The winter 2015/2016 floods in the <scp>UK</scp>: a hydrological appraisal

Abstract

The flooding in winter 2015/2016 was one of the most extraordinary hydrological episodes witnessed in the UK in recent decades. The winter was defined by a succession of severe storms, bringing extreme rainfall and associated widespread flooding across northern and western regions. For the UK, December 2015 was the wettest month (for any calendar month) and it was the second wettest winter (December–February) in a series from 1910, while a number of new records were established for rainfall accumulations over shorter timescales (McCarthy et al., 2016; Burt, 2016). The hydrological response was remarkable; period of record maximum peak flows were established across northern areas of the UK, and the high flow response was notable in terms of its spatial extent, duration and frequency. The flooding had severe impacts on properties, infrastructure and livelihoods. Inevitably, the flooding was at the forefront of public, political and media attention through much of the winter and, occurring only two years after the major flooding of 2013/2014, led to a significant debate around flood management in a warming world. This paper describes the hydrological characteristics and widespread impacts of the winter 2015/2016 floods, places the event in historical context and briefly considers the wider narratives surrounding the events. Companion papers in this issue describe the meteorological events (McCarthy et al., 2016) and the meteorological records broken (Burt, 2016). Together, it is hoped that these papers provide a comprehensive overview of the exceptional meteorological and hydrological conditions and their impacts in the winter of 2015/2016. The term ‘winter floods’ has been widely adopted, and reasonably so given the focus in the winter months of December and January. We use that terminology here, although the hydrological conditions before and after the meteorological winter (December–February) are important for the wider context so are considered in this paper where appropriate. Data and information used in this paper are primarily sourced from the National Hydrological Monitoring Programme (NHMP). Supplementary data and information was provided by the UK Measuring Authorities. The NHMP collates data for 104 index river gauging stations and 37 index groundwater boreholes in the UK alongside rainfall, soil moisture and reservoir data to produce the monthly Hydrological Summary (http://nrfa.ceh.ac.uk/monthly-hydrological-summary-uk). Data submitted to the NHMP by the UK Measuring Authorities are provisional, and any records reported in this paper may be subject to change. The total outflows for Great Britain mentioned in this paper are routinely used by the NHMP to identify national scale runoff trends and to make comparisons between extreme events. Details of their calculation method are outlined in Marsh et al. (2015). Figure 1 shows the location of selected catchments referred to throughout this paper. Through the first half of 2015, river flows in the UK were generally rather unremarkable and remained within the normal range in the majority of catchments. The early autumn was very dry in northern and western parts of the UK. Consequently, contrary to the typical seasonal response, October saw steep recessions in some northern and western catchments; in northwest Scotland, the Carron and Naver recorded new seasonal daily flow minima. The drier conditions continued into early November, and estimated outflows from Great Britain were less than half of the average for the time of the year. Thereafter, the month was dominated by mild and unsettled weather, including a sequence of Atlantic low pressure systems. Following a Met Office/Met Éireann pilot initiative to name winter storms, November saw the first three named storms in the British Isles (Storms Abigail, Barney and Clodagh; McCarthy et al., 2016). The associated rainfall prompted a dramatic hydrological transformation, and many of the catchments that recorded below average flows in October recorded notably high flows in November. By mid-month, Flood Warnings were widespread across much of northern Britain – severe warnings were issued in Kendal and Egremont on the 14th/15th, prompting property evacuations. A number of major rivers draining the Pennines registered new November peak flow records (e.g. the Ribble, Lune and Wharfe). Average flows for November were exceptionally high in a band from north Wales to the Scottish borders, exceeding twice the average, and the Wharfe recorded its highest November runoff in a series from 1955 (Figure 2(a)). Outside this area, flows were notably high across southern Scotland and Northern Ireland, while flows in the rest of UK were average or below. Following the wet November, soils in many northern and western catchments were saturated (as is typical for the time of year), and river flows were already increasing entering December (Figure 3). The stormy weather continued, and on 5th/6th December, Storm Desmond brought heavy and persistent rainfall across much of Ireland, northwest England and southern Scotland. New maximum 24 h (341 mm at Honister Pass) and 48 h (405 mm at Thirlmere) rainfall records were established in Cumbria (McCarthy et al., 2016; Burt, 2016), and new peak flow maxima were registered in many catchments across northern regions of the UK as a devastating flooding episode unfolded (the impacts of which are discussed in a later section). Due to the exceptional rainfall and river flows associated with Storm Desmond, large volumes of sediment and debris were transported in watercourses and deposited when flows began to recede, drastically reducing channel conveyance. This contributed to several communities in the Lake District (e.g. Glenridding) being flooded three times before Christmas, when further intense rainfall events affected northern England. While the passage of Storm Eva on 23rd/24th December brought strong winds but only modest rainfall, a slow-moving low pressure system brought further exceptional rainfall totals on the 25th/26th, and 100–120 mm fell over the southern Pennines (McCarthy et al., 2016). By Boxing Day, the number of Flood Warnings across Wales and northern England totalled 500 (including the severe warning, warning and alert categories), with widespread warnings across Scotland. A large swathe of northern Britain recorded exceptional river flows and floodplain inundations were witnessed in many locations. Following the record peak flows in November, several rivers draining the Pennines recorded new peak flow maxima on the 25th/26th (e.g. the Nidd, Wharfe, Aire, Calder and Irwell). With more rainfall falling on already saturated ground between Christmas and New Year, high flows persisted. The arrival of Storm Frank on 29th/30th December saw flows on the Scottish Dee exceed their previous maxima (in a series from 1929; Figure 3(b)) by a considerable margin, and a new maximum peak flow was recorded on the Cree (in a series from 1963). Due to the persistent and exceptional rainfall during December, monthly mean river flows established new records for any month in the majority of catchments in a band from southwest Wales to northeast Scotland (Figure 2(b)). Of these catchments, the Tyne, Tees, Wharfe, Conwy and Eden recorded more than three times the December average (in record lengths of more than 45 years). Entering 2016, soils remained saturated and river flows remained exceptionally high across much of northern and western Britain. With many catchments vulnerable to even moderate rainfall, river flow records were once again established in January as the UK recorded its fourth wettest January on record (in a series from 1910). From the 4th to the 7th, new January peak flow records were registered in catchments across northeast Scotland. The Deveron recorded a new January maximum (in a series from 1960), the Don a new maximum for any month (in a series from 1971), and the Ythan a new maximum for any month by a wide margin (in a series from 1983). Furthermore, new daily maximum flows were established on the Ythan and Don for 12 consecutive days, illustrating that both the individual events and duration of high flows were exceptional. With more settled conditions mid-month, recessions became established, and flows on some rivers fell below average for the first time since early November. However, these were interrupted towards month-end as river flows rose again in response to frontal systems, including Storm Gertrude on 28th/29th January. Over 100 Flood Alerts and Warnings were issued in northern and western catchments, but peak flows were not as remarkable as those recorded in the previous month. January was also very wet in parts of central and southern England, particularly the far south – places that had seen more typical winter rainfall. Some seasonally high flows were recorded in Wales and southern England, including the second highest flow on record on the Lymington (in a record from 1976). New record monthly mean flows for January were recorded in catchments across eastern Scotland (e.g. the Scottish Dee recorded more than three times the average in a series from 1929) and in parts of Wales (Figure 2(c)), some of which also set new records in December. February was the driest of the winter months, but still registered above average rainfall in the majority of regions in the UK (McCarthy et al., 2016). River flows were high entering the month as a result of the substantial late-January rainfall, and more peak flow records were established during the first week with the passage of Storms Henry and Imogen. On the 1st, Flood Alerts and Warnings were issued across Scotland following Storm Henry; the Naver registered a new February peak flow maximum (in a series from 1978). Over 100 Flood Alerts and Warnings were issued on the 6th, following further rainfall; flows rose sharply in rivers in Wales and southwest England, and the Tone registered a new February peak flow maximum (in a record from 1961). Warnings were issued again on the 8th/9th during the passage of Storm Imogen, and seasonally high flows were recorded on some rivers in central and southern England (e.g. the Trent, Blackwater and Dorset Stour). From mid-month, drier conditions prevailed across the UK, heralding steep recessions; by month-end some responsive catchments (many of which established several records over the winter) registered flows half of the average for the time of year. Despite the drier end to February, monthly mean flows were above average across most of northern and western Britain (Figure 2(d)), and more than 150% of average in a number of catchments. Following the exceptional winter, the spring was dry in northern and western catchments, prompting a marked shift in river flow response. Although interrupted by some seasonally high flows, long and often steep recessions were observed on many rivers. By the end of flows on the Eden were notably with new seasonal daily flow recorded on 12 consecutive and on in a from a new maximum peak flow months following Storm this the exceptional of the river flows of winter 2015/2016 are and The duration and spatial of the November runoff first be at a scale by national On December, daily outflows from Great Britain the previous maximum daily December by more than following the exceptional rainfall of Storm outflows from Great Britain on 6th, and December and in a series from exceptional are moderate flows across much of the south and of England. The intense and of the winter flooding is in the average outflows from Great Britain Average flows across two in December and January in the including the highest on for the 2015/2016 events were the on record for of to days, exceeding those from recent notable events as and mean flows for individual months from November to January new maxima across much of northern and western Britain (Figure new monthly maxima into new records for average flows in catchments in a band from southwest Wales and Northern to and northeast Scotland (Figure many of these or exceeding of their average many catchments across the north and the average flows previous maxima for any period of record – by more than on the Scottish Tyne, Wharfe, and The of the flooding in winter 2015/2016 is most by the peak flows on flow for individual catchments, and their associated were estimated the Flood method et al., The provided a of maximum records to which was This not for the use of or historical flood shows the peak flow and estimated period range for selected large catchments across northern and western areas of the UK, illustrating the wide of new maximum peak Although this is a it was reported at the time by the that new river flow maxima were recorded across northern England the spatial of flows when river flows New maximum peak flows in the for England and Wales of were established on 5th/6th December on the Eden and the Lune in northwest England, and the in northeast England, following Storm shows that the peak flow recorded on the Eden the previous maxima in January by both events were associated with severe flooding in The estimated period for the peak flow on the Eden for the Lune and are within the is important to that these records are from series that in the and it is that flows been witnessed in the The flow on the is to be the highest since the Great Flood of 2016), which had estimated flood peak of on historical flood et al., on flood for catchments be to the recent events in a catchments recorded new maximum peak flows for November, December, January or for any month. For these to events of exceptional with estimated of more than years for the rivers Dee Wharfe and of which to areas by Although the peak flows recorded on the not first for any of the winter months, daily flows on in only so times in the The in flows in the extreme high flow range – for to of flows, of or at gauging stations – to substantial in the recorded peak Furthermore, the from sediment and debris may in the between river and the peak flows and hydrological in this paper be with and may also be subject to following of their By autumn 2015, groundwater were generally or below average, and seasonal had in parts of the far south of England. The most extreme rainfall that through winter 2015/2016 was across areas of northern and western regions of the UK where are less but the to which groundwater was a in the winter 2015/2016 flooding was some boreholes in northern England, southern Scotland and Northern to the exceptional rainfall. the of southwest Scotland (e.g. Figure and the of eastern Northern (e.g. were notably high in November, and new maxima were established in December. in also sharply in December to as notably high in both the (e.g. and the (e.g. groundwater in at were exceptionally and a new January maximum was recorded at in a Although groundwater were exceptionally high at times in some parts of northern England, were of flooding from parts of the of the of groundwater the of outflows at high river and groundwater to flooding in river in parts of northern Britain. the unsettled weather had groundwater generally fell across northern England but remained above seasonal recessions remained in the normal range or above throughout the spring for the most of the UK. continued to be exceptionally high throughout the spring and early (Figure in many months new maxima in the period of conditions across the and early autumn reservoir for England and Wales to below average for the first time in more than a by the end of in northern and western Britain were particularly several as Northern and were more than below in reservoir were by the very wet conditions that during the The most extreme rainfall across areas of the north and of the UK, reservoir to sharply in the regions with that most northeast England, by in a week at reservoir in response to Storm 2016). the reservoir in the UK by also registered its in in a series from 2016). The in UK reservoir between October and November was the monthly (for any month) in a record from By the end of November, in most were already above average, and further substantial rainfall in December and January many to in the Northern between October and December (Figure Thereafter, in northern and western parts of the UK remained to through the winter and generally were or above average throughout the normal seasonal series from average reservoir for the winter (December–February) for the UK were the second highest winter and for Scotland were the on make important to the of but only in those when the a of the of northern and western where the impacts of rainfall and flooding in winter 2015/2016 were many to be in the areas of catchments, to the of the rivers. below average in the early this was for by the end of November following substantial rainfall. the of was in the flood peak from the Tyne, it to that of the 2016). with this in peak flow on the Tyne, the gauging at still registered one of the highest flows recorded in England and river flows been and more The winter storms substantial impacts across northern and western Britain and Northern The period was defined by flooding with some communities being flooded were flooded in England in December with more flooding in January 2016), to in the et al., and in the winter et al., these events saw flooded than the affected during the floods, event that saw a far of flooding and A of flood events by impacts is to a number of as the of flood conditions and 2015/2016 flooding was by a of that generally from flooding in England during December 2016). While was some of their provided time for to or be the spatial scale and the duration of flooding over winter 2015/2016 in northern and western regions of the UK considerable impacts on and the time of this being the of impacts is still being – more comprehensive The focus of the flooding from Storm Desmond was in with Cumbria a major on December in response to the widespread flooding of and was the with the its in several in were from their and strong winds of Storm Desmond also impacts in were in as the and a major was in and as the flooded The of the flooding on Boxing was particularly in river draining the a major flooding affected large including and were also flooded in and including in in the flooded in the number of affected been had the not been With the passage of Storm Scotland the of the including widespread flooding in and around and The impacts in Scotland continued into with substantial flooding in on the and further flooding in the winter, flooding affected communities across northern and western Britain – notably so in some of the major across and were affected by the December storms, including the and the in and the in property flooding was also seen in and also impacts on from the winter storms – were in Cumbria were and was floodplain to and had substantial impacts on including Following Storm Desmond, were in and in Scotland during Storm some were as the at flooded to or be – of in had for several over the Christmas The of the event had significant on as and were in northern Britain and Northern of were and several notably in and both The of and in some and months to – in the Lake District the was after Storm Desmond, a were in were to to flooding and infrastructure including the north of was also major to the the Calder and and the and Calder on December. The flooding a response from the UK and Following a of were between Christmas and New to and and in across and and the time of of has been to – for those affected by Storm Desmond, to England for the and of and and for the to for affected and including to flood A provided to to by the of British that flood has in for more than properties, which are to to of 2016). The average of is around which is than previous events 2016). Of floods, the winter 2015/2016 episode had a in terms of impacts on and that any While by of infrastructure had the of these events on and communities is to be and the the winter 2015/2016 was extreme hydrological in terms of spatial and the 2015/2016 winter only years after the winter flooding of et al., is the more were the wettest two on record for the UK as a (in records from McCarthy et al., 2016). terms of these episodes in the Great Britain series (Figure these events to of flood events in the early flooding also in while less exceptional in terms of winter over shorter outflows were also and flooding was also witnessed in the of that et al., national scale flood events also since the of the (e.g. in the of and years also seen flood events that the areas of northern Britain affected in winter 2015/2016. for devastating flooding in and et al., flows for the were the highest on record for England, while the event in a 24 h rainfall record – records which were by Storm Desmond in December This of flood events has led to that flooding is increasing in the UK. While are substantial in for UK flood are for in flood in many in the winter the of et al. and The of recent are of these is a more The UK has over the and the with is The to which this has flooding of long records of rainfall and river is for in heavy rainfall in the UK over recent et al., et al., but in rainfall not to in A of has trends in river flooding in the UK the of the that is for any in long records of flood or in the UK. However, the north and the of the UK has seen in winter runoff and high river flows since the early associated with in This is particularly the with the Atlantic which in its as it was in in in the of the in used records to the early years of the some of the most severe flood events of the national scale to be but Figure that a towards increasing winter runoff has of maximum peak flow trends (in this on daily as are not widely are in Figure for the catchments as in Figure as further the exceptional of the winter 2015/2016 the significant from around the on the and the Eden (the most as as over a period for the Wharfe and catchments with in northwest While trends in high flows be it is not to the recent of in or are associated with or – as is most – a of UK river flow records are in the or The Scottish Dee is a of a long record and Figure that the is far less steep and is when to a from the previous in to to and by the and For the been in records to and to any event or of events to records is a on and that use to the of extreme events. Following the widespread flooding in autumn these events were more of on the et al., et al., et al., et al., A for the winter et al., et al., 2016) the exceptional winter rainfall was more and also of high flows and property on the to warming – although these were more A was within a week of the early December 2015 that the Storm Desmond rainfall was more as a result of warming et al., et al., However, this was on very be which on this While it is very to identify and trends in the extreme seen in the early with increasing in event the that are of warming on high flow in the UK. this a for further of the recent the range of recorded with significant for flood this and its is important for been a number of high flood events in the early years of the and, as flooding has a subject of intense public, political and media While many of these events the of to some in terms of flow spatial is to the events of winter 2015/2016. individual Storm Desmond the previous significant events of and The flooding around Boxing was the most severe on record in many major rivers of northern England. parts of Scotland saw and flooding in January. of these events was their over a period has set a new in terms of flood at in the of typical gauging records Of historical records be for a wider historical with of severe flooding (e.g. in the winter of 2015/2016 further the of the UK to persistent episodes of severe This paper has provided a of these their and their and has also considered the historical context and recent hydrological with the floods, these events were and the subject of media on flood of flood management and flood many A focus of the in winter 2015/2016 was around the of management flooding and on the of the flood A of flooding and a more debate of these is although it is that a significant the very scale given the scale and of the rainfall. previous (e.g. the of et al., et al., that use and management impacts on flooding are only at or for low events. in where even a modest in peak flow had the range of to be important for is in the and of in UK this the flooding prompted a number of major including a national 2016) that to the UK from extreme weather in The from National to for Figure and and for the flood in also to and for data and flooding The UK Measuring Authorities are for their of the the of with the flood of winter 2015/2016.