As surfers we develop something of an inbuilt tide clock over the years. With that comes a better than average understanding of the way tides work. So we all know that the movement of the ocean is basically caused by the gravitational pull of the moon on the earth’s water. And we know that the sun also exerts a slight gravitational pull, so when the moon lines up with the earth, either in a full or new moon, the pull on the oceans is greater, which is why we get the larger, spring tides. Most importantly, we know exactly what tide our local beachy works on best.
The huge tidal range means waves like this don't last long on the South Wales coast
But what many of us (OK, me) might never have considered in enough detail is what creates tidal range. So why, for example, are the tides in South Wales the second largest in the world at over 14m on the biggest springs, while in Cornwall, less than 50 miles away as the crow flies, the average tide is more like 7 or 8 meters? From my few trips to the tropics, where the tides barely seem to move, I had lazily (and wrongly) assumed that the size of the tide gets larger the further away from the equator you move. Not so.
While in other places the low tidal range means the surf pumps all day long!
Tidal range is actually created by something far more interesting. That something is Amphidromes.
These are fixed points around the world’s oceans around which water rotates. There are 15 main amphidromic points around the world, and it is these, not proximity to the equator that determine tidal range.
How do they work?
Imagine you fill a frying pan with a layer of water, and swill the water in a circular motion. The water on the outside of the pan is moving a lot more than the water in the middle. This is roughly the same as what occurs around an amphodromic point. So a coastline located at the centre of a point, like Tahiti, will experience almost no tidal range at all. And somewhere situated on the edge of a point’s rotation, like the UK, will experience a far greater tidal range. In the northern hemishpere, water moves ani-clockwise around the points, in the southern hemishpere the water is rotating clockwise.
Map showing the major amphidromes around the world. The closer a coast is to an amphidrome, the less tidal range it will experience
This still hasn’t answered the question of why the tides in South Wales are so much larger than those in Cornwall. The answer is that the South Wales coast is a perfect topographical storm.
The funnel effect caused by the Bristol Channel and the extremely flat sea bed are the two main factors here. Water pushes up the channel, and encounters very little resistance before it meets our coast. In fact, there’s only one place in the world where the conditions come together to create a larger tidal range. Answers on a postcard…
The information in this post, as well the frying pan metaphor is the consequence of having been enlightened by the superb Wave Watcher’s Companion‘ by the wonderfully named Gavin Pretor-Pinney’. If you like waves, you’ll like this book.