Heavy rainfall in Western Europe in July 2021 triggered billions in losses

[#1]

Flooding is a natural phenomenon that is part of the water cycle. Abundant precipitation or snowmelt in areas where there is a flood formation leads to increased runoff that is collected in the rivers. But there are also human influences: Due to climate change, extreme weather, such as heavy rain, is becoming more and more frequent. In densely populated Central Europe in particular, the resulting extreme floods can be destructive.

A major storm system known as Bernd stalled over Western Europe on July 13-14, 2021, leading to record rain over Belgium, Germany, Luxembourg, Switzerland, France and the Netherlands. Some parts of Germany received the equivalent of two normal months’ worth of rain in less than 48 hours, causing rivers to overflow their banks, producing flash floods, and resulting in hundreds of people injured, missing or dead. The floods triggered €1bn in losses and is expected to recur every three to five years, according to a Cologne-based actuarial consultancy.

Amateur footage from July 14 shows the waters rose rapidly in Altenahr, overwhelming emergency services and leaving residents racing to avoid the flood.

Using JMP visualization, I wanted to see the rainfall location and impact in Germany for July 2021. I found data on rainfall measurement over the month of July in all German states on mtwetter.de. I explored this data in Graph Builder in JMP and then customized a contour plot with a clipped shape using German states. It is readily apparent where the most rain fell. In particular, parts of Nordrhein Westfalen and Rheinland-Pfalz had more than 150 mm in 24 hours on July 14 (in blue).

The next two graphs show the total amount of rainfall accumulated per day in July 2021 in the different German states. The two states that were most heavily impacted are Nordrhein Westfalen (NW) in red and Rheinland-Pfalz (RP) in light red. Particularly heavy rain started on July 13 in those two states, with the maximum falling on July 14, as shown in the bar chart below.

Since custom shapes can be used in bubble plots, I’ve chosen umbrellas whose size and color signify the amount of rainfall in millimeters. My colleague Dan Schikore (@danschikore) wrote a very useful add-in, the Bubble Shapers Editor, that allows SVG files to be imported directly and then opens those files from the interface. I have used this umbrella SVG file from the freesvg.org website.

Experts say several factors led to the disaster, including a wet spring, which meant the rain in July just ran off the landscape, dragging debris and silt with it. Entire trees that were uprooted by the water, as well as cars and caravans, were washed downstream, clogging the bridges and creating dangerous backlogs of water.

However, climate change played its part, too. A recent scientific study found that such intense rain events are now up to nine times more likely in Western Europe as a result of global warming.

"The warmer it is, the more water the air can absorb. About one degree warmer air can hold about seven percent more water vapor, and what is in the atmosphere sooner or later comes down again as precipitation,” says Tobias Fuchs, Director of Climate and Environment from Germany’s Meteorological Service (DWD).