Recent years' extreme rainfall events due to climate change have left Himmelskibet (The Celestial Boat), a large 130-children daycare institution,exposed to repeated flooding. The daycare institution is situated in the municipality of Gentofte, a suburban municipality to Copenhagen. It lies at the bottom of a river valley immediately adjacent to a watercourse which collects water from two drainage ditches leading excess water from Gentofte Lake and Utterslev Bog to Emdrup Lake and onward to the Sound.
Furthermore, the daycare institution is located alongside a railway embankment which, serving as a barrier in the landscape, contributes to the accumulation of water on the daycare institution's property during cloudbursts. The daycare institution has been flooded twice over the past two years; on both occasions completely flooding the basement.
"On both occasions, we experienced property damage and general nuisance - also with regard to having to rehouse everyone while the buildings were being repaired.The situation was unacceptable for children and staff as well as for parents.Also, we had to ask ourselves whether it would be possible to have the buildings insured in the long term.So we began considering what to do to solve the problem," said Sven Hartmann Andreasen, from Gentofte Ejendomme, who managed this climate protection project for the municipality.
"Of course, the long-term solution would be to reduce the inflow of water and to adjust the capacity of the drainage system out to the Sound in order to enable the system to cope with the type of 100-year rainfall events that we are now beginning to experience almost every two years.However, that would take too long.We therefore asked a consultancy firm to help us explore other possibilities," Sven Hartmann Andreasen explained.
A 60cm-high dyke around the property now effectively protects The Celestial Boat against future flood events.
The location is challenging because very large volumes of rainwater accumulate in close vicinity to The Celestial Boat during heavy downpours. This is one of the lowest points in the municipality, and the water table in the Gentofterenden drainage channel, which runs straight past the daycare institution, is only ½-1 meter below the foundation of the buildings during normal water levels.
The contractors initially performed a hydraulic modelling, which made it clear that the total water volumes from surface water run-off and overflow from sewers during cloudbursts were far greater than the site could manage.
The dyke under construction. For practical and aesthetic reasons, the fence was placed in the middle, but it should probably have been placed on the inside, so as to protect the dyke from toddlers digging it away.
"First, we always look at whether it is 'our own water' or 'someone else's water' that we have to manage.In this case it was 'someone else's water', in that the channel drains a large urban area with thousands of homes; from Vangede in the North to Søborg in the West.First we looked at whether we could detain the rainwater before it reached the daycare," said Henrik Lynghus from the consultancy firm NIRAS, who was involved in the case.
A little upstream from the daycare institution, the municipality and the Nordvand utility company had already launched a project aimed at increasing the storage capacity of Gentofterenden. However, even more capacity was required to prevent flooding of the channel downstream at The Celestial Boat. On the opposite side of the channel are green playing fields which, in practice, already serve as a retention basin during flooding. However, it would entail a far more extensive construction project to establish the required retention capacity here, and such a project would potentially take much longer and be much more comprehensive in scope.
Other possible technical solutions, e.g. raising the buildings by 50cm, were also considered. That would have been an easy solution had the buildings not been made from brick. A pump solution alone was also considered, however to obtain the required pumping capacity the pumps would have to be impractically large.
The solution had to be a natural part of the landscape and look 'as if it had always been there'. This ruled out simple solutions such as driving a sheet pile into the ground around the buildings, erecting a wall or building walls from flower-bed boxes. Another requirement was that it had to be possible to fence off the whole property.
"We ended up saying:Let's build ourselves a 'hydraulic island', entirely shielding off the property from 'everyone else's water' so that we only have to deal with our own water.This could be done by building a dyke around the entire property," said Henrik Lynghus.
The final solution was a simple, grass-covered earthen dyke with a clayey core; 60-cm high with a 35-degree slope. At the entrance to the property, stairs and a zig-zag ramp provide for access over the dyke. The sides of the ramp are made from rustic COR-TENsteel and this accentuates the entrance section.
The daycare institution is more or less at level with the Søborghusrenden drainage channel.
As opposed to sheet piles or a wall, the dyke has the disadvantage that it takes up quite a lot of space. As a consequence, the reduced area of the property due to the dyke influences the calculation of the number of children and, thus, resources which the municipality allocates to the daycare institution. In the final analysis, however, the calculations showed that the area was still large enough for this not to pose a problem.
With the dyke in place, The Celestial Boat is indeed a boat, or perhaps rather an island, which is hydraulically isolated from its surroundings Now, the daycare institution only has to manage the rainwater that falls on its own property. This requires pumps, as infiltration is not an option due to a high water table.
A pump has been running 24-7, pumping the daycare institution's sewage, as well as drainwater, from the perimeter drainage system around the building and, out into the public sewer system. This pump has now been supplemented by a pump to manage normal rainfall flowing from rainwater gullies and the ground. Finally, an extra pump has been installed which will be activated during heavy rainfall, and which is large enough to cope with any water ending up inside the dyke during a cloudburst.
All of the pumps are gathered in a pump well at the centre of the property, so that they can easily be inspected and serviced. This is the only part of the solution requiring regular maintenance. The pump house is equipped with a non-return valve preventing the intrusion of sewage from the public sewer system. It was therefore not necessary to install a backflow blocker inside the main building.
In connection with installation of the solution it was necessary to re-establish all paved areas at ground level in order to remove depressions so that all surface water is now diverted way from buildings and on to drains.
The solution won backing, not just from The Celestial Boat and the owner of the building, Gentofte Ejendomme, but also from many other stakeholders, including the Nordvand utility company and the relevant municipal authorities. However, the specific design of e.g. the entrance section was the object of much debate and several alternative proposals were discussed.
The entrance section above the mini dyke was somewhat of a challenge. It has to be presentable, safeguard easy and safe access to the building for prams etc. as well as for goods deliveries. The chosen solution was a ramp with a wide 'hairpin bend'. The sides of the ramp are made from COR-TENsteel.
Due a location in vulnerable, protected natural surroundings at the very back of the local park, Dyssegårdsparken, there was much focus on the project. It was therefore decided to involve the various stakeholders in a number of 'star meetings' at which possible solutions were presented and debated.
"Naturally, it made the process a little more cumbersome, with several meetings and different project proposals, however, all in all, it meant that we found solutions for which stakeholders feel a sense of ownership," said Henrik Lynghus.
The new flood protection solution for The Celestial Boat daycare institution was established in 2013-14 and stood the test during a cloudburst in August 2014. While water once more built up outside the dyke, inside the dyke the water was under control. Dykes and pumps have been dimensioned to withstand a 100-year rainfall event. The owner of the building, Gentofte Ejendomme, is very pleased with the result.
"Simple solutions are often the best solutions.There is broad agreement that we have found a good solution to the problem.Overall, it was the best solution of all the solutions that we looked at, because it made the best use of public funds," said Sven Hartmann Andreasen. DKK 3.4 million (including consultancy fees) was the price of protecting The Celestial Boat from future flood events.
The more radical alternative, i.e. to tear down the institution and relocate it, had also been examined by the owner.
"Ultimately, it’s a cost-benefit trade-off: do you want to invest in climate protection or give up on the buildings?Tearing down the building and building a new one would cost around DKK 10 million.To this amount would have to be added derived losses and extra costs of closing down, rehousing and moving the entire daycare," explained Henrik Lynghus, who therefore gives a thumbs up to the chosen solution.
"Once you've decided to hold on to an exposed building, this solution is an obvious choice.The pumps are the only facility requiring inspection and service to ensure that they work.The rest takes care of itself.
In the case of a daycare institution, however, this is not completely true. At The Celestial Boat the children are fond of playing on the dyke, which is used as a sandbox and for sliding down. Although the heavy clay core makes the dyke relatively robust, the problem of protecting it from the wear and tear of the playing children has not yet been completely solved.