Rik Thijs, deputy mayor for public space, greenery and water in nearby Eindhoven [Netherlands] said private and public initiatives were needed to adapt to the changing weather. “Our sewage system cannot cope with the rainfall that is coming, and we cannot increase its capacity, so we need to do things differently,” he said. “This means that you need to capture as much as possible on the surface.”
This might mean plans to bring an old river, the Gender, back to the surface, “wadi” pools that can hold water during heavy rain, and incorporating green roofs and rainwater storage into housing developments. “The Netherlands is very vulnerable because we are, of course, one large delta,” he said.
Pretty much al water in te Netherlands eventually flows out to sea. The problem the country is experiencing is that during periods of heavy rainfall the rivers are already at capacity because of water that comes from upstream so they can’t pump all excess rainwater from the polders into the rivers. If you can temporarily store all that rainwater in the polder (by using wadi’s or infiltration systems and letting it slowly dissipate into the groundwater) instead of immediately funneling it into sewers and into the rivers you can lower the peak excess by spreading it out over a longer duration into moments where the rivers do have the capacity.
Tldr, it still goes to the same place, just at a later date by temporarily storing the excess water.