depth. For experienced swimmers this isn’t a problem, but for those less confident or people with young kids, it is safer to swim at low tide when the water stays shallower. The depth change also explains why waves behave differently through the tide; generally waves break aggressively over a steeply-sloping seabed, and passively over a gently-sloping seabed. On a beach like Brighton this means you’ll get an aggressive shorebreak at high tide, and passive peeling waves at low tide, despite the swell being exactly the same.
Perhaps the most powerful effect of the tide on swimmers is the currents it creates. To best understand how these work, think of water draining from a beach when a tsunami is approaching. This is the trough of the tidal wave and water is essentially being ‘sucked’ towards the approaching peak. When it arrives, water then surges with the wave up the beach. Now turn that tidal wave into a tide wave and spin it 90 degrees so it’s travelling along the coast. This explains why, when I go swimming in Deal at low tide, I am being sucked towards Cornwall – because that’s where the peak of the tide wave is coming from. But at high tide, I get pushed up the coast towards Margate and I’m literally riding the wave. And because there is more water travelling between the peaks and troughs of tide waves during Springs, the currents can flow 25% faster, adding 3 knots to my speed – or creating a natural treadmill that keeps me in one place if I were to try and swim against it.