Cloudburst mitigation

Canada could learn some lessons from Denmark about climate change adaptation.


Henrik Stener Pedersen

T: +45 51618124
Article by Martin Vilhelmsen. Originally published in Water Canada’s September/October 2015 issue.
The world is facing many severe issues, such as climate change, pollution, droughts, and floods and these problems are not limited to specific regions. As a result, it is imperative countries learn from each other through best practices and success stories.
From August 2010 to August 2011, the City of Copenhagen—the capital of Denmark—was hit by three devastating cloudbursts in a 12-month period. Major roads and other infrastructure were flooded. The total damage from the most destructive of the three events cost Copenhagen more than €800 million (or $1.18 billion). As a consequence, the city decided it had to do something to protect the city from future damage.
An initial economic analysis indicated the cost of doing nothing would triple in 100 years due to climate change affecting weather patterns, so the city decided it had to do something to protect the city from future damage.

The plan

The cloudburst master plan is based on a few simple principles, the main one being to keep the water on the surface and control it rather than making large expensive pipes underground. New infrastructure will be used for separating rainwater from smaller events to take the load off the sewers and wastewater treatment plants. From a socioeconomic perspective, this proved more feasible compared to conventional pipes, underground retention volumes, and a completely new separate system.
Cloudburst streets collect and transport the water away from the vulnerable areas. Retention streets are typically located a bit upstream from a low-lying vulnerable area and retain the water through the large storage volumes created. Adjacent to the cloudburst streets, areas with secondary streets will be transformed into green streets with swales or permeable pavements that retain the water in the area, and to some extent, infiltrate it, thus helping to recharge the groundwater aquifers.
Central retention will be created in public spaces like parks and parking zones. In areas where the water simply cannot be handled on terrain, large underground cloudburst tunnels up to three metres in diameter will be built instead of cloudburst streets.
The master plan intends to create synergy for the city as a whole, achieved by using water-sensitive solutions to increase the overall livability of the city, with the water on the terrain used as a resource in the city space. The benefits are many, such as increased recreational value from the upgrading of parks and meeting places, improved microclimate, and synergy with traffic planning.
More specifically, the concept is to retain the water in the higher-lying areas of the city and slowly release it when the peak of the storm has passed. In addition, the plan aims to create robust solutions that drain the low-lying areas. Where possible, the water should be handled locally. An extensive hydrogeological assessment of the whole city was conducted to identify the effects of infiltration on the groundwater table.

Detailing the vision

Copenhagen has been divided into eight areas, and a concretization plan for each of those catchments has been developed. Ramboll group prepared four of the eight concretization plans with detailed illustrations of how cloudburst streets, retention streets, and green streets can be designed, and how these solutions support the overall goal of the city to increase livability.
Multifunctional spaces are key elements in the concretization plans, such as parks and playgrounds that can be flooded during heavy rainfall but in dry weather serve as recreational spaces. In one example of a multifunctional space on a cloudburst boulevard (see illustration), the boulevard is wide enough to have a substantial retention volume in order to both store the water and transport it away.
As it stands currently, the street is a traditional boulevard with a green strip in the middle, which is common all over the world. The green strip is elevated a bit and has no other function than adding some green space to the city and providing space for the citizens to walk the dog. During cloudbursts, the water is likely to run from the green area and onto the street. The whole road profile is sloping toward the buildings and does nothing to prevent the cellars under the houses from flooding.

The vision Ramboll created for this boulevard is to change the whole road profile to a V-shaped profile, creating a large retention volume in the lowered green area in the centre of the profile.
When it rains, the water can run away from the houses and the street into the green area. The capacity of the urban river created during cloudbursts can carry up to 3.3 cubic metres of water per square metre. During normal rain and dry weather, the lowered green strip can serve recreational purposes.
Central retention is also a key element in the plan. One of the more radical suggestions is to transform one of Copenhagen’s three inner-city lakes, Saint Joergens Lake, into a beach park by lowering the water level in the lake. This creates a vast area for the collection of rainwater while also improving the recreational value of the city. The alternative would be to construct a gigantic and expensive cloudburst pipe to divert the expected half a million cubic metres of water away. The recreational solutions above ground will save approximately €44 million compared to the construction of an underground stormwater pipe.


A socioeconomic analysis was done for the master plan, and the result was that the benefit from this approach exceeds the costs of construction and maintenance. Even though the budget is €1.3 billion ($1.9 billion) over a 30-year investment period, the benefits from prevented flooding and reduced damages far exceed the investments. In 2015, several hundred projects were approved and tendering has already commenced.

The concretization plans help Copenhagen maintain its position as one of the most livable cities in the world. The city is a showcase for the importance of long-term city planning and holistic and sustainable solutions, as well as an example to follow for many other cities in the world that are facing the same issues. Adapting to climate change in this way is good business for cities because of the many socioeconomic benefits.

Canada should worry about adaption, too

Article by Jonathan Leonardsen. Originally published in Water Canada’s September/October 2015 issue. 

In the past, insurance claims and damages for extreme weather events have been roughly $400 million per year. However, in recent years, these costs have increased rapidly and are now more than $1 billion per year according to the Insurance Bureau Canada’s 2014 fact book. The Toronto flood in July 2013 cost insurers more than $850 million, and the devastating floods in Calgary in 2013 cost more than $5 billion.

It’s clear that we cannot afford the cost of doing nothing.

However, the costs of investing in cloudburst adaptation are also hard to finance. Canadian cities are limited in their income and many local governments rely on real-estate taxes and national
government funding for their infrastructure investments. And no government grant is currently directed at “climate adaptation.”

It’s therefore paramount to look for ways to engage the private sector to attract funding and to work across departments to finance stormwater adaptation. Bluegreen infrastructure creates places that handle water and are, at the same time, attractive public spaces.

Blue-green infrastructure helps increase the value of nearby real estate by as much as five per cent, which makes it interesting for real-estate developers to participate in adaptation investments. In order to attract private investors and work fully across departments to raise finances to avoid the cost of doing nothing, cities must understand the full benefits of doing something and capitalize on these benefits, as Copenhagen has done.

Related projects and services


Cloudburst Concretization Masterplan

In 2011, large areas of the city of Copenhagen were flooded. Copenhagen is on the front line of climate change, with a vision of transforming their city into a sustainable, C02 neutral city by 2025.

Climate adaptation adds value to Copenhagen suburbia

The suburban area around Gentofterenden, north of Copenhagen, has suffered from severe flooding during cloudbursts. Ramboll has assisted in improving the water management to also enhance the quality of life of local citizens.


Climate adaptation & flood risk management

The present situation requires adaptation to the inevitable impacts of climate change. Ramboll is among the world’s leading consultants for climate adaptation and flood risk management. Read more about the service here.

Ramboll Group A/S

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


Danish CVR numbers

Danish CVR numbers

Ramboll Group

Rambøll Danmark

Ramboll Energy

Rambøll Management Consulting

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