Stormwater grates have been installed along the sides of all streets leading down to the canal area around Slotsholmen. From here, the water is directed via underground drains down into an opening in the quay where the water can run out. The drains are covered with cast iron grates.
At the outlet, there is a non-return flap valve, which ensures that the water can only run out and not in. During very high water levels, the valve shuts and protects the city from flooding from the sea.
Each outlet drains an area of around 1 hectare and is dimensioned to cope with a massive rainfall event likely to occur less than every ten years.
"Previously, water from the roads contained dirt from houses and animals. Water from the rooftops wasn't clean either, because the houses used to be coal-fired. Today, the stormwater from roads and rooftops is fairly clean, so leading it into the harbour is no longer problematic," said chief advisor at HOFOR, Jes Clauson-Kaas.
The national bank building has been designed with the downpipes running inside the building. Rainwater from the roof used to run into the basement, where a pump pumped it up into the sewer.
During heavy rainfall, the sewer ran full. Therefore, the water was pushed up into the street just beside the ramp leading down into the carpark basement, which was then flooded.
This has now been changed. At the national bank, both surface water from the street and rooftop water from the building is now led directly out into the harbour.
Each outlet can manage a rainfall event likely to occur less than every ten year.
The installation has an expected lifetime of 75 years.