National Hydrological Monitoring Programme The winter floods of 2015/2016 in the UK - a review by Terry Marsh, Celia Kirby, Katie Muchan, Lucy Barker, Ed Henderson & Jamie Hannaford National Hydrological Monitoring Programme The winter floods of 2015/2016 in the UK - a review by Terry Marsh, Celia Kirby, Katie Muchan, Lucy Barker, Ed Henderson & Jamie Hannaford CENTRE FOR ECOLOGY & HYDROLOGY l [email protected] l www.ceh.ac.uk [i] This report should be cited as Marsh, T.J.1, Kirby, C.2, Muchan, K.1, Barker, L.1, Henderson, E.2 and Hannaford, J.1 2016. The winter floods of 2015/2016 in the UK - a review. Centre for Ecology & Hydrology, Wallingford, UK. 37 pages. Affiliations: 1Centre for Ecology & Hydrology; 2British Hydrological Society. ISBN: 978-1-906698-61-4 Publication address Centre for Ecology & Hydrology Maclean Building Benson Lane Crowmarsh Gifford Wallingford Oxfordshire OX10 8BB UK General and business enquiries: +44 (0)1491 838800 E-mail: [email protected] [ii] The winter floods of 2015/2016 in the UK - a review THE WINTER FLOODS OF 2015/2016 IN THE UK – A REVIEW This report was produced by the Centre for Ecology & Hydrology (CEH), the UK’s centre for excellence for research in land and freshwater environmental sciences, in collaboration with the British Hydrological Society (BHS) which promotes all aspects of the inter-disciplinary subject of hydrology – the scientific study and practical applications of the movement, distribution and quality of freshwater in the environment. Funding support was provided by the Natural Environment Research Council. CEH and BHS are extremely grateful to the many individuals and organisations that provided data and background information for this publication. River flow and groundwater data were provided by the principal hydrometric measuring authorities in the UK: the Environment Agency in England, the Scottish Environment Protection Agency, Natural Resources Wales (Cyfoeth Naturiol Cymru) and the Rivers Agency in Northern Ireland. The Met Office National Climate Information Centre (NCIC) supplied the majority of the rainfall and climatological data featured in the reporta. The reservoir stocks data were provided by the water service companies, Scottish Water and Northern Ireland Water. Additional data and information has been provided by the Canal & River Trust and a range of individuals and organisations. The provision of the basic data, which provides the foundation both of this report and the wider activities of the National Hydrological Monitoring Programme (NHMP), is gratefully acknowledged. Note: as is the case with the documenting of most extreme hydrometeorological episodes a proportion of the data featured in this report – maximum river flows in particular – is necessarily provisional and may be subject to future revision. The National Hydrological Monitoring Programme This report is an output from the NHMP which is operated jointly by CEH and the British Geological Survey (BGS). The NHMP was set up in 1988 and aims to provide an authoritative voice on hydrological conditions throughout the UK and has a particular obligation to document major contemporary hydrological events and to identify and interpret long-term hydrological change and variability. Monthly Hydrological Summaries for the UK are routinely published as part of this programme. This document, and a full set of the publications in the NHMP series, including the monthly Hydrological Summaries, are available from the NHMP website: http://nrfa.ceh.ac.uk/nhmp. aThe Met Office data are Crown Copyright – they may not be passed on to, or published by, any unauthorised person or organisation. CENTRE FOR ECOLOGY & HYDROLOGY l [email protected] l www.ceh.ac.uk [iii] CONTENTS INTRODUCTION 1 METEOROLOGY OF WINTER 2015/2016 3 Climatological and physical background 3 Rainfall 3 Synoptic background 3 November 4 December 4 January 7 February 8 Raingauge, regional and national rainfall 8 Temperatures 9 HYDROLOGY OF WINTER 2015/2016 11 Soil moisture deficits 11 River flows 11 Summer and autumn 15 November 15 December 15 January 16 February 17 National and regional overview 17 WATER RESOURCES OF WINTER 2015/2016 20 Reservoir stocks and flood mitigation 20 Groundwater 20 IMPACTS OF THE FLOODING 22 MAJOR NATIONAL RUNOFF EPISODES 25 HYDROMETEOROLOGICAL TRENDS IN THE UK 26 DISCUSSION 30 CONCLUDING REMARKS 32 Acknowledgements 32 Location map 33 References 34 CENTRE FOR ECOLOGY & HYDROLOGY l [email protected] l www.ceh.ac.uk [v] [vi] The winter floods of 2015/2016 in the UK - a review INTRODUCTION A remarkably persistent and exceptionally mild cyclonic episode beginning in early-November 2015 and lasting around fourteen weeks brought severe, extensive and protracted flooding which impacted most damagingly on northern Britain, Northern Ireland and parts of Wales. Many existing rainfall and seasonal temperature records were eclipsed during this period and, most notably, maximum recorded river flows were exceeded over a substantial proportion of the country. At the national scale, previous maximum daily and monthly outflows were clearly eclipsed and four relatively discrete episodes of extreme runoff can be recognised. In many areas, the magnitude, persistence and repetitive nature of the flooding had major adverse impacts on communities, infrastructure, agriculture and a host of other sectors of the economy. The extent and duration of the flooding has very few close parallels in the historical record and, overall, it was a hydrometeorological episode which ranks alongside the 1975/1976 drought and 1947 floods as the most extreme broad-scale events captured in observational records during the last 100 years at least. The truly exceptional runoff patterns experienced in 1947 and 1976 had a major impact on flood and drought management strategies but occurred before the exacerbating impact of climate change was generally recognised. The 2015/2016 flooding, together with the impact of other protracted flood events in the 21st century thus far, has underlined the need to adapt engineering design and flood management strategies to accommodate the recent extension in the range of recorded runoff variability. This report documents the 2015/2016 flooding within a hydrometeorological framework. The extreme conditions are reviewed in the context of lengthy rainfall and river flow time series, and a brief review is included of their magnitude relative to other major flood episodes. In addition, rainfall and river flow patterns through the early part of the 21st century are examined within the context of hydrological trends emerging over the post-1960 period. CENTRE FOR ECOLOGY & HYDROLOGY l [email protected] l www.ceh.ac.uk [1] [2] The winter 2015 - 2016 floods in the UK - a review METEOROLOGY OF WINTER 2015/2016 Climatological and physical Rainfall background Synoptic background The UK is exceptionally diverse in terms of its climate, geology and land use. Each, to differing The early autumn of 2015 was notably dry across degrees, can influence vulnerability to flooding. much of the country – the combined September The climate is temperate and, whilst notably and October rainfall was the third lowest for the UK capricious in the short term, sustained periods of since 1914. However, from early-November 2015, a very dry or very wet weather are relatively rare. persistent sub-tropical airflow from the south-west Rainfall across the UK derives predominately from became established and dominated weather Atlantic low pressure systems and on average, is conditions until well into the following February2. well distributed throughout the seasons but with Crucially, in relation to the flooding, the Jet Stream a tendency towards an autumn and early winter guided a succession of very vigorous low pressure maximum, particularly in western areas. By contrast, systems along similar south-west to north-east tracks spatial variations in rainfall are large; the annual across the UK. This synoptic pattern coincided with average in the wettest upland areas, mostly in the above average sea surface temperatures, a strongly west, is around six times greater than in the driest positive phase of the North Atlantic Oscillation and areas of the eastern lowlands. Aside from a few a strong El Niño (other teleconnections will also mountainous regions, the Cairngorms in particular, have been influential). These driving mechanisms snow is generally a very minor component of total provided the backcloth to a 14-week period that precipitation but snowmelt can be an important was truly remarkable in both meteorological and exacerbating factor in relation to flood risk, hydrological terms3,4,5 – many existing rainfall and particularly across northern Britain. Historically, temperature records were exceeded and, in relation snowmelt has contributed to a substantial number to runoff patterns, this episode has very few close of major floods events throughout the UK. historical precedents. At a national scale, around half of the annual rainfall Several of the major flood events were associated is accounted for by evaporation losses which peak with named storms (Table 1). The naming of during the late spring and summer. Evaporation storms was introduced in September 2015 by the demands are typically of minor significance in Met Office and Met Éireann to designate those moderating any particular flood episode, but expected to cause disruptive impacts in the UK, primarily through their impact on antecedent soil to raise awareness of severe weather and improve moisture conditions, they exercise an important control on flood seasonality. Whilst intense rainfall Table 1 Named storms in the winter of 2015/2016. can produce flash flooding in any season, the Those with the greatest hydrological impacts frequency of fluvial flooding is much greater are marked in bold. Source: Met Office Storm Centre. during the winter half-year with around 80% of major events occurring within the October - March Name Date of impacts (UK/Ireland) timeframe. However, extreme flow events do occur Abigail 12th - 13th November 1 during the summer half-year (for example in 2007 ) Barney 17th - 18th November particularly in upland and urban catchments.