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

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).

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.

Comparison

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.

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.

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.

400 ppm

I dessa dagar kan man läsa i tidningen att mätningarna vid Mauna Loa nu visar att vi har 400 ppm koldioxid i atmosfären. Det är skrämmande hur mycket koncentrationen stigit på få år. När jag läste till civilingenjör vid Lunds universitet (2003-2008) lärde vi oss att atmosfärens koldioxidkoncentration är 360 ppm. På bara några år har koncentrationen ökat drastiskt, vilket onekligen ger en viss respekt för människans samlade dumhet och avsaknad av framtidsperspektiv. Det är på tiden att klimatfrågan tas på allvar på alla nivåer, från varje enskild individ till stater och internationella maktcentrum. Vi måste sluta subventionera fossil förbränning och börja använda våra resurser med respekt för framtiden.