Select Committee on Environment, Food and Rural Affairs Minutes of Evidence


Memorandum submitted by the Natural Environment Research Council (NERC) (FL 112)

  1.  The Natural Environment Research Council (NERC) is one of the UK's seven Research Councils. It funds and carries out impartial scientific research in the sciences of the environment. NERC trains the next generation of independent environmental scientists. Its three strategic research priority areas are: Earth's life-support systems, climate change, and sustainable economies.

  2.  Details of NERC's Research and Collaborative Centres are available at www.nerc.ac.uk.

  3.  NERC's comments are based on input from the British Geological Survey (BGS), the Centre for Ecology and Hydrology (CEH), the National Centre for Atmospheric Science (NCAS), the Proudman Oceanographic Laboratory, and Swindon Office staff.

INTRODUCTION AND SUMMARY

  4.  NERC welcomes the opportunity to contribute to the Committee's inquiry into the issues raised by this summer's flooding in England and Wales, and what steps public authorities should take to address them. NERC and its Research and Collaborative Centres endeavour to ensure that their research findings reach potential users to facilitate appropriate decision-making, and the area of environmental hazards is a particular concern.

  5.  Several NERC-funded scientists have been examining the factors involved in the recent flooding, particularly scientists at BGS and CEH, and other scientists involved in NERC's Flood Risk from Extreme Events (FREE) programme[1]. NERC issued a briefing note on 27 July summarising its contribution to flooding research in general and to immediate analysis of the summer's events[2].

  6.  The first section of this memorandum covers BGS's and CEH's analyses of those events.

  7.  The work conducted by BGS is described, supported by Annexes 1-7. The response of BGS to the 2007 flooding events represents one part of its long-term surveying and monitoring role. This has involved the production of detailed geological and geohazard map and Geographic Information System (GIS) series for the whole of the UK, with two sets of information especially relevant to flooding issues: "Geological Indicators of Flooding" (Annex 1) and "Groundwater Flooding Susceptibility" (Annex 2).

  8.  CEH conducted a broad-scale appraisal of the floods. The text is available on the CEH website, but it is reproduced below because of its high relevance to the inquiry (the web address is provided for publication purposes).

  9.  The second section of this memorandum outlines research that BGS, CEH and others pursue or intend to pursue that will help to improve our understanding and prediction of future flooding events and our ability to quantify risk.

  10.  Annex 7 contains further details of BGS's research into the processes involved in groundwater flooding, and some information about the NCAS Weather programme. It also contains specific contact details for BGS and NCAS scientists.

SECTION 1: ANALYSIS OF THE 2007 FLOODING EVENTS

Activities and observations of the British Geological Survey (BGS)

BGS response to the June/July 2007 flooding

  11.  The BGS Geohazard Rapid Response Team conducted aerial photographic surveys that acquired more than 500 photo images and 87 minutes of video footage. On June 26th, the team recorded the extensive fluvial (river) flooding along the valleys of the Derwent, Don (Sheffield) and Rother (Catcliffe-Bentley); smaller catchments, in Nottinghamshire that had experienced sudden rapid flooding ("flash-flooding") were included. A photographic record was also made of the devastation in Hull, caused by flooding from land (pluvial flooding) unable to cope with the large volume of rainwater. A second flight, on July 24th, monitored the extent of fluvial flooding in the Cherwell and Thames rivers (Banbury, Oxford, Abingdon areas), and the Severn and Avon rivers (Gloucester, Tewkesbury, Worcester, Stratford and Evesham). Ground surveys of flooded areas were also conducted, around Oxford (see below), and in Nottinghamshire.

  12.  The flood extents recorded by aerial survey will be digitised and compared with the Geological Indicators of Flooding dataset. Preliminary results show that in the majority of cases, the June—July inundation limits correspond well with the distribution of "natural" floodplains, as defined geologically and depicted in the dataset (see Annex 1). The digitised flood extents will be incorporated within a sub-project of NERC's FREE programme, providing a record that can be used for future flood modelling.

  13.  Oxford was one of the urban areas most severely affected by flooding in southern Britain. In the few days prior to the peak of the recent floods the existing network of automatic water level recorders measuring on a 15-minute interval (see Annex 7) was expanded by BGS to monitor other key flooding locations. In addition to the collection of these data, BGS undertook walking surveys of the floodplain areas of the city to make observations of the flooding processes. These observations and the data collected will be collated, written up and incorporated into the overall documentation on the floods in Oxford being prepared by the Environment Agency.

BGS assessment of the issues raised

  14.  Important insights were obtained in Oxford about the complex interaction of river overbank flooding, groundwater flooding and the role played by subsurface drainage. Observations have confirmed that a significant number of properties in Oxford were flooded by groundwater alone. This occurred due to the emergence of groundwater above ground level but also due to the inundation of basements. Where both groundwater and overbank flooding occurred, groundwater flooding significantly extended the overall period of flooding. It is clear from subsequent discussions with the Agency's Oxford Flood Risk Management Team that understanding these interactions is key to developing effective risk management measures.

  15.  Some of the information provided to local residents in Oxford about the timing and nature of the flooding was incorrect and confusing and resulted in unnecessary distress. The inability of the Environment Agency to accurately predict the timing of flood peaks shows that significant work is required to be able to understand and adequately model flood events. This should include, where appropriate, incorporating the groundwater dimension into flood prediction models.

  16.  Further evidence bearing on the "lessons learned" debate is provided by studying geological maps showing the natural extent of floodplains (ie before human modification) in relation to building, construction and infrastructural development placed on them over the past decades and centuries. Such developments are potentially at risk from flooding, and examples from the 2007 summer floods included the damage to industrial and commercial properties sustained along the River Don floodplain (e.g at Meadowhall, Sheffield, Annex 3). Similarly, the floodplain of the River Rother is now extensively covered by raised-up made ground, possibly exacerbating flooding at places such as Catcliffe (Annex 4). In southern England, the function of vital installations such as the Walham substation and Mythe water treatment plant (Annex 5) was compromised by their location on the natural course of the Severn floodplain.

BGS provision of flooding-relevant national geological data

  17.  The two sets of data referred to earlier: "Geological Indicators of Flooding" and "Groundwater Flooding Susceptibility", now form part of the Survey's digital geohazard information system. The Geological Indicators data set (Annex 1) shows the extent of natural floodplains in the UK and is therefore considered to complement (though not to replace) the flood maps produced by the Environment Agency. It has been shown to the EA (Eastern Area), who have found it to both justify, and refine, their flood outlines in places where ambiguity formerly existed. More such meetings are planned, at local and national EA level, and it is possible that the Geological Indicators dataset will eventually be incorporated into the EA flood mapping programme.

  18.  Geological data can reveal the vulnerability of areas that may not be included in conventional flood risk mapping. For example, in parts of the UK settlements are developed on relatively higher ground formed by alluvial fans. These features lie off the floodplains, but as they may be prone to debris flow and flash-flooding, they have been incorporated into the "Geological Indicators" data set.

  19.  Research at BGS has involved the construction of 3-dimensional models of floodplains and estuaries; for example, showing the relationship between different types of substrate and shallow water table rest levels in the Manchester area (Annex 6 and 7). These methodologies are likely to be prime tools for resolving issues such as "sewage flooding", and the sustainability of urban drainage systems during severe rainfall events of the type recently experienced in Hull and parts of London.

  20.  In areas such as Toll Bar, near Doncaster, there has been a history of mining-induced subsidence that may, in places, have exacerbated the extent and duration of flooding. Such issues could be clarified by satellite ground movement surveys (radar interferometry technique), of the type undertaken by BGS across London and other parts of the UK.

  21.  BGS has developed groundwater flood susceptibility maps for the UK based on process-based models of groundwater flooding. These currently include models of typical "Chalk valley" type groundwater flooding and groundwater flooding in alluvial environments. The susceptibility map is produced using the permeability characteristics of mapped geology (at 1:50,000 scale) and groundwater level data. It will be possible to upgrade this map on the basis of observations made during the recent floods. BGS plans to continue to refine these process-based maps by including additional groundwater flooding scenarios and to improve the base data upon which the maps are produced. Links between these maps and those required by the Environment Agency for flood risk management are being explored.

  22.  In addition to the groundwater flood susceptibility maps, BGS continues to undertake process-based research into groundwater flooding, as detailed below.

CEH appraisal of the July 2007 floods in England and Wales [available on the CEH website at http://www.ceh.ac.uk/news/BriefingnoteJuly2007Floods.html]

  23.  The weather conditions experienced across much of the UK throughout the summer of 2007 have been exceptional. The jet stream (which influences the paths taken by weather systems in the North Atlantic) has followed an abnormally southerly track and the extension of the Azores high pressure cell across the UK—which brings settled weather conditions in most summers—has failed to become established. Correspondingly, a sustained sequence of rain-bearing low pressure systems has produced outstanding 12-week rainfall totals, and a series of flood events culminating in widespread severe flooding in late July.

Rainfall

  24.  The combined May and June rainfall total is the highest on record for the UK (in a series from 1914) by a considerable margin and the exceptional weather conditions continued into July. Provisional data indicate that the May-July period is likely to have been the wettest for England and Wales in a series from 1766 with many areas registering more than twice the long term average.

  25.  As warm and very moist air moved north from France, the volatile July weather patterns culminated in an extremely wet episode on 19-20 July. Outstanding storm rainfall totals were reported across much of southern Britain. These included:

    145 mm in Pershore (Hereford and Worcestershire)

    111 mm at Chieveley (Berkshire)

    c120 mm at Brize Norton (Oxfordshire).

  26.  Statistical analyses confirm the extreme nature of such storms—on the basis of historical data they would be expected to occur, on average, only once in several hundred years (longer in the case of the Pershore event). Thundery interludes contributed to substantial spatial and temporal variations in rainfall intensity but catchment rainfall totals were exceptional over wide areas. Localised storms of tropical intensity are a feature of many English summers but a distinguishing characteristic of the July 2007 storms was the spatial extent of the extreme rainfall totals.

Characteristics of the flooding

  27.  Normally, flood risk during the summer is substantially diminished by dry soil conditions. Following the record late spring and early summer rainfall, accompanied by widespread flooding in June, soils were close to their wettest on record (for mid-summer) in early July across much of England. This rare circumstance left many catchments vulnerable to further significant rainfall.

  28.  The exceptional rainfall on 19-20 July triggered a sequence of relatively distinctive flood episodes: localised (mostly urban) flash floods, extremely high flows in small responsive (impermeable) catchments and, subsequently, extensive floodplain inundations as the runoff concentrated in the major rivers of southern Britain (including the Severn, Warwickshire Avon, Bedford Ouse, Trent and Thames).

  29.  Initially, the intense rainfall overwhelmed many urban drainage systems producing localised but severe flash floods. The emergency services were widely deployed to rescue stranded individuals and organise evacuations from the most severely affected localities. These contributed to massive and extensive transport disruption across southern Britain, exacerbated by the volume of holiday traffic. Subsequently, floodplain inundations caused extensive crop damage and the need to move livestock to higher ground. The sustained high levels in the major rivers also hampered the drainage of flood waters away from the urban areas inundated on 19 July.

River flows

  30.  Preliminary data suggest that a significant minority of rivers across southern Britain exceeded their previous maximum recorded flow and many eclipsed previous maxima for the summer half-year (April-September)—often by very wide margins. In the worst affected areas (e.g. in the lower parts of the Severn and Warwickshire Avon basins and some upper reaches of the Thames catchment), flood flows may have exceeded those of March 1947—the most severe flood in southern Britain in over 100 years (note however that the 1947 event was primarily the result of rapid snowmelt over still-frozen ground and its overall impact was substantially more severe than the current flooding).

An historical perspective on the July floods

  31.  An indication of the rarity of the hydrological conditions experienced this summer is provided by the recent increases in groundwater levels in some parts of eastern and southern England. Generally, groundwater levels decline over the May to September period, due to an absence of natural replenishment (recharge). This summer, groundwater levels in the Cotswolds rose rapidly in mid-July and by 24 July stood above normal winter levels; this is reflected in the exceptionally high recent flows reported for many spring-fed streams. In the 19th century, significant summer recharge was recorded in a number of years (eg 1860 and 1879) but examples of significant and widespread summer recharge in the 20th century are very rare.

  32.  Episodes of extensive summer flooding may be found in the historical record (eg in 1875)—particularly in the nineteenth century when summer half-year (May-October) rainfall often exceeded that for the winter half-year. There are, however, no close modern parallels to the scale of the summer flooding experienced this year. It has served to underline our continuing vulnerability to very exceptional summer rainfall and to, as yet poorly understood, changes in the position of the jet stream.

Outlook

  33.  In the medium term (2-4 months) it is likely that soil conditions will remain wetter than the seasonal norm. This will encourage an early onset of the seasonal recovery in river flows and groundwater levels in the autumn. Correspondingly, an extended flood season throughout the autumn and winter of 2007-08 may be expected.

How influential is climate change?

  34.  By their nature, individual extreme flood events cannot be linked directly to climate change. If they form part of a developing pattern or emerging trend, then a causative association becomes more plausible. In England and Wales, evidence for long term increases in fluvial flood magnitude is elusive—one factor here is that in a warming world snowmelt (a primary cause of the 1947 flood) is very likely to decline as an exacerbating factor as temperatures increase. Warmer, drier summers would also tend to produce very dry soil conditions which, unlike this year, should help moderate fluvial flood risk. On the other hand, more intense summer rainfall would increase the risk of localised flash flooding—with associated drainage problems, particularly in urban areas.

SECTION 2: ONGOING AND FUTURE FLOODING-RELEVANT RESEARCH

BGS

  35.  Further upgrades will be made to the Geological Indicators and Groundwater Flooding Susceptibility data sets as increasingly accurate data become available through the BGS survey and monitoring functions. The upgrades will also take account of the examples provided by flooding events as they occur. Further projects examining groundwater flooding processes in a range of hydrogeological settings will contribute to the national flood risk management programme. The expertise gained from this work, and also from studies into the sustainability of urban drainage systems, are directly applicable to issues such as pluvial flooding of the type experienced in Hull and London.

CEH

  36.  Hydrological research is carried out at CEH on a continuous basis: the organisation has extensive and long-term experience of processes of inland flood generation, the development of quantitative methods and modelling systems to predict river flows, and analysis of how any current conditions compare with the historical record.

  37.  Major aspects of research at CEH which have been initiated and which it is CEH's intention to pursue include:-.

    —  quantification of the separate effects on inland flood generation of land use practices and climate drivers, in both rural and urban river catchments

    —  analysis of inland flood risk from river, pluvial and groundwater sources on a true spatial basis (that is, extending beyond the quantification of arrays of point risks)

    —  quantification of the relationship between magnitudes of river, pluvial and groundwater flooding and their frequencies of occurrence for very extreme events (in excess of 1000 years average return period)

    —  quantification of inland flood risk under transient conditions of changing natural and man-induced conditions

    —  extending beyond the quantification of uncertainties in flood generation systems to their reduction in flood management.

POL

  38.  As mentioned in NERC's briefing note of 27 July, POL is collaborating with the Met Office Hadley Centre in a project that forms part of the Environment Agency's Thames Estuary 2100 programme. Using computer models the project will address the issue of how to protect the Thames from flooding this century by indicating whether or not extreme water levels will change in the future.

  39.  This summer's flooding events did not have a (significant) coastal element, but on occasion, fluvial flooding of low-lying areas can be made worse if outflow to the sea is impeded, at high tide or by sea-level raised above the predicted tide by low atmospheric pressure and/or strong winds, and POL will be able to contribute to predictive analysis in this area.

NCAS

  40.  NCAS funds a Weather research programme which contributes to improved weather prediction, particularly concerning small-scale processes and severe weather (e.g. flash floods, thunderstorms). It is also used to better represent small-scale processes in global climate models. Further details appear in Annex 7.

Flood Risk from Extreme Events (FREE)[3]

  41.  NERC's FREE programme aims to improve the prediction of pluvial, river and coastal flooding occurring as a result of extreme events. BGS, CEH and POL, as well as a number of university-based researchers, have all received funding through the programme.

  42.  A dedicated fund allows the programme to support research in the event of an extreme event during the lifetime of the programme. The Steering Committee has decided to use the fund to support a project gathering and processing data from the summer's flooding events, with the aim of making the data freely available and producing a report regarding the lessons learnt.

  43.  The report may address issues such as the management of risk through joined-up modelling, the importance of land use in rural areas, and the need to reconsider infrastructure-design-specifications in the context of a changing climate.

Flood Risk Management Research Consortium (FRMRC)[4]

  44.  NERC is a member of the FRMRC Consortium led by the Engineering and Physical Sciences Research Council (EPSRC). The research priorities of the Consortium are coincident with many of the issues raised by the summer's flooding events.

Natural Environment Research Council

September 2007



1   http://www.nerc.ac.uk/research/programmes/free/ Back

2   http://www.nerc.ac.uk/press/briefings/2007/14-floods.asp Back

3   http://www.nerc.ac.uk/research/programmes/free/ Back

4   http://www.floodrisk.org.uk/ Back


 
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