2015/02/01 by Katie Muchan, Melinda Lewis, Jamie Hannaford +1 · 3 citations
Environmental Science · #Flood Risk Assessment and Management #Hydrology and Watershed Management Studies #Hydrological Forecasting Using AI
paper · pdf · doi:10.1002/wea.2469
Throughout the winter of 2013/2014, a succession of vigorous low pressure systems crossed the UK. This resulted in the wettest winter on record for the UK (since records began in 1910), by a considerable margin, and the stormiest for the UK and Ireland (Matthews et al., 2014). The persistent heavy rainfall, combined with strong winds, high tides and storm surge conditions, severely impacted many parts of the country. The winter was distinctive for the occurrence of multiple types of flooding. Pluvial, fluvial, groundwater and coastal flooding all affected the UK, sometimes simultaneously, although the relative importance of these sources of flooding varied geographically and over the course of the winter. The winter was also notable for the exceptional duration of flooding and the high profile that the severe weather impacts commanded. From late December until late February, flooding was at the forefront of the media spotlight and received a high level of public and political attention. This paper outlines the hydrological aspects of the 2013/2014 winter flooding in the UK, as well as the impacts. The episode is considered in a long-term historical context and wider issues raised by the flood events are discussed briefly. Companion papers focus on the meteorological characteristics (Kendon and McCarthy, 2015) and coastal dimensions (Sibley et al., 2015) of the winter storms. Taken together, these papers provide a comprehensive overview of the winter weather patterns and their impacts on the UK. The National Hydrological Monitoring Programme (NHMP) forms the source of much of the material discussed in this paper. The NHMP collates data for 104 index river flow gauging stations and 37 index groundwater boreholes in the UK, and produces monthly Hydrological Summaries (http://www.ceh.ac.uk/data/nrfa/nhmp/monthlyhs.html). The data submitted to the NHMP are provisional, and therefore any records mentioned in this paper may be subject to change. In addition, this paper refers to total outflows for Great Britain and its constituent countries (as featured routinely in the monthly summaries), calculated using a method described in Marsh and Dixon (2012). At the end of November 2013, flows in most rivers across the UK were declining and runoff rates fell well below the seasonal average by month-end. Flows continued to recede in early December, with some rivers in England and Wales approaching their period-of-record minimum for the time of year. These recessions, however, were interrupted by sharp river flow responses, triggered by the onset of the storms in mid-December. As a result of intense rainfall on the 23rd, the wettest day of the month, new December daily flow maxima were registered at five index stations as a severe flood episode unfolded in parts of southeast England. Flows on the Medway (Figure 1(a)) were the highest on record (from 1957), exemplifying the rapid response of many rivers draining impermeable catchments. The unprecedented December rainfall recorded in Scotland (Kendon and McCarthy, 2015) resulted in high spate conditions across much of the country. Despite a high prevalence of fluvial and tidal flood warnings, impacts were relatively localised (e.g. evacuations in Dumfries and Galloway as the Nith recorded its highest December flow in a series from 1957). In contrast, parts of the Midlands, Yorkshire and East Anglia were substantially drier during December. Correspondingly, some rivers in these regions experienced below average flows for the time of year, with the Lud in Lincolnshire recording only a third of the average December flow. During the last week of December, as larger rivers responded to the persistent heavy rainfall, floodplain inundations became widespread and overall outflows from Great Britain approached their previous maximum for the time of year. With further rain falling on already saturated ground, flooding continued into the New Year. Several index rivers across southern England established new January runoff maxima, with remarkable increases in flows in many groundwater-fed rivers across the Chalk outcrop (e.g. Figure 1(b)). Substantial increases in river flow were also witnessed away from southern England – many rivers in central and eastern England recorded above average flows every day in January, and the Trent recorded twice the monthly average flow. Conversely, after the wettest December on record in Scotland, some rivers in the far northwest (e.g. Naver, Ewe and Carron) recorded below average flows on most days in January. Elsewhere, by late January, the exceptional accumulated rainfall totals resulted in extensive inundation of floodplains and other low-lying areas. The Somerset Levels and Moors were particularly badly affected as floodwaters inundated an estimated 6500 ha of land, with drainage hampered by high tides. By February, there had been little respite in the storms since mid-December. Several major storms early in the month brought further increases in river flow, and floodplain inundations remained widespread. The risk of flooding extended across much of the UK, and for England and Wales flood warnings totalled around 500 (embracing the severe warning, warning, and alert categories) early in the month. Flows continued to rise in the UK's larger rivers (e.g. Figure 1(c,d)): the Thames peak was the highest since November 1974, leading to major flooding in some middle and lower reaches, and flows on the Severn (at Bewdley) exceeded those recorded in the summer floods of 2007. Northern Ireland recorded twice the average February rainfall (Kendon and McCarthy, 2015) and new monthly flow records were established on five of the seven index rivers. Average flows for February were the highest on record for 18 index catchments across central and southern England, including several groundwater-fed rivers as a result of the volume of aquifer recharge. With nearly twice the average rainfall, February concluded the wettest winter on record for the UK (Kendon and McCarthy, 2015). Correspondingly, a substantial majority of index rivers, both in responsive and slowly responding catchments, eclipsed their previous winter runoff records (Figure 2). For Great Britain as a whole, the previous average winter runoff was exceeded by a considerable margin (Figure 3). Despite this sustained runoff, few significant peak flow records were attained during the winter of 2013/201411 It should be noted that this is based on the 104 index catchments. More records may have been established across the wider gauging station network. and new period-of-record maximum flows were registered at only a handful of index stations. In some responsive catchments in southeast England, the December peak flows were notable. New period-of-record maximum flows were established for the Medway in Kent and Mole in Surrey, although in the latter case the Kinnersley Manor record begins in 1972 and the 1968 flood was greater at other stations in this catchment. In permeable catchments across southern England, new maxima were registered in February, for example on the Coln, Itchen and Lambourn. The winter was more notable for the duration of high flows and floodplain inundation, demonstrated by flow patterns on the Thames, where at Kingston flows exceeded 250m3s−1 for 76 consecutive days22 The threshold of 250m3s−1 corresponds to the flow at which spilling would have occurred in the Kingston reach before the channel reprofiling that took place as a result of the 1947 flood. (Figure 1(d)), more than twice as many as any previous period in the record from 1883. The last major storm occurred in mid-February, and the following dry spell allowed river levels to recede. In the second week of March, outflows from Great Britain fell below average for the first time since the onset of the storms in mid-December, bringing an end to the immediate risk of flooding in many areas. A continuation of seasonally exceptional flows in many spring-fed streams across southern England, however, meant that some flood alerts on rivers draining Chalk catchments were to remain in place until the early summer. At the end of November 2013, groundwater levels across the country were generally in the normal range, with levels still declining in some boreholes in Yorkshire and parts of the Chilterns. A month later, however, steep recoveries had been triggered in southern England. Rises of over 25 m were recorded in the Chalk index wells at West Woodyates Manor (Dorset) and Chilgrove House (West Sussex) during December, and some groundwater flood alerts were issued on Boxing Day. In contrast, levels remained low or close to the seasonal average in the slower responding Chalk in eastern England. This contrast continued into January, where levels in the Chalk of East Anglia, Lincolnshire and Yorkshire remained in the normal range or slightly below average (e.g. Figure 4(a)), while six index boreholes in the southern Chalk recorded new monthly maxima (Figure 5). The Chilgrove House borehole overflowed (one of around six similar artesian episodes in a record dating back to 1836), and Tilshead (Wiltshire) was also artesian in January and February. Groundwater also emerged above the ground in bournes, e.g. South Winterbourne (Dorset), Aldbourne (Berkshire), Caterham (Surrey) and Nailbourne (Kent). In February, levels fell at all six of the Chalk index boreholes that registered new maxima in the previous month. However, three of these (plus an additional borehole in Kent) again recorded monthly maxima in February. After two months of contrasting behaviour, levels in the slower responding eastern Chalk began to rise steeply in February, although they still remained at or near average. Further north, where the rainfall had not been exceptional, levels also started to rise, although due to the notably low starting point at Dalton Holme (East Yorkshire), levels were below average throughout the winter and remained so until July 2014 (Figure 4(a)). Into the spring, levels fell by more than 20 m from February to April at West Woodyates Manor, Compton House and Chilgrove House (Figure 4(b)). In contrast, water levels peaked from mid-March onwards in the Chilterns, and remained exceptionally high in the North Downs. By the end of April, levels in the Chalk were consistently falling from Dorset to Kent, and had mainly returned to within the normal range. Although the falling levels meant that by the end of April most of the flood alerts had been removed, areas of Buckinghamshire, the Berkshire–Oxfordshire border and Hampshire were still affected by groundwater flooding. Here, groundwater flood alerts remained in place until late May or early June; however with much of the water still to drain away, sewers continued to surcharge, with a few basements still being pumped and some minor roads remaining submerged in mid-July. Although the majority of high groundwater levels during the winter of 2013/2014 were recorded in the Chalk, other aquifers in the UK also experienced periods of notably high water levels. In the Permo-Triassic sandstones, levels became exceptionally high in December/January in southwest and northwest England and were still high at the end of August 2014. Record monthly maxima were established in the northwest at Skirwith (Cumbria) for 8 consecutive months (January–August 2014) and Newbridge (Dumfries and Galloway) for 6 (January–June 2014). No groundwater flooding was reported however, due to the high storage coefficient of the aquifer and hence small overall rises in water level – Nuttalls Farm (West Midlands) rose by only 1.1 m between the end of January and the end of June, but for the last four of these months, levels were exceptionally high. New monthly maximum levels were recorded in the Upper Greensand at Lime Kiln Way (Somerset) for a sustained period (February–September 2014) and in the Lower Greensand, between Haslemere and Godalming (Surrey), groundwater flood alerts were issued on 9 January and remained in force until 12 March; however little flooding occurred. Some flooding occurred from the Jurassic (Great Oolite) limestones of the Cotswolds (e.g. Barnsley, Gloucestershire and Blenheim Park, Oxfordshire) and exceptional levels were recorded in January in the Ampney Crucis borehole (Gloucestershire). However, any groundwater flooding was of short duration and affected only a small number of properties. In the responsive Carboniferous limestone, levels at Alstonefield (Peak District) were exceptional in January in response to the persistent rainfall. Levels in the Magnesian Limestone of northeast England remained high throughout 2013, a legacy of the exceptional 2012 rainfall and therefore the 2013/2014 winter recharge started from a high baseline. As a result, levels were above average for the time of year throughout the winter at Swan House (Durham), reaching exceptionally high levels in June and July 2014. A defining characteristic of the winter was the spatial extent and persistence of floodplain inundations, rather than the magnitude of peak river levels. Accordingly, this shaped the nature of the flood impacts. In total, more than 7000 properties were flooded during the winter (Environment Agency, 2014), comparable with the 2012 floods (Parry et al., 2013) but significantly less than the 55 000 impacted properties during the summer 2007 floods (Marsh and Hannaford, 2008). Comparisons of flood events based on impacts are made problematic by a number of factors, including the role of flood defences, but broadly the 2013/2014 floods were not outside the range of recent nationally significant flood events in terms of properties flooded. However, the prolonged nature of the flooding ensured that its wider impacts were substantial. A particular focus of flood impacts throughout the winter was the Somerset Levels and Moors, where a major incident33 A major incident is declared where the situation could not be dealt with by the local council and could threaten lives, disrupt the community or damage property. The local authority can organise emergency evacuations, set up rest centres and mobilise voluntary organisations. http://www.bbc.co.uk/news/uk-england-somerset-25876309 was declared on the 25 January following several weeks of inundation. The prolonged flooding some for flooded to the and a major emergency including to away the By early February, were of flood water of the the UK was much media and political around the of channel and in the of the a to 8 of the and began (Environment Agency, 2014). England was of the winter in Surrey, Kent and Dorset the of flooding around and New Year. In early February, the focus of the flooding to the lower Thames, in widespread in the Thames and significant flooding of properties in areas as and in central and southern England, although flood warnings were widespread and major were to river flooding had relatively the major from risk the Agency, emergency and the were in to floodwaters and provide and to those With the prolonged flooding other impacts of the severe storms (e.g. widespread and the situation became as a major at the highest level of the and were for emergency flood as well as long-term and was from with in the 2014 2014). was by a of that flood across the UK (e.g. the Thames and It was estimated that properties were the although this coastal areas from the early December storm surge (Environment Agency, 2014). It was that the of fluvial as the prolonged inundation to land, winter and evacuations of 000 ha of were inundated in a week in February, most significantly across the Somerset Levels and Moors, extensive of the Thames and Severn floodplains and in the southern of England 2014). Although the of the winter floods to to be have from the and other to with the and with the flooding 2014). The nature of the fluvial wider impacts from the to particularly severe impacts on the network. The flooding before major across much of the country during of the periods of the year. In early January, major roads were in every of southeast England and South was with and England was from the rest of the UK network. from to were by inundation of and across the Somerset Levels and Moors the and at the the which the only into was away by coastal in (Sibley et al., 2015). The impacts of fluvial flooding throughout the winter of 2013/2014 were further in some areas by high groundwater levels. In the Thames (e.g. and the Thames below the river were saturated by late January, with high water levels drainage back into the river and in flooding that much than that from the river In for flooding from the river started and continued for a few weeks the fluvial flooding. Groundwater flooding also had impacts on many (e.g. West in and in and some areas (e.g. in parts of and in The duration of the flooding was a major – some were to for many The high levels also sewers (e.g. in the and Chalk in and the Jurassic limestones in and resulted in floodwaters (e.g. at in The high groundwater levels also brought of to the (e.g. at in Oxfordshire) and that had remained in the for The winter also an in the of as the in the first in this for and reported across England in February. These where in as or or e.g. due to at are by material that is away by the in rainfall and The intense heavy rainfall also on both and as with reported in Great Britain between December and February 2014 (Figure The 2013/2014 winter floods only a year after the last major flood of and widespread severe flooding was in which was all the more remarkable the (Parry et al., flooding occurred the but winter flooding was also particularly in November and December. In with winter 2013/2014, the of 2012 was notable for the persistence of high flows and the spatial extent of floodplain inundation, rather than flood In terms of England and Wales winter was more severe over (e.g. winter groundwater levels were also in across the major aquifer with new maxima recorded in several Chalk boreholes in the winter and Permo-Triassic boreholes from winter to the following summer. In terms of duration and extent of high flows and groundwater the only close is (Marsh and This was a winter with widespread and fluvial flooding as well as unprecedented groundwater flooding. The period of high runoff across England and Wales was more in with exceptional fluvial flooding in early and groundwater flooding in a few well into the following summer. The winter months of 2013/2014 were much however, and over the outflows from Great Britain were (Figure 3). winter 2013/2014 a greater of severe storms into a time period and within that period flows remained particularly in major rivers, permeable catchments and low-lying However, several fluvial flood events in were more severe and river flooding was more with maximum records established from to Yorkshire (Marsh and the 2013/2014 fluvial floods in a much Figure the water year maximum flow record for the Thames, the flow series on the UK National and a for the winter Although the winter 2013/2014 maximum flow was the highest since 1974, and than the winter flooding of there were larger events in the with 1947 and being substantially of these events were with as flooding and The also in there is of a flood in levels have (Marsh and In other major rivers with records in England (e.g. the and the peak flows of 2013/2014 not the events on and were significantly less severe than other flood as 2007 and In terms of with maximum levels for 2013/2014 were generally not as high (Figure and groundwater flooding was less In the and however, levels occurred in 2013/2014, groundwater flooding to in areas not affected in and hence outside areas where flood alert systems (e.g. At Compton of the groundwater level records in the UK, the peak water level was the second highest since records began in but m lower than in (Figure all five of the highest levels recorded have been since with the three highest being in and At Chilgrove House the record back to 1836), however, the only with artesian flow recorded before January was December This exceptional high water levels have occurred in the but more than In other where the period-of-record is significantly new record levels in the Upper Greensand Kiln record from were recorded for 8 consecutive months in In the Permo-Triassic of southwest and northwest England and southwest Scotland, levels started above average due to the rainfall, but remained exceptionally high for more than 6 months at Skirwith and Newbridge from and In both the exceptionally high levels for a duration in 2013/2014 than in although is by levels being estimated for some months in the period due to The 2013/2014 winter was a period of severe storms that few (Matthews et al., and McCarthy, 2015). The flood response was remarkable – high flows and groundwater levels were extensive and with a range of severe impacts. Although the winter was exceptional in terms of the total rainfall and runoff the fluvial flooding events were generally not in terms of magnitude a few notable in the Thames and rivers in southern and groundwater flooding was widespread but not as severe as in A defining of the winter was the occurrence of multiple types of flooding. The of fluvial and groundwater flooding in winter is not but its and the winter of 2013/2014 was The occurrence of multiple types of flooding and other weather a major for the emergency The between flood were also – fluvial flooding in the lower of major rivers was made by where fluvial flows are from an by high groundwater flooding from the river in major river the The weather patterns that to heavy rainfall and high can also to storm and previous on the of these (e.g. and The winter 2013/2014 the occurrence of multiple types of flooding in 2013/2014 eclipsed all other recent flood events in terms of the media and political As with other recent occurred in an of media and Although the floods severe impacts in some the damage was relatively The (e.g. of the Somerset Levels and however, of the of the particularly of from high and coastal flooding. The events occurred a of intense political around flood as of a wider public As would be not all of the was or but in to an around flood risk including river and coastal As with other weather events of the recent a focus of public The to which the hydrological of the early of the is to is a focus of is not to but and are et al., 2014). a the events of winter 2013/2014 are with of in and risk due to levels. The of of heavy rainfall are from winter 2013/2014, and previous events as 2012 and flood maxima not in duration or of heavy winter rainfall events could have major impacts on and would the risk of groundwater flooding. the early of the major and seasonal patterns – most with persistent in the and of the which may be by a range of is a to the relative of these in hydrological et al., 2014). The from National to Marsh and for on of the paper. also to and for fluvial flooding groundwater flooding and The UK are for their of the the of with the flood situation of winter