Source: Pappalardo, V., La Rosa, D., Campisano, A., La Greca, P.(2017). The potential of green infrastructure application in urban runoff control for land use planning: A preliminary evaluation from a southern Italy case study. Ecosystem Services. 26(B): 345–354. DOI: 10.1016/j.ecoser.2017.04.015.
This study investigates how green roofs and permeable paving contribute to flood mitigation. Using a hydraulic model technique, the research found that, in particular urban cases, green roofs were more effective than permeable paving. Policies to promote the adoption of sustainable urban drainage systems (SUDS) could prove more effective under certain circumstances, and policymakers should look at ways to promote SUDS where suitable.
A worldwide trend in an increase in impervious surfaces (impermeable to water), coupled with precipitation extremes, are contributing to a rise in urban flooding. Current drainage systems may struggle to cope with the amount of water run-off during heavy rainfall events, which are predicted to increase under climate change1. The effectiveness of green roofs and permeable paving for stormwater management in an urban location was assessed.
SUDS refers to a range of drainage technologies that are more sustainable than conventional solutions2, and may include types of green infrastructure, for example green roofs, permeable surfaces, and purpose-built ponds and wetlands. These techniques use landscape features and natural processes to slow flows of water, increase evaporation and encourage infiltration into the ground. As a co-benefit, SUDS strategies can enhance ecosystem services, such as wildlife habitat, carbon sequestration, recreation and education. Conventional drainage, meanwhile, focuses on channelling water into drainage areas as quickly as possible.
To evaluate the effectiveness of two types of SUDS, the researchers compared three scenarios in the city of Avola, Sicily — an area which underwent rapid urbanisation in the 20th century. First, they identified the amount of land falling into different use categories, for example ‘residential-semi-intensive’, with less than 50% permeability, ‘road and parking areas’ and ‘bare soil’, and obtained plans of the sewer system. Together with rainfall data going back to 1940, these inputs were used with modelling software (the US EPA Storm Water Management Model3) to analyse water flows and run-off under each scenario, for a sub-catchment (part of the wider watershed) in the city.
Modelling was conducted over two-, five- and 10-year timeframes, taking into account the number of peak-flow events (e.g. storms) that would be likely to occur during this time, known as a ‘return period’. In the first scenario, with no SUDS measures in place, modelling showed that the existing drainage system would not cope adequately with peak flows over five and 10 years — therefore flooding would occur in several places.
In the second scenario, 150 areas of permeable paving, covering 15 m2 each, replaced impermeable surfaces in public spaces. In the final scenario, green roofs were installed on 110 buildings, covering about 3.3 hectares. These measures were placed upstream of the area that experienced flooding in the non-SUDS scenario.
Results showed that green roofs were the most effective at mitigating run-off and flooding, but efficacy depended on the return period considered. Over two-year periods, both permeable paving and green roofs exhibited improvements over the first scenario, and over five years they reduced, but did not prevent, flooding. Green roofs halved the volume of flooding over the 10-year period, while permeable paving only slightly reduced the incidence.
Surface run-off, meanwhile, was reduced from 34.7mm (non-SUDS) to 34.3mm (permeable paving) and 30.7mm (green roofs), in this time period, indicating limited benefits in this category.
The researchers attribute such limited benefits to the small area covered by the SUDS measures and the fact that the area was limited to public space, compared to the large impervious surface area over the whole catchment. They also acknowledge that the SUDS modelled were only designed to achieve a general reduction in run-off peak discharge, to relieve downstream areas during heavy precipitation.
The results are also subject to some uncertainty. There is a lack of field data on the performance of real-life SUDS in southern Europe and the Mediterranean region, but such data would help to improve modelled results. Such validation would also allow urban planners to use this type of model to inform the best positioning of SUDS, say the researchers. The researchers also underline that results obtained from SUDS simulations are strongly dependent on site-specific characteristics of the urban catchment, which limit the possible location of permeable pavements and, therefore, favour green roofs. In this study, the researchers highlight that more substantial mitigation of peak flow was achieved by green roofs, which are located on private buildings, than permeable paving in public areas. This implies that policies incentivising private adoption of SUDS measures are important. Demonstration projects and subsidies may be used to drive adoption, they suggest, as well as compliance-based instruments such as building-code requirements.
1. Fratini, C. F., et al. (2012). Three Points Approach (3PA) for urban flood risk management: A tool to support climate change adaptation through transdisciplinarity and multifunctionality. Urban Water Journal, 9(5): 317–331.
2. Fletcher, T.D., Shuster, W., Hunt, W.F., et al. (2014). SUDS, LIDS, BMs, WUDS and more – The evolution and application of terminology surrounding urban drainage. Urban Water Journal, 12(7): 525–542
3. Rossman, L.A., 2010. Storm Water Management Model User’s Manual. Version 5.0.
For technical information, guidance and advice, CIRIA provides a comprehensive range of resources on SuDS through its Susdrain network. It has also produced detailed best practice guidance on the planning, design, construction and maintenance of SuDS in ‘The SuDS Manual’ and an accompanying ‘Site handbook for the construction of SuDS’, which provides guidance on the construction of SuDS to facilitate their effective delivery.
The Landscape Institute’s Technical Committee published Management and Maintenance of Sustainable Drainage Systems (SuDS) Landscapes (March 2014) which includes a selection of water management case studies.
Department for Environment, Food & Rural Affairs and The Rt Hon Michael Gove MP published their ‘A Green Future: Our 25 Year Plan to Improve the Environment’ which sets out what the government will do to improve the environment. The plan seeks to deliver an end to plastic waste, create new habitats for endangered species, a ‘green Brexit’, create nature-friendly schools, and lead the way for other countries to tackle environmental destruction. The 25-year plan also confirmed that the government will consult on plans for the development of a statutory body – a “world-leading” and independent environmental watchdog – to ensure standards on clean air, water and soil will be maintained post-Brexit.
The government has pledged to expand the use of natural flood management solutions, put in place more sustainable drainage systems and make at risk properties more resilient to flooding. Although £15M has been set aside for natural flood management up to 2021 in the 2016 budget but no further flood investment was announced in this report. Sustainable drainage systems (SuDS) such as permeable surfaces, storage tanks and ponds, to reduce the risk of surface water flooding will be encouraged and the government said it would consider changes to the national planning policy framework and building regulations.
Reaction to the launch of the report has been mixed with WWF stating,
“This could be a turning point for the UK’s relationship with the environment, where we begin to restore nature rather than destroy it. The plan is an important first step, but the commitments will only become a reality if they are backed by the force of law, money and a new environmental watchdog with the power to make sure the government lives up to its promises.”
The plan also describes plans to investment £10 million in a schools initiative, called the Nature Friendly Schools Programme, to target children in the most disadvantaged areas of the UK. This funding will come from the Department for Education budget and be spent over five years.
Much coverage of the report’s launch focused on reducing single-use plastics from supermarket food packaging – an initiative described as a major part of the Government’s plan to make Britain a global leader on tackling plastic waste. But the government’s plans for plastic waste do not go far enough, say environmentalists. Tanya Steele, CEO of WWF, said: “We welcome any step to reduce the plastic waste we produce, and policies like this can spur change. But if we really want to solve this problem, we need to think bigger and ultimately move towards an end to single-use plastics.”
Conservation bodies felt that a lack of legal underpinning was a fundamental flaw and environmental groups criticised the lack of proposed legislation and the lengthy timescales for dealing with problems.
“It has been (rightly) criticised for being vague and lacking detail regarding how it will be implemented, but it can’t be criticised for lack of content. Within the 151 pages are a wide range of promises, from the creation of a northern forest and a network of green corridors, to new green infrastructure arising from development.
There is a lot here to welcome, however the lack of specifics is concerning, and the dissonance between some of the aspirations and the context in which they need to delivered (austerity, the weakness of the planning system and the crisis facing our parks and open spaces) is jarring. It takes a leap of considerable faith to see some of these welcome words being translated into effective action.”
Common land and uplands appear to have been ignored with the Government’s failure to mention them in its environment plan following earlier extensive consultation.
The Environment Agency (EA) has published a variety of documents, ‘The evidence behind natural flood management’ which contain data, case studies and evidence about the role of natural flood management in reducing flood risk. This is the first time all the evidence has been brought together, with the intention of enabling more uptake.
The report contains more than 60 case studies from across England and explores how successful the approach is, how it could be used elsewhere and what research may still be needed.
Natural flood management is when natural processes are used to reduce the risk of flooding and coastal erosion. Examples include: restoring bends in rivers, changing the way land is managed so soil can absorb more water and creating saltmarshes on the coast to absorb wave energy.
The Environment Agency has developed a Working with Natural Processes (WWNP) Evidence Directory which looks in detail at the effectiveness of different measures at reducing flood risk. This is supported by maps which help practitioners think about the types of measure that may work in a catchment.
To view a presentation to give you an overview of the Working with Natural Processes – the evidence behind Natural Flood Management project click on image below.
The data is presented across 3 areas –
• Evidence Directory that summarises the effectiveness of Working with Natural Processes measures from a flood and coastal erosion risk (FCRM) perspective as well as the wider ecosystem service benefits they may deliver.
• mapping the potential for WWNP which is intended to be used alongside the Evidence Directory to help practitioners think about the types of measure that may work in a catchment and the best places in which to locate them.
• research gaps that need to be addressed to move this form of FCRM into the mainstream are identified in the Evidence Directory
The report highlights examples of different forms of natural flood management, describes the level of confidence in the flood management examples, areas and actions the EA feels still need to be actioned or explored, benefits accrued from using the management scheme plus further reading, case studies and maps. The report also comments on the degree of scientific confidence for each topic i.e. the level of confidence in the science that underpins the individual measures is given a confidence level (high, medium or low) based on the potential effectiveness of each measure at reducing flood risk.
The report looks at the following key areas:
River Restoration – River restoration reintroduces meanders to rivers and restores physical process. Making a river more sinuous can reduce flood peaks, water velocities and attenuate flow by slowing and storing flood water. The extent of this flood risk effect depends on the length of river restored relative to the overall size of the river catchment.
Floodplain Restoration – River floodplain restoration restores the hydrological connectivity between the river and floodplain, which encourages more regular floodplain inundation and flood water storage. This can decreases the magnitude of the flood peak and reduce downstream flood depths especially for high frequency, low return period floods. The extent of this flood risk effect depends on the length of river restored relative to the overall size of the river catchment.
Leaky Barriers – Leaky barriers are usually formed of wood and they are either formed naturally or are installed across watercourses and floodplains. They reduce flood risk by intercepting the flow of water in a river, this can can help restore river-floodplain connectivity which can reduce flood peaks, slow water velocities and attenuate flow by storing water on the floodplain.
Offline Storage Areas – Offline storage areas, are areas of floodplain which have been adapted (with a containment bund, inlet, outlet and spillway) to store and then release flood waters in a controlled manner. They provide temporary flood storage which can reduce peak flow. The extent of their flood risk effect depends on the number of storage areas provided throughout a catchment and their total storage volume.
Catchment Woodland – Catchment woodland can intercept, slow, store and filter water. This can help reduce flood peaks, flood flows (from 3 to 70%) and flood frequency. Largest reductions in flood risk have been seen for small events in small catchments, the extent of this reduction decreases as flood magnitude increases.
Cross-slope Woodland – A cross-slope woodland is a woodland which is planted across a hill slopes. It intercepts the flow of water as it runs down the hill reducing rapid runoff and encouraging infiltration and storage of water in the soil. There is an absence of measured data to show the flood risk impact of cross-slope woodland at the catchment scale.
Floodplain Woodland – Woodlands in floodplains can slow floodwaters and increase water depth on the floodplain. This can help reduce flood peaks (0-6%), delay peak timing (2 hours or more), desynchronise flood peak and reduce peak height. It can also enhance sediment deposition on the floodplain. Floodplain woodlands have greatest flood risk effect in the middle and lower river reaches of medium to large catchments.
Riparian Woodland – Riparian woodlands are planted on land immediately adjoining a watercourse, they can slow flood flows and can help reduce sediment delivery to the watercourse and reduce bankside erosion. They also have high evaporation losses and can create below ground water storage. Largest reductions in flood risk have been seen at the reach scale, in middle and upper catchments
Soil and Land Management – Soil and land management techniques can reduce peak flow by slowing and storing surface water runoff and encouraging infiltration with the soil. They can include a wide range of different measures as shown in the following flow chart.
Headwater Management – Headwater drainage management techniques can delay and flatten the hydrograph and reduce peak flow locally for small flood events by intercepting, slowing and filtering surface water runoff and encouraging attenuation and infiltration with the soil.
Runoff Management – Run-off pathway management techniques can delay and flatten the hydrograph and reduce peak flow locally for small flood events by intercepting, slowing and filtering surface water runoff. They can include a wide range of different measures as shown in the following flow chart. They usually work best as a cluster of features working as a network throughout the landscape.
Saltmarsh and Mudflats – Saltmarsh and mudflats reduce and dissipate wave and tidal energy in front of flood defences and can extend their design life. They can reduce the forces impacting on flood defences, and also reduce tidal surge propagation and lead to slightly lower water levels at defences.
Sand Dunes – Beach-dune systems form a natural barrier that reduce the risk of tidal inundation landward of the dune, they also act as reservoirs of sand to nourish beaches during storms. They act as a buffer protecting flood defence structures or cliffs behind from direct wave attack and erosion, this in turn enhances the design-life of other flood risk management infrastructure. They can also protect estuaries and lagoons through restricting the passage of storm surges and waves (Pye et al., 2007).
Beach Nourishment – Beaches provide an effective form of coastal defence, but only if they are of sufficient width and level. Where beach systems become depleted this affects their flood risk management value. Beach nourishment is the process of adding material to the shoreline. It is undertaken to improve or restore beach and their coastal defence function, it helps retain the standard of flood protection to the section of coast where implemented. To be effective it is a long-term maintenance activity usually repeated annually.
All the ‘Working with natural processes to reduce flood risk’ documents, maps and case studies can be read here
The world’s delta cities face challenges as a result of climate change. One of the main difficulties for Jakarta, and other megacities located in the Southeast, South, and Mainland Asian, is flooding. Of these megacities, 14 are situated in river deltas and 18 have experienced flooding in the past decade. (Ceola et al., 2014) (Brakenridge et al., 2006). In his paper, ‘Inundated Infrastructure: Jakarta’s Failing Hydraulic Infrastructure’, Frank Sedlar contends that much of Jakarta’s annual flooding episodes are not only linked to heavy precipitation but are also associated with direct human interference in the hydrologic-hydraulic systems of the city. Sedlar points to a novel approach of accessing and utilising new sources of data as a possible way improve the operation of hydraulic infrastructures in cities such as Jakarta.
By connecting urban infrastructure networks to crowd-sourced, social media-based data and linking this information and analysis you can increase the potential of each by producing an innovative, open framework for citizen-participation co-monitoring and management of urban systems. Jakarta’s provincial government has developed a Smart City Platform – Jakarta Smart City – that increases citizen participation. The platform consists of a smartphone app that known as Qlue – allowing users to report problems that occur in their neighbourhood in real time by clicking a photo, geo-tagging the location, offering a brief status report and report it to the local authority. Citizen engagement is seen as crucial to improving services, improving transparency in government and holding local leaders accountable.
In community meetings, known locally as musrenbangs, taking place at both districts and sub-district levels, community forums have become a primary path for citizens to express concerns and demand better services for their neighbourhoods. Now a digital component has been added to the process, known as e-musrenbang. Proposals decided upon at the local-level meetings can be submitted to city government through this web-based application. This bottom-up process works in tandem with the existing top-down planning and budgeting systems of the local government agencies.
Smart Environment is one of 6 pillars included in the Jakarta Smart City platform – others include Smart Governance, Smart People, Smart Living, Smart Mobility and Smart Economy. Smart Environment is supported by PetaJakarta.org (Map Jakarta) – a web-based, crowd-sourcing data collection platform developed to capture data from social media used to gather, sort, and display information about flooding for Jakarta residents in real time. The concept focuses on using a GeoSocial Intelligence Framework to explore Jakarta’s existing and complicated hydrological systems by mapping data mined from social media onto the existing drainage systems to inform knowledge about urban infrastructure and the city’s conditions related to flooding and inundation. The PetaJakarta.org pilot study was developed by Tomas Holderness, Etienne Turpin and Rohan Wickramasuriya of University of Wollongong, Australia who employed the power of existing social media networks, such as Twitter, to provide critical, real time information about the city’s infrastructure and flooding. To read further – Crowd-sourced data harnessed to improve flood response in Jakarta
- Brakenridge, R, and E Anderson. “MODIS-Based Flood Detection, Mapping and Measurement: The Potential for Operational Hydrological Applications.” Transboundary floods: reducing risks through flood management. 2006: 1-12.
- Ceola, Serena, Francesco Laio, and Alberto Montanari. “Satellite night time lights reveal increasing human exposure to floods worldwide.” Geophysical Research Letters 41.20. 2014: 7184-7190.
- Sedlar, Frank. ‘Inundated Infrastructure: Jakarta’s Failing Hydraulic Infrastructure.’ Michigan Journal of Sustainability. Volume 4, Summer 2016.
- Holderness, Tomas, Turpin Etienne and Wickramasuriya, Rohan. ‘A GeoSocial Intelligence Framework for Studying & Promoting Resilience to Seasonal Flooding in Jakarta, Indonesia’
The Landscape Institute funded a trip to Minnesota, USA for PhD researcher, Dawn Purves, here at the School of Architecture and Landscape, Kingston University. Dawn is a graduate of Architecture and Landscape Architecture and has a MA Sustainable Place Making and Urban Design. She is a practicing landscape architect with a particular interest in sustainable water management and urban design. At present Dawn is writing her PhD in relation to Ecological Citizenship in identifying the constraints to and acceptance of ecological solutions to environmental problems and in particular flooding events.
Reflecting on her research and the conference, Dawn discusses community resilience and the ways in which low-impact sustainable urban drainage as a means of preventing localised flooding can be encouraged at multiple scales as climate change adaptation. The following report of her experience is courtesy of LI website:
Dawn presented aspects from her PhD research illustrating the scope for broad Ecological Citizenship to facilitate low-impact sustainable urban drainage through re-framing issues, facilitating solutions through active participatory social learning engagement and motivating collective responsibility and actions.
The focused was environmental decision-making: in particular how to inform decisions in a changing climate, and the role of motivating people to take more responsibility for global complex issues such as localised flooding, increasing populations and altered lifestyle patterns due to population migrations, causing densification and reductions in permeable surfaces that leads to increased instances of flooding. Dawn’s talk illustrated various roles where citizens could be encouraged to be involved in the process of planned climate change adaptation to localised flood prevention and in particular drew upon current research undertaken with Transition Cambridge in establishing a Learning to Stay Dry ‘Community of Practice’ sub-group, one that aims to facilitate a grass roots bottom-up engagement communication to the complex issues around flooding.
The conference illustrated wider research that is looking at the roles of environmental identity and participation success, exploring how identities alter through the lifetime of the project, a key attribute of my research. It also illustrated the role of ‘learning by doing’ or ‘learning from others’ through communities of volunteers undertaken at micro and macro levels, illustrating ‘groupiness’ of the project and its effect in influencing social norms (again aligning my research) which looks at the role and scope of social learning in overturning apathy and lack of responsibility to flood prevention in favour of LISUD, and fostering ecological norms. Finally, it looked at ‘framing’ and the way the issues are framed, illustrating how rhetoric presentation determines desired outcomes.
The research: a brief overview
Evidence (base-adaptation.eu, 2015) points to small-scale bottom-up actions being important in flood prevention. However empirical work carried out by Transition Cambridge sub-group ‘Learning to stay dry’ and other recent literature (Somerville & Hassol, 2011) point to indifference. Column inches and top-down action follow large-scale disaster – but small-scale prevention is a ‘Cinderella’ relative.
Flooding is increasingly seen as a significant problem around the world, costing billions each year to rectify. The causes of flooding are well known. These include increased populations, and population migrations, which cause densification and losses of permeable surfaces, and ‘global weirding’ (Lovins, 2002), where altered rainfall patterns lead to instances of heavy rain.
‘Super-wicked’ problems (Lazarus, 2010) such as climate change have become a significant problem in our cities causing increased property and neighbourhood flooding. If flooding is to be tackled in a sustainable way and increased community resilience as called for by the European Water Framework Directive 2015, putting the citizen back at the forefront of sustainability; then each of us needs to refocus our behaviour rather than being reliant on over stretched Local Government Authorities of Municipalities to solve those issues for us. Current legislation in the UK recommends maintaining pre-development flow-off rates for all new developments through a promotion of both hard-engineered solutions and green sustainable urban drainage measures. These measures are enforced and guided through the planning system. But in parts of the world where these regulations and controls are less evident, or within existing urban areas flooding is still unresolved. In these areas, community resilience is vital. Many local organisations exist which bring communities together to resolve specific issues, such as residents’ associations, transition groups, business investment districts (BID) and flood groups.
This research looks at which is more successful at lessening localised flooding and demystifying water management to increase community resilience. Either top-down approaches of legislation, policy, taxation and incentivisation, or bottom-up grass roots broad ecological citizenship. Community resilience is defined as ‘The ability of community members to take meaningful, deliberate, collective action to remedy the impact of a problem, including the ability to interpret the environment, intervene, and move on.’ (Pfefferbaum, 2005). Broad ecological citizenship is a concept that we are all an integral part of our environment, recognition that our future depends on how we care for our ecosystems, and a sense of responsibility that lead to action on behalf of the environment.
The proposition of the research is that issues surrounding flooding that currently restrict engagement and motivation could be re-framed around broad ecological citizenship and a moral judgement system focusing on our values, beliefs and attitudes, so that citizens could be motivated into taking more collective responsibility towards devising appropriate low impact sustainable urban drainage measures. It proposes that a different approach to communication is needed drawing on the wider issues associated with climate change, identification, social justice, ecological footprints and ecosystem services. By communicating the issues through active participatory social learning within ‘communities in practice’, meaningful participatory planning could occur. Through surveys and focus group interviews undertaken for this research in 3 different flood-prone areas across the UK these ‘communities of practice’ – ‘groups of people who share a concern or a passion for something they do and who interact regularly to learn how to do it better’ (Lave & Wenger, 1991). can also play a further role in facilitating and motivating community resilience, enabling practitioners to take collective responsibility for managing the knowledge they need.
Can the profession adapt in light of these findings?
Flooding affects us all. In the UK, we have recently witnessed devastating examples of flooding from the Somerset levels in the winter 2013-2014 to Penrith in Cumbria in the winter of 2015-16. But this is not only affecting the UK, with many instances around the world re-emphasizing the urgent need for action. With changes in climate and global warming it is not enough to wait for the next flood, we need to undertake greater planned adaptation measures in the form of retrofit low impact sustainable urban drainage.
Transition Cambridge, Property Level Chelsea Fringe Raingarden. © Dawn Purves 2016
These measures have been seen to be effective, but are currently not being installed in great enough numbers to have significant effect. As Landscape Architects and built environment professionals we have great opportunities to promote these measures, both on new developments and existing areas being regenerated.
Current policy facilitates participation in decision making through the Localism agenda and the Flood and Water Management Act via the promotion of community flood groups. If more of these LISUD measures are to be implemented rather than hard engineered solution, measures that demystify water management and provide better collective understandings around the issues and solutions, then Landscape Architects need to facilitate greater collective responsibility for planned adaptation and in particular at a grass roots bottom up level, through property, street and neighbourhood level LISUD measures as seen at Cloudburst Copenhagen, Rainproof Amsterdam and Climate proof Zoho, Rotterdam.
It is no longer acceptable to assume others will protect us, either financially or morally. We all need to do our bit, and as Landscape Architects we are ideally placed both to motivate community groups and local organisations to understand the issues around super wicked problems and, via active participatory social learning with stakeholders, develop LISUD solutions that provide a legacy for future generations.
To read further information on work connected to Dawn’s PhD research read – Influencing “social norms”: promoting climate change adaption to minimize flooding
- Somerville, R.C.J and Hassol, S.J., 2011. Communicating The Science of Climate Change, [Online] Available at:< https://www.climatecommunication.org/wp-content/uploads/2011/10/Somerville-Hassol-Physics-Today-2011.pdf >[Accessed 10 December 2014].
- Lave, J & Wenger, E., 1991. Situated Learning. Legitimate Peripheral Participation. Cambridge: Cambridge University Press.
- Lazarus, R.J., 2010. Super Wicked Problems and Climate Change: Restraining the Present to Liberate the Future, [Online] Available at:<http://scholarship.law.georgetown.edu/cgi/viewcontent.cgi?article=1152&context=facpub> [Accessed 20 January 2013].
- Pfefferbaum, B. J. et al., 2005. Building Resilience to Mass Trauma Events. In L. S. Doll, S.E. Bonzon, J.A. Mercy & D. A . Sleet, eds., Handbook on Injury and Violence Prevention Intervention. New York: Kluwer Academic Publishers.
Regulating urban surface runoff through nature-based solutions – An assessment at the micro-scale. Zölch,T, Henze, L, Keilholz, P & Pauleit, S. Environmental Research, Volume 157, August 2017, Pages 135–14.
- Runoff after heavy rain events accounts for approx. 95% of total precipitation in highly sealed urban areas.
- By enhancing water storage capacities green infrastructure reduces runoff by max. 14.8% compared to the baseline.
- Green roofs and trees both show to be effective but due to different functions.
- The reduction of runoff is larger with higher shares of green cover in the case area.
Urban development leads to changes of surface cover that disrupt the hydrological cycle in cities. In particular, impermeable surfaces and the removal of vegetation reduce the ability to intercept, store and infiltrate rainwater. Consequently, the volume of stormwater runoff and the risk of local flooding rises. This is further amplified by the anticipated effects of climate change leading to an increased frequency and intensity of heavy rain events. Hence, urban adaptation strategies are required to mitigate those impacts.
A nature-based solution, more and more promoted in politics and academia, is urban green infrastructure as it contributes to the resilience of urban ecosystems by providing services to maintain or restore hydrological functions. However, this poses a challenge to urban planners in deciding upon effective adaptation measures as they often lack information on the performance of green infrastructure to moderate surface runoff. It remains unclear what type of green infrastructure (e.g. trees, green roofs), offers the highest potential to reduce discharge volumes and to what extent.
Against this background, this study provides an approach to gather quantitative evidence on green infrastructure’s regulation potential. We use a micro-scale scenario modelling approach of different variations of green cover under current and future climatic conditions. The scenarios are modelled with MIKE SHE, an integrated hydrological modelling system for building and simulating surface water and groundwater flow, and applied to a high density residential area of perimeter blocks in Munich, Germany. The results reveal that both trees and green roofs increase water storage capacities and hence reduce surface runoff, although the main contribution of trees lies in increasing interception and evapotranspiration, whereas green roofs allow for more retention through water storage in their substrate. With increasing precipitation intensities as projected under climate change their regulating potential decreases due to limited water storage capacities. The performance of both types stays limited to a maximum reduction of 2.4% compared to the baseline scenario, unless the coverage of vegetation and permeable surfaces is significantly increased as a 14.8% reduction is achieved by greening all roof surfaces.
The authors conclude that the study provides empirical support for the effectiveness of urban green infrastructure as nature-based solution to stormwater regulation and assists planners and operators of sewage systems in selecting the most effective measures for implementation and estimation of their effects.
Urban green infrastructure as nature-based solution to regulate surface runoff becomes increasingly important, as climate change and urbanisation alter the urban water balance. The present study assessed the performance of two urban green infrastructure (UGI) types, trees and green roofs, on relevant hydrological processes, especially surface runoff. The two measures were applied in scenarios of different greening quantity and for heavy rain events of different intensities as projected for the future. This scenario approach revealed that both trees and green roofs contribute positively by interception, evapotranspiration and infiltration. Differences in their performance showed to be dependent on the greening quantity, share of permeable surfaces leaf area index (LAI) and finally, intensity of the rainfall event. Generally, their effectiveness remains low under heavy rain events, unless a significant proportion of the case area is greened to provide sufficient water storage capacities (Artmann, 2014).
For urban planning the presented results have practical implications for the selection of UGI types to reduce surface runoff volumes and in consequence reduce discharge loads, the sewage system has to handle. An effective nature-based solution increases the storage capacities within the area of interest as much as possible, while using open spaces that have not been used previously and/or while providing benefits to other areas of urban planning. Trees increase the storage capacity mainly by intercepting and evapotranspiring rainwater, their infiltration capacity is limited to the tree pits. But trees can normally not be implemented in large quantity in dense urban areas due to their requirements of open space. Green roofs on the other hand, provide storage capacity mainly by retaining rainwater in their substrates and can be implemented at larger scale on previously unused roof surfaces.
Furthermore, both types are multifunctional and can provide co-benefits for urban planning. The approach represents a first step in allowing planners as well as operators of sewage systems to estimate reductions in runoff volume when locally implementing UGI measures. These estimations could be further improved by integrating additional stormwater management practices and the drainage system in more detail into the modelling setup. Thus, conducting a larger systematic study of UGI scenarios would allow for including e.g. more UGI types, different species and LAI values as well as planting conditions. Such a study could enhance the provision of empirical evidence for climate resilient urban planning.
In March 2013 the Landscape Institute published an updated Mission Statement, ‘Green Infrastructure: An integrated approach to land management’. The document was described as,
“An opportunity to showcase a range of successful strategic GI work and completed
projects. The aim is to give public and private sector bodies, clients and natural and built environment professionals fresh insights into the benefits GI can bring by creating multifunctional landscapes and show how people can collaborate to deliver it.”
Research: Collentine, D. & Futter, M.N. (2016). Realising the potential of natural water retention measures in catchment flood management: trade-offs and matching interests. Journal of Flood Risk Management. DOI: 10.1111/jfr3.12269.
Natural water-retention measures, which ‘keep the rain where it falls’, have great potential to be used as part of flood-risk management plans. But their benefits for downstream urban areas can bring costs to the upstream agricultural areas where they are installed. The researchers suggest that we need new and/or improved policies and institutions to oversee the trade-offs and benefits for agriculture and flood management, and a better scientific understanding of the measures’ likely impact on urban flood risk.
The analysis, conducted by academics in Sweden, draws on various studies to discuss the many benefits of natural water-retention measures and how to encourage their uptake. Floods, which have caused annual economic losses of €4 900 million in Europe, are a serious problem. Natural water-retention measures could play an important role in managing floods, the researchers say, and help meet the goals of the EU’s Floods Directive and Water Framework Directive. These nature-based solutions store rainfall and allow it to evaporate back into the atmosphere to help prevent water flowing into urban areas, where it can cause the greatest damage.
Nature based water-retention measures are a key example of how green infrastructure can work alongside traditional grey infrastructure (e.g. flood gates). They take many forms, from small green roofs to large forests, and bring multiple benefits. As well as reducing flood risk, they can have value for biodiversity, recreation and water quality because they reduce soil erosion and can prevent agricultural pollution running into rivers.
However, this multi-functionality may also act as a barrier to their uptake. Because they bring benefits that fall under different policy areas, it is not necessarily easy for decision makers to determine who should pay for these measures. For some measures, the effect on flood risk may be an additional, secondary benefit — as in the case of agricultural buffer zones, which are primarily intended to reduce fertiliser run-off.
Alongside flood-management benefits, measures installed on upstream agricultural land can also lead to trade-offs, notably reduced crop yield. This loss occurs either because cropland is replaced by the measures or because soil in fields becomes too wet. A payment-for-ecosystem-services (PES) model could, therefore, be used to finance natural water-retention measures, possibly through national agri-environmental schemes. Research from the USA has indicated that it is less costly to pay farmers to temporarily flood their land upstream, than it is to pay for urban damage downstream.
Other barriers to widespread uptake of natural water-retention measures include specific knowledge gaps. There is plenty of evidence to show the effectiveness of individual natural water-retention measures, particularly in small catchments. Computer-modelling studies suggest natural water-retention measures could contribute to downstream flood risk reduction, but there is a lack of real world studies which show how to incorporate such measures into flood-risk management plans and their likely impact on downstream urban flood risk. Such studies are needed to encourage further uptake by decision makers and land managers.
The exact area of responsibility of decision makers with regard to flood-risk management also remains unclear. As such, water policy at the national level is accountable to EU water directives and national decisions must be consistent with the decisions made at the EU level. This is also true for regional-level policy, which is based on national policy and, similarly, local decisions are founded on regional policy. Unfortunately, this set of embedded decisions seems to break down with flood-risk management and has led to a lack of distinction between jurisdictions. Each jurisdiction attempts to reduce flood damage within a limited area and thus tends to focus on the use of preventive structural measures, such as dams or retaining walls, rather than measures to reduce flooding (1).
The authors recommend that institutional structures and mechanisms are created or enhanced to oversee the urban/rural trade-offs that natural water-retention measures may bring, to match potential costs with benefits and ensure an appropriate compensation scheme is put in place; for example, one which links PES with flood-risk management plans.
1. This shortcoming should normally be addressed by the inclusion of a river basin approach within the flood risk management plans: http://ec.europa.eu/environment/water/index_en.htm