Reconstruction of a flood in Limburg: a climate disaster can strike anywhere | NOW

It is July 12, 2021, and afterwards day 0 of the flood. Not a drop of rain falls. But the KNMI models have had a crazy storm complex in their minds for a few days now. The institute gives for the next day code yellow specifically for the province of Limburg.

Even on July 13, it seems rather unremarkable summer weather. With about 20 degrees it is not remarkably warm, and it is again largely dry in the Netherlands: in De Bilt a total of 0.5 millimeters of rain falls in 24 hours.

Almost 200 millimeters in Vaals, not a drop in De Bilt

But in Maastricht it is already a completely different story that day: 24.4 millimeters of rain. Then you have to deal with heavy downpours. It turns out to be only a harbinger, because in Limburg and in Belgium and Germany the rain will not stop.

The next day there is nothing to notice in the center of the country: 21 degrees and barely 1 millimeter of rain. But the weather station in Maastricht recorded 40.5 millimeters on 14 July. Those statistics show two things: how unexpected the rain came, and most importantly, how local it was.

But what the measurements of the large KNMI stations do not yet fully show is how extreme the precipitation is slightly higher up in the hills.

Limburg got a graze, full layer over the border

This is because there are very large differences even within South Limburg. Maastricht has to process almost 70 millimeters of rain in three days. That’s almost a monthly average. But in the eastern part of the hill country – between Heerlen and Vaals – there is two to three times as much precipitation in just two days.

That amount then pales in comparison to that in the town of Jalhay, 20 kilometers south of the Dutch-Belgian border in the Ardennes. There, 271.5 millimeters of rain falls from the sky in 48 hours. And in the neighboring part of Germany too, there is locally as much precipitation in two days as the average in a full summer.

Limburg, for example, does not get the full brunt of precipitation, but it does have to do with two important consequences: flooding of streams, and extreme discharges in the Meuse and the Roer, due to even more extreme rainfall just across the border.

More water than ever in the Maas, and that in the summer

The water damage from locally fallen rain seems bad enough. This is how the Geul overflows its banks. As a result, 2,300 houses are damaged and a bridge collapses in Valkenburg. The total damage in that place is estimated at 400 million euros.

Shortly afterwards, a large amount of water enters from the Ardennes via the Meuse, and also from Germany via the River Roer. That water does not pose a threat to the high-lying hill country, but it does pose a threat to larger cities such as Maastricht, Roermond and Venlo downstream, where ten thousand people are being evacuated.

The Maas drains more water than ever measured before. The exceptional thing about this is especially the season. All previous records were set in the winter months, when the water level in the Dutch rivers is usually much higher than in the summer. When the absolute peak is finally reached on 15 July, no less than eighty times as much water will flow through the Meuse as in the same time a year earlier.

Near Maastricht a dam threatens to fail and a canal dike later collapses, but most of the water can drain away thanks to measures taken after an impending flood in the 1990s. The river was given more space to collect water, and that will pay for itself in July 2021.

Triple link with climate change

Soon after the disaster, it is suggested that there may be a link with climate change. Scientific research shows that for every degree of global warming, showers transport an average of 7 percent more water, as a result of increased evaporation.

In addition, the rising airflow in storm complexes is increasing due to climate change, causing the total precipitation intensity to rise as much as 14 percent per degree. As a result, the number of summer downpours in the Netherlands has approximately doubled in a century.

But during the extreme rainfall of July 2021, another phenomenon plays a role: the small, powerful low-pressure area that brings all the precipitation hangs over one spot for a long time. Research into such ‘persistent weather’ is still in full swing, but climate models expect it to occur more frequently in our region in the summer months as the Earth warms further.

Westerly winds decrease with us in the summer months, so that we can have the same type of weather for longer. Think of prolonged drought and heat, or persistent rainfall.

Chance of extreme rainfall has increased eightfold

Ultimately, the climate link with the 2021 flood will also be officially investigated by climate researchers from the KNMI and the German meteorological service DWD. They have climate models imitate the weather conditions of 13 and 14 July and conclude that the chance of extreme rainfall in the border area between Belgium, Germany and Limburg has increased by eight times due to global warming.

The fact that the low pressure area rained empty right there, and not above Brussels, Amsterdam or Berlin, is largely a coincidence. And so the highest part of the Netherlands was eventually hit by a flood, with a warning for an even warmer future.