Probability of flooding increasing due to increasing river discharge volumes

Climate change increases the probability of flooding from rivers. Higher water levels increase the probability of flooding from inland waters, such as tributaries and streams. This probability is highest in winter, as river discharges increase in winter. But the probability of river flooding also increases in summer. The paragraphs below provide more details on how the probability of river flooding is increasing.

How does the probability of river flooding increase in winter?

According to the KNMI'23 climate scenarios, average winter precipitation in the Rhine and Meuse basins is increasing. Consequently, the rivers will discharge more water in winter. Concurrently, average summer precipitation will decrease in the basins, resulting in lower average river discharges in summer. This is also reflected in research from 2023 in which Rijkswaterstaat calculated what the KNMI scenarios mean for river discharges in 2050, 2100 and 2150.

How does the probability of river flooding increase in summer?

Due to climate change, precipitation decreases in March to August. As a result, rivers discharge less water in summer. However, according to international research the probability of extreme summer precipitation will increase. If a large area suddenly experiences extreme downpours, it could lead to flooding like in July 2021 in parts of Limburg, Germany and Belgium.

Other factors also affect river discharges

Besides increasing precipitation, river discharges also increase due to other factors: melting snow, evaporation in the catchment area, and water management.

What can we learn from the flooding in July 2021?

In July 2021, parts of Belgium, Germany, and the Dutch province of Limburg were flooded. The cause: extreme rainfall on 13 and 14 July in the Meuse and Rhine basins. These floods have prompted a range of studies:

• According to a study by an international team of scientists, the probability of flooding as a result of severe rainfall like in July 2021 is increasing. Their study focused on the region between the North Alps and the Netherlands.
• Deltares has researched whether the highly extreme precipitation that fell in the Rhine and Meuse basins in July 2021 could also occur in the central part of the Netherlands, and what the consequences would be. The conclusion: yes, this may happen, even though it is slightly less probable. Such precipitation would have a major impact, as it has had in Belgium, Germany, and Limburg. According to the study, we need to prepare better, in several ways, to prevent a disaster in such situations. The researchers have presented seven recommendations in this respect, such as: conducting stress tests at a supra-regional scale; assessing disaster management capacities; and boosting the exchange of risk information across the borders.