FLOODED FUTURE: WHAT DENMARK CAN TEACH THE UK

The UK remains too reactive when it comes to flood defence and Copenhagen illustrates the benefits of a long-term response to a catastrophic event.

As rainfall records are broken in Cumbria and we hear yet more painful stories of flood damage to the communities there, we're once again reminded of our vulnerability to natural forces.

 Climate models produced by the Intergovernmental Panel on Climate Change suggest that extremes of flood and drought will become more common this century and researchers at Oxford University and the Royal Netherlands Meteorological Institute have stated that climate change had made Storm Desmond's torrential rain 40 per cent more likely.

The economic costs of clean-up from floods are high and the latest reports estimate that damage caused by this past month's flooding will rise to £500m. Fortunately, we're not alone in facing the challenges of extreme flooding and there are lessons to be learnt from our near neighbours in Denmark in how we can build resilience.

On 2 July 2011, 150 mm of rainfall fell on the city of Copenhagen in just two hours, leaving swathes of the city under a metre of water. Insurance claims from this flood exceeded €800m and the total socio-economic loss has been estimated to be double this figure.

The city describes 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.

Cloudburst mitigation planning

In recognition of the costs of the 2011 flood, the city produced a Cloudburst mitigation plan. This, 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.

 The overall principles of the strategy are to retain rainwater in the higher elevated areas; to provide robust and flexible drainage of lower-lying areas; and to focus on ???green and blue' solutions for existing projects.

Cloudburst ???fingers' to convey runoff were located between the major roads into the city centre (see map below). Various roads connected to these Cloudburst fingers are then transformed into green retention roads. City catchments were subdivided by topography and sewer network to assess practical solutions on a local scale.

 Other solutions in the toolkit include green streets and central areas of retention in existing squares and lakes. Road profiles and cambers are adapted to provide surface-level storage in cross-section, while keeping a 'dry lane' to maintain movement across the city.

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 that will also provide surface-level storage.

This network of blue-green infrastructure aims to replicate the natural water cycle that has been disrupted through modern urban development. As well as the flood relief and water management functions, the solutions also contribute to the amenity and liveability of the city.

UK relevance

So how transferable is Cloudburst thinking for UK cities? Well, perhaps more transferable than we may think. Sustainable drainage systems (SuDS) are increasingly mainstream, and since April 2015 the National Planning Policy Framework has been strengthened in this area to make them a material planning consideration for major applications.

In our experience SuDS have been most successfully implemented on a smaller project-by-project basis rather than a catchment basis, perhaps due to funding pressures. What is more challenging is finding a willingness to implement more integrated catchment solutions like the Cloudburst approach taken by Copenhagen.

The solutions Copenhagen found could be applied to many of our cities, converting urban streets into Cloudburst or green streets instead. 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.

 Public spending on flood defence schemes and watercourse maintenance is increasing. However, the impression is that we are still too reactive to flood events, rather than adopting proactive measures to make our towns and cities more flood resilient - which can also bring economic, amenity and liveability benefits.

While a welcome contribution to the design toolkit, SuDS on their own can't solve our flooding issues, especially when located in lower parts of the catchment.

Upland management

As recent events in Cumbria have shown, upland land management is a crucial issue that needs to be integrated into catchment planning, and the sense is that Lead Local Flood Authorities are overly focused on surface water run-off from new development.

We can't be certain where or when the next flood will hit our cities. One thing is certain though: the economic costs of doing nothing are mounting, and our cities can learn from the good practice seen across the North Sea.

Source:ConstructionNews 

 

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