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A 50-year storm? Just how unusual is this run of huge waves? EXPERT VIEW
12:40pm Saturday 8th February 2014 in News
So just how unusual is this massive storm? Here is what forecasting gurus Magic Seaweed have to say.
"The huge swell hitting Europe tomorrow is quite probably going to create larger waves in coastal waters than the previous three in 2014. The question we’re being asked by both surfers and the media is ‘how unusual is it?’ The answer depends on how long your memory is, but we conclude ‘infrequent but not unusual’. Here’s why."
"It’s tempting to assume we are witnessing something unprecedented. With waves of 35ft+ forecast to hit the English coastline tomorrow we have the data to put that to the test.
"Tomorrow’s storm is currently forecast to peak at 37ft in deep water around Western Cornwall. If these values are confirmed by the wave buoy then we are looking at an event moving right into that 50 year return period range – that is to say ‘infrequent’ but not necessarily ‘unusual’."
The Sevenstones Light Vessel, sitting in deep water, right in the path of the incoming storm, gives data about incoming swell, with the ‘Significant Height’ of the waves the most important for wave size measurement.
This is an average, over a typical time frame of 20 minutes, of the largest one third of all the waves and gives an idea of the general size of the larger waves.
The averaging "removes the kind of data errors that typically creep in when trying to show just the one very largest wave".
Magic Seaweed says: "The Sevenstones Light Vessel has been reporting on every storm and swell since the early 1960s. As well as giving us the ability to pick out some specific storms in a similar range to tomorrow’s, we can also use some fairly simple and elegant maths, and this data to look at what sort of swell extremes might be likely to occur here.
"This technique is a cornerstone of coastal engineering. If you’re building an expensive coastal structure you need to ensure it’s constructed to withstand whatever nature throws at it. For this it’s typical to calculate what’s called a 50 or 100 year return period. This is simply the size of the largest waves you could expect will definitely occur at least once in that timeframe.
For the Sevenstones Light Vessel, with our long historic record, we can do this with some accuracy. In fact analysis as early as the 1970s had already identified these values in the 36-40ft range. Subsequent analysis on a larger data set pushes these further still, possibly as large as 40-45 feet.
"Western Cornwall could reasonably expect swell to 15m/50ft at least once in a century. Well in excess of the latest forecast for tomorrow.
"The last couple of swells have hit maximum significant height values of almost exactly 30ft at their peak - well within what we might reasonably expect over a 50 year period.
"However tomorrow’s storm is currently forecast to peak at 37ft in deep water around Western Cornwall. If these values are confirmed by the wave buoy tomorrow then we are looking at an event moving right into that 50 year return period range - that is to say ‘infrequent’ but not necessarily ‘unusual’."
With thanks to Magic Seaweed who gave us kind permission to use the article. Visit their website on www.magicseaweed.com
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