The citadel in Copenhagen can now cope with a 1,000-year rainfall

Seven cast-iron manhole covers on the ground are the only sign of extensive and very complex flood protection for the Citadel in Østerbro, Copenhagen. The Citadel is around 350 years old and is one of the best-preserved fortification works in Northern Europe. The fortification is a listed ancient monument. It also functions as military barracks and as a workplace for hundreds of staff in the Danish Defence. Furthermore, the Citadel is much visited by tourists, and Copenhageners use the ramparts around the Citadel as a green escape from the busy capital. In recent years, the Citadel has been exposed to several torrential flood events following cloudbursts. Following major underground work, the Danish Defence Estates and Infrastructure Organisation has now secured the Citadel against a 1,000-year rainfall event.

Status: Area within the ramparts has now been protected against cloudbursts

Protecting the Citadel from flooding has taken 11 years, from the initial discussions with the City of Copenhagen and others, to the inauguration in May 2022. The actual construction work took two years and was completed on schedule and within budget.

A full 11 kilometres of new pipes have been laid underground, of which almost four kilometres are rainwater pipes. A retention basin and a pumping station with three pumps have also been established underground, leading the rainwater into Copenhagen Harbour.

An area of 26,000 square metres of cobblestone have been removed and re-laid exactly as they were originally – a total of 780,000 cobblestones!

Everything was carried out under careful supervision by, and in collaboration with, archaeologists from the Museum of Copenhagen, the Agency for Culture and Palaces, other authorities and other stakeholders.

The Danish Defence Estates and Infrastructure Organisation administers the area and also funded the entire project with around DKK 140 million.

Background: Extensive and expensive flood events in 2011 and 2014

The severe cloudbursts that hit Copenhagen in 2011 and 2014 flooded the entire area within the ramparts of the Citadel.

Low-lying buildings and floors were under water, and multiple parts of the steep rampart areas collapsed.

Copenhagen experienced torrential cloudbursts in 2011 and 2014. Costs of clean-up and repairs amounted to millions of Danish kroner.

The Citadel is located at the end of a stream system, which in most stretches runs in pipes beneath Copenhagen. The City of Copenhagen refers to the stream system as Copenhagen's blue circuit.

It extends from the Damhussøen lake around eight kilometres west of the Citadel and ends in the Citadel moat, from where an overflow construction leads to the Copenhagen Harbour.

During heavy downpours, water from neighbouring municipalities also runs into the circuit.

Large amounts of rainwater run onwards from the city to the Citadel and only a small amount of this water could previously flow onwards into the harbour. Furthermore, the Citadel's own sewer system was worn-out and under-dimensioned.

The solution: Larger pipes, large pumps and an underground retention basin

A new and much larger sewer system and a retention basin now lie hidden below the Citadel's 780,000 re-laid cobblestones, as well as three large pumps that can lead rainwater into the harbour.

The Citadel stands as an island in the middle of a lake. Before the flood protection, only very little rainwater could flow away from the Citadel.

For instance, rainwater from the Citadel was led into the Citadel moat. The capacity was very low, and the water level in the moat would rise during heavy rainfall.

Now, cobbles and green areas at the Citadel have been dug up to establish flood protection.

A retention basin has been established, consisting of two 70-metre-long pipes with a diameter of 140 cm and a total capacity of around 120 cubic meters of water.

A pumping station with three pumps has been established. Together, they can pump 3,000 litres/3 cubic metres of water per second.

The pumps will automatically turn on when the retention basin is full. There is access to the pumps via shafts hidden under cast-iron manhole covers.

The pumps are directly connected to the harbour 250 metres away. Water is pumped from the retention basin via large pipes into the harbour.

The connection to the harbour was made via controlled under-drilling, thereby avoiding having to dig up the ramparts around the Citadel and the Langelinje harbour promenade.

The 250-metre-long pipes were pulled into the drilled hole from the harbour basin.

The parts of the ramparts that had previously been damaged by heavy cloudbursts have been reinforced with coconut matting, which will slowly disintegrate as the earthen ramparts regain their strength.

Normally, geotextile would be used to reinforce the ramparts. However, since the insides of the ramparts are also listed, the contracting authority used a degradable material, i.e. the coconut matting.

The City of Copenhagen allows a maximum of 10 cm of standing water after rainfall. The Citadel decided to impose an even stricter requirement: 0 cm of standing water.

According to the Danish Defence Estates and Infrastructure Organisation, this ambitious goal was set because the Citadel is listed and is a military area.

The Citadel can now cope with a 1,000-year flood event. The Citadel will still be vulnerable to storm surges that can push water in from the harbour.

[...]