HOW TO: Tides For Swimmers, Part 2 – Local Effects

Part 1 – Theory

So, why a different post for what we can call the observed local effects?

Well, depending where you are in the world, this may not be needed. There are some places where the observed local effects match those predicted by theory.

However, why isn’t that the case for everywhere?

Simply, the oceans aren’t uniformly shaped basins. They have obviously irregularly shaped coasts, and differing sizes and depths, all of which interfere with the tidal wave. Gravitational effects (lunar and solar) cause waves. But inertia and drag also interfere with the tides.

This is a map of global tides. We need to look at the Global situation to understand the local situation.

world tidal constituent

Whoa there.

Okay, how about this simpler-to-read one?

global tides

All right, that second one’s a bit easier to read. Forget everything else and have a look at the Atlantic. Notice there is a node, a black point, in the centre of the North Atlantic, south of Greenland, east of Newfoundland, from which lines radiate out?

That’s the Atlantic Amphidromic Point.

This means it’s the point in the Atlantic where the tidal range is zero, i.e. it has no tide, it’s all at the same tide height. The tides radiate out and away from that.

The further you go the greater the tidal range.

Each of those lines reaching indicate different tide heights. The further away from the amphidromic point the greater the tide height. Each of those lines also has a number associated which is the (very) approximate tide range.

Now go back to the coloured chart. See how one you get up to Europe the tidal range (colour) gets higher (a deeper colour)? Compare the difference between California (and the Catalina Channel) and the Atlantic Isles and the English Channel.

Now lets back up again. Imagine the Atlantic is a rectangular basin. You pick it up sloosh the water from on side to another. Assuming you are not spilling it out, and keep it moving, there’s a point somewhere toward the middle where the water is hardly moving up and down, just backwards and forwards. The water at the ends though will pile up against the basin for the “high tide”.

Now also think of the moving water. All the water doesn’t move at the same time and most of the water from either side will never get to the other side. So this affects the locality of the tide even within the basin.

Now imagine swishing the water and trying to add a circular motion. It gets more complicated.

This also then applies on a global scale. The tidal wave doesn’t all go from A to Z and back to Z every 12 hours. A to Z is a long way with lots of intermediate points. So lets the water from A goes to C, the water from B goes to D etc. Or better yet. The water from Kerry makes it to West Cork, the water from West Cork makes it to Cork, the water from Cork makes it to Youghal etc. So that means each area’s tide is at a different time, as you already know.

So that’s the whole Atlantic situation: Here’s an animation of the simple and complex (real) models.

Imagine this applying to Ireland. Think of that tide coming towards Ireland. It’s coming from west. So imagine it arriving as a single wavefront. Some flowing into the west coast, some flowing up around Donegal, across the north coast, some flowing across the south coast, around Carnsore Point and up the east coast.

Ok, the tide wave doesn’t cover all the area. As said previously it’s actually water moving back and away from the originating point. And then what? See it?

Ireland is an island, with water flowing around it. Logically that flow must reach around the coast and each flow start to interfere with other. Which is does. There’s actually an amphidromic point north of Ireland near Skye, and in the Irish Sea just south of the Isle of Man and yet Lancashire and Cumbria, on the NE coast of England, not that far away, have the highest tides at eight metres and Carnsore in Wexford, is just 1.75 metres. According to the Irish Cruising Club, there is an amphidromic point close to the east coast 25 miles north of Carnsore.

But we’re not finished. So you’ve heard of Capes, Cape Horn etc. Ok you channel swimmers, your favourite, Cap de Griz. Well in geographical terms, a Cape is a headland that significantly affects the ocean currents around it.

It does this, and also other headlands to a lesser extent, by essentially forcing the tide to flow through a smaller opening like putting you finger over the end of a tap. (Hello Channel tides!) Or maybe the coast “falls away” froma tide flow, having the opposite effect.

And then there’s the estuarine effect where a tide flowing into a estuary gets higher as it is forced further in. The tide here outside my door is about 7 metres, 20 miles from the sea, where is about 5m normally.

The Irish Sea, because of the narrow North Channel, also acts as estuary, forcing higher tides further north along it. In fact there are really varying effects in the Irish Sea.

Courtown for example sometimes has 5 tides instead of 4, with a tiny variation between them. The tides around Kilmore Quay also can be 8 knots. Some other places have similar very strong tides, while a few miles away there are negligible tides.

So lots of effects. In the case of the south coast of Ireland, the only bit I feel qualified and experienced enough to talk about, all this combines to produce a “lag” effect, where the slack portion of the tide is NOT necessarily at the high and low water marks, as we saw is the theoretical case in the first post. Instead, depending where along the coast you can be, slack water may be the middle of the tide or other times.

The charts to determine this are published by the South West Cruising club and printed every year in Reed’s Almanac, the yearly sailing almanac, so I can’t really post them here. PM me if you want them and i’ll send them on, though they are a bit complicated apparently first time you see them.

All of which serves to demonstrate that adhering, as so many swimmers do, to a theoretical tide idea should be tempered with acknowledging that it’s necessarily the truth.

For good tide knowledge, sailors and kayakers are the best. Like us they are at the mercy of tides, but are further out so their knowledge is often more extensive and their literature is far more developed.

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