‘How can we learn to live with floods? Challenges for science and management’: guest editorial introduction

In the winter of 2015/16, the UK recorded its highest ever 24-hour and 48-hour rainfalls, mainly across Northern England and much of Scotland, resulting in catastrophic flooding. The resulting impacts across the UK were devastating, with hundreds of homes evacuated, roads and bridges swept away and total damage estimated by the UK insurance industry at £1.3 billion (Association of British Insurers, 2016). In Scotland, severe impacts were suffered in Aberdeenshire, the Scottish Borders, and Dumfries and Galloway. The Rivers Dee, Ythan and Don in Aberdeenshire all recorded their largest ever flows, in each case with a probability exceeding once in 200 years (Marsh et al., 2016).

Triggered in part by these extreme floods, the Royal Society of Edinburgh held a one-day conference on 15 March 2017 entitled ‘How can we learn to live with floods? Challenges for science and management’. The meeting addressed the following key questions:

1. What is the link between individual extreme events and climate change?

2. How can we include climate change in estimating future flood risk?

3. How can we improve predictions of extreme rainfall?

4. How can we improve our mapping of flood hazards?

5. How can we make our cities more resilient to flooding?

6. How can we manage catchments to reduce flood risk?

7. How can we improve our flood warnings?

8. How can flood insurance be made more affordable and effective?

9. How can we better engage with communities at risk?

The papers by McLaughin, Wilkinson and Penning-Rowsell which follow this introduction are based on their conference presentations and address questions 4, 6 and 8 respectively. Based on the papers presented at the conference, the answers to each of the nine questions are summarised below, together with follow-up references for each speaker.

• The link between individual extreme events and climate change is complex. Many have sought to determine whether extreme flows, such as those in Aberdeenshire in January 2016, are statistically significant. But as Chris Kisby reported, trend detection using flood data is ‘crippled by uncertainty’ as only small time-series samples are available. The problem in attributing individual events, and especially record-breaking events, to climate change is what period of record to use. Statistical analysis on its own is insufficient; more research is needed on the background weather conditions which produce extreme rainfall and associated flooding. This challenge has been taken up by Hayley Fowler whose convection-permitting models (PCM) give better representation of convective processes during rainstorms and thus more accurate estimates of hourly rainfall. Unlike coarser resolution climate models, this high resolution (1.5 km grid spacing) weather forecasting type of model can simulate realistic hourly rainfall characteristics. In terms of UK climate change, it suggests increases in hourly rainfall intensities in winter and intensification of short-duration downpours in summer. More detailed accounts of these issues can be found in Serinaldi, Kilsby, and Lombardo (2018), Serinaldi and Kilsby (2018), Chan, Kendon, Roberts, Blenkinsop, and Fowler (2018) and Blenkinsop et al. (2018).

• In addressing the challenge of incorporating future climate change into flood risk modelling, Nick Reynard echoed some of Chris Kilsby’s caution over trend analysis. Although it appears that over the period 2000–2016 the UK appears to have entered a ‘flood-rich’ period, with increased winter rainfall consistent with predicted climate change, natural variability must also be taken into account as inferred trends may be misleading. Hitherto, flood risk managers have added 20% to peak flows to allow for the impact of climate change, but such a ‘one-size-fits- all’ approach is ill-advised on account of contrasting catchment characteristics. Reynard and colleagues at the Centre for Ecology and Hydrology have derived a range of responses to climate change based on individual catchment characteristics for floods of varying probabilities or return periods. The allowances for climate change can vary between 20% and 40% depending on the region being considered. See Kay, Crocks, Davies, and Reynard (2014) for a more detailed account of Reynard’s presentation.

• Surface water flooding is a major hazard for towns and cities. In light of this, David Balmforth (MWH Global) posed the question ‘How do we make our urban conurbations more resilient to flooding?’ The standard approach has been to focus on improved infrastructure by, for example, trying to increase the capacity of storm drains. But such an approach is not only costly but also unsustainable: instead, urban areas need to be made more resilient. Each of the three elements of drainage (the source of the flow, the receptor area and the pathway down which the water runs) must be managed. Methods include placing water barrels outside homes to collect run-off from gutters, or, as in the Netherlands, housing developments with additional green space with channels to take away excess water and housing on stilts, or on platforms that can float, when flood water pours into an area. The resultant ‘blue-green’ cities that emerge also promote biodiversity and enhanced health and well-being for their inhabitants. In the context of climate change, solely building physical defences no longer works: building resilient urban infrastructure is the solution. For further details, see Balmforth, Kenney, and Digman (2014) for examples of more integrated urban drainage networks.

• A key component for successful flood risk management is the provision, and more importantly, the acceptance and use of flood warnings. For many years the Scottish Environment Protection Agency has been providing a flood warning service, initially on its own and more recently in close collaboration with the UK Met Office. The presentation by Mike Cranston was based on a report that addressed the question ‘How effective is Scotland’s flood warning system, and how can it be made better?’ Flood warnings to householders who register with SEPA’s Floodline take a number of forms. A Flood Alert – ‘flooding is possible’ is sent out 36 hours in advance. Flood Warnings – ‘flooding is imminent’ are sent out typically three to six hours in advance. Severe Flood Warnings are sent out when there is a significant risk to life, destruction of properties or local communities. The results from a widely distributed questionnaire indicated that respondents wanted and valued warnings. In terms of responses to warnings, 71% of registered users of Floodline deployed property level protection, 62% of landowners moved livestock and 42% moved vehicles. Most customers were satisfied with the service and act on warnings, but would value information more closely tailored to their local area. For the full report on which this presentation was based see Geddes, Cranston, Ambler, and Black (2017).

• Engineered defences, natural flood management and the promotion of greater resilience all have a place in managing flood risk, but as Ioan Faizey argued, greater resilience also depends on effective engagement with the communities at risk. The Scottish Borders Climate-Resilient Communities project was developed to understand some of the critical factors that help shape community resilience to the impacts of climate change, focussing on three communities with a history of flooding. Nine workshops (3 per community) examined issues and developed actions designed to build community resilience. The ensuing tenth workshop summarised the outcomes of work within each community and made proposals for a more integrated national policy of promoting community flooding resilience. For the full report on which this presentation was based see Fazey et al. (2017).

McLaughin’s paper in this issue highlights the rapid technical advances that have been made since the first flood hazard maps of the 1900s. The use of airborne radar (LiDAR) to define topography, plus major advances in computational power and the modelling of the pathways and destinations of river, coastal and surface water floods has provided flood risk managers with better hazard maps – essential tools for enhancing resilience in light of projected climate change.

Wilkinson reminds us that natural flood management – often seen as a panacea for managing flood risk when engineered defences are too costly – whilst locally effective, has yet to prove its value at the catchment scale. Whilst the use of linked ponds, soil treatment measures and re-connected wetlands yield measurable benefits locally, not least because of associated benefits, major challenges remain in up scaling such measures to the catchment scale.

Penning-Rowsell issues a timely warning that flood insurance penetration for low-income families, often living in rented accommodation, is worryingly low in Scotland. Given that such households do not take out flood insurance, more affordable cover via Flood-Re will not prove effective. Specific recommendations, targeted on the most vulnerable communities, include group insurance for social housing tenants, tailored insurance for low income families and use of local intermediaries for specific cultural groups. But for such measures to be effective they require strong local community engagement, potentially assisted by the Scottish Flood Forum, and explicit central government support.

Following the meeting, the Royal Society of Edinburgh produced a Flooding Advice Paper on the key policy implications of the conference findings aimed at the key stakeholders who manage flood risk across Scotland (Royal Society of Edinburgh, 2017).