Cloudbursts: What can we learn from Copenhagen?

Luke Strickland and Stuart Divall of Ramboll UK review some of the flood mitigation measures implemented in the Danish capital since its cataclysmic flood in July 2011. Can such an approach be transferred to the UK?

Hydraulic model of flooded areas after cloudburst in Copenhagen


Paul Stubbington

Team Director
T: +44 7939 001 621

Originally published by Environtec Magazine, July 2015 

How do we best respond to a changing climate? That’s the question many people are asking, from homeowners seeking to defend their properties against flooding, governments seeking to best improve their infrastructure and decision makers at every level in between.

Around the world, the manifestation of a changing climate varies. Sea level rise is most critical to Pacific island communities for example, whilst drought and desertification is a big issue in many African countries. In the UK the evidence is that we’re facing more extremes of weather. Reports of river, coastal, groundwater and urban flooding across the country are increasingly in the news. Extreme rainfall events deluge our cities and overwhelm our drainage systems, with the subsequent clean-up having significant economic consequences.

We’ve disrupted the natural water cycle through extensive urbanisation over the past two centuries in particular, contributing to flooding and also disconnecting our citizens from the water environment. We have succumbed to a Victorian mind-set that rainfall runoff is a nuisance, something not to be seen or heard and we’ve buried our drainage networks and many of our urban watercourses underground. What we’ve found, however, is that our ageing infrastructure isn’t as robust as we think when faced with increasing weather extremes, and when our drainage systems are overwhelmed and flood waters erupt from their subterranean networks, the damage and consequences are severe.

Climate models produced by the Intergovernmental Panel on Climate Change suggest that extremes of flood and drought will become more common this century, and we face the major challenge of making our national infrastructure more resilient to this changing climate. Fortunately, we’re not alone in facing these challenges, and there are lessons to be learnt from our near neighbours in Denmark.

What happened in Copenhagen

On the 2nd July 2011 150mm of rainfall fell on the city of Copenhagen in just two hours, leaving swathes of the city under up to a metre of water. The city described this as a “cloudburst” event, from the Old Danish word Skybrud, and it seems a fitting term for the extremely intense storms that are becoming more frequent across Europe. Insurance claims from this flood exceeded over 800M Euros and the total socio-economic loss has been estimated to be double this figure. In recognition of the significant impact on society and the economy, the city produced a Cloudburst Mitigation Plan. This plan, and the subsequent catchment level plans (prepared by Ramboll among others), identifies the parts of the city most at risk from future cloudburst events, and proposes a toolkit of solutions to increase the city’s resilience to flooding.

The overall principles of the strategy are: to retain rainwater in the upper catchment; to provide robust and adaptable drainage of lower lying areas; and to focus on implementing green and blue solutions in existing projects. A “finger strategy” has been adopted – cloudburst “fingers” to convey runoff are located between the major roads into the city centre. Various roads which connect to these cloudburst fingers are thereby transformed into green retention roads.

The underlying modelling has been undertaken using an integrated hydraulic model of the sewers and watercourses, and it employs a digital elevation model of the city. Each catchment has been further subdivided by topography and sewer network to assess practical solutions on a local scale.

Streets addressed as a hierarchy

These solutions take the form of a toolkit – a portfolio approach towards public realm solutions – including the cloudburst-adapted streets and retention streets mentioned above, as well as the provision of other green streets and central areas of retention in existing squares and lakes. Road profiles and cambers have been adapted to provide surface level storage in cross-section, whilst also keeping a “dry lane” to maintain movement across the city. The solutions reflect the hierarchy of streets across the city. So where the streets are wider, such as Sonder Boulevard, a grassed channel can be incorporated for storage, treatment and conveyance. Narrower streets such as Korsgade have additional channels installed in the existing paving, and benefit more from adaption of the camber to facilitate storage. Other streets have a mix of hard and soft features in proportion to their width and use.

In St Jorgen’s Lake, an existing lake in the city centre, it is proposed that the water level will be lowered to provide a central storage area for flood waters, along with designated city squares which will also provide surface level storage. Taken together, this network of blue-green infrastructure aims to replicate the natural water cycle that has been disrupted through modern urban development. A by-product is that water is increasingly brought to the surface where it is more visible, blockages can be more easily identified and managed, and the overall consequences of failure are reduced. As well as the flood relief and water management functions, the solutions also contribute to the amenity and liveability of the city by increasing planted areas. Benefits such as additional habitat and urban cooling are also additional positive outcomes.

Relevant to the UK?

Some may ask the question, that while these kinds of solutions are all very well in Scandinavia, how transferable are they over here in our crowded, urbanised and creaking cities? Well, perhaps they are more transferable than we may think. Sustainable Drainage Systems (SuDS) have become increasingly mainstream, although more widespread uptake in England has been held back by the five year delay by Government for implementing the relevant parts of the 2010 Flood and Water Management Act. Happily since April 2015 the National Planning Policy Framework (NPPF) has been strengthened in this area and SuDS are now a material planning consideration for major applications.

Our experience suggests that SuDS have been most successfully implemented on a smaller project by project basis rather a catchment basis, perhaps due to funding pressures. We are used to including a portfolio of SuDS solutions on a site by site basis; what is more challenging is finding a willingness to implement more integrated catchment solutions like the Cloudburst approach taken by the city of Copenhagen. The range of solutions they have identified could just as easily be applied in our own post-industrial cities. Surface Water Management Plans undertaken by Lead Local Flood Authorities have gone part of the way to a more holistic approach, but it’s questionable how much change these have brought – especially with reductions in Government spending and Council budgets in recent years.

In response to recent flooding, public spending on flood defence schemes and watercourse maintenance is increasing. However, the impression is that we’re still too reactive to flood events rather than taking pro-active measures to make our towns and cities more flood resilient, enjoying the economic, amenity and liveability benefits at the same time. Arguably, many of the suggestions and solutions within the Pitt Report remain aspirational and not widespread practice.

A further driver for change is the Water Framework Directive, adding the requirement for our drainage and flood management solutions to improve water quality as well as provide the necessary storage and attenuation. With the need to report this year on the progress made in improving our water bodies, and with more stringent chemical and ecological targets ahead, the argument for integrated catchment solutions is stronger than ever.

Some progress has been made already

Maybe this is too bleak a picture though, and perhaps this joined-up thinking is closer than we think. Cities like Birmingham already have established Blue and Green city plans, aiming to capitalise on the existing water networks and green spaces for the amenity and health benefits of their citizens. Hard and soft SuDS, blue/green spaces, retrofit SuDS and resilient buildings are all becoming more common and should become ever more so as Lead Local Flood Authorities work out the finer detail of their strengthened position with regard to SuDS and urban flooding. Making our homes, communities and national infrastructure more resilient to extremes of weather and flooding in particular is doable but the challenge is for it to become the norm rather than the exception.

A lesson from Copenhagen is that as well as funding, the key issue is cross-department co-operation between the roads, water, parks and environment authorities. It’s long been identified that the responsibility for water in England is spread unevenly between local authorities, drainage boards, water companies, highways departments and others. Joined-up thinking has often fallen between the cracks between organisations. This is slowly changing as a result of the Flood and Water Management Act, the recent changes to the NPPF and the growing role of Lead Local Flood Authorities.

However the implementation of the latest prescription for SuDS in the NPPF goes, there’s no doubt that it’s a step in the right direction. But in the face of a changing climate one thing is for certain, the economic costs of doing nothing are mounting, and our cities can learn from the good practice, joined-up thinking and cloudburst approach in Copenhagen. When disaster strikes, as it did in Copenhagen, the hidden costs of not taking proactive action are painfully realised. This is a valuable lesson we would do well to learn from.


St. Jørgens sø in Copenhagen

Cloudburst Concretisation Masterplan

On 2 July, 2011 large areas of the city of Copenhagen were flooded. Copenhagen is on the front line of climate change, experiencing increasingly heavy rain events, as they are also on the leading edge of urban innovation with a vision of transforming their city into a sustainable, C02 neutral city by 2025.

Ramboll Group A/S

Ramboll Group A/S
Hannemanns Allé 53
DK-2300 Copenhagen S
Tel:+45 5161 1000
Fax:+45 5161 1001


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