Urban flooding

Stormwater inlet in street

Two concepts in urban storm water management

By Johanna Sörensen (johanna.sorensen@tvrl.lth.se), Water Resources Engineering, Lund University

In this article, two concepts in urban storm water management, the Three Point Approach (3PA) and Sustainable Urban Drainage System (SUDS), are presented and discussed. While SUDS is well known and has been used for many years, the 3PA is new and not so well documented. SUDS could be called a Best Management Practice (BMP), while 3PA is a tool for discussing urban storm water management.

According to IPCC (2007) it is “very likely that … heavy precipitation events will become more frequent” in the future (during the 21th century) because of climate change. The expected life time of a conventional urban drainage system is 50-100 years (Winther, 2011) and consequently it is important to consider climate change effects in all urban storm management, even though the precise effect are uncertain. The uncertainty calls for a more resilient design (Godschalk, D. R., 2003).

Three point approach, 3PA

Frantini et al. (2012) have developed the Three Point Approach (3PA), which is a tool for urban flood risk management. The idea is to give managers and operators a tool for discussion and reflection when planning a new drainage system or redesigning an old system. The three points are presented as follows:

  1. Domain of technical optimisation: This is the domain for design storms, which are typically described by national guidelines. In this domain, system optimisation from a socioeconomic perspective is discussed. The idea of national guidelines is often to prevent cities from storm floods to occur more often than with 2-20 years return period.
  2. Domain of urban resilience and spatial planning: Bigger storms, with return periods higher than 2-20 years, will occur and therefore city needs to be resilient toward flood risk from such event.
  3. Domain of day-to-day values: When building a resilient city, as mentioned in point 2, big above ground areas are often needed. These areas should be useful also in the daily life for people in the city, which calls for an inclusion of social participation. The political and public participation is also important for the public awareness of flood risk.

Sustainable Urban Drainage System, SUDS

Urban drainage systems are built to transport waste water from households and industries to treatment plants, to drain and to prevent cities from being flooded. But, there are three major problems connected to urban drainage: pollution from Combined Sewage Overflow (CSO) and from paving, roofs, etc., high peak flows causing erosion in receiving waters and flooding when the system is overloaded. These problems calls for better design and many authors suggest Sustainable Urban Drainage System (SUDS) as a solution. Poleto (2010) presents the following goals for SUDS:

  • Quantitative control of surface runoff;
  • Improvement in the quality of water from surface runoff;
  • Conservation of natural characteristics of bodies of water;
  • Balance of hydrological variables in watersheds.

Another goal with SUDS is to achieve a more resilient urban drainage system, as the SUDS solutions are more flexible than using a conventional pipe system only.

In many cities today, we are spending a lot of money on transportation and treatment of storm water. In combined systems the storm water are often pumped a couple of times before reaching the waste water treatment plant, WWTP. The WWTPs need a huge extra capacity to be able to take care of the water load from heavy rainfall. During the rainfall, treatment processes will run in an inefficient way because of the extra load. When the limit of the WWTP and the sewage system is reached, combined sewage overflows causes pollution in the receiving waters. In separated system the storm water are often led directly to receiving waters, without any treatment. Therefore a separated system is not good enough to solve problems with heavy metals in lakes, etc. According to Berndtsson & Bengtsson (2006a, 2006b, 2007) storm water from urban areas is the highest contributor for heavy metals to smaller rivers. Therefore also storm water needs to be treated, for example in such a storm water treatment facilities as described in Vollertsen et al. (2007).

Another problem, caused by building more impermeable surfaces in the cities, is that the water cannot reach the ground water to the same extent as in natural systems. This has an effect on the water supplies where ground water is used as drinking water. In addition to this, the pollution from the cities lowers the possibility to use ground water as drinking water (Winther 2011).

Poleto (2010) suggests following types of techniques for sustainable urban drainage:

  • Permeable pavement;
  • Semipermeable pavement;
  • Detention and retention reservoirs;
  • Infiltration trenches;
  • Infiltration gullies;
  • Infiltration wells;
  • Microreservoirs;
  • Rooftop reservoirs;
  • Green roofing;
  • Underground reservoirs; and,
  • Grassed strips.

Many of the suggested techniques above are both treating the water by sorption or degradation of pollutants and delaying the water flow and thereby minimising risk for erosion and flooding. While the permeable pavements only are effective until infiltration capacity is reached, they can be combined with other uses, like being used as a school yard or a parking lot. Infiltration trenches can successfully be combined with many other functions as they are built underground, if the hydraulic capacity of the soil is big enough. But they share one problem with conventional pipe systems; they are not flexible in their design. Green roofs are only effective for smaller rain falls, but they are useful for many other tasks, like energy saving and making solar power more efficient by lowering the temperature in cities/on the building, obtaining noise reduction, isolating buildings, providing richer ecosystems to the cities and helping degradation of air pollutants (Berndtsson 2010). Green roofs have also a big effect in lowering the annual runoff by evapotranspiration (Berndtsson 2010). Different kinds of reservoirs are very effective, but needs big areas for construction. However they can often be combined with other uses, in the same way as permeable areas. Grass areas in parks can be used as storm water reservoir while raining and as a recreational area in sunny days.

One SUDS technique not mentioned in the list above from Poleto (2010) is the synthetic wetlands used for instance in Malmö, Sweden (Stahre, 2006). This technique is very effective, both for flood control, detention and remediation, if the required area are present.

Discussion

We are facing a climate change, which means that we can expect more extreme weather, including more extreme rain fall. The investments for adaption to more heavy rainfalls are huge. As we would like to develop our cities to more liveable places, we need to cleverly use every penny two times: both for storm water control and for other things such as for recreation, better ecosystems in our cities, energy saving and so on. The Three Point Approach helps us to discuss the different needs in a systematic and holistic way.

One thing that Frantini et al. (2012) do not mention is the importance of taking care of pollution from storm water and sewage system. The focus in Frantini et al. (2012) is on flood risk. I suggest that the third point, day-by-day values, in an extended version of the 3PA also should include also the pollution problem. By discussing pollution as a part of the day-to-day values domain, a more sustainable system would probably be built. A slow runoff is important for both degradation of pollutants and a more controlled outflow to receiving waters. In a combined system a slow runoff would secure better remediation at the treatment plant and fewer sewage overflows.

I have tried to find literature on two other subjects of interest, by searching in online libraries and talking to professors and practitioners in urban storm water planning. I have found little concerning the following questions and I would therefore suggest further research in these areas:

  • Catchment effects from SUDS – how is the downstream system effected by implementation of Sustainable Urban Drainage System in one part of the city? Is it needed to reconstruct the drainage system of the whole city to have a well functioning system, meeting climate change in the future? Or is it enough to reconstruct and develop SUDS in only some parts of the city?
  • SUDS effect on extreme events – can SUDS protect against flooding from extreme events (point 2 in the three point approach)? Or is a conventional pipe system with large basins needed in addition?

Comment

Note that this text was written in December 2012. For instance, I have published a paper on how SUDS affect flooding during extreme events. However, I still find it relevant and hereby publish the text for anyone to read.

References

Berndtsson, J. C. & Bengtsson, L. (2006a) Vattenöversikt i tre skånska åar, VA-forsk rapport nr 2006-22

Berndtsson, J. C. & Bengtsson, L. (2006b) Stadens inverkan på vattenmiljön i avrinningsområden, VA-forsk rapport nr 2006-23

Berndtsson, J. C. & Bengtsson, L. (2007) Vattenöversikt i små avrinningsområden i Skåne, VATTEN-3-07

Berndtsson, J. C. (2010) Green roof performance towards management of runoff water quantity and quality: A review, Ecological Engineering, Volume 36, Issue 4, April 2010, Pages 351–360

IPCC (2007) Climate Change 2007: Synthesis Report: An Assessment of the Intergovernmental Panel on Climate Change

Frantini et al. (2012) Three points approach (3PA) for urban flood risk management: A tool to support climate change adaptation through transdisciplinary and multifunctionality, Urban Water Journal 2012, 1-15

Stahre, P. (2006) Sustainable in urban storm drainage, Svenskt Vatten, ISBN 91-85159-20-4

Vollertsen et al. (2007) Treatment of urban and highway stormwater runoff for dissolved and colloidal pollutants, conference paper, Novatech’2007

Winther L. et al. (2011) Afløbsteknik 6. udgave, Polyteknisk Forlag, ISBN 978-87-502-1015-3

Godschalk, D. R., (2003) Urban Hazard Mitigation: Creating Resilient Cities, Nat. Hazards Rev. 2003.4:136-143

Dissertation on Friday

Karl_writing

Karl helping his mother with thesis writing.

On Friday I will finally defend my thesis! I have been looking forward to this day for long time now. All summer we have lived together, me and my thesis, and it has been so much joy to write it, to search for the connection between my five articles, to shape my theoretical background in words, sentences, and paragraphs. I have always enjoyed writing in Swedish and now I have finally begun to enjoy writing in English. The final product, the thesis, has its mistakes and shortcomings, and I guess any researcher has things to add for the next study that was not included in the one before, but with the five studies I base my thesis on, I am satisfied with the final result. I hope that others, more senior researchers, in the field like it as well.

If you would like to read it, you can find it online for free or write to me and I will send you a paper copy as soon as possible. You are also welcome to come and listen to the open defense in Lund on Friday.

 

PhD thesis, Johanna Sörensen

PhD thesis, Johanna Sörensen, Water Resources Engineering, Lund University

Abstract This thesis investigates urban, pluvial flooding and if blue-green infrastructure, for handling of stormwater in urban green spaces, can be used as a strategy for resilient flood risk management. Spatial analyses of flood claims from insurance companies and the water utility company of Malmö are used to better understand the mechanisms and characteristics of pluvial flooding and how blue-green infrastructure impacts flood risk. It was found that flooding during intense rainfall often is located closely to the main overland flow paths and the main sewers, while flooding during rainfall with longer duration seem to be more randomly distributed. Combined sewers are more affected by flooding than separate sewers. Blue-green infrastructure can reduce urban, pluvial flooding. The large-scale spatial distribution of flooding with respect to urban flow paths and drainage system are discussed in relation to the small-scale impact of surface water detention in e.g. detention basins and concave green spaces. Based on transition theory, socio-technical transition towards wide-spread implementation of such measures are examined through interviews with municipal and water utility officials. Legal, organisational and financial changes are suggested. A framework for management of spatial data in the strategic planning of blue-green infrastructure is also presented. The thesis consists of a summary and five appended papers, where the first paper serves as a background for the thesis.

First article published

My first article was recently published in the journal Water. I wrote this article together with 12 co-authors in our research project Sustainable Urban Flood Management (SUrF) and it gave me new insights in our different fields of research. We represent nine different affiliations and my job was to coordinate the writing process and of course to write my own parts. It is wonderful to work with this group of researchers. Thanks to all of you for your contributions!

Article: Sörensen, Johanna, et al. “Re-Thinking Urban Flood Management—Time for a Regime Shift.” Water 8.8 (2016): 332.

The article can be found here: http://dx.doi.org/10.3390/w8080332. The article is freely available online.

Abstract

Urban flooding is of growing concern due to increasing densification of urban areas, changes in land use, and climate change. The traditional engineering approach to flooding is designing single-purpose drainage systems, dams, and levees. These methods, however, are known to increase the long-term flood risk and harm the riverine ecosystems in urban as well as rural areas. In the present paper, we depart from resilience theory and suggest a concept to improve urban flood resilience. We identify areas where contemporary challenges call for improved collaborative urban flood management. The concept emphasizes resiliency and achieved synergy between increased capacity to handle stormwater runoff and improved experiential and functional quality of the urban environments. We identify research needs as well as experiments for improved sustainable and resilient stormwater management namely, flexibility of stormwater systems, energy use reduction, efficient land use, priority of transport and socioeconomic nexus, climate change impact, securing critical infrastructure, and resolving questions regarding responsibilities.

Flooding in Copenhagen 31st of August 2014 – Taxi

 

 

Paper about severe floods in Malmö

Finally, I have finished my paper about severe floods in Malmö. It is a detailed, qualitative studie of the three most severe flood events –5th of July 2007, 14th of August 2010, and 31st of August 2014– and a comparison with smaller floods, where I have look closer at the causality behind flooding. Now I am eager to continue with statistical analysis of the same data set. I will work together with department of mathematical statistics. With their help, I will hopefully increase my knowledge in the field of extreme values modelling, and they are in constant need of interesting data sets – what a good base for collaboration!

paper_finished2

Finished paper together with some of the references.

 

Vad Malmö behöver är ingen avloppstunnel

Rainy day in Lund

Vilken väg ska vi välja?

Idag skriver Sydsvenskan att avloppstunneln i Malmö, som VA Syd har föreslagit, beräknas kosta 2 miljarder kronor. Det är mer än vad Malmö Live, Malmös nya konferens-, hotell- och koncerthus, kostade. Det är vansinnigt att gå den vägen och plöja ner så mycket pengar på en statisk och kortsiktig lösning när Malmö är världskänt för att arbeta med betydligt modernare metoder.

Vad Malmö stad minst av allt behöver är att gräva ner 2 miljarder under jord. Satsa på mer grönt och blått i staden istället. Det ger minskade föroreningar (luft och vatten), lägre risk för översvämning och högre biodiversitet. Om vi arbetar med blå-gröna lösningar kan vi samtidigt se till att skolgårdar, parker och bostadsområden blir trevligare och roligare att vara i. Det finns många ställen i Malmö som skulle kunna rustas upp och samtidigt bli en del av ett blå-grönt nätverk genom staden.

En tunnel kan endast lösa en fråga: minskade föroreningar i vattnet. Modern VA-teknik har kommit betydligt längre än så. Vi måste använda pengarna bättre och lösa flera frågor samtidigt. Inom akademin idag är två honnörsord multifunktionalism och resiliens. Multifunktionalism betyder att en lösning ska ha många funktioner på samma gång, så vi får ut så mycket som möjligt av varje spenderad krona och varje bit mark vi bygger på. Resiliens betyder bland annat att vi ska vara beredda på framtida förändringar. Därför måste vår stad och all infrastruktur vara flexibel, så att den kan förändras när staden eller klimatet förändras. Tunneln som VA Syd har skissat på tycks vara allt annat än flexibel. Det vore bra om VA Syd berättade hur de tänkt sig att tunneln ska kunna förändras i framtiden och vad det i så fall skulle kosta att bygga om den.

Sydsvenskans har skrivit en för- och emot-lista för tunnelbygget. Listan ger en hyfsat bra bild av frågan, men de nämner inte ens blå-gröna lösningar som ett alternativ, trots att Malmö är världskänt för just detta. Där kan vi prata beprövad teknik.

I VA Syds utredning nämns att “.. de gröna och blå kvaliteterna ska utvecklas. Malmös parker, grönområden och vattenmiljöer ska utökas, värnas och ha höga rekreativa och biologiska kvaliteter.” Trots det föreslås alltså en tunneln, vilken inte direkt bidrar till höga rekreativa och biologiska kvaliteter. Om hållbar dagvattenhantering (blå-gröna lösningar) skriver de att “.. denna typ av åtgärder kan vara mycket effektiv för att minska risken för källaröversämningar och generellt bidra till att toppflöden i ledningsnätet minskas.” Trots det har de inte beräknat vad lösningen kostar och hur mycket staden tjänar på att förbättra både rekreativa och biologiska värden. De nämner ytterligare tre möjliga lösningar, men inte heller här några beräkningar av plus/minus för ekonomi, samhälle och miljö gjorts. Om vi ska kunna ha en förnuftig debatt kring detta, krävs att alternativen utreds lika noga som tunnelförslaget. En utredare med uppgift att se till hela stadens intresse, måste studera detta i detalj. På åtta år och med 2 miljarder i budgeten kan Malmö genomföra många projekt som förbättrar både vattenkvaliteten och staden som helhet. Tänk så många områden som skulle behöva förbättras ordentligt och vilka möjligheter en så stor budget ger. Och samtidigt slipper man sitta fast i ett statiskt och oflexibelt system.

Malmö behöver förmodligen ingen avloppstunnel när det nu finns väl beprövade, bättre alternativ – men om vi ska kunna diskutera frågan krävs det att alternativen utreds ordentligt av en utredare som ombeds att se på staden som helhet. Förmodligen behövs en grupp av experter från olika discipliner, eftersom modern avloppsteknik involverar många olika frågeställningar och går på tvärs av staden både fysiskt och organisatoriskt.

One year since the severe flood in Malmö and Copenhagen 2014

Flooding in Copenhagen 31st of August 2014It is today one year since Malmö (Sweden) and Copenhagen (Denmark) were severely flooded on the 31st of August 2014. Copenhagen has already been challenged by floods in 2010 and 2011, while the severity of this flood event was new to Malmö and south-western Scania. The flood has been like a wake-up call for southern Sweden when it comes to how fragile the modern society are to severe stress like from this flood.

Flooding in Copenhagen 31st of August 2014 – destroyed manhole One year later, this flood event still effects the city: several house owners have not been able to move back yet (!), a group of employees at the utility company VA Syd are working full time to assess all flood reports from the citizens, at the university we will in few weeks start a big project on urban flood resilience, and Malmö municipality will write their first cloud-burst plan. I write on an article about the flood event in Malmö, where I look into how the city was affected. I compare this flood event with the much smaller, but still severe, flood events in 2007 and 2010, and discuss how we can learn from these flood events in the future city planning. Hopefully, I will submit the article to an international journal in October or November.

The local newspaper, Sydsvenskan, writes today about the flood event (in Swedish): Ett år efter översvämningarna i Malmö. They have also collected video clips from the event: Se videoklipp från regnkaoset.

Before and after – it takes a flood to understand

Leave a comment

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

Leave a comment

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.

Flooding in Copenhagen 31st of August 2014 – pumping
Flooding in Copenhagen 31st of August 2014
Flooding in Copenhagen 31st of August 2014 – flooded cars
Flooding in Copenhagen 31st of August 2014 – rainy people at bus stop
Flooding in Copenhagen 31st of August 2014 – pumping
Flooding in Copenhagen 31st of August 2014
Flooding in Copenhagen 31st of August 2014
Flooding in Copenhagen 31st of August 2014
Flooding in Copenhagen 31st of August 2014 – destroyed manhole
Flooding in Copenhagen 31st of August 2014 – Taxi

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

Leave a comment

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.

Development of stormwater systems

Leave a comment

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.

 Reference

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.