The Copenhagen Cloudburst Formula: A Strategic Process for Planning and Designing Blue-Green Interventions


Analysis & Planning

Copenhagen, Denmark | Ramboll and Ramboll Studio Dreiseitl | Client: Municipality of Copenhagen


Following a 2011 Cloudburst that caused damage of approximately USD $1 billion, climate change mitigation solutions became an urgent focus for the city of Copenhagen. The flood’s consequences transcended jurisdictional boundaries, necessitating a truly collaborative effort be established between planners, engineers, economists, citizens, utility providers, politicians, and investors to integrate Climate Adaptation within regulatory planning.

The result is the Copenhagen Cloudburst Formula, a flexible, universally adaptable model for mitigating increasingly common extreme flood events – or Cloudbursts – through Blue-Green solutions that integrate urban planning, traffic, and hydraulic analysis with sound investment strategies to improve the quality of cities’ Liveability.

Existing city space is valuable. A cost-benefit analysis, conducted on the 10km2 catchment, concluded that the potential of implementing a surface-first approach to mitigating Cloudbursts over solely pipe-based systems reduced investment costs by over $200 million.

The trickle-down effect has been the identification of over 300 citywide pilot projects, the incorporation of flood management design guidelines within local developer requirements, and the testing of the Copenhagen Cloudburst Formula applicability throughout Europe, the Americas, and Asia.


Globally, the impact of climate change is an issue that can no longer be ignored.

On the 2nd of July 2011, in less than two hours, Copenhagen was hit by an extreme 1000-year storm event – or Cloudburst – where 150mm of rain left large areas of the city under up to one meter of water. The 2011 event had been preceded by a 100-year storm in August 2010 and was hit again in 2014. Copenhagen realized that Cloudbursts were not a one-off occurrence; the threat compounds as harbor sea levels are predicted to rise one meter by 2110. In a city where many buildings and services are located below street level and where stormwater and sewage are in a combined pipe system, contaminated floodwater penetrated buildings and city infrastructure.

Traditional drainage solutions such as underground reservoirs are becoming less viable as utilities occupy more underground space. Extreme weather events cannot be managed by conventional pipe systems and their occurrence becomes more difficult to predict.

Conventional infrastructure is considered to be generally technical, underground, hidden elements while Blue-Green solutions are low-tech, on the surface, and interactive. The Blue-Green Approach develops a synergistic relationship between the two, integrating climate adaptation solutions within the limited confines of urban space, encouraging a solution utilizing the best of both techniques.

The Copenhagen Concretization Plans were commissioned to combat climate change following 2011’s flood. These integrated, multi-disciplinary plans bridge the gap between planning and site-specific solutions through the application of a typology-based Cloudburst Toolkit.

The process was formalized as the Copenhagen Cloudburst Formula, a six-step procedure for integrating the Blue-Green Approach:

  1. Data and Investigation: The city investigated, identified, and ranked areas according to their overall threat due to Cloudburst risk indicators, their potential to stir investment and influence property value, and the viability of implementation affecting adjacent developments.
  2. Modeling and Mapping: Municipalities divided their regions into stormwater catchments, undertaking large-scale hydrological models (including GIS, surface water, sewage, landscape character, risk assesments) to map vulnerable areas. The conclusion - traditional piped solutions alone were not enough. The result - public water utility companies began financing solutions that integrated Cloudburst events.
  3. Cost of Doing Nothing: An analysis undertaken by the city and consultants calculated that the effect of climate change was so large, that the cost of doing nothing would amount to approximately $60-90 million a year from now to 2110.
  4. Design and Qualify:  Hotspots were identified, transferring strategic planning to human-scale experiences as a model for how other cities can mitigate Cloudbursts and daily rain events. The “Cloudburst Toolkit” was developed as a palette of universally applicable, multi-functional, flexible elements.
  5. Involvement and Iteration: Cloudbursts would influence each area of Copenhagen; an overall strategy for a public participation program was established to gauge the requirements of the citizens who would be affected.
  6. Cloudburst Economics: A detailed socio-economic Cost-Benefit Analysis (CBA) tested two masterplan options. The option with the highest percentage of Blue-Green solutions and also the least additional infrastructural pipe improvements created a potential savings 50% greater than Conventional solutions alone. Additional qualitative social benefits, such as health, environmental, and urban spatial quality improvements resulting from the enhancements would potentially push this number even higher.

Demonstrating the relevance of the Cloudburst Toolkit palette of retrofit techniques on the first selected Hot Spot of the 10 km2 catchment of Lådegåds-Åen (a priority catchment set in the center of Copenhagen and at a high risk to flooding) investment opportunities in the form of Blue-Green techniques were created on the surface where they are visible, interactive urban components. The solutions are based on real situations, aligned to preexisting underground infrastructure.

Two masterplan variations were developed to assess potential advantages and disadvantages – Option 1 Conventional and Option 2 Blue-Green. Crisscrossed by a number of constructed urban barriers (such as streets, sidewalks, buildings, or train tracks) and with only minimal grade differences that prevent floodwaters from positively draining towards the main Copenhagen harbor outlet point into the sea, the key difference between the two options is the strategy for mitigating Sankt Jørgens Lake.

The Conventional Masterplan Option retained Sankt Jørgens Lake as it exists; there is cultural value in keeping the historical layout yet the lake currently lies above the surrounding street level and floods during rain events, requiring the creation of a new 5m diameter pipe to funnel flood water to the harbor. The technical engineering investment is calculated at twice the size and cost of the pipes required for the Blue-Green Option, and also creates limited new public green space.

In contrast, the Blue-Green Masterplan Option lowered the lake level from +5.8m to +2.8m, creating a new Cloudburst storage volume of 40,000m3 and a revitalized lakeside connection which had previously been only partially accessible. The lake overflows into a 2.5m diameter tunnel to the harbor as a reduced sized pipe. The solution combines the Blue-Green with the Grey (conventional) piped solution to result in a harmony between infrastructure and green space. But what do the numbers say?

The Conventional Masterplan has a total net performance value (NPV) €78 million while the Blue-Green Option is almost twice as effective at an NPV of €142 million. This estimate can even be considered conservative when the qualitative health benefits or the macro-scale loss of GDP caused by extreme flooding are further calculated. The insurance damage savings and the increase in real estate value are two of the highest socio-economic benefits from Cloudburst adaptation.

Today we are seeing Cloudburst solutions implemented in local plans where synergy projects are encouraged between municipalities, water utilities, and philanthropists as catalysts for development. Public participation workshops encourage and allow citizens to actively shape their municipality's Cloudburst strategy. Blue-Green is the future for establishing urban ecological waterscapes while balancing sound investment and economic opportunities with social benefit improvements.

The Copenhagen Formula provides a structure for integrating built, existing context with retrofit Blue-Green solutions.

The implementable, pragmatic tools mitigate extreme storm events and improve our cityscapes. Private developers and homeowners alike become champions for local solutions where a multi-disciplinary, cross-agency collaboration engaged designers, planners, sociologists, economists, biologists, geographers, information specialists, and communication experts interacting with public utility companies, stakeholders, interest groups, local politicians, and investors.

Cloudburst solutions are often left out of upstream area planning where residents see no flooding problems. Yet water has no boundary. Municipal borders must be lowered to develop a common vision across disparate districts. A recent interactive workshop led by the Engineer and Landscape Architect in a suburb of Copenhagen engaged residents through a series of interactive sessions designed to raise awareness and survey desired citizen interests. Hydraulic function was presented in an engaging, educational sequence that involved the public interest with private development goals.

Cloudburst solutions can provide much more than just stormwater management. The strategic flood masterplan is the opportunity to safeguard Copenhagen while providing the foundations for a high quality city environment. Resilient urban ecological waterscapes are the foundation for vibrant public realm spaces that are culturally and socially significant and contribute to the economic longevity, quality of life, and well-being of cities. 

Blue-Green Infrastructure represents the next generation of water infrastructure considerations where nature, city and recreational space are rolled into a holistic package. Cities around the world can look to the Copenhagen Cloudburst Formula as a model for implementing innovative, pragmatic, feasible measures within existing urban fabric. It's time to test the method and take on the Cloudburst Challenge!

“An excellent planning document that communicates skillfully to a wide range of audiences. Based on a solid and robust analysis, which was refreshing to see.”

- 2016 Awards Jury


General Cloudburst Master Planning Initiatives:

  • Administered by Mayor for technical and environmental affairs, Morten Kabell

General cloudburst masterplan with catchments of Copenhagen and Frederiksberg (basis for the specific  master planning and project identification):

  • City Copenhagen and Greater Copenhagen Utilities (HOFOR) assisted by COWI

Approach, Vision and concepts of 3 masterplans covering more than half of 300+ projects in Copenhagen and Frederiksberg:

  • City Copenhagen
  • Greater Copenhagen Utilities (Hofor)
  • City of Fredriksberg
  • Fredriksberg Utilities
  • Ramboll
  • Ramboll Studio Dreiseitl

City of Copenhagen:

  • Palle Dannemand Sørensen, Jan Rasmussen and Lykke Leonardsen

Greater Copenhagen Utilities (HOFOR):

  • Margit Lund Christensen, Nis Fink and Jes Clauson-Kaas

City of Frederiksberg:

  • Susanne Jakobsen (now HOFOR)

Frederiksberg Utilities:

  • Mikael Koopmann


  • Camilla Julie Hvid, Jesper Rasmussen, Christian Nyerup Nielsen

Ramboll Studio Dreiseitl:

  • Dieter Grau, Hendrik Porst, Stefan Brückmann, Jessica Read