Where does all the rainwater go?

In Zuidas, we are therefore increasingly seeing water-retardant green strips in the streetscape, also known as water drainage channels. These water drainage (and infiltration) channels, called WADIs in Dutch, consist of a large type of plant container around 35 cm in depth that holds the rainwater temporarily. As it gradually sinks into the ground, the water is purified and then it drains into the groundwater and sewer system.

Sometimes, a water drainage channel is not enough. This is something we learnt from the one in Kop Zuidas – the first water drainage channel in Zuidas in 2016. Ter Mors: ‘According to the plan, during heavy downpours, the rainwater from Hogelandplein was supposed to end up in the water drainage channels in the Southern Promenade (Zuidelijke Wandelweg). These channels are connected to each other and form a drainage route to the surface water some distance away. Although the water reached the first one, it didn’t reach the next one, probably because of a blockage. As a result, the water remained in the channel, causing the groundwater level to rise too far.’ This clearly shows that extra measures are sometimes needed: ‘When installing new water drainage channels, such as the ones in Prinses Irenestraat, we therefore always include drainage in the form of underground pipes. This enables the excess groundwater underneath a full water drainage channel to drain away to a nearby sewer or watercourse, such as a ditch or canal.’

When renovating Europaplein-Oost, we opted for an underground water storage system with a capacity of 400 m³. ‘It’s a large, concrete container where the rainwater can be stored. It’s even big enough to stand in.’ According to Ter Mors, this actually serves as back-up: ‘In Amsterdam, the public spaces, together with the sewer system, must be able to drain away a rain shower at a rate of 70 mm/h. To give you some idea: 25 mm of rain is what you would call a heavy downpour, and more than 50 mm amounts to extreme precipitation that happens once every century on average. Things will have to be really bad for it not to work.’ So, why take all of these measures? Ter Mors: ‘In Amsterdam, we’ve agreed that we need to be sure that there is no water damage or flooding and that the streets are always fully accessible for the emergency services. This policy also applies in Zuidas.’