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Report Task Force Fact Finding on the High Water & Flooding in NL

16 September 2021
IVM researchers of the Water and Climate Risk and Environmental Economics departments contributed to a recently published report by the Task Force Fact Finding on the High Water and Flooding in the Netherlands.

This Task Force, coordinated by Deltares and TU Delft, consists of various Dutch knowledge institutes, including IVM researchers, Jeroen Aerts, Wouter Botzen, Kees van Ginkel, Elco Koks and Hans de Moel, who contributed to the chapters on the meteorology, hydrology, evacuation and damage of the flood events.

Extreme rainfall occurred on 13 and 14 July, with many locations receiving record amounts of rainfall in measurement series of over 100 years, and estimated return times in the order of once every 1000 years. The core of the low-pressure system responsible for the rainfall became stagnant, and remained over the same region for several days leading to excessive rainfall amounts. From 11 July onwards, the weather forecasts predicted heavy precipitation, but the extremity had to be revised upwards until just before the event itself. The probability of such rainfall extremes has increased due to climate change, although quantitative estimates show a wide range from +20% to +800%. The long persistence might be related to the observed weakening of the summer Jetstream (westerly flow) which is expected to weaken further under future climate change. Still, the uncertainties are large, but also the potential risks. IVM has an active research line to better understand the role of climate change on such stagnating extremes weather events.

Discharge in the river Maas reached the record level of almost 3300m3/s at the gauging station St Pieter. Not only the discharge itself was extreme, but also the timing in the summer period was exceptional. The magnitude and timing of this event was not anticipated for in any statistics (e.g. GRADE) available to test flood management in the region. Further, the water levels upstream in the Dutch part river Meuse were up to 60cm higher than in the forecast. This is probably related to the fact that hydrological models  mostly simulate discharge for winter conditions, without much vegetation and thus roughness in the riverbed during the summer, which may have enhanced water levels. Despite the extremes, the main levee system could withstand hydraulic pressures, and no casualties were reported. The main damages occurred in the side rivers, such as the Geul and Geleenbeek.

Besides record rainfall and discharge, the event also caused a record  in flood damage along the Meuse river. It is estimated that over 2,500 homes, 5,000 residents and about 600 businesses in the area were affected. The fact finding study estimated that total damages are approximately €350-600 million. Structural damage and destroyed content, business failures, damage to infrastructure, and agriculture constitute the largest impacts. The estimated damage for the total flooded area is substantially larger compared to the floods of 1993 and 1995. Damages of these two events were approximately €200 million and €125 million converted to 2021 prices, excluding damage caused by business failure. The main difference is that this time most of the damage occurred in the side rivers (particularly the Geul flooding municipalities of Valkenburg and Meerssen) instead of along the Meuse itself. The measures taken along the Meuse (e.g. Maaswerken) since the floods in the ‘90s likely contributed to this.

An main recommendation made in the report is to conduct a survey among households and businesses in the areas affected by the high water to obtain more detailed insights into experienced impacts and implemented risk reduction measures. The survey for this follow up research has been prepared by IVM.

The report can be downloaded here: https://www.enwinfo.nl/publicaties/

The study received wide spread media coverage, such as this article in De Volkskrant: https://www.volkskrant.nl/nieuws-achtergrond/watersnood-limburg-brak-records-qua-neerslag-afvoer-en-schade-in-belgie-en-duitsland-nog-catastrofaler~b1c3bcb9/