Just How Unlucky Was Britain to Suffer Desmond, Eva, and Frank in a Single December?

Usually, it’s natural disasters occurring elsewhere in the world that make headlines in Britain, not the other way around. But you’d have to have been hiding under a rock to have missed the devastation wrought by flooding in the U.K. last month, thanks to the triple-whammy of storms Desmond, Eva, and Frank. Initial analysis from the Association of British Insurers suggests that the damage done could run to the region of £1.3bn.

But just how unlucky was the U.K.to suffer not just one, or two, but three big storms in one December, and for these three storms to interact in such a way as to produce the chaos that followed?

First it’s worth pointing out that floods in the U.K. are—as is usually the case elsewhere—subject to important seasonal variation (see chart below). The winter months bring the highest number of events, and December does in fact come out (slightly) on top, especially for flooding events of the sort seen last month, which tend to follow heavy rainfall leading to soil saturation (November 2015 received about twice the average climatological rainfall for November in the U.K.).

The reason this matters is that, when soil is sodden following an extended period of heavy rains, further rains can more easily run off the surface, exacerbating the risk of pluvial flooding. The water will then follow natural and artificial drains until it reaches the closest river network, in which it can accumulate, potentially triggering river or “fluvial” flooding. The runaway effect of the masses of water can also cause what is known as ground-water flooding. This cumulative phenomenon means that—as we saw in December—flooding can persist for a significant amount of time, leading to several flood events in close succession.

A flood CAT model that properly captures these sorts of interactions between rainfall events and hydrological systems will allow not just for an assessment of the likelihood of a single severe event, but also a better understanding of the compounding factors that can lead to the sort of flooding seen in the U.K. last month. And based on our latest RMS pan-Europe flood model, the chances of having three rainstorms lead to major inland flooding over a single December are far from negligible.

The chart above shows the probability of one, two, three, and four flood events for the month of December. What it means is that, on average, every second December in the U.K. has at least one flood event, and every third December has only one flood event. Around every eight years there are two flood events, and a cluster of three flood events happens once every quarter-century.

Now, this does not mean that flooding on the scale just witnessed happens on average every 25 years—just that this is the average period for seeing three flood events in one December. Even if it did, it wouldn’t mean that the U.K. can rest on its laurels until 2041… this is just a statistical average. It is quite possible for clusters to hit several years in a row—a so-called “flood-rich period”.

This gets to the real nub of the issue. The question of how often this sort of flooding takes place in the U.K. is almost by-the-by. The point is that it isn’t rare as hen’s teeth, and so the U.K. needs to be prepared. And what was most shocking about December wasn’t the flooding itself, so much as the sheer lack of resilience on display. A media storm has understandably been whipped up regarding the level of investment into flood walls and so on, but protective infrastructure is only part of the equation. What is needed is not just flood walls (which can actually be counterproductive on their own), but a wider culture of resilience. This includes things such as flood warning systems, regular evacuation drills, citizens having personal plans in place (such as being ready to move furniture to upper levels in the case of an alert) and, critically, the ability to respond and recover should the defences fail and the worst happen (which is always a possibility). The U.K. is the world’s sixth richest country—it has the resources to cope with flood events of this magnitude… whether they happen every five, ten or 25 years.