Water system characteristics
Key to understanding the impact of sea level rise is the knowledge of the water system in and around Amsterdam and how it is managed. Amsterdam has a central position in a large, low lying polder area, in which the main drainage canals are connected to the main artery formed by the Amsterdam-Rhine Canal and the North Sea Canal. The water level in these canals – including all canals in the center of Amsterdam – is carefully maintained at a level of 40 cm-NAP. All excess water in this area has to be drained via the sluices at IJmuiden. At low tide at sea, a natural drainage is possible, however, at high tide the water has to be pumped out.
Sometimes, the water level in the canals rises more than 20cm, for instance when a storm and associated prolonged high tides at sea prevent a natural drainage at IJmuiden. During such an event, the connections of the polder drainage canals to the North Sea Canal are closed and a second, but far smaller, pumping station Zeeburg starts discharging water to lake IJmeer/Markermeer.
Impact of sea level rise
Sea level rise will have the same impact, but much more pronounced. Pumping stations need to take over the role of natural drainage at IJmuiden, which make the system more vulnerable. Problems will arise during periods of storms, coinciding with intense rainfall or failure of pumping stations. In the long run the capacity of the pumping stations might not be sufficient any more at all. The consequences – long periods with excessive water levels – will be cascading from the North Sea Canal and Amsterdam Rhine Canal, via the polder drainage canals, to the polder areas because all connections between these water bodies must be closed one after the other during high water events. It remains uncertain when this will occur, because sea level rise scenarios vary a lot, but as a rule of thumb the frequency of occurrence of high water events (0 cm NAP in the main drainage canals) is calculated to increase from once in 100 years at present to once in 25 years when sea level rises 50cm and once in 10 years when sea level rises 100 cm.
Maintaining the resilience of the water system requires anticipation by the water managers. Two strategies may be envisaged. The first strategy is to increase the capacity of the pumping stations at IJmuiden and Zeeburg. Technically, this can be done and is similar to the building of a new pumping station in the Afsluitdijk near Den Oever, which was decided a few years ago. The second strategy aims at creating water storage capacity in all parts of the drainage basin, that is along the North Sea Canal and Amsterdam Rhine Canal, along the polder drainage canals and in the polders as well. This solution involves the revision of building and infrastructure regulations too.
Research needs
Further research can underpin these options or combinations thereof. Possibilities for increasing the drainage to the IJmeer/Markermeer depend heavily on the salt intrusion in the North Sea canal as brackish water is not allowed to be drained or pumped into a fresh water reservoir. Where to build water storage facilities in the entire drainage systems requires a solid planning study incorporating socio-economic aspects.