Cheap and effective climate change adaptation in Hareskoven

Climate change adaptation is not only a cause for specialists. The public are most often faced with the problems, and occasionally it is the public who find a solution to the problems.

When the Copenhagen area was affected by the 'worst downpours of the century' on 2 July 2011, the town Hareskovby north-west of Copenhagen was hit hard. Shortly after, a citizen submitted photos to the Danish Nature Agency and the Municipality of Furesø of water pouring out from the forest. The citizen suggested a solution to delay and retain rainwater in the forest.

The Danish Nature Agency and the municipalities affected by the downpour liked the idea. A quick investigation showed that the conditions of the ground would make it relatively easy to establish a solution to delay the water. The solution was even in line with the Danish Nature Agency's wish to convert the forest for greater diversity. The costs were no higher than they could be included in ongoing budgets, so with good intentions from all parties and with the necessary permits, the contractor could start digging in October 2012. About a month later, the construction was ready.

The solution is as simple as it is cheap: Across the existing drainage trenches, small embankments have been constructed in suitable places in front of hollows in the ground which retain water. During the year, most of the water will then stay in the forest and slowly sink into the ground to become part of the groundwater.

In the wetland created behind the embankment, a "monk" has been installed; a mechanism which, with a simple system of pipes, regulates the water level in the reservoir and ensures even drainage of the surplus water to the trench downstream of the embankment. In rainy periods, the construction retains the water in an ever-increasing lake, until the water reaches a level at which any surplus rainwater can run off through a larger pipe.

In this way, eight new wetlands with a total area of 7-8 hectares will be created by the embankments in Store Hareskov. Seven of these areas totalling 4 hectares (corresponding to eight-nine football fields) will probably have gravitational water table all year round. The total reservoir can retain up to 24,000 m³ of water. This corresponds to a large retention basin.

The construction in Store Hareskov has not yet been tested during a proper flood. However, Kim Søderlund, forester at the Danish Nature Agency and responsible for the construction, has no doubts that it will serve its purpose:

"With regard to mechanics, the construction itself prevents water from coming out of the forest in volumes that will cause problems in Hareskovby, even during heavy cloudbursts. We are not yet able to evaluate the impact on biodiversity of the forest.

The new wetlands have been partially filled with water, especially downstream. We know from experience that it easily takes 2-3 years before they are filled entirely. But there is no doubt that it will also work for biodiversity," said Kim Søderlund.

Wetter nature will automatically attract new life into the forest, which is a popular destination for an outing for many Copenhageners. The Danish Nature Agency expects the area to be filled with dragonflies, water damselflies, caddis flies and mosquitoes, which again will attract swallows and bats, as well as a wealth of frogs, newts and grass snakes.

Everything has a price, and in Store Hareskov, the price is primarily that the new wetlands take up space which has previously been used as production forestry. Around the new forest lakes, the forest will be more or less unaffected.

"Seen in isolation, it is not particularly good for the forest to become wetter. Trees can drown if they stand under water for too long. This is why the drainage is there. But that the forest floor becomes a little softer in a week after a downpour, only helps increase diversity and the dynamics of forest life.

The terrain in Hareskoven enables us to collect water in some smaller areas which then become more or less permanent wetlands. This is in line with the trend to return smaller productive areas to nature. Taking some marginal, smaller productive areas out of forestry, only has a marginal impact on the operating economy of the forest," said Kim Søderlund, who does not hesitate to call this a brilliant solution, also because it is cheap. Total construction costs in Store Hareskov were DKK 430,000.

"This is peanuts compared to what it would cost to establish a 24,000 m³ detention basin in Hareskovby instead. We could have made it even cheaper if it was not located in a local forest with a lot of paths and forest roads that we have had to elevate," said Kim Søderlund.

The detention installation in Hareskoven will not break operating budgets. With the water intake at the bottom, the monks have been constructed so that the pipes cannot be blocked by leaves or other organic material, meaning the construction is almost maintenance-free," said Kim Søderlund.

The costs have been divided between the parties, so that the Municipality of Furesø (under which Hareskovby belongs) and Furesø Spildevand A/S paid the contractor and materials. The Danish Nature Agency has contributed with project management and guidance. The Municipality of Gladsaxe has also contributed to the project, as the affected areas of Hareskoven are located on Gladsaxe's side of the municipal boundaries.

With the right conditions, the Hareskov model is a solution which is likely to have potential in many other places, Kim Søderlund assessed. "In cases in which the 'problem' as in this case is upstream in relation to a building, this is a perfect solution. I believe that there are many places where this solution could be considered, especially places with undulating terrain as in this case. In such cases, this would be a very good and cheap way to deal with the problems."

"Other solutions may be necessary in other places. For example, in completely flat terrain it will be more difficult to gather the water in limited locations. I believe that it's about incorporating such possibilities as a train of thought. This is not something we have worked with before."

The Municipality of Furesø is also enthusiastic about the solution: "In this specific case, it was difficult to find any alternatives. It's difficult to see how we could find room to construct a detention basin in Hareskovby and the alternative - laying larger sewer pipes - is a cost-intensive solution. This was a cheap solution to the problem. And as the water can also benefit nature, the whole project makes good sense," said Dorthe Tiersgaard, environmental official in the Municipality of Furesø.