New Water Maps Help Determine Measures Against Flooding

In cities, prolonged water nuisance can occur and streets may flood. This is evident from research on so-called stress tests conducted by 4 water boards and Rijkswaterstaat, in collaboration with the provinces of North Holland, Utrecht, and South Holland. The insights from the research help determine which measures are necessary to limit the consequences.

In July 2021, Limburg was severely affected by large-scale, extreme rainfall, causing flooding in multiple locations. This resulted in significant damage to homes and businesses. The floods demonstrated the potential impact of a large-scale extreme downpour. The chance of such a downpour occurring in North Holland is small but is increasing due to climate change. This calls for good preparation throughout the Netherlands. For this reason, the Northwest Netherlands region has been mapped to show the potential consequences of large-scale extreme downpours. These maps provide valuable insights into possible bottlenecks and how long the water remains.

Effects on polders and cities The report shows that during an extreme downpour, the lowest parts of the polders mostly flood up to knee depth (about 60 centimeters, with some peaks up to 1 meter or more) and that the water often remains for 1 to 3 weeks. The water system cannot discharge such large amounts quickly enough. By increasing the capacity of water drainage and providing more space for water, possible water nuisance can be reduced. In cities, prolonged water nuisance can occur. This affects accessibility and can also cause damage to buildings. The biggest challenge lies in areas where important facilities are located, such as electrical cabinets, telephone connections, and main roads. The next step is to accurately determine the consequences.

What is a stress test? 

To prepare the Netherlands for large-scale water nuisance, so-called supra-regional stress tests are carried out in 13 regions. Researchers from Sweco have used computer models to examine what happens with different amounts of precipitation. For example, as in Limburg, where 200 millimeters of rain fell in 48 hours in 2021. As a first step in this process, maps (water images) have been created showing how high the water will rise and how long it will remain. For the Amsterdam-Rhine Canal North Sea Canal area (ARK-NZK), these water images are now ready. This area covers the entire province of North Holland, part of Utrecht, and the northern part of South Holland.

No street-level detail

The maps in the stress test do not show details at street level. Because the sewer system was not included in this study, this is difficult to assess. Probably more urban areas are vulnerable than the maps show, because the effects can be greater when sewer problems are taken into account. A blue area does not automatically mean a problem but does provide a clear general picture of how vulnerable the area is to water nuisance. The water images can be viewed via the Water Images ARK-NZK Region report (pdf). 

Mapping consequences

The next step is to create a consequences map. For this, provinces will consult with, among others, safety regions and water boards to map the possible consequences of failure of vital and vulnerable functions, such as hospitals and dikes. Then it will be discussed which consequences are acceptable and where measures are needed.