CEG8526: Hydrosystems modelling and management
W2: Climate Modelling and Decision-making Under Uncertainty
Aim and learning outcomes
The purpose of this practical is to use some simple city-scale climate change information to understand the uncertain nature of future projections. The practical encourages you to think about the limitations of climate models and the challenges posed by uncertainty for real-world decision-making.
After completing this practical you should be able to:
• Explain how the uncertainties of climate model projections affect decision-making in hydrological applications
• Identify additional information that would be required to aid decision-making
• List at least three potential implications of your decision and ways in which the resultant risk can be managed
Practical summary
Climate models provide important information on future climate for use by planners, policy- makers and engineers. However, as discussed in the lecture, there is inherent uncertainty from several sources in the projections they provide. In this practical you will undertake a simplified decision-making scenario to highlight the implications of these uncertainties on real-world decisions.
Task
Imagine that you are an urban drainage engineer appointed to upgrade parts of the Victorian sewerage system of Newcastle-Upon-Tyne. You are aware that heavy downpours in summer 2012 overwhelmed the sewers causing widespread flooding and disruption across the city. You also know that any new or upgraded infrastructure could be in place to the end of the 21st century and most possibly beyond.
You know that the Department for Food and Rural Affairs (DEFRA) provides projections of future change in peak rainfall intensities that show changes to peak rainfall by management catchment. These are derived from the latest UK Climate Projections. The Environment Agency and Lead Local Flood Authorities use peak rainfall allowances to inform their advice on flood risk and drainage in their roles as statutory consultees. Allowances are included for two future time-frames, labelled 2050s and 2070s. It is recommended that you use the 2050s for development with a lifetime up to 2060 and use the 2070s for development with a lifetime between 2061 and 2125. The range of allowances included is based on percentiles. A percentile describes the proportion of possible scenarios that fall below an allowance level. The 50th percentile is the point at which half of the possible scenarios for peak flows fall below it, and half fall above it. The:
• central allowance is based on the 50th percentile.
• upper end allowance is based on the 95th percentile.
This is a plausible consultancy project. Engineers and planners recognise that the climate does not stand still and that climate change safety margins need to be factored into long-lived infrastructure design, at least in the UK. But, is it reasonable to expect a high-resolution climate model to provide accurate predictions of extreme summer rainfall likelihoods for the 2080s at the city scale? Does the climate model even simulate realistic convective storms? How might the results change if different greenhouse gas emissions scenarios or climate models are used? Was the cost and time invested in producing the scenarios really worth it? These are some of the important questions being raised by both users and suppliers of high-resolution climate change scenarios. See information from previous lectures on climate modelling.
What should the consultant engineer do about the state of the sewers in Newcastle? What advice should he/she give? There are basically four options.
1) Apply the same design event as in the past. This is the cheapest solution in the short- term but there is a risk of greater costs in the future if storms do become more intense.
2) Turn to the Environment Agency guidance on climate change allowances for flood risk assessments and look up the relevant climate change uplift factor. These have been derived from the output from very-high resolution convection permitting models. The guidance for the Tyne Management Catchment is shown in Figure 1. Now there is a danger of spending money unnecessarily on enlarging the drainage capacity if storm intensities do not change that much.
3) You take a more sophisticated approach by looking at the trade-off between the size of the adjustment for climate change and the cost of construction. (You might still end up choosing the most precautionary value (45%) because once the roads have been dug up the additional cost of wider diameter sewers is relatively low.)
4) You think about all the other ways of managing the surface runoff from source, such as green roofs, permeable parking areas, ponds and water retention features woven into the urban fabric. This seems an appealing option until you realise how many organisations would have to be involved and how long it would take to implement a joined-up action plan.
Questions:
1) What solution(s) would you choose?
2) What are the uncertainties that make decision-making difficult?
3) What are the limitations in the information provided and the climate models used to produce them?
4) Is there any additional information (including non-climatic) that you would find useful for decision-making?