Moving towards greener resilience
Jonathan Simm of HR Wallingford explains how nature-based solutions can help us to see the big picture when building in system resilience
The UK construction industry has made great strides forward in recent years. We have seen improvements to time, cost, quality, safety and environmental impact issues, and significant innovation in technology and delivery. Innovation brings danger, however, if new ideas, terms and developments come into our dialogue and practice, but are not integrated into a realistic and holistic view of the problems we are trying to solve. In flood and coastal erosion management, resilience and nature-based solutions illustrate
It is commonly agreed that system resilience has something to do with both robustness in the face of extreme events and time to recovery where an event exceeds the capacity of a system to deal with it without damage. A genuine attempt to create and assess resilience needs to reflect both the complexity of the system being considered and a realism about the performance of that system under everyday events, design events and greater-than-design events. We need to make a genuine attempt to assess and, where possible, quantify the uncertainties which face us.
We face considerable challenges here. Design standards, such as Eurocodes, might acknowledge variability and uncertainty, but they remain firmly linked to the concept of design events. Designs based on these codes are essentially deterministic and focus on particular threshold values. This does not sit well with the concept of risk analysis, where the focus is across a much broader range of possibilities. If the two approaches are to be combined, we need to put much more effort in to looking at performance above and below these design events, and avoid them becoming our sole focus.
The idea of ‘design for exceedance’ focuses on above-design events. Even where it is present, it does not always find its way properly into the decision-making process. My own
organisation, when carrying out physical modelling of coastal projects, has for years adopted the principle of sensitivity testing with greater than design events. Even that modest concession to design for exceedance is still not always welcomed by those who worry (wrongly in my view) about the risk of eroding confidence in the design concept. In our tests we explore and quantify both elements of resilience to greater-than-design events: the robustness of a structure or system (e.g. a sea defence or breakwater) to these events, and the extent of damage under these events so that the time and cost of recovery can be assessed.
The importance of testing
The idea of evaluating serviceability limit states is intended to focus on everyday, less-than-design, events. This is often paid lip service but can be narrow in its implementation and insufficient attention is given to the whole system at risk. The now-fashionable approach of using nature-based solutions is forcing a more integrated approach to evaluating performance under both everyday and extreme events. This is because a number of nature-based approaches can have a dramatic impact on consequences such as flooding. This is the case under events of order not exceeding about a 10% annual exceedance probability.
When events become much greater than this, nature-based solutions may have the capacity to survive themselves, but their ‘engineering’ effectiveness (e.g. in ‘slowing the flow’) may reduce to negligible proportions. The solution in such a situation may well be a composite approach, using a combination of conventional grey engineering along with the nature-based approaches. Along with the multiple environmental and social benefits, an additional win for the engineering community in adopting green infrastructure alongside conventional solutions is that it may finally force us to think much more carefully about performance across a spectrum of design events.
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