Cloudburst-proof urban redevelopment in Aalborg

With the ongoing redevelopment of the city's former railway freight yard, Aalborg is showing how a city can redevelop its abandoned industrial landscapes into new and thriving urban areas in a way which integrates all aspects of sustainability while also paying tribute to the legacy of the area. The project integrates climate change adaptation in the overall plan as well as in individual buildings.

The 19 hectares of former freight yard is located next to the rail yard, bus terminal and upcoming light railway in the middle of the city's 'axis of growth', the backbone of Aalborg's urban development plans, as well as a green wedge running through the area and along the partially culverted Østerå watercourse to the Limfjorden sound.

The area had been left undeveloped for 15 years. In 2008 a private stakeholder group joined with a number of local educational institutions to air the idea for a common 'campus of education' in the area. The owners of the land, DSB Ejendomsudvikling and the City of Aalborg, took on the idea. In cooperation with stakeholders and future users, a joint vision for redevelopment was drafted, and in 2009 three architecture firms were invited to compete for the best project design.

The Polyform architecture firm won the competition and their plan became the basis for the local development plan adopted by a unanimous City Council in 2010. Two years later, the upper-secondary school, Aalborg Studenterkursus, was the first educational institution to move into newly built buildings in the area. More have followed since. Over the coming years the area will become gradually more built-up.

Furthermore, one of the two new local urban parks on the former track area has already been opened to the public. So, while the freight yard area previously constituted a barrier, today, with the redevelopment, it is helping connect the city, with new foot paths and bike lanes and with new life and activity from the many new homes, educational institutions and meeting points.

Climate change adaptation was a key aspect of the planning process from the onset of the project. The local development plan included a requirement that rainwater be managed locally in the former freight yard area. Discharge of stormwater to the public sewerage system must not exceed one litre per second per hectare. This applies even in the event of cloudbursts, and constitutes a mere hundredth fraction of the natural stormwater discharge from the area.

The soil in the area is unsuited for infiltration because of a high water table, so local rainwater drainage solutions have to be based on local evaporation, retention and accumulation in basins, until the water can be diverted onward to a receiving water body in a controlled manner.

The engineering consultants, NIRAS, have developed and dimensioned the selected local rainwater drainage solutions. According to project manager, Jan Scheel, the project is inspired by a similar project in the Swedish city of Malmø.

He emphasised that the idea of interconnecting different local rainwater drainage solutions through a system of open distribution channels at ground level is what differentiates the project on the site of Aalborg's former freight yard.

"95% of annual precipitation comes as low-intensive rain. The volumes are not larger than what can be absorbed and evaporated through green roofs on buildings, the infiltration beds in the built-up area, and the areas in the parks designated as retention basins," he said.

"The challenge comes when we experience heavy rainfall, especially when we experience several consecutive heavy downpours.  When roofs and infiltration beds have absorbed all of the water they are able to retain, in principle, we still have to be able to cope with the same amount of rain as we would without these solutions. In combination with the very small amounts of water which the local development plan allows us to discharge to the receiving water body, this places huge demands on our ability to divert and store stormwater from cloudbursts," explained Jan Scheel.

The areas for retaining and storing stormwater have been found; they are located outside the built-up area in low-lying local urban parks on the former track areas. Sports grounds and recreational park areas serve as flood retention basins. There is nothing new nor revolutionary about this. In Aalborg, they have merely prepared for the most extreme event by allowing the total park area to serve as rainwater reservoir in extreme rainfall events.

In the event of particularly powerful cloudbursts with a statistical likelihood of occurring one in every five years, or even less often, the systems will not be able to divert and transport all of the rainwater. Simulations with MIKE Flood have helped consultants identify risks as well as areas at risk of flooding during cloudbursts.

"The local development plan prescribes even-level access for the disabled  to all buildings, and this obviously makes the buildings vulnerable to flooding during heavy rainfall. According to current standards, the buildings must be established with a so-called moat solution which diverts stormwater to the buildings' perimeter drainage system. In combination with the channels, this will solve the problem in most situations, however in some places there will be a risk of surface water entering the buildings. The computer modelling tool helps us identify these places. Establishing a raised lip around the building can easily prevent problems in these places, according to Jan Scheel.

The costs of establishing climate protection for the built-up area have been distributed among the individual building plots. The individual landowner will only own the area of the building's footings on their property. Public areas, parks and joint surface water management will be owned by the land owners' association. Landowners will be responsible for jointly operating and financing the required maintenance.

According to architect Karin Højlund, who is the City of Aalborg's project manager on the project, these costs have not discouraged developers; rather the opposite.

"As a pilot project, the freight yard project has helped define and realise the ideas which Aalborg had for tackling climate change adaptation in our planning efforts. It has happened as a dynamic cooperation with different stakeholders, who from the very beginning have agreed with the basic principles. We developed a common framework for understanding and for cooperation built on trust, which allowed everyone to work toward finding the best solutions," Karin Højlund explained, and she continued:

"One of the most important experiences that we can draw from this process is that it is possible for stakeholders, local government and developers to develop a project like this jointly and to have them take joint ownership, if the process is based on dialogue and backed by a strong vision".