Blue-green spaces (bio-retention and infiltration) for flood reduction

HOFOR is usually the entity responsible for financing flood reduction and mitigation measures. However, as the City and HOFOR set out to implement the Cloudburst Management Plan, they encountered a legal roadblock. National legislation at the time (2011) clearly distinguished between mandates for ‘below the ground’ (largely grey infrastructure) and surface projects. HOFOR had the mandate to finance traditional grey water infrastructure, but any surface measures—such as parks, swales, and roads—had to be financed and owned by the City of Copenhagen, even if those measures were to reduce flooding. In close collaboration, the City and HOFOR approached national authorities to request a change in the law to allow the city to plan and build the surface projects and still have the water utility pay. The petition was successful, especially given existing political will at the national level and the fact that catastrophic floods were still very present in everyone’s memories. The legislation was amended in 2013 to allow 100 percent funding by HOFOR until 2015; from 2015 onwards HOFOR’s funding would be reduced to 75 percent and the city would be expected to contribute the remaining 25 percent. 

Following this legal change, nature-based solutions in public areas were implemented jointly by the city and HOFOR, with HOFOR financing the flood protection measures. In a park, for example, the city finances tree planting and playgrounds and HOFOR finances a swale or drainage hole. To offset the costs, HOFOR can raise the water tariffs by 10 to 15 percent after conducting a political meeting and a public hearing to garner support. 

Since 2015, Danish municipalities have had to pay 25 percent of the hydraulic cost for new surface projects. This has made the implementation of the Cloudburst Management Plan very difficult as the program competes with other municipal responsibilities. The city has therefore again asked for a revision, and the decision is currently being revised with the hope that 100 percent funding by HOFOR will be allowed again.
 

In 2010, the City of Copenhagen conducted feasibility and economic assessments of different measures for flood protection (conventional, blue-green, and a combination of both) as part of its newly prepared Climate Change Adaptation Plan to decide on the most appropriate way to protect the city. The plan was further developed in a Cloudburst Plan in 2012, addressing the decisions and actions that can be undertaken at a services level for protecting the city against a statistical 100-year storm event. 

This provided the basis for preparing the Cloudburst Management Plan, which addresses flood protection for seven central city catchment areas through the integration of existing and new grey infrastructure and a selection of blue-green solutions. The plan includes 300 surface projects to be implemented based on priority over 20 years through 2035.

In 2018, Copenhagen’s seven catchments were further divided into 60 sub-catchments based on the natural water catchment areas to facilitate project implementation. Each of the sub-catchments is to have its own master plan with respective projects for flood protection: of these plans, three had already been completed and nine were under development by mid- 2019. At that date, eight surface projects, along with 25 pipes and tunnels projects, have been completed. The surface projects range from bigger parks to retention ponds and squares. One of the projects in the central part of the city uses Enghave Park as a reservoir for rainwater during a cloud burst. 

The city monitors progress every year and adjusts the Cloudburst formula to increase accuracy. A new and more scientific calculation and modelling is now available for the entire city, including projections of the numbers of citizens affected in each area.