Water and Effluent Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/1057079
20 WET NEWS DECEMBER 2018 | wwtonline.co.uk Moving towards greener resilience T he UK construction indus- try has made great strides forward in recent years. We've seen improvements to time, cost, quality, safety and environmental impact issues, and significant innovation in technology and delivery. Inno- vation brings danger, however, if new ideas, terms and develop- ments come into our dialogue and practice but are not inte- grated into a realistic and holis- tic view of the problems we are trying to solve. In flood and coastal erosion management, resilience and nature-based solutions illustrate this issue. 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 sys- tem being considered and a realism about the performance of that system under everyday events, design events and great- er-than-design events. We need to make a genuine attempt to assess and, where possible, quantify the uncertainties that face us. • INSIGHT RESILIENCE Nature-based solutions can help us to see the big picture when building in system resilience, writes Dr Jonathan Simm, chief technical director for resilience at HR Wallingford We face considerable chal- lenges here. Design standards, such as Eurocodes, might acknowledge variability and uncertainty, but they remain firmly linked to the concept of design events. Design based on these codes are essentially deterministic and focus on par- ticular threshold values. This does not sit well with the con- cept 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 into looking at performance above and below these design events, and avoid them becom- ing 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 car- rying out physical modelling of coastal projects, has for years adopted the principle of sensi- tivity testing with greater than design events. Even that modest concession to design for exceed- ance is still not always wel- comed 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 ele- ments of resilience to greater than design events: the robust- ness of a structure or system (e.g. a sea defence or breakwa- ter) to these events, and the extent of damage under these events so that the time and cost of recovery can be assessed. The idea of evaluating ser- viceability limit states is intended to focus on everyday, less than design, events. This is o‰en 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 inte- grated approach to evaluating performance under both every- day and extreme events. This is because a number of nature- based approaches can have a dramatic impact on conse- quences such as flooding under events of order not exceeding between about a 10 per cent 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 propor- tions. The solution in such a sit- uation may well be a composite approach, using a combination of conventional grey engineer- ing along with the nature-based approaches. Along with the multiple environmental and social benefits, an additional win for the engineering commu- nity in adopting green infra- structure alongside conven- tional solutions is that it may finally force us to think much more carefully about perfor- mance across a spectrum of design events.