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.
Here’s some food for thought…….The following commentary blog was posted on the World Design Summit’s ‘Designing the Future’ website – world design summit taking place over 10 days in Montreal in October, 2017. The event is described as, ‘an international gathering of diverse disciplines with a common focus: how design can shape the future.’
“Green has gone mainstream. But as green walls, rooftop farms, and tree covered skyscrapers become the norm in cities around the world, critics ask if these developments are simply the latest face of ‘greenwashing’. Environmental advocates such as Naomi Klein have long warned of corporations that attempt to mask questionable records by emphasizing green elements. Klein has challenged industrialists who partake in flashy, eco-conscious campaigns but do little to alleviate the impact of their businesses on the climate. Many companies spend more money convincing consumers that they are green than they do on actual green initiatives; some corporate green initiatives actually harm the environment.
Urbanists worry that greenwashing has spread to urban design and architecture by literally covering up buildings with plants. The vertical farms, plant-covered towers, and eco-villages that once seemed far-fetched are now not only possible but all the rage. Bringing greenery into the city may reduce emissions and improve air quality, but it does little to address the deeper causes of urban and environmental stress. Greening initiatives increase the economic value of places, risking displacement of economically vulnerable residents. Does a greenroof on top of a superstore help uninsured workers? What difference does a green tech-campus make if it requires hundreds of parking spaces?
One architect who questions greenwashing is Alejandro Aravena, director of the firm ELEMENTAL in Chile. At last year’s Venice Architecture Biennale, Aravena exposed the wasteful side of design by exhibiting 90 tons of detritus from the previous year’s event. He also tackles underlying challenges, by collaborating with communities to empower them to design affordable and sustainable housing for themselves. In this age of tree-covered skyscrapers, Aravena’s commitment to modest and even monotonous design stands out.
Questioning greenwashing demands that designers ask whether the green city must be a luxury for the privileged and whether sustainability initiatives accelerate inequality. Designers in all disciplines must look beyond rhetoric and aesthetics to evaluate the impact of environmental design. How can we, as designers, reconcile our role, which is closely linked with the market economy and over-consumption, with environmental and social awareness? Designers invested in truly sustainable practice are encouraged to submit a proposal for the topic “Questioning Greenwashing” at the Congress for the World Design Summit.”
The article is not attributed to any particular author but it would be interesting to hear your thoughts? What’s your opinion in relation to #greenwashing? The article is illustrated by Stefano Boeri‘s proposals to build “forest cities” in Shijiazhuang, China plus an image of Bosco Verticale, Milan although the latter is not credited – (neither design is referred to by name in the article). Both designs incorporate vegetation on the outer construction with the twin towers of Bosco Verticale – constructed with a $2.5 billion public-private investment as part of the redevelopment of Milan’s Porta Nuova district – housing 800 trees between 9 and 30 feet tall, over 4,000 shrubs, and 15,000 ground cover plants including vines and perennials.
The summit focuses on 6 key themes including ‘Design for Earth’ and 108 topics including ‘Questioning greenwashing’,
“Designers are sensitive to environmental issues and announce their willingness to conform their practices. However, beyond appearances and words, critical scrutiny is essential. How can we, as designers, reconcile our role, which is closely linked with the market economy and over-consumption, with environmental and social awareness?”
To read Naomi Klein’s, ‘The hypocrisy behind the big business climate change battle’ . Source: The Guardian, Sept, 2014.
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.”
Living roofs play an important role in helping to achieve London’s target to increase green cover in central London by 5% by 2030. They can improve the city’s resilience to the impacts of climate change by reducing the amount and speed of storm water run-off and helping to keep buildings, and the surrounding areas, cooler during increasingly frequent hot spells. They also provide much needed outdoor living space, improving life for both residents and adding to the beauty of the local neighbourhood.
The illustrated publication, City of London green roof case studies provided to the City of London Corporation includes a number of green roof applications in The City describing their context, format, drivers, barriers, benefits and notes on biodiversity.
It is estimate that there are around 700 green roofs in central London alone, shown on the map below. New green roofs need to be built to meet the greening targets, installed through both new development proposals and through the retrofit of existing buildings, to deliver as many of the following objectives as possible:
- Reduction of Urban Heat Island – Research in Tyndale Centre for climate change suggests we need a 10% increase in green space in our cities to combat climate change. This is particularly relevant to the reduction in the Urban Heat Island [UHIE]. Green roofs are recognized to have a positive effect on reducing the UHIE
- Biodiversity – Green roofs can provide important refuges for wildlife in urban areas. Research in Switzerland and the UK has demonstrated that green roofs can provide important refuges for rare invertebrate populations.
- Water – Green roofs can significantly reduce the surface run off volumes and rates of rainfall leaving roofs. As a source control mechanism in the Sustainable Urban Drainage System green roofs can help reduce flash floods as a consequence of intense rainfall events. This will become increasingly important as a consequence of climate change. Green roofs also improve the quality of water and although the amount of water is reduced it is possible to rainfall harvest from roofs that have been greened.
- Thermal Performance – Green roofs cannot be given a U-value at present. However they have been shown to significantly reduce the need for air conditioning in summer and can provide a degree of insulation in winter.
- Sound Insulation – The combination of soil, plants and trapped layers of air within green roof systems can act as a sound insulation barrier. Sound waves are absorbed, reflected or deflected. The growing medium tends to block lower sound frequencies whilst the plants block higher frequencies.
- Protection of Waterproofing – The original green roofs in Germany stem from covering wet bitumen with 6cm of sand, which became vegetated. This covering was to protect the wet bitumen from fire. Green roofs have now been shown to double if not triple the life of waterproofing membranes beneath the green roof.
- Air Quality – airborne particles and pollutants are filtered from the atmosphere by the substrates and vegetation on a green roof.
- Amenity Space – in dense urban environments there is often a lack of green space for residents. Roof Gardens and roof top parks provide important green spaces to improve the quality of life for urban residents.
- Urban Agriculture – in the form of Urban Rooftop Food Growing – roofs, where strong enough provide a space for urban food growing. Although many large flat roofs may not have the loading capabilities to hold food growing some roofs will and the many balconies in are urban areas are ideal.
Transport for London have installed green roofs at St James’s tube station and West Ham bus garage and a green wall at Edgware Road tube station.
This green roof map of London below shows there are around 700 green roofs in central London alone, covering an area of over 175,000m2. That’s 17.5 hectares or around 25 football pitches. The green roof map was produced by the GLA and the Green Roof Consultancy by studying aerial images of London taken in 2013 (by The Geoinformation Group).
The map currently contains 678 known green roofs, but there are many which are missing, including those that have been installed since the summer of 2013. If you would like us to add a green roof to the map please email, GIMap@london.gov.uk. Or, if you would like to add more information about a green roof currently on the map – such as a photo, or links to a website with further details of the roof – then please click on the green roof on the map and the ‘email us’ link, which will open a new email window
Image sources: City of London Corporation