Slotsholmen in Copenhagen is now well protected from the next cloudburst

After almost all the streets around Slotsholmen have been equipped with stormwater grates which lead the rainfall directly into the harbour during heavy rainfall, events with flooding have been prevented.

The project was decided in the winter of 2011-2012 after an intense cloudburst in July 2011.

The utility company HOFOR began work in spring 2012, so that the first stage could be finished before the cloudburst season in the summer of 2012.

Today, the project is almost completely finished. The remaining work has been planned in the investment budget for 2016.

The solution has been tested. Neither the storm Bodil in 2013, nor the cloudburst in the summer of 2014 led to flooding of the area.

During the 2011 cloudburst, very large volumes of water collected in the area around Slotsholmen and flooded shops, hotels, restaurants and organisations, etc.

Stormwater from the city's streets naturally runs towards the harbour. Here, it was previously blocked by the quay walls protecting the city against seawater during high water levels.

During the cloudburst in July 2011, the water could not run into the harbour and stood 0.5 to 1 meters high in natural depressions around Slotsholmen.

The water also found its way into many buildings. Stormwater penetrated Holmens Kirke church, flooding the church floor, and the printing press in the national bank building also stood under water.

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.

It cost DKK 25 million to install the stormwater grates, outlets and non-return flap valves in the area around Slotsholmen.

The costs are included in the annual budget of the utilities company HOFOR, which is approved by the City of Copenhagen.

The price of water paid by consumers is calculated on the basis of the annual budget.

"The money is well invested when comparing the costs with the losses the city has incurred due to flooding," said chief advisor at HOFOR, Jes Clauson-Kaas.

Businesses in the area, local residents, owners of quay properties and the local water authorities have all been very keen to find an effective solution to the flooding.

The flood protection solution was projected by the utility company HOFOR, who is the project owner. The company has oversight of the project and is responsible for maintenance.

Construction work was carried out by the two contractor and engineering companies,  MT Højgaard and M.J. Eriksson.

The work required a discharge permit, which was obtained without any problems. Furthermore, it required separate permits to establish the outlets in the quay. These had to be applied for from a number of different local and private property owners.

There was no standard dictating the type of long stormwater grate to be used along the sides of the streets. HOFOR obtained a separate permit to establish these. A standard is currently being prepared, which will apply in the future.

The extraordinary flooding in 2011, which cost many millions of DKK, illustrated how vulnerable the area around Slotsholmen was to flooding during cloudbursts.

"The project received enormous backing. Everyone - both the authorities and private property owners - agreed that something had to be done and that it was in the city's best interest," said chief advisor in HOFOR, Jes Clauson-Kaas.

The roadworks in the area did not cause problems to traffic beyond what could be expected and no budgets were exceeded.