February 18, 2020, by Blue-Green team
Perspectives on Urban Flood Resilience
Dick Fenner (University of Cambridge) introduces the work of the Urban Flood Resilience research consortium that forms a core of papers for a special themed edition of the Royal Society’s Philosophical Transactions A Journal on urban flood resilience, published in February 2020. Details can be found on the special issue homepage.
The edition brings together the approach to urban flood resilience in the UK with contributions describing similar initiatives in other countries including the Netherlands, China, Australia, New Zealand and India. The challenges and goals in these countries are similar, requiring the restoration of the benefits of pre-development hydrology in rapidly growing urban centres using Blue-Green Infrastructure solutions to manage flood risk. So the need to develop ways of adapting to the increasing number, frequency and magnitude of damaging flood events in ways that are resilient and sustainable is the underlying premise behind this themed issue.
These approaches can provide multi-functional assets which create a range of other benefits, contribute to wider practices of urban greening, and present real opportunities for enhancement of urban environments.
The papers have been contributed by authors from a wide range of disciplines including engineering, geography, planning, social science, hydrology, economics, and architecture. Whilst each reflects the norms and understanding of flood problems from the perspective of these different backgrounds, it is clear that solutions need to embrace all these various dimensions and never has the need for such a multi-disciplinary approach been so apparent.
Paradigm shift in flood risk management?
A paradigm shift is needed in flood risk management, whereby the solution is framed as an opportunity rather than seeing the problem as a risk. If approached from this perspective stormwater can be treated as a huge potential resource which can be used in some contexts for energy generation as well as providing an essential source of future water supplies. Using waterit in this way makes perfect sense rather than merely draining it “away”. Beyond that, the use of Blue-Green Infrastructure to manage stormwater provides a range of other important benefits from enhancing urban biodiversity through creation of new habitats, sequestering carbon and trapping air pollutants, and providing recreation and amenity opportunities for local communities.
Currently, urban flood risk management can present many messy problems where parts of both the problem and potential solutions are owned by a diverse range of stakeholders ranging from water utility companies, regulators, planners, and property owners, leading to complex and often fragmented responsibilities. Moreover, for flood schemes to be resilient they have to be acceptable to the local communities in whose locale they are situated. Thus solving urban flooding is no longer a solely technical problem, where solutions are imposed by specialist engineers and scientists responsible for understanding the physical responses of the systems. Instead, responses must be framed within a wide socio-technical system where many actors interact in often muddled ways. Hence the papers included in this special edition attempt to deal with all these critical aspects of urban flood resilience ranging from modelling and evaluating the performance attributes of flood resilient solutions, to perspectives from planning, governance and even the social psychology of the public’s awareness of the solutions available.
What is urban flood resilience?
The term “resilience” doesn’t have a generally consensual definition, despite it being regularly used in relation to Flood Risk Management. One definition of resilience provided by Sayers et al.  is: “ The ability of an individual, community, city or nation to resist, absorb or recover from a shock (such as an extreme flood) and/or successfully adapt to adversity or change in conditions (such as climate change or economic downturn) in a timely and efficient manner”. Extending this further, community resilience is based on damage prevention, speedy recovery and preservation of community functionality .
In the Safe and SuRe Framework resilience is defined as “the degree to which the system minimises level of service failure magnitude and duration over its design life when subject to exceptional conditions” . This is an example of engineering resilience where the key features are the resistance to disturbance and the speed of return to equilibrium. Other formulations refer to ecological resilience which sees resilience in a more dynamic way where the capacity to absorb the magnitude before changing its structure is the main feature . Thus, engineering resilience focuses on efficiency, constancy and predictability while ecological resilience focuses on persistence, change and unpredictability .
Overall Urban Flood Resilience can be captured in the following three important aspects :
- The capacity of maintaining resistance over a period of time
- The capacity of the affected communities to recover from material losses, and
- The capacity of the drainage system to recover its functions and keep operating after the storm, guaranteeing basic conditions for urban services to return to normality.
Information about the spatial distribution of resilience is particularly valuable since well-targeted projects can enhance the surrounding areas considerably . The different forms of resilience can be reflected by contrasting the engineering fail-safe approach with the ecological safe-to-fail response, noting that the challenge for flood management is to find more environmentally sound materials and technologies, whilst in the long term recognising the necessity to change our habits and lifestyle . In reality, achieving urban flood resilience requires co-ordination across multiple levels of government. Multiple flood risk management strategies are required – coordinated across multiple governance layers – achieved by proactive policy entrepreneurs, bridging concepts, clear rules, and the provision of the necessary resources .
The focus of the themed edition is on encouraging the practical planning, design and implementation by practitioners of SuDS, Blue-Green Infrastructure and other related techniques, as these have the potential for transformative change of urban water systems.
Papers from the Urban Flood Resilience research consortium
From the Urban Flood Resilience consortium O’Donnell and Thorne contribute the first paper which reassesses the current drivers of urban flood risk, first established by the UK Flood Foresight Project over ten years ago. They suggest five drivers have strengthened and they introduce two new drivers relating to loss of floodable urban spaces, and indirect economic impacts. This reappraisal frames the overall problem in the light of recent advances in flood risk science, technology and practice.
Two papers deal with different modelling approaches to examining aspects of urban flood resilience. Ferguson and Fenner use a novel coupled model linking Dynamic TOPMODEL, HEC-RAS and Infoworks ICM to explore the effect of Natural Flood Management (NFM) interventions in the Asker catchment (Dorset UK). Their paper investigates if moderating water levels in the urban receiving watercourse can be achieved by NFM to allow free drainage at frequently submerged drainage outlets, in this case from a housing estate in Bridport. A parallel systems approach is taken by Dawson, Vercruysse and Wright who combine hydrodynamic modelling with spatial information on infrastructure systems to explore how flood management interventions can be inter-operably connected. Applied to the urban catchment of Newcastle-upon-Tyne, their findings illustrate the benefits of combining data sources in a systematic and spatial way, highlighting the interactions between flood source areas (where most intervention are required) and flood benefit areas (where most of the reduction in flooding is achieved).
Two more papers from the consortium follow which examine the planning and performance of Blue-Green Infrastructure and SuDS systems. Kapetas and Fenner present an adaptive pathways approach to answer the question: what is the most suitable mix of grey and blue-green solutions to urban flooding at any given location and at any future time. A methodological framework is applied to a small sub-catchment in south London using a Storm Water Management Model (SWMM), a SuDS opportunity selection tool, and an adaptation pathways generator. The CIRIA B£ST tool is then used to monetise and compare the multiple benefits of the alternative pathways generated (using combinations of grey pipe expansion, bioretention cells, permeable pavements and storage ponds). Krivtsov, Birkinshaw et al. report the performance of a historic pond in the Royal Botanic Garden Edinburgh which regulates surface runoff, using the CityCAT hydrodynamic model and the Shetran hydrological model, as well as assessing the ecology and biodiversity of the pond and adjacent area, giving insights into the benefits such a facility can provide.
The edition then moves on to considering diverse insights on the impacts of urban flood resilience measures from planning, community stakeholder, recovery, governance, regulatory and economic perspectives. Potter and Vilcan examine how resilience thinking can be implemented despite the realities of English planning procedures. They find three institutional factors constraining the implementation of SuDS, namely the lack of institutional backing, the power of private commercial interests and the severe lack of resources in local authorities which, if not addressed, will ensure resilience approaches remain largely aspirational. A novel application of the Implicit Association Test is used by O’Donnell, Maskrey, Everett and Lamond to investigate unconscious perceptions of SuDS, which can help inform future SuDS design to increase their public acceptance.
Provision of traditional flood control measures follow a logical progression starting with hydrological studies, selection of a design storm of a suitable return period, and the design and subsequent implementation of an engineered system to convey the flow generated. However, there will always be the risk of the system being overwhelmed by a hydrological event which exceeds the design storm, in addition to the eventually of the physical failure of the assets. Disregarding these residual risks can lead to a false sense of security and create increased exposure to hazard in urban environments. These traditional, inflexible, often centralised and increasingly deteriorating assets are failing more frequently as climate and other drivers of flood risk rapidly strengthen, whilst responsibilities are complex, convoluted and opaque.
The special themed edition closes by calling for a new paradigm in which “an integrated approach to managing the water cycle begins by seeing potential opportunities, exploiting resources, adding to the quality of urban areas and preferring nature-based approaches, seeking multi- value and multi-functional infrastructure” . To do otherwise may be seen as a transgression from the inevitably of natural laws, attempting to hold back the tide – if you will.
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- Ashley R, Gersonius B, Horton B. 2020 Managing flooding: from a problem to an opportunity. Phil. Trans. R. Soc. A 378, 20190214. (doi:10.1098/rsta.2019.214)
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