FloodsWhat Caused Heavy Rainfall Which Led to Western Europe’s Severe Flooding

Published 26 August 2021

Mid-July flooding resulted in at least 184 fatalities in Germany and 38 in Belgium and considerable damage to infrastructure, including houses, motorways and railway lines and bridges and key income sources. Road closures left some places inaccessible for days, cutting off some villages from evacuation routes and emergency response. What was the cause of these devastating floods?

From the 12th to the 15th of July, heavy rainfall associated with cut-off low-pressure system “Bernd” led to severe flooding particularly in the German states North Rhine-Westphalia and Rhineland-Palatinate, as well as in Luxembourg, and along the river Meuse and some of its tributaries in Belgium and the Netherlands.

At the time of the heavy rainfall event, soils were in part already saturated. Some valley sections are very narrow with steep slopes leading to funnel-like effects in the event of extreme floods. These factors were modified locally also by differences in land cover, infrastructure and water management alleviating or increasing the devastation of the extreme flooding. At the Ahr river the flood is estimated to be a 500 year event or rarer according to preliminary data.

The flooding resulted in at least 184 fatalities in Germany and 38 in Belgium and considerable damage to infrastructure, including houses, motorways and railway lines and bridges and key income sources. Road closures left some places inaccessible for days, cutting off some villages from evacuation routes and emergency response. The worst affected areas were around the rivers Ahr, Erft and Meuse.

The World Weather Attribution notes that scientists from Germany, Belgium, the Netherlands, Switzerland, France, Luxembourg, the US and the UK, collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the heavy rainfall causing the severe flooding.

Using published peer-reviewed methods, we analyzed how human-induced climate change affected maximum 1-day and 2-day rainfall events in the summer season (April-September)  in two small regions where the recent flooding has been most severe in the Ahr-Erft region (Germany) and on the Meuse (Belgium) and anywhere over a larger region including Germany, Belgium and the Netherlands.

Main Findings

·  The severe flooding was caused by very heavy rainfall over a period of 1-2 days, wet conditions already before the event and local hydrological factors. While river discharge and water levels are the physical components most directly linked to the impacts of the event, we focus our assessment on the main meteorological driver, the heavy rainfall event. This is due to the fact that some hydrological monitoring systems were destroyed during the flood and data of sufficiently high quality and quantity is not currently available.