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.