Tag Archives: fetch

There’s no such thing as a freak wave – coastal safety is your own responsibility

I was reading a letter to Tramore Town Council, about the new Guillamenes cattle-crush, from someone who’d been involved in International Water Safety for many years. In the letter the person pointed out how someone had been “swept off the diving platform by a freak wave”. I said the Club Secretary Aidan Farrally that his points were valid but subsequent to that assertion, his arguments became suspect.

Now despite my deliberately somewhat provocative title, yes there are such things as freak waves. However while they are long reported, they are an only recently confirmed open ocean phenomenon, and more commonly known as rogue waves, (twice the height of the significant wave height around them).

But here we are discussing so-called freak waves at the coast, the interface of land and sea.

Every year we read or hear stories of coastal drownings caused by freak waves, where people at the coast are caught by a wave seemingly out of scale or size to the preceding waves, and swept off rocks, out to sea or suffering a fatal concussion from rocks. These are tragedies, but I have long had a problem with this reporting because it perpetuates a myth about the sea, and somewhat shifts the responsibility of care away from the person into a force-majeure situation, an “act of god“.

If there are freak waves, how can an average person realistically protect themselves except to stay away from the coast? If there aren’t freak waves, then the responsibility shifts to people themselves to be more vigilant.

The phrase freak wave implies that what has happened is unusual and unforeseen, neither of which are the case. Ask any surfer. So we need once again to talk about waves and safety at the coast.

Wind & WavesMost waves are caused by wind. Wind blows over the water surface and the friction pushes the water. The distance of water over which the wind blows is called the fetch, and the longer the fetch, and the longer the time and stronger the wind that blows, the bigger the initial waves. If the wind continues to blow, as the waves grow, they present even more surface for the wind to push. The waves continue to grow.

Waves are an energy pulse that travels through the water and will continue to travel unless something stops them. That something from our point of view, is land.

I said initial waves above, and that’s really important. Wind that blows in a Western Atlantic storm cause waves, which if unimpeded by other contrary weather, may blow those winds toward Ireland and Europe, across 2000 miles of open water. 

We need to think about the fundamentals of a wave.

Imagine the wind blowing and causing waves.

  • The height of a wave is called the amplitude.
  • The greater the amplitude the more energy in the wave.
  • The number of waves in a particular time is called the frequency.

Waves of different amplitudes usually have different frequencies. The higher the amplitude or height, the lower the frequency.

The bigger the wave, the longer the time between them and the less frequently they appear.

Waves travel at different speeds. Wind that cause waves nearby can have lots and lots of chop, small waves really close to each other.

Now the critical things about waves of any kind, not just water, is if they are even fractionally different heights they will travel at different speeds. What happens if two things, in case waves, are travelling the same direction at different speeds? The faster one will pass the slower one.

The further away the initial wind, the further the waves will travel. If a faster waves catches a slower one, the basic physics means that they will be added together. The amplitude, the height of the waves, will become the two combined heights.

A one and half metre wave catching a one metre wave, will become a two and half metre wave.

This wave, in surfer’s parlance, is called a Set Wave. Surfers don’t see them as freaks, but as normal aspects of the ocean’s behaviour.

Set waves at Tramore pier
Long groundswell  waves at Tramore pier

This doesn’t happen quickly but when you have hundred or thousands of miles or kilometres in which it can happen, it doesn’t have to be quick.

Waves reaching Hawaii from the Aleutian Islands
Waves reaching Hawaii from the Aleutian Islands

The result is that you have a wave that is now a third higher than the higher of the two previous waves. And that may be higher than all the surrounding waves. And the bigger it is the faster it’s travelling but the longer time between them, so there can up to many minutes between large waves like this. Any experienced surfer can tell you that the period between the largest set waves could be up to 15 minutes.

Now we’ve explored the formation and irregularity of waves, we’ve seen that these are not freak waves, but normal ocean behaviour.

The important thing becomes that the responsibility for understanding this lies with the person on the shore, just as it lies with a pedestrian crossing a road, except the ocean can’t see or react to you like a motorist can even when you are in the wrong.

Therefore the most important action to watch the water. Always.

This is NOT expect the unexpected. This is how the ocean works.

It should be noted, unlike beaches facing directly into the swell, at reefs and rocks, especially those that are not directly facing the oncoming swell, set waves can be difficult to see. if the water around a reef or rocky shore is deep, the first indication you may have is the wave actually breaking onto where you are standing

One wave seems to appear out of a flat surrounding sea at Tramore. easy to see on a beach, not easy to see elsewhere.
A wave seems to appear out of the flat surrounding sea at Tramore. Easy to see on a beach, not so easy to see elsewhere.

Watch the water for twice as long as the waves are high (in imperial measurement).

If the waves look to be two feet high, watch the water for four minutes before venturing close to the shore.

If the waves are two metres high on initial appraisal, you should (approximately) convert that to feet, and watch the water for six to seven minutes before getting close. Do this regularly and you will start to gain a better appreciation for the sea and its rhythms. And more importantly, you and the people around you will be safer.

Coastal safety is your responsibility!

Tramore Bay

HOW TO: Understanding Rough Water: Force Three

When writing the article on how to swim in rough water, I acknowledged that it’s an area I need to do a bit more about. And the popularity of the post caught me a bit by surprise, as it always does when such happens. But I wasn’t sure what I could do next. After all while I love writing about cold swimming, and it continues to provide me new avenues of investigation and expression, rough water is just rough water.

On last Saturday’s visit to the Guillamene, a swim I didn’t want to do, because two days previously I’d had a bad asthma attack while pool training, (for the first time in two years), and I didn’t feel I was recovered. Also, though the start of the fourth week of May, the water temperatures were still down, only nine degrees the previous weekend, worse the air temperature was low, only ten degrees, with a north-easterly wind. When you live at fifty-four degrees latitude, northerly winds are always cold. It felt just like winter and when I measured the temperature at the Guillamenes, it was on a par with the air temperature, ten degrees, yes a rise over the previous week, but with the chilly wind and the lowering dull grey sky and no-one around, it was less than inviting.

And as I said, the wind was north-easterly. As this point it’s useful to show you (again) how the Guillamene is situated.

Tramore Bay

The bay faces south-west (directly up is North in this image), the Guillamenes is on the west wide, so looking out from the platform you are facing south-east. It’s sheltered from the prevailing south-westerlies, but exposed to south-easterlies, easterlies and even north-easterlies.

The bay is just under five kilometres wide from Newtown Head to Powerstown head, a not-insignificant distance. Therefore a Force Three north-easterly wind will have about four kilometers of water to blow over before it reaches the Guillamenes. You may recall we discussed this in Understanding Waves for Swimmers a long time ago. The distance of water over which water blows is called the fetch. The greater the distance over water which wind blows, the greater the waves and chop that can be pushed up.

Apart from the fetch, the strength of the wind is important. I have long thought the ability to observationally measure wind using the Beaufort Scale should be an essential skill for serious open water swimmers.

Force Three on the Beaufort Scale is also known as a “gentle breeze”, a more pleasant title. Force Three is however a critical measurement for open water swimming because at Force Three scattered whitecaps start to appear, as crests start to crumble from the wind. This means an increase in rough water to the swimmer. (If you see choppy-ish water with no whitecaps, it’s Force Two). Below is a very boring picture of all these facts! A Force Three wind, blowing onshore, across a four kilometre fetch. It’s full size if you click on it and want a better look at what Force Three looks like, but it’s pretty unexciting. If you look around you’ll see those occasional whitecaps.

You can of course swim in Force Three, but you’ll be slowed down. It took me almost twenty-five minutes to pass the pier, where it would normally take twenty. I call Force Three critical because it’s the transitional point into rough water.

And because video is better for this, here’s a short clip. (With the wind blowing across the microphone, you might want to turn your volume down first).

.

This second video is taken from the water. However, this was taken underneath Doneraile Head, and the fetch that wind was blowing across was less, so though the wind was the same, and blowing the same direction, the waves were less.

.