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.”
‘Towards an EU Research and Innovation policy agenda for Nature-Based Solutions & Re-Naturing Cities’ – the Final Report of the Horizon 2020 Expert Group on ‘Nature-Based Solutions and Re-Naturing Cities’ published by the European Commission’s Directorate-General for Research and Innovation, Climate Action, Environment, Resource Efficiency and Raw Materials.
- Nature-based solutions – what are they?
Nature-based solutions aim to help societies address a variety of environmental, social and economic challenges in sustainable ways. They are actions which are inspired by, supported by or copied from nature. Some involve using and enhancing existing natural solutions to challenges, while others are exploring more novel solutions, for example mimicking how non-human organisms and communities cope with environmental extremes. Nature-based solutions use the features and complex system processes of nature, such as its ability to store carbon and regulate water flow, in order to achieve desired outcomes, such as reduced disaster risk, improved human well-being and socially inclusive green growth. Maintaining and enhancing natural capital, therefore, is of crucial importance, as it forms the basis for implementing solutions. These nature-based solutions ideally are energy and resource-efficient, and resilient to change, but to be successful they must be adapted to local conditions.
- Key opportunities for research and innovation policy on nature-based solutions. The expert group identified four goals:
- Enhance sustainable urbanisation – Currently, 73% of Europe’s population live in cities and this is projected to increase to 82% by 2050, resulting in over 36 million new urban citizens . This will pose a range of challenges for cities, including resource availability and equitable economic growth. The quality of urban environments is also at risk, necessitating their sustainable development and regeneration in order to provide citizens with healthy and liveable conditions.
- Restore degraded ecosystems – In Europe, significant areas of ecosystems are being lost or degraded as a result of human activities. For example, between 60% and 70% of European wetlands have been completely destroyed.The drivers of loss and degradation vary according to the ecosystem and location, but the key pressures include agricultural intensification, grey infrastructure expansion, pollution of brownfield sites, hydrological modifications to water bodies, the intensification of forestry practices and, generally speaking, climate change
- Develop climate change adaptation and mitigation – Addressing climate change is a challenge as its impacts on Europe are likely to increase and it affects all aspects of the environment, economy and society. For example, the annual damage of climate change to the EU economy, measured as GDP loss from today’s conditions, could be between €20 billion for a 2.5°C scenario and €65 billion for a 5.4°C scenario with high sea level rise.
- Improve risk management and resilience – Europe is exposed to a range of natural and technological hazards, including drought, extreme temperatures, floods, industrial and transport accidents, landslides and avalanches, storms, volcanoes and wildfires. In the EU, between 2002 and 2012, numerous such events generated 80,000 fatalities and €95 billion in economic losses.
The restoration of the floodplain of the Noordwaard polder, the Netherlands, will provide climate change-related flood protection, improve the environmental quality for people and nature, increase recreational facilities and boost the economy. Both photos show the situation after depoldering, which has left more room for the river.
- Seven priority nature-based research and innovation actions to meet societal challenges in the above four goals have been identified:
- Urban regeneration through nature-based solutions – Changes in land use, neglected land and abandoned areas are challenges for many cities. Urban regeneration through nature-based solutions offers a context for innovative interventions for green growth.
- Nature-based solutions for improving well-being in urban areas – With millions more people needing housing, services, workplaces, infrastructure and institutions by 205020, the potential impacts of development decisions are unparalleled. By integrating nature based solutions into urban design and planning, increasingly large and dense cities can improve human health and well-being, while offering ecological and economic co-benefits.
- Establish nature-based solutions for coastal resilience – Coastal habitats are iconic and of considerable economic and social importance across the EU, protecting against floods and erosion, while providing livelihoods for many individuals through tourism and fishing.
- Multi-functional nature-based watershed management and ecosystem restoration – Watershed management and restoration using nature-based solutions can help to reduce the risk of floods and droughts, while improving water quality and quantity.
- Nature-based solutions for increasing the sustainable use of matter and energy – Nature-based solutions can decrease resource demand through energy and matter-efficient processes. In cities, green spaces and green roofs provide natural cooling or insulation.
- Nature-based solutions and the insurance value of ecosystems – The insurance value of ecosystems has to date been largely overlooked in research and practice and mostly discussed in relation to its role as a metaphor for the value of resilience.
- Increase carbon sequestration through nature-based solutions – Over the last 30 years, terrestrial and freshwater ecosystems have stored about a quarter of human generated CO2 emissions.