Regional stress tests demonstrate the effects of large-scale extreme rainfall and assist in preparation
Expert(s) |
Bart Maas
The severe flooding in Limburg in 2021 underscored the urgent need for the Netherlands to better prepare for large-scale extreme rainfall events. In response, Deltares, commissioned by the Ministry of Infrastructure and Water Management, has developed a national water impact assessment. This study explores the consequences if an extreme rainfall event—200 mm in 48 hours—were to occur simultaneously across other provinces. In some areas, floodwaters could persist for several weeks, while in others, localised depths could exceed one metre.
Marked differences between high and low-lying regions
The national water impact assessment reveals significant regional variation in the effects of large-scale rainfall. In low-lying areas, widespread but shallow and prolonged surface water flooding is expected. These polder areas, which rely on pumping stations, have limited water storage capacity and cannot discharge such volumes quickly. As a result, extensive pumping restrictions may be necessary, prolonging the duration of flooding and increasing its impact on surrounding areas.
In contrast, higher-lying regions are likely to experience more localised flooding, but with greater water depths—particularly along streams with limited discharge capacity. Infrastructure such as bridges, sluices, and narrow passages in built-up areas can become bottlenecks, potentially leading to inundation of adjacent zones.
Large-scale extreme rainfall produces different effects than localised cloudbursts, which have been studied previously. When water accumulates across a broad area and converges into larger watercourses, water levels rise and exceed the system’s discharge capacity. This leads to prolonged standing water, affecting both urban environments and surrounding rural areas.
Moreover, such rainfall events are not confined by national borders, meaning that cross-border water inflows must also be considered.
Bart Maas, researcher at Deltares, explains: “For the first time, the national water impact assessment provides a coherent overview of potential flooding caused by widespread extreme rainfall across different types of landscapes. In low-lying areas, we see extensive zones with predominantly shallow but long-lasting flooding—sometimes lasting several weeks. In higher regions, deeper water accumulations are more likely around streams. This assessment is a crucial first step in improving our preparedness for such events.”
How the study was conducted
This national assessment marks the initial phase of the regional stress tests, which follow recommendations from the ‘Policy table on flooding and high water’. The aim is to better understand the Netherlands’ vulnerability to large-scale extreme rainfall. The national overview was compiled using regional assessments from 13 stress test regions. These were developed by provinces in collaboration with water authorities and engineering consultancies, and provide insight into maximum water depths, flood duration, and in some cases, flow velocities.
To ensure consistency, all regions used the same assumptions regarding rainfall duration, intensity, and spatial extent. Deltares then compared and integrated the results using its hydrodynamic modelling tools, Delft3D and SOBEK.
The findings were refined through an iterative process within the ‘Community of practice for regional analyses of large-scale extreme rainfall’, which includes ministries, Rijkswaterstaat, provinces, water boards, safety regions, municipalities, utility providers, and research institutions.
The results from all regions are presented in the interactive map storyLarge-scale extreme rainfall, published by the Climate Adaptation Services Foundation (Stichting CAS).
Next steps
The next phase of the regional stress tests will focus on impact assessments, identifying potential damage, affected populations, disruptions to critical infrastructure, and accessibility issues. These insights will feed into the National approach to flooding, due by the end of 2026.
