• Matthew Blackett

Why are recent floods so flashy (and so common)?

It will not have gone unnoticed that in recent weeks, several (some devastating) #flash #floods have been witnessed globally:

London: both on 13 and 23 of July, intense rainfall flooded roads and #underground/#DLR stations, causing severe disruption but fortunately no casualties.

NW Germany & E Belgium: from 14 July, large areas were flooded as the Meuse, Rhine and Ruhr (plus their tributaries) burst their banks, resulting in over 200 deaths, many more injuries many as yet unaccounted for.

Blessem, Erfstadt, Germany, July 16, 2021 (Rhein-Erft-Kreis, AP, from France24).

Schuld, Germany (source: Reuters, Aljazeera).

Henan, China: dozens of cities in this vast province have been flooded since 17 July, killing at least 70, displacing over a million and affecting nearly 10 million.

Footage from BBC of Chinese floods.

So what is going on here? Are these freak, one off events that are all part of the norm or do they reflect a wider set of changing climate patterns? The answer to these questions is not easy to come by, given that we don't have a whole lot of data to compare today's climate with (reliable climate records are little more than 100 years in length) and that it is near impossible to directly relate such one-off events to a changing climate.

One thing we can do with more certainty however, is to understand why urban areas are increasingly affected by flash floods. Impervious surfaces (tarmac, concrete etc) replacing grass and woodlands, plus efficient drainage systems, mean water rapidly accumulates rather than more slowly infiltrating into the ground, resulting in these flashy situations (see below for a comparison of urban vs. rural river responses to rainfall). In effect, urban environments recreate the more natural conditions of flash flooding: steep sided valleys with impervious geology. These effects however, do not account for the seeming increase in intense rainfall events - this is likely where #climate #change is coming in.

Flood hydrographs for urban and rural areas, following rainfall (source: DOI:10.14288/1.0300042).

A simple rule of physics is that a body of air can hold more water vapour as its temperature increases (7% more so with each 1 °C temperature rise). As such, a warming atmosphere has been shown to result in increased volumes of rainfall and, if temperatures continue to rise, this trend will likely continue. Researchers have crunched the numbers and have even been able to predict those areas of the planet likely to see more substantial rainfall events (see below), with the UK and Germany 20-80% more likely to and central China >80% more likely. We should also remember that associated with the storms that may deliver flash flooding, are thunder and lightning which, themselves, have caused significant damage in the UK of late.

So if this is the case, what can we do? Well, short of stopping the climate warming any further (that's a topic for another day), we could make our urban areas more resilient to heavy downpours. Things such as green roofs and walls and permeable pavements will go someway to helping the problem (so please don't pave over your driveway!) while flood barriers (such as London's Thames Barrier) will largely be useless. What we've seen recently, should be regarded as a wake-up call to populations and to goverments: we need to ensure we do all we can to stop the global climate becoming increasingly warm and to create environments that we live in that are in tune with the extremes in climate that may inevitably be on their way.

An example of a green roof. Here, the vegetation will take up some rainfall and the roots + soil will act as sponges, reducing the time it takes to enter the surrounding environment (source).

The Thames Barrier in London (source: author).