The challenge of unprecedented floods and droughts in risk management – publication in Nature

Are we able to manage severe floods and droughts? In a pair-event study published in Nature, researchers from all over the world have gathered information on flood & drought management for hazardous events. Based on a big data set with detailed information on 45 cases of paired events, we could see that if the second event was much more hazardous than the first, its impact was almost always higher. One would wish that after a first, severe event, the management would lead to improvements that can handle a second hazard in a better way. However, in reality, this is hard to achieve. Our case, Malmö is used as an example in the paper: “… the unprecedented pluvial flood in 2014 in Malmö, Sweden (ID 45) exceeded the capacity of the sewer system and the unprecedented drought in Cape Town (ID 44) exceeded the storage water capacity. This means that infrastructure is effective in preventing damage during events of a previously experienced magnitude but often fails for unprecedented events. Non-structural measures, such as risk-aware land-use planning, precautionary measures and early warning, can help mitigate the consequences of water infrastructure failure in such situations, but a residual risk will always remain. Second, risk management is usually implemented after large floods and droughts, whereas proactive strategies are rare. Part of the reason for this behaviour is a cognitive bias associated with the rarity and uniqueness of extremes, and the nature of human risk perception, which makes people attach a large subjective probability to those events they have personally experienced.” We are, in a different project called GlobalHydroPressure, using this knowledge and are designing decision support indicators that relate for instance water level in a river to the water level experienced during a hazardous flood event in the past. We believe that both early warning systems and urban planning can be made more understandable for both public and decision-makers in this way.

Two success stories were examined in the study in Nature. In these cases, the damage was less, despite a higher hazard, in the second event. Three success factors were identified: effective governance of risk and emergency management, high investment in structural and non-structural measures, and improved early warning and real-time control systems. Read more about the study in Nature.

Article in Nature: Kreibich, H., Van Loon, A.F., Schröter, K. et al. The challenge of unprecedented floods and droughts in risk management. Nature 608, 80–86 (2022).

Stormwater inlet in street

Two concepts in urban storm water management

By Johanna Sörensen (, 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.


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?


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.


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

IPCC Grand Seminar

I had the great honour to be panellist of the IPCC Grand Seminar arranged today at Lund University by MERGE, BECC and LU Land. I talked about pluvial flooding and how urban areas can be adapted to climate change. With me I bring new ideas on how we can mitigate and adapt to climate change. There are three main points that I want to share from the seminar:

  • The last report from IPCC, the AR6 report on the physical science base of climate change is great. Read it! I think it is great for two reasons: 1) there conclusions are more clear than ever and 2) they have included illustrative figures that very useful in order to explain historic and future climate change, as well as the effects on a wide variety of drivers, like extreme heat, landslides, fire weather, and coastal erosion. With 35 different drivers, there is valuable information independent on your specific interest.
  • The group of climate change deniers is relatively small, about 10% in the U.S. for instance. We should therefore focus on the broad public and use our time to discuss with them.
  • And finally, it is important how we vote in national elections. The elections are one of the most important ways to influence the politics and the system that steers what measures that are taken.
Me at the IPCC Grand Seminar together with skilled researchers, including Markku Rummukainen (Swedish representative in IPCC) and Deliang Chen (one of the authors).

Cities, rain & risk

Cities, rain & risk

Today our large project on sustainable urban flood management (SuRF) is presented at a conference in Malmö. We learn about reasons for flooding and what we can do about it.

Jens Thoms Ivarsson from Gothenburg started out by stating that Gothenburg aims to be the best city in the world when it rains. He gave an inspiring speech about art and rain – a very good start for a conference like this. I look forward to present my research tomorrow. Then I will speak about two studies from my thesis and hopefully get feedback on my last study on rain and flooding.


Dissertation on Friday


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.

Pris från Föreningen Vatten

Igår fick jag New Generation-priset från Föreningen Vatten. Jag är mäkta stolt!

“Priset skall delas ut till en yngre person eller grupp av personer, verksamma i Sverige, som genom hög kompetens, engagemang, initiativkraft, nytänkande och framåtanda bidrar till en sund och hållbar utveckling inom vattenområdet.”

Efter prisutdelningen höll jag föredrag om hållbar hantering av översvämningar. Jag presenterade min forskning från Malmö där jag tittat på vilka mekanismer som styr var översvämningen drabbar spatialt. Dessutom pratade jag om vad som kan göras med blå-gröna lösningar för att minska risken för översvämningar. Det senare har jag studerat i Ekostaden Augustenborg där det finns ett utvecklat system för hållbar dagvattenhantering med dammar, kanaler, svackdiken och gröna tak.

Dagvattenkonferens hos Svenskt Vatten

Svenskt Vatten samlade till konferens om dagvatten i Stockholm i veckan. Konferensen samlade avloppsingenjörer och forskare från hela Sverige. Som vanligt var det bästa med konferensen alla nya, spännande personer som jag träffade. Det är otroligt intressant att höra vad folk jobbar med i olika sammanhang, speciellt de som arbetar med mest annorlunda vinklar i förhållande till en själv. På denna konferensen mötte jag till exempel en person från ett byggföretag som arbetar med hur de ska inkorporera dagvattenhantering i byggprocessen på ett bra sätt mycket intressant. Själv presenterade jag min forskning kring vilka faktorer som påverkar översvämning vid kraftiga regn, t.ex. topografi, typ av avloppssystem, närhet till ledningsnät, lokal hantering av dagvatten, osv. Jag fick många värdefulla kommentarer från både forskare och praktiker, vilket jag uppskattar mycket. Dessutom presenterade Maria, Salar och jag forskning kring dagvattensystemet Augustenborg, där min del handlade om hur Augustenborg påverkades vid extremregnet 31 augusti 2014. Även denna presentation gav upphov till många intressanta diskussioner att ta med sig hem.

Här kan du se min presentation Vilka regn orsakar översvämning och var?

Och här kan du se min, Salar och Marias presentation Blå-gröna lösningars effektivitet i urban dagvattenhantering– erfarenheter från Augustenborg i Malmö



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: The article is freely available online.


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



Hard for small municipalities to adapt

It seems like the process adaptation to climate change is slower in Sweden, compared to Denmark. My experience is that Danish municipalities have worked more intensively with climate change adaptation during the last decade. There might be many reasons for this, but one cause that I have discussed with my engineering colleagues is that Swedish municipalities in general are much smaller than Danish municipalities.


Histogram of population is Swedish and Danish municipalities

To clarify the difference between the size of Danish and Swedish municipalities, I made a histogram that illustrates the size of population. While most Danish municipalities have a population of between 30.000 and 70.000, most Swedish municipalities are smaller than 20.000. These municipalities have a hard time to make sure that they have the right competence for such a difficult task as climate change adaptation. Expertise is needed that can include the perspective of far-distant, future development in master plans for urban areas. One possible solution for Sweden is to ensure even stronger coordination between neighbouring municipalities. This is however a demanding task in the already overstretched organisations.

Danish data comes from Danmarks statistik and Swedish data comes from Statistiska centralbyrån, SCB. Data from end of 2014/beginning of 2015.

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!


Finished paper together with some of the references.