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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