Flood CLD – at the first try

Last Friday, I made a causal loop diagram (CLD) for urban flooding. It is a very simple version that points out the problem that occures when pipe systems are used as the only solution to handle stormwater in the city. I will continue to develop CLDs for my research, both general ones like this and more specific ones.

Overview of the drivers behind construction of a stormwater system based on pipes. "Flooding" means here any unwanted stormwater that leads to a cost for the society.

Overview of the drivers behind construction of a stormwater system based on pipes. “Flooding” means here any unwanted stormwater that leads to a cost for the society.

Due to climate change and urbanisation, there is a need to change the stormwater infrastructure in the city. This is not possible if the stormwater system is not flexible.

Due to climate change and urbanisation, there is a need to change the stormwater infrastructure in the city. This is not possible if the stormwater system is not flexible.


Forskning.se skriver om vår senaste rapport

Idag skrev forskning.se om vår senaste rapport: Identifiering av extrema händelser och dess översvämningskonsekvenser i tätort. Själva rapporten kan laddas hem från SVUs hemsida. Jag har bidragit med två artiklar till rapporten. Båda artiklarna är konferensartiklar och de handlar om risken för översvämningar när det både regnar mycket och havet samtidigt tränger sig på. Förrutom mina artiklar omfattar rapporten forskning som bland annat Lars Bengtsson (min doktorandhandledare), Arun Rana (vår tidigare kollega, nu disputerad och verksam på ett universitet i Portland, USA) och Karolina Berggren (disputerad från Luleå tekniska universitet) har gjort. Rapporten ger en del intressant läsning till den som intresserar sig för extrem nederbörd och hur översvämningar påverkar våra städer.

Mina artiklar

Sörensen, J., & Rana, A. (2013). Comparative Analysis of Flooding in Gothenburg, Sweden and Mumbai, India: a Review. International Conference on Flood Resilience, Experiences in Asia and Europe. Exeter.

Sörensen, J., & Bengtsson, L. (2014). Combined effects of high water level and precipitation on flooding of Gothenburg, Sweden. 13th International Conference on Urban Drainage (pp. 7–12). Sarawak, Malaysia.


Before and after – it takes a flood to understand

Pluvial flood in Copenhagen August 2014

Pluvial flood in Copenhagen August 2014

When I started my PhD study in 2012, everyone said: I see, you live in Copenhagen – over there floods are of great interest. Copenhagen was flooded in August 2010, July 2011 and once more in August 2011. My PhD topic (urban flooding in a changing climate) was of course partly inspired by these three floods, but also by the need to create better cities in the future. We need greener cities and more resilient stormwater systems. This was in 2012 and the interest of flooding in Sweden was rather weak.

Then, in 2014, the rain fell. After a few hours of heavy rainfall, floods where suddenly on everyone lips. Malmö was hit by a costly and frightening flood event. Now people had their own experience of what a flood could be. It is not any longer a problem “over there”, in Copenhagen. It takes only 35 minutes to go from central Copenhagen to Malmö. Nevertheless, it took the flood risk awareness more than three years to cross the bridge.

Cars flooded in Copenhagen 2014

Cars flooded in Copenhagen 2014

Gothenburg have seen similar flood events and they seem to be aware of the problem. During a conference in Gothenburg, someone discussed why the national authorities in Sweden still seems so unaware of the pluvial flood risk. As this person mentioned: “We have still not seen a major flood event in a bigger Swedish city – read: Stockholm!” I guess this summarises the problem with risk awareness. It is unbelievable hard to understand the risk of flooding before you have seen one yourself in your own town.

Floods in Malmö, Sweden

On the 31st of august, heavy rainfall hit Malmö in southern Sweden. The city has never before seen such volumes of water in the streets, basements and backyards – at least not in modern time. At Lund University, we decided to take a closer look at the flood event. We try to understand how all the green spaces and open stormwater solutions, that Malmö city is well known for, did affect this flood event. The open stormwater systems in Malmö were not specifically designed to prevent from flooding, but this could be an important, positive side effect of the systems.

My student, Joanna Theland, have contacted the utility company (VA Syd), insurance companies and house owners in Malmö, to collect information about consequences of the flooding. Everyone have been very helpful and we have had interesting conversations with both VA Syd, Länsförsäkringar Skåne (an insurance company) and others about this flood event and flooding in general. I would say that Swedish authorities are getting more and more focused on floods and flood prevention. We are going from an idea that floods are a natural catastrophe that we only can act upon after the disaster already has happened, to a more proactive view were we see possibilities to prevent areas, important buildings, as well as our own basement from flooding. There is a great interest in our study and in how to build cities in a better way in the future.

Joanna Theland will present the results from this study in March. After this, I will continue with more analyses. One idea is to compare the situation in Malmö with other cities that have seen recent flood events. One might be Helsingborg, as my contacts there (at NSVA) have been very helpful.

The photos beneath are all from Copenhagen. The same rainfall as in Malmö, on the 31st of august 2014, stroke Copenhagen and I took a trip with my bike to document some of the consequences in the city.

Radical redesign of our cities will lower the flood risk – II of IV, turn the dikes

Conventional dike in Gretna, Mississippi. Photo: http://commons.wikimedia.org/wiki/User:Infrogmation

Conventional dike in Gretna, Mississippi.
Photo: Infrogmation

Turn flood dikes perpendicular to the coast, so they point at the sea. Stop to fight against the nature and let nature rule your city. This was the radical message when Anuradha Mathur and Dilip da Cunha visited Malmö in March 2014. The new ideas are now discussed among city planners and architects.

There is a saying that ‘there are two kinds of dikes – the ones that did break down and the ones that will break down in the future’. We design dikes with standards that should lower the risk of failure and we choose a safety level for the dike. But, there is always a limit for the construction. One day the dike will break down or water will overtop it during an extreme storm. The clear message from Mathur and da Cunha is that we should stop to fight the nature with dikes.

Their idea is to turn the dikes or levees around and use them for evacuation when a flood comes. On top of the levee buildings could be constructed. Here is a good and safe place for vulnerable buildings, but also for schools. During a flood – because flood will come, these buildings will be used as evacuation centre.

Rising sea water level will flood cities

In the future, we will see higher sea water level due to climate change. Mathur and da Cunha claims that the future sea level rise challenge our understanding of the sea. We need to rethink the urban design fundamentally, as the sea will eat a lot of urban areas along the coast lines. It is not possible to compete with nature.

As the city will be flooded more often with the new turn-around dikes, people will get more aware of the risk and therefore construct the city in a smarter and less risky way. This awareness contributes to flood resilience for the city.

Nature is not like engineering

Sometimes people claim that we should ‘construct natural dikes’. This is an engineering approach to nature, says Mathur and da Cunha. The nature works in systems and not in functions. In Norfolk (USA), where they currently are working with the sea level rise problem, the military base is dominating and the military way of thinking is wide-spread: Conquer the fronts. Conquer the land from east to west. This thinking also transfers to the cities fight against the rising sea. Mathur and da Cunha explained how they work with this idea on conquering and that they could see a shift in people’s idea when they gave workshops about it. In their workshop people would learn how natural systems could be used in an effective way for protection. We should work together with nature and see the sea as a friend, they say. It is time to stop fight against nature.

Radical redesign of our cities will lower the flood risk – I of IV, the dotted coastline

Think about the coastline as a dotted line and turn flood dikes perpendicular to the coast, so they point at the sea. Stop to defend the nature and see the rain as a citizen rather than a foreigner in your city. Anuradha Mathur and Dilip da Cunha gave a talk about all this on 15th of March 2014 in Malmö for researchers, city planners and architects in Lund University Urban Arena. This blog post is number one in a series of four posts related to their talk in Malmö.

I of IV, the dotted coastline

Coastlines from SOAK

No coastline are clear and well-defined. Examples from Marthur and da Cunha’s work about Mumbai (published in SOAK).

Mathur and da Cunha started out telling us that we shall think about the coastline, not as a line, but rather as a series of dots where land and sea can interact in between. During their work in Mumbai, after the huge floods in 2007, they have noticed that there is a clear line that divides land from sea, and sea from land, on all our modern maps. The line makes us believe that the sea and the land is fundamentally separate. This is not true in the real world around us. Imagine the beach where waves and tide makes the water flow back and forth. Imagine a coastal wetland, like mangrove forest or marshland, where the water goes deep into land and where there are no clear line between sump and land or between sump and sea. Mathur and da Cunha told us that in Mumbai, salt water can be found in boreholes on land and fresh water can be found in boreholes under sea. In other words, it does not make sense to divide land and sea by a distinct line.

Mathur and da Cunha showed us how maps from older days had a more unclear coastline, just like the true world around us. They claimed that the way we show coastlines on our maps influences the way we plan and build our cities. I definitely believe they are right about how maps influences the way we create our cities, but I am not sure that this is the main driver to reclaim land in Mumbai. Because of the clear coastline marked on the map, city planners in Mumbai started to reclaim land from sea by filling out areas between the seven previous islands, according to Mathur and da Cunha . I have recently read Daniel Brook’s book, A History of Future Cities, and according to Brook, the land reclamation had more economic causes. The reclamation started because the Mumbai economy bloomed in the mid-19th century during the American Civil War. Mumbai became the most important port for cotton trading and the real estate market expanded fast. When there were no more land to build on, investors started to reclaim land from sea and sell it for construction. I guess, the idea to see land and sea as two separate worlds came in the same time, but the market must have been a very influential driver. Still, I will try not to make sharp coastlines on my maps in the future.

Development of stormwater systems

I am very thankful to ÅForsk, who gave me the opportunity to go to the International Conference on Flood Resilience (ICFR) in Exeter, Great Britain. I am also glad that my paper about the storm water systems in Gothenburg and Mumbai got accepted to be presented at the conference. It gave me a great chance to dig deep into the development history of the stormwater system in Gothenburg. Arun Rana and I made a comparison between the Gothenburg system and the stormwater system in Mumbai and learned a lot from reading about how the systems have developed.

Stormwater inlet in street

Stormwater inlet in street

The sewer system of yesterday strongly influences the system of today and thereby also the system of tomorrow. Because stormwater and sewage structures last for a long time, and the price to reconstruct the systems are very high, the old system will have strong influence on all future decisions. Cettner, Söderholm and Viklander (2012) did write an interesting article about this in Journal of Urban Technology. This was one of the important learnings from the article writing and my conference preparation.

Conference lunch at ICFR in Exeter

Conference lunch at ICFR in Exeter
Photo: Johanna Sörensen

At the conference, I enjoyed to meet peers from all different universities, working with flood related issues in European and Asian cities. We discussed how the urbanisation and city development affect flood risk in growing cities and how climate change can aggravate the risk. We discussed how to cope with the flood risk, both from a technical and a social point of view. How far can we come with technology? What are the possibilities to protect our cities from floods? Flooding is one of the most wide-spread disasters, which can hit cities in various climates all over the world. A big concern for the future is the rising sea level, due to heating of the globe. In addition to rising sea level, we will also see more high-intense storms in many places. How can we construct our cities in a smart way to handle this? The main idea from the conference, which I took with me home, was that we will not be able to totally avoid flooding. When cities are hit by the most extreme events, there will be floods of such a magnitude that we cannot prevent them. Therefore, we need to build cities in a resilient way, with flexible systems, flexible public organisations, and flexible citizens that are prepared to cope with floods. A big flood event must not be a catastrophe for the city if the preparation is good and the technology is adaptable.

The conference gave me a complete view of ongoing flooding research. There are four main driving forces behind severe pluvial flooding in cities today: higher precipitation due to climate change, urbanisation, land use change, and higher sea level due to climate change, which can aggravate pluvial flooding. In some parts of the world, only one or the other driving force is seen, but in many places are several of these processes ongoing.

Comparison between stormwater system in Gothenburg and in Mumbai

There are several similarities between the stormwater system in Gothenburg and in Mumbai. Both systems where constructed in the late 19th century with strong influences from Great Britain. India was at the time under British control and the British engineers led important infrastructure projects in Mumbai, among those building of the early sewer system. In Gothenburg, Swedish engineers went to London to learn about the new technology and were in this way strongly influenced by the British engineers. Later on, the German engineers led the technological development in this field in Europe. Today the systems in the two cities are very different, despite the fact that the first parts where built in the same time and in the same way. After the British Empire left India, the infrastructural development of Mumbai stagnated. Things have happened since then, but at a slower pace compare to Gothenburg, were the development continued. Also the urbanisation has been considerably stronger in Mumbai, which is the biggest centre for trade and commerce in India.

Solid waste in stormwater system in Mumbai

Solid waste in stormwater system in Mumbai.

It is obvious that the problems related to flooding are much bigger in Mumbai, compared to Gothenburg. The monsoon period comes every year with heavy rainfall and the stormwater system does not have capacity enough to handle the runoff. The solid waste system in the city is not satisfactory, meaning a lot of plastic bags with solid waste lie in the watercourses instead of landfills and cause clogging of the stormwater system during the monsoon period. The municipality aims to clean all watercourses before monsoon, but often the jobs is not done careful enough to keep the watercourses free from clogging. Therefore, a better solid waste system is very important to improve flood control in Mumbai. Another problem in Mumbai is settlements on flood plains along the river. Many people in Mumbai are very poor and the city is overloaded with people. Because there is no housing for all people in Mumbai, many informal settlements are built on the floodplain and people of the floodplain lives in a high risk of flooding. As this is the poorest people in the city, they also have least possibility to protect themselves.

The ship Götheborg in Gothenburg harbour.

The ship Götheborg in Gothenburg harbour. Götheborg is a copy of a 18th century ship.
Photo: Mikael Tigerström.

One similarity between Mumbai and Gothenburg is that both cities were built on former marshland along the coast. Both cities were built as an important port and economical centre for their region. They are both low-laying and situated close to the sea, meaning the water cannot easily leave the area during storm. Gothenburg is known as one of the rainiest cities in Sweden, while Mumbai is situated in an area with monsoon climate. Both Gothenburg and Mumbai lay on the west coast with mainly westerly winds, meaning they are influenced by the sea.

Mölndal River

Mölndal River at Lackarebäcksmotet.
Photo: Johan Jonsson.

In Gothenburg the area around the central station, Gullbergsvass, is low-laying and in high risk of getting flooded. There are far-reaching plans to develop this area into a housing and shopping area in the future, despite the high flood risk from Mölndal River. When it comes to flooding, this is one of the main problems in Gothenburg, together with rising sea level and landslides along Göta River. When reading about this, I learned that high risk of flooding not always is enough argument to leave an area free from expensive investments.


Cettner, A., Söderholm, K., and Viklander, M. (2012) An Adaptive Stormwater Culture? Historical Perspectives on the Status of Stormwater within the Swedish Urban Water System. Journal of Urban Technology, 19(3), 25–40.