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.”
Following our last post featuring the publication of results of the London iTree Urban Forest Survey, this week’s post focuses on an in-depth analysis of the green assets of the Victoria in the Borough of Westminster, London highlighting the wide range of benefits (or ecosystem services0 green infrastructure provides in urban settings.
The Green Benefits in Victoria Business Improvement District Report – An analysis of the benefits of trees and other green assets in the Victoria Business Improvement District. The following is a summary of the full report.
Victoria, London Borough of Westminster, is a major transport hub, tourist destination and centre for business and commerce. Like many urban areas Victoria suffers from the effects of a high density built environment with an overall lack of natural features and this contributes to overheating in the summer through the urban heat island effect as well as surface water flooding, when the drainage system cannot accommodate the volume of water running off of hard surfaces. The Victoria Business Improvement District (BID) acts as a platform for business to take a lead in enhancing the overall urban environment of Victoria and to help shape the area for future development.
Trees intercept rainwater thus reducing the burden on the drainage system; they cool the air and provide invaluable shade on a sunny day, and, of immediate benefit to Victoria in the face of major redevelopment, they can contribute to cleaning the dust and pollution from the air we breathe. So clearly Victoria’s street trees and those in public and private green space have a central role in mitigating these environmental challenges and so contribute to making Victoria a better place to do business. The Victoria Business Improvement District considers trees to be a core component of the local infrastructure and has therefore commissioned the research presented in this report to provide a clearer understanding of the financial benefits of trees in a specific location so that we can say with confidence that our trees are saving the business community thousands of pounds per annum.
Green infrastructure can deliver benefits to urban areas. Trees in particular can provide a wide range of benefits (or ecosystem services) such as storing carbon, reducing the urban heat island effect and improving air quality. Understanding the structure, function and value of Victoria Business Improvement District’s green infrastructure can inform decisions that will improve human health and environmental quality. This report presents a baseline quantitative assessment of the air pollution, amenity, carbon storage and sequestration benefits of trees as well as the storm water and surface temperature benefits of existing green infrastructure in the Victoria BID.
This report also estimates the additional benefits that could be generated if the 5% canopy cover increase target set by the Mayor of London for Greater London, by 2025, and the potential green roofs and ground level green spaces investments identified by the BID are both realised in Victoria. This assessment was conducted using the i-Tree Eco model (also known as UFORE1 ), as developed by the U.S. Forest Service, Northern Research Station, Capital Asset Valuation for Amenity Trees (CAVAT) and tools within the Green Infrastructure Valuation Toolkit. It considers the impact of public and private trees as well as other types of green infrastructure assets including green roofs and gardens.
Existing trees, green spaces and other green infrastructure assets in Victoria divert up to 112,400 cubic metres of storm water runoffs away from the local sewer systems every year. This is worth between an estimated £20,638 and £29,006 in reduced CO2 emissions and energy savings every year. The total structural value of all trees in Victoria, (which does not constitute a benefit provided by the trees, but rather a replacement cost) currently stands at £2,103,276. The trees in Victoria remove a total of 1.2 tonnes of pollutants each year and store 847.08 tonnes of carbon. London plane currently dominates the treescape within Victoria BID, storing 59% of all carbon, filtering 67% of all pollutants and making up 29% of the tree population.
However the London planes represent an ageing population and in order to maintain the current level of tree benefits to Victoria BID more trees capable of attaining a larger stature will need to be planted. When implemented, the green infrastructure opportunities identified by Victoria BID have the potential to:
- Divert up to 67,500 additional cubic meters of storm water runoff every year, representing an estimated extra £12,392 in avoided CO2 emissions and £17,417 in energy savings annually. Future design choices – particularly in relation to green roofs – will have a determining impact on the scale of water management benefits realised.
- Reduce peak summer surface temperatures by up to 5.1˚C in the area surveyed. This will moderate local air temperatures, helping to ensure that the BID remains an attractive and comfortable environment for residents, visitors and workers alike. It will also reduce the need for air conditioning in office buildings, lowering energy costs and carbon emissions.
- The high CAVAT value of London plane in particular justifies the investment required to establish and maintain very large trees in the urban environment, yet equally points to the vulnerability arising when such a high proportion of value resides in a single species.
Public and private trees in the Victoria BID provide valuable benefits. Realising the Mayor’s target for increasing canopy cover together with the creation of additional green roofs and ground level green space would dramatically enhance the scale of ecosystem services the area enjoys. These conclusions highlight some key recommendations in order to ensure that benefits currently arising from the BID’s green infrastructure are sustained and future green investments fulfill their potential for generating returns:
- Conduct succession planting for London planes. The local tree resource is characterised by a good diversity of species, which makes it more resilient to pressures such as pests and diseases or climate change. However, most of the canopy area and therefore most of the benefits arising from the Victoria BID’s treescape are provided by London planes. Many of these trees are mature specimens, as has already been highlighted in previous studies (Kelly 2012). Provision should be made to ensure that adequate succession planting is carried out in order to maintain benefits at the current levels.
- Target private trees owners for awareness raising and best practice on tree management. Many of the BID’s trees are in the private realm and in the buffer area. Therefore, a reduction in privately owned trees will reduce this benefit. Strategies and policies that will serve to conserve this important resource (through education and community engagement for example) would be one way to address this.
- Focus on large canopy trees both for planting and maintenance. The amount of healthy leaf area equates directly to the provision of benefits (or ecosystem services). Canopy cover can be increased through new tree plantings. However, the most effective strategy for increasing average tree size and tree canopy is to preserve and manage the existing trees within the BID so that a good proportion can grow to maturity. Where new trees are planted the Westminster Trees and the Public Realm SPD (Westminster City Council 2009) recommends using the tree species with the largest canopy a site can accommodate. Ensuring this is well-enforced in the future will be key to maintaining tree benefits at their current level.
- Engage with Thames Water to ensure the benefits from proposed green investments can more directly reach BID members. Trees currently make a positive contribution to the management of storm water in Victoria. The proposed investment to increase canopy cover, create new ground level green spaces and dramatically extend the area of green roofs and will significantly alleviate the local drainage system. This will unarguably result in public benefits, by contributing to reducing the prevalence of flooding events which regularly damage and interfere with the local infrastructure. It will also reduce the amount of water that will need to be treated by the local water company, Thames Water, thus offering real savings. All efforts should be made to ensure local owners and occupiers in Victoria who, as part of their water bill, pay for the drainage of the area get access to their share of this saving. Thames Water grant a rebate to its customers on drainage fees based on pipe diameter rather than based on volume reduction. This doesn’t provide a strong incentive for green approaches such as those envisaged for Victoria. Direct engagement with Thames Water on this issue could result in an innovative pilot – offering a model that could be rolled out to other parts of London.
The Landscape Institute featured Greening for Growth in Victoria as a case study in their 2011 publication ‘Local Green Infrastructure: helping communities make the most of their landscape’
“ The Victoria BID is championing a sustainable green agenda for the area and is looking to become the leader in the development of sustainable business environments through the retrofitting of green infrastructure. We are working closely with public and private sector partners to achieve a positive change in the physical landscape in the area. The increase in green infrastructure will not only benefit the environment but also the workers, residents and visitors that come into Victoria every day.” Ruth Duston, Chief Executive Officer Victoria BID
The results of London’s i-Tree Survey are now available. The results of the London iTree urban forest survey were published on the 2nd December 2015 in the House of Lords. Attending was the host Lord Framlingham, Environment Minister Rory Stewart, Sir Harry Studholme (Forestry Commission Chair) and other senior representatives from London, the tree sector and the built environment sector.
The report has identified that the tree population of inner and outer London holds nearly 2.4 million tonnes of carbon and is sequestering an additional 77,000 tonnes per annum. This is equivalent to the total amount of carbon generated by 26 billion vehicle miles.
The report estimates the Capital Assessment Value (CAVAT ) – the functional, visual and social contribution value of London’s urban forest as £43.3 billion. London’s canopy cover is calculated at 21% and the most common species in inner London are Birch, Lime and Apple and in outer London it’s Sycamore, Oak and Hawthorn. It is also interesting to look at the sheer variety of species – 126 different tree species which is the highest recorded species diversity of any urban forest analysed with i-Tree in the UK.
“The importance of trees in the urban environment is unquestionable but is often minimised and lost amongst the myriad of other competing factors involved in urban space making, creation, management and maintenance.” Source: Sir Terry Farrell, CBE
“The millions of trees and shrubs in London’s parks, gardens, woodlands and open spaces are collectively described as London’s ‘urban forest’. This urban forest is part of London’s green infrastructure. It provides a range of ecosystem services that delivers multiple environmental benefits to Londoners. The scale and effectiveness of these benefits, such as air quality improvement, carbon sequestration or temperature reduction, are directly influenced by the way we manage the resource and decisions and actions that affect its structure and composition over time.” Source: London i-Tree Eco Project Report Executive Summary.
“This project demonstrates just how much can be achieved when we engage with the largest stakeholders of our urban forest – the public. Without them this study (the world’s largest urban forest survey using citizen science), this report, and what it reveals, would simply not have been possible. In addition, London has also developed a core group of trained and skilled i-Tree surveyors from all walks of life. They are now able to carry out further i-Tree Eco (and other tree) surveys, thereby helping to raise awareness of the benefits of London’s trees.” Source: London i-Tree Eco Project Report
The i-Tree Eco survey methodology was developed by the US Forest Service to assess the ecosystem service value of urban forests across the world. It has been used in Los Angeles, New York, Chicago, Barcelona, Melbourne and, closer to home, in Edinburgh and Glasgow. However, the London survey uniquely teamed professional volunteers with ordinary members of the public who all received training in the i-Tree Eco methodology. The survey element of the project could then be delivered solely by volunteers.
Trees in Hard Landscapes: A Guide for Delivery is the companion document to Trees in the Townscape: A Guide for Decision Makers. It explores the practical challenges and solutions for integrating trees in 21st century streets, civic spaces and surface car parks. These are arguably the most challenging environments for growing trees, but are also the areas that can derive great benefits from their inclusion.
The report was developed by the Tree and Design Action Group (TDAG) who have joined forces with the Chartered Institution of Building Services Engineers (CIBSE), the Chartered Institution of Highways and Transportation (CIHT), the Institution of Civil Engineers (ICE), and the Institute of Chartered Foresters (ICF). Trees in Hard Landscapes: A Guide for Delivery has benefited from the input of over 100 built environments professionals and organisations from wide ranging disciplines and sectors who have taken part interviews, contributed case study materials and/or offered feedback on earlier drafts.
To download the report: http://www.tdag.org.uk/trees-in-hard-landscapes.html
Starting from the point where the policy decision to retain or plant trees has been made, this guide explores the key building blocks to success through:
- Collaborative Process – From project initiation to maintenance and monitoring, when, how and with whom joined-up working needs to happen. Weaving natural resources, especially trees, into the built environment requires a cross-disciplinary collaborative approach from project initiation through to design, implementation, maintenance and monitoring. This guide looks at when, how and with whom joined-up working needs to happen.
- Designing with Trees – Practical strategies to ensure trees best contribute to the delivery of the design objectives of a project. Achieving long-term benefits from trees requires a concerted strategy. This guide explores current evidence on the enabling factors for trees to bring value to the hard landscapes in which they grow.
- Technical Design Solutions – The available technical below-ground solutions to achieve lasting overall success. Design of the below-ground environment is key to achieving long-term compatibility between trees and the built infrastructure that surrounds them in towns and cities. This guide examines innovative and available technical solutions to help build lasting success from investing in trees.
- Species Selection Criteria – The frame of reference to use as a basis for tree selection. While tree species selection alone cannot make up for a poor design strategy or inadequate underground growing conditions, choosing the right tree for the right place is an essential final ingredient for success. This guide offers a five-step process for making the best shortlist of available options and achieving resilient and successful tree choices.
Audience for the report – Highway engineers, civil and structural engineers, highway contractors, construction site managers, project managers, designers and tree specialists are the primary audience for this guide. It will also be of value to developers, planners, elected members, local communities and all involved in hard urban landscapes and their design and management.
The need for a sustainable integrated infrastructure – Two themes are expressed throughout the guide. Firstly, the importance of innovation, an element of experimentation, so that improvements can be made in response to changes in available techniques as well as in constraints and expectations for the public realm. Secondly, the mindset to achieve multiple benefits. Both are essential for successfully integrating trees in high-performance hard landscapes in a 21st century context.
There is much evidence on the wide range of benefits that can be gained by integrating trees with other infrastructure. However research also shows that, while trees need to reach a degree of maturity to fulfil their potential and deliver returns on investment and benefits to their communities, many urban trees in hard landscapes are not living as long as they should.
The context in which trees can thrive in hard landscapes is in a state of flux, with new challenges and opportunities for success. Many urban trees, especially the larger growing trees, were planted from the mid-19th to mid-20th century when there was less crowding, complexity and compaction beneath our highways and public spaces.
There are now new uses and quality expectations for the public realm. Streets are increasingly regarded as ‘places’, not just as vehicular movement corridors. The same ‘space’ needs to accommodate cycles and a public transport system as well as private vehicles and better conditions for pedestrians to encourage walking. These challenges will need creative solutions.
Climate change and increasingly erratic weather patterns including heatwaves and extreme localised rainfall also put new pressures on the infrastructure of our towns and cities. Urban trees have a lot to offer towards urban cooling and surface water management. Modelling conducted for Manchester has shown that increasing the canopy cover by 10% would keep summer peak temperatures at their current level until the 2080s. In terms of Manchester this would mean an increase in average canopy cover from about 15% to 25% but it is important that canopy cover is as evenly distributed as possible across the urban area.
Images: Tree and Design Action group (TDAG)
Lord Framlingham advocates the role of our urban trees in the UK’s House of Lords:
“Are we making the most of this incredible asset?”
The Landscape Interface Studio blog recently reported on the ‘Trees in the Townscape A Guide for Decision Makers Report’ – now Lord Framlingham has made a strong case for urban trees at the House of Lords on Thursday 15 January 2015 during a Natural Environment debate initiated by Lady Bakewell. Lord Framlingham called for a nationally coordinated approach to enhance the integration of trees in the design and management of hard landscapes. He commended the Tree and Design Action Group’s latest two publications and urged Lord de Mauley, Parliamentary Under-Secretary of State for the Department for Environment, Food and Rural Affairs (DEFRA) to “lend his weight to the distribution of these guides, or more particularly their contents, so as to co-ordinate and encourage the most enlightened and best practice everywhere”.
Less satisfactory was the lack of clear answer to Lord Framlingham’s question on who has responsibility for urban trees at a governmental level. It would appear that, despite the interest and good intentions towards urban trees demonstrated during the debate, the central question as to who is actually responsible for ensuring the urban forest and all the benefits it bestows is managed in the present and is sustained and developed into the future, remains unanswered.