The area of Augustenborg in Malmö, southern Sweden, was retrofitted with a green open stormwater system in the late 90s. When a major rainstorm hit the city of Malmö in August 2014, Augustenborg was less affected by flood damage than other nearby areas. With the help of a coupled stormwater and surface flooding model, DHI evaluated the flooding extent for two scenarios – one with the existing open stormwater system and the other with the former traditional system. Based on the results, we concluded that the new and open stormwater system had a clear effect on the flood risk and helped control the flooding extent to avoid damage on buildings and infrastructure.

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Challenge

Ever since the stormwater system in Augustenborg was reconstructed, it had been assumed that the system helped protect against flooding. However, this had not been proven, and it was also unknown if the new system was efficient for extreme rainfall events with a 100-year or higher return period.

Solution

The stormwater system in Augustenborg – a Sustainable Urban Drainage System (SUDS) – consists of open canals, swales, ponds and green roofs as well as adapted levelling of green areas to ensure controlled flooding.

A model of the system was set up in MIKE FLOOD using MIKE URBAN (now MIKE+) and MIKE 21, including the rain on grid and infiltration modules in MIKE 21. This enabled correct estimation of stormwater infiltration in green areas. Two scenarios were created. One for the current (green, open) stormwater system and the other for the former (traditional, pipe-based) system. This methodology allowed us to evaluate the flood protection efficiency of the new open stormwater system compared to the old pipe-based system.

The simulations showed that the retrofitted green stormwater system had a substantially lower risk of flood damage compared to the former traditional system for the Augustenborg area during the rainstorm event in August 2014.

Results

Proves that the new stormwater system protects against flooding

Shows that a SUDS can be efficient also during extreme rainfall

Quantifies the effects of the retention and detention processes in the system (infiltration, storage etc.)

'Being a PhD student at Lund University at the time this research was conducted, I had the opportunity to closely collaborate with DHI and their experts within the field of urban hydrology and urban drainage modelling. This project, beside its interesting results, was also a very fruitful partnership as it led to more collaborations in research and innovation between DHI and Lund University in the following years. In addition to DHI’s robust and handy modelling software, I found the collaborating professionals of the company very smart, helpful, ambitious and with high problem-solving capacity.'

Salar Haghighatafshar, Researcher
Lund University

About the client

The project was carried out together with two partners:

VA SYD – Water utility for the region of Malmö, Sweden. The utility is the owner of the stormwater system and provided data to the study.

Lund University – Water and Wastewater Engineering, Dept. of Chemical Engineering. The department served as a collaboration partner and main project initiator.

Software used

MIKE URBAN (now MIKE+)
MIKE 21
MIKE FLOOD

The results of this project have also been published in the journal ‘Efficiency of blue-green stormwater retrofits for flood mitigation – Conclusions drawn from a case study in Malmö, Sweden.’

Case story - Highlights flyer

Evaluation of flood management in Malmö, Sweden

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