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 following data we conclude 'infrequent but not unusual'. Here's why.
On the back of a run of such significant swell and the consequent coastal damage caused in areas of Northern Europe which typically see full exposure to storms of this size, 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 just shy of that 50 year return period range – that is to say 'infrequent' but not necessarily 'unusual'.
The first issue we have to remove is subjective observation. Fortunately with waves we have a direct system of measurement. Sitting in deeper water, just away from the coast, the wave buoy or light vessel measures (by movement from the surface or changes in pressure from the seabed) the movement of water up and down above the sea bed. These monitoring stations give us a huge amount of data about the make up of an incoming swell. The most useful parameter, for this discussion, is the 'Significant Height' of the waves. This is simply an average, over a typical time frame of 20 minutes, of the largest one third of all the waves. This is the same number you'll typically see on a forecast for waves (eg. here on MSW). It's tremendously useful because it gives a real idea of the general size of the larger waves in a swell and that process of averaging removes the kind of data errors that typically creep in when trying to show just the one very largest wave.
In this case we're very fortunate that right in the path of tomorrow's swell we have one of the longest running monitoring stations providing wave data, the Sevenstones Light Vessel. This ship will not only provide us with essential data about this storm as it passes, but 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.
The last couple of swells have hit maximum significant height values of almost exactly 30ft at their peak with Feb 1st briefly showing 34ft - 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 just shy of that 50 year return period range - that is to say 'infrequent' but not necessarily 'unusual'.
TRACK the storm on our all new wave buoy interface HERE
*DO NOT miss our animation of the North Atlantic winter in 23 seconds HERE