Here is good news for those of us concerned about climate change and our profession’s current lack of guidance on when and how to consider potential climate change impacts when making flood-related designs or decisions with long lasting implications.
Engineers Australia’s Australian Rainfall & Runoff (ARR) Revision Team has just released a discussion paper on an interim guideline for considering climate change in flood-related planning, design and operations.
Revision of ARR began in 2007 and is due for completion by the end of 2015. Release of the draft interim guideline is therefore timely because the need for advice on how to consider climate change in flood estimation is a key driver for revising ARR. It was also the main reason why the former Federal Government provided several millions in funding.
Globally, climate change is expected to cause more intense individual rainfall events and fewer weak storms, increasing the risk of flooding over time at many locations. The guideline draws on the most recent climate science, as set out in the IPCC’s Fifth Assessment Report (September 2013) and new climate change projections for Australia (CSIRO and BOM, 2014). It is intended to be applied to the design standard adopted for the structure, infrastructure or plan. Possible impacts of climate change on other flood-influencing mechanisms, such as antecedent wetness, base flow, tail water levels and oceanic processes, are not considered in the interim guideline.
Importantly, the interim guideline outlines an approach to address the risks from climate change to projects and decisions that involve estimation of design flood characteristics. It has been developed to respond to the question: ‘is climate change a significant issue for the location or project at hand?’ If the answer is ‘no’ the guideline recommends that climate change be disregarded and the design flood or decision only be based on the updated historical rainfall statistics to be published in 2015 in ARR Book II – Rainfall Estimation.
Under the interim guideline, explicit allowance for climate change in the adopted design or decision would only be made if the following four criteria are all satisfied (a flow diagram is available upon request):
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long service life of the proposed asset or planning horizon of activity (>20 years), AND
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design standard is less than the Probable Maximum Flood (if PMF use latest BOM estimate) AND
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high consequences of failure (of flood-related aspects) and high cost of retrofitting for the design annual exceedance probability (AEP), AND
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high impacts and consequences of failure and high cost of retrofitting for events rarer than the design AEP, as shown by a sensitivity analysis;
and then only if one of the following criteria is satisfied:
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low cost of modifying the design for climate change relative to the benefits of reducing the residual risk OR
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if the cost of modifying is high, that there is a statutory requirement to account for climate change, OR
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detailed cost-benefit analysis of the original and the potentially changed design supports adopting the changed design.
The interim guideline is expected to be replaced over time as new research findings are released. An example is the research underway to quantify possible changes and uncertainties in design rainfall Intensity-Frequency-Duration (IFD) curves due to anthropogenic climate change for the Greater Sydney region and for south-east Queensland. The discussion paper recommends that the interim guideline be reviewed following release of IPCC’s Sixth Assessment Report, expected in 2020/21.
The interim guideline can be downloaded at http://www.arr.org.au/wp-content/uploads/2013/Projects/Draft_ARR_interim_guidance_Format.pdf
The ARR Revision Team is seeking feedback from the engineering to help determine whether the proposed guideline will be included in the final version of ARR. IPWEA members are encouraged to read the interim guideline and submit their comments, even if only to support inclusion of the interim guideline in ARR 2015.