Memorandum submitted by Professor Howard
Wheater (FL 119)
SOME OBSERVATIONS ON UK FLOODING
EXECUTIVE SUMMARY
Is UK flood hazard increasing?
— urbanisation increases flood risk,
but the problem is well known and mitigation measures are normally
put in place
— agricultural intensification increases
flood risk at local scale—effects at larger catchment scales
are not yet known
— land use effects have greatest impact
for frequent floods
— there is potential to mitigate flood
risk by changing land management and the return of floodplains
to washland; the magnitude of effects is uncertain, but research
to quantify effects is ongoing
— climate change is expected to increase
the intensity of storms and to increase flood risk, but quantitative
estimates are highly uncertain
— there is currently no guidance concerning
climate change impacts on extreme events such as the Boscastle
storm.
Does the UK have the policy, resources and technical
tools in place to manage flood risk effectively?
— DEFRA's "Making Space for Water"
provides a far-sighted basis for flood management policy, but
tools for implementation are not yet available
— Responsibilities for flood management
in urban areas are unclear. Technical advances will shortly provide
new tools for urban flood design
— There is legacy of inappropriate
floodplain development that requires policy attention
— Technical developments are needed
for groundwater flooding, but research is in hand. Other technical
needs were discussed above and include modelling tools for land
use management. There is a particular need to represent and reduce
uncertainty in estimates of climate change, and to provide some
guidance on climate change effects for high return period extreme
floods
— Human resources are limited by a
lack of numerate graduates
— If improved standards of flood protection
are to be implemented in a reasonable period of time, substantial
additional financial resources will be required.
MEMORANDUM
1. Floods will always occur and are part
of the natural functioning of river systems. Floods cannot be
prevented, but people and infrastructure can be protected. Key
strategic questions are:
a. What level of flood hazard is acceptable?
b. Is flood hazard increasing?
c. Does the UK have the policy, resources
and technical tools in place to manage flood risk effectively?
2. This note attempts to provide an overview
of the UK situation and a personal perspective. I would be happy
to provide amplification and further information as appropriate.
3. What level of flood hazard is acceptable?
4. The term "return period" is
commonly used to describe flood frequency, and is easily misinterpreted.
A return period of 10 years means that such a flood will occur
on average once every 10 years, or more usefully, that there is
a 1 in 10 chance of such a flood occurring in any given year.
Thus a 100 year flood has a 1 in 100 chance each year of occurrence.
The cumulative risk over a period of time can easily be calculated;
for example, over a 70 year period (a human lifetime, or a design
lifetime of a structure or facility, perhaps), there is a roughly
50:50 chance of a 100 year flood occurring.
5. Floods occur across the full range of
frequencies. The Boscastle flood (August 2004) was associated
with an extreme rainfall (181 mm in 5 hours), estimated to have
a return period of the order of 1000 years or more. It is inconceivable
that protection could be provided nation-wide for such an extreme
event—the costs would be huge, and the landscape altered
unacceptably. The Carlisle flood (January 2005) is a different
story. Damage was estimated at £450 million, and 2 lives
lost. The flood had an estimated return period of 150 years, whereas
the defences were designed for levels of protection from 20 years
to 70 years. This raises the question of what is an acceptable
level of risk for an urban area, which is essentially a political
question. Conventional practice in the funding of flood protection
schemes weighs the costs of protection against the economic benefits
from preventing flooding for a range of frequencies. This ignores
wider issues of social equity (protecting the rich is economically
more justified than protecting the poor), as has recently been
recognised by DEFRA, and a target level of the 1 in 100 year flood
has been identified. However, very few UK towns and cities achieve
this level of protection. For example the Jubilee River, recently
built to protect Maidenhead from Thames floods, was designed for
an approximately 1 in 60 year event (similar to the 1947 Thames
flood). Clearly where major assets are at risk, higher levels
will be appropriate, but is 1 in 100 years (with a roughly 50/50
lifetime chance) an appropriate target level of risk as a baseline
for our towns and cities?
6. There are two important footnotes to
the above discussion. One is that any level of flood protection
has a finite risk of being exceeded, and there are important issues
of (lack of) public awareness of that risk. The second is that,
for certain facilities, lifetime risk is important. In a recent
case, consultants unthinkingly suggested a 1 in 100 year level
of flood protection for an important facility with a lifetime
of 70 years. As noted earlier, that gives a 50/50 chance of flooding—which
in that case was completely unacceptable. Where lifetime risks
are important, much rarer floods must be considered.
7. A final point concerns extreme flood
hazard. Where facilities such as reservoirs have significant risks
associated with potential dam failure, such as substantial loss
of life, under UK practice rare extreme events are considered,
for example the 1 in 10,000 year flood, and the Probable Maximum
Flood (the largest flood that is considered possible). I return
to this point, below.
8. Is UK flood hazard increasing?
9. Flood risk is generally considered to
be a product of a) flood hazard (the chance of flooding) and b)
the consequences of flooding. Nationally (and globally) flood
risk is increasing, as more properties are built in vulnerable
areas, and as the value of properties and infrastructure at risk
increases with economic growth. Here I consider just flood hazard.
A brief discussion follows—more detail can be found in Wheater
(2006).
10. Land use change
11. Man's activities continuously change
the environment, and hence flood risk. New build replaces vegetated
sites with impermeable surfaces—roofs, roads, car parks,
etc. This changes the runoff—more surface runoff, less soil
storage, with runoff usually collected in storm drains and channelled
rapidly to watercourses. This increases river flood peaks and
may reduce low flows. It may also change flood seasonality—rural
streams mainly flood in winter, urban areas generate high runoff
from intense summer storms. These effects are well known and hence
urban development usually comes with a requirement to provide
temporary storage (for example in detention reservoirs) to mitigate
these effects. There is also interest in SUDS—sustainable
urban drainage systems—where for example soakaways, permeable
pavements and in-pipe storage can be used to reduce the rate of
runoff. Confused responsibilities for flood management and maintenance
in urban areas have inhibited uptake of these methods in England
and Wales. Nevertheless, effects of urbanisation are well known
and design solutions are commonly provided to mitigate the effects.
(Flooding within urban areas is of increasing concern, not least
to the water utilities responsible for sewerage in England and
Wales and the insurance industry. Flooding may occur due to intense
rainfall within an urban area overloading the drainage systems
(sometimes called pluvial flooding), due to a failure in the sewer
system, or due to interactions with river or coastal flooding.
I return to this issue below.)
12. While urban development is a clear and
dramatic example of changes to the environment, other changes
are more subtle. There has in recent years been much concern about
the effects of agricultural intensification. Changing arable land
management practices include changing cropping patterns (with
increased working of bare soils in Autumn and Winter) and the
use of contractors (with increased size and weight of machinery,
and constraints on time available for land access). There is evidence
of associated degradation of soil structure and mainly anecdotal
evidence of associated "muddy floods". In the uplands,
there has also been intensification. In Wales, for example, sheep
numbers increased by a factor of 6 from the 1970s to the 1990s,
and changing breeds led in some cases to a doubling of weight
per animal. The DEFRA/EA Flood and Coastal Defence R&D programme
recently funded a definitive review (project FD2114) which concluded
that there has been a lack of hard evidence on the local scale
effects, and a lack of methods to predict with reasonable confidence
the effects at the scale of river systems. Under the Flood Risk
Management Research Consortium (FRMRC), led by the Engineering
and Physical Sciences Research Council, but co-funded by DEFRA,
the EA, the Scottish Executive and the Northern Ireland Rivers
Authority, a research programme was established in 2004 to develop
the science base in this area. Current results suggest that agricultural
intensification has led to locally-increased flood risk and demonstrate
that changes to upland land management practices can reduce flood
runoff, at least at local scale (individual fields up to 20km2
river catchments). Effects at larger scale are the subject of
continuing studies, funded variously by DEFRA/EA, the FRMRC and
NERC'S FREE (Flood Risk from Extreme Events) programme.
13. For both urban development and agricultural
intensification, impacts are greatest for frequent flood events.
As the severity of the rainfall increases, so the relative effects
of the land use change decrease. For urban areas, it is speculated
in UK practice that impacts are minimal for the 500 year event.
Research to quantify rural effects is yet to report.
14. The previous discussion in this section
has focussed on runoff generation. River flows are routed downstream
in river channels and, under flood conditions, in the associated
flood plain. Flood protection works often have the effect of protecting
flood plain areas from flooding—either to protect properties
built on the floodplain, or to allow agricultural development
in the floodplain. This involves disconnecting the river channel
from the natural floodplain storage, so that the natural storage
and attenuation is lost. This has the effect of transmitting increased
flood peaks downstream. This issue is of major concern in Europe,
for rivers such as the Rhine. Flood risk for downstream towns
and cities on the Rhine has significantly increased as a result
of river engineering and floodplain disconnection and efforts
are being made to re-establish some floodplain storage. In the
UK, creation of washland storage may be considered an option in
the design of flood relief schemes, and there is considerable
interest in the potential use of floodplain agricultural land
to provide flood storage, at the expense of a loss in agricultural
flood protection.
15. Climate Change
16. A detailed discussion of climate change
and flooding issues can be found in the 2002 Proceedings of the
Royal Society (Phil. Trans. R. Soc. Lond. A, Vol 360, 2002).
17. Scenarios of climate change for the
UK, developed from Global Climate Models and embedded Regional
Climate Models, suggest a change in the South to warmer, wetter
winters and hotter drier summers, and in the North, to wetter
summers and winters. It is important to note that Global Climate
Models are impressive in explaining global temperature change,
but are poor at representing rainfall—for example they fail
to capture the correct daily cycle of rainfall in the tropics.
And as the scale of interpretation is reduced, the uncertainty
in rainfall estimates increases. It is recognised that there are
large differences in quantitative estimates of response to emissions
scenarios between models, and even between the same model, with
different initial conditions. Hence while there is reasonable
consensus between models concerning the direction of climate change
for the UK, quantitative estimates are highly uncertain. Recent
work by NERC's Centre for Hydrology and Ecology, Wallingford,
has shown that when Hadley Centre climate model scenarios are
combined with hydrological models, results can be mixed. In many
catchments flood risk is increased, but in some it is reduced,
as a result of the effects of the drier summers. The main message
is that estimates of future rainfall and consequent flooding are
extremely uncertain. Recent work funded by DEFRA and the EA (e.g.
project FD2113) has sought to develop new methods to improve the
confidence in estimates based on Global and Regional Climate Models,
and this task remains an important research challenge.
18. Climate models provide a basis for estimating
extreme rainfall with fairly frequent occurrence (return periods
of say a few decades). However, there is no guidance currently
available to quantify climate change impacts on extreme events,
such as the Boscastle storm, for example, or the rare events considered
for dam safety. This is a strategically important gap in knowledge.
19. An alternative to modelling to evaluate
climate change is to look at the historical record to detect change.
However, since extreme events are by definition unusual, there
are major technical difficulties in determining whether they indicate
changing response. My understanding of the literature is that,
although short records may indicate change, there is as yet no
evidence for the UK from long term data that floods have increased.
However it seems likely that within the long term natural variability,
flood occurrence is currently greater than 30 years ago, which
was a relatively dry period with respect to flood occurrence.
20. Does the UK have the policy, resources
and technical tools in place to manage flood risk effectively?
21. Making Space for Water
22. DEFRA has recently developed a vision
for water management "Making Space for Water" (MSW)
which is a visionary and far-sighted document. MSW embodies a
radical change in perspective for flood risk management from earlier
approaches that focussed on local assessment of hard defences.
MSW emphasizes the need for integrated management of flood risk
at the spatial scale of the whole river catchment or the whole
shoreline. This requires consideration of both structural and
non-structural measures, including rural land use solutions, and
a more integrated approach to specific issues such as urban drainage,
coastal flooding and erosion. MSW also emphasizes the need to
"deliver the greatest environmental, social and economic
benefits consistent with the Government's sustainable development
principles," which requires broadly-based multi-criterion
assessment. Implicit in this new perspective therefore is the
need for new and broader approaches to decision support systems
and modelling. MSW must also be seen in the context of European
developments, in particular the Water Framework Directive, which
has wide-ranging implications for water management and the protection
of ecological quality, and the forthcoming Floods Directive.
23. There are, however, major technical
challenges in taking forward such broad-based assessment. The
DEFRA/EA R&D programme has recently funded an expert group,
which I led, that has mapped out a technical vision to achieve
this (project FD2118) over a 5 to 10 year timescale. In the meantime,
the EA has been developing Catchment Flood Management Plans (CFMPs),
with the aim of providing more integrated assessment with the
more limited tools currently available (these tools are unable
adequately to represent features such as land use change, or changes
in river geomorphology). It is however unfortunate in this context
that the recent reorganisation of the EA has removed the river
catchment as a basic management unit—a structure that had
been in place for 30 years or more and viewed internationally
as a flagship example of how to organise river management.
24. MSW recognises that land use management
and flood management are intimately related, and the current structure
of DEFRA combines these responsibilities. However, a truly holistic
approach has yet to be achieved. For example, changes to rural
land use to mitigate flood risk are likely to have associated
benefits for diffuse pollution, but a framework to account for
such multiple benefits has yet to be achieved.
25. Another issue of coordination within
DEFRA was that until recently, responsibility for reservoir safety
(and the associated issue of design for very extreme floods, discussed
in para 7 above) was divorced from fluvial flood protection. This
led to a situation where a new code of flood design practice was
introduced for fluvial flooding in 1999, which when extrapolated
beyond the return periods for which it was intended, gave very
large flood estimates, in some cases bigger than the previously
considered Probable Maximum Flood values. This left the dam owner
community in a difficult position, with potentially large costs
to be incurred to meet new standards of protection, which might
be incorrect. Work was eventually funded by DEFRA to reconcile
this issue, but in 2007 it has yet to report. Recently these responsibilities
have been integrated, however.
26. Urban flood management
27. In the urban situation, as noted above,
there has, in England and Wales, been divided responsibility for
flooding between the EA (responsible for main rivers), the water
utilities (responsible for sewer flooding) and Local Authorities
(responsible for lesser watercourses). DEFRA has been aware of
these problems, which inhibit the development of integrated solutions
to flooding problems and implementation of methods such as SUDS,
where responsibilities for maintenance have been unclear.
28. For urban flooding there has also been
an important technical gap. Storm sewer design has been based
on models to simulate flow within the piped system, and criteria
for the acceptable frequency of pipe-full flow (return periods
of just a few years). The relationship between sewers flowing
full or under pressure (surcharge), and surface emergence of flows
is site-specific, and the tools have not been available to represent
surface flooding for the sort of return periods commonly considered
for river flooding (eg 20-100 years). New technology is changing
that, and within the FRMRC, colleagues are using detailed remote
sensing topographic data (from airborne LiDAR) to identify surface
flow paths in urban areas. These can then be used with models
that can represent the interaction of storm drain and surface
runoff in urban areas. This provides the potential for a completely
new approach to the design and management of urban flood infrastructure
in the future.
29. Groundwater flooding
30. Current fluvial flood estimation methods
focus on surface water flooding. Extensive and prolonged groundwater
flooding occurred in the wet Autumn and Winter of 2000-01, affecting
areas such as the Chalk landscape of South-East England where
streams are predominantly fed by groundwater. In these areas,
normal river flows are low (typically just 2% of rainfall appears
as rapid runoff), but under exceptional periods of prolonged wet
weather, dry valleys begin to flow, springs break out in new areas,
and much larger volumes of runoff occur. A notable example is
the flooding of Chichester in 1994. New technical methods are
needed to asses and manage groundwater flooding. NERC's FREE research
programme has commissioned some of the first studies in this area,
led by me, which began in 2007.
31. Floodplain management
32. There is a major legacy of inappropriate
floodplain development, approved by local authorities against
the advice of the EA (and its predecessors). Recent strengthening
of the EA's voice in floodplain planning issues is welcome, but
as the floods of this Summer have demonstrated, there is a major
issue of legacy development and inappropriate siting of buildings
and other infrastructure in floodplains. Apart from the recent
example of Severn-Trent's water works, it is not uncommon to find
hospitals, old peoples' homes and the bases for emergency services
located on sites at risk of flooding. This is clearly an unsatisfactory
situation.
33. Human resources
34. As Director of the UK's oldest MSc programme
in Hydrology (which started in 1955), I should report that there
is a severe shortage of technically competent and experienced
hydrological specialists in industry and the EA, which is reflected
in demand for the engineering-based students which we produce.
However, in particular over the last 5 years, we have struggled
to recruit suitably qualified students to both MSc and PhD programmes,
despite the availability of funding from UK Research Councils
for UK students. There are large numbers of students wishing to
follow careers in this area, but lacking the necessary background
in Mathematics to enable them to develop the appropriate technical
skills. One consequence of this is that many overseas graduates
from our MSc course take up UK posts, another is that funded training
opportunities remain unfilled, a third is that employers use underqualified
staff. It would in my opinion be most helpful to the provision
of skilled manpower if Research Council grants could be made available
in full to EU residents.
35. Financial resources
36. In my opinion, much of the UK's flood
infrastructure provides levels of protection that are incompatible
with society's expectations of risk. This has implicitly been
recognised by DEFRA, in establishing guidelines of 1 in 100 risk
as appropriate for urban areas. Raising defences nationally to
this level (or better) within a reasonable time frame will require
very significant expenditure.
REFERENCE:
Wheater, H.S. (2006) Flood hazard and management:
a UK perspective. Phil Trans R Soc A, 364, 2135-2145.
Professor Howard Wheater FREng
Department of Civil & Environmental Engineering
Imperial College London
September 2007
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