Memorandum submitted by HR Wallingford
(FL 108)
FLOODING LESSONS LEARNED REVIEW
1. BACKGROUND
TO OUR
SUBMISSION
1.1 HR Wallingford has been at the forefront
of providing research and specialist advice on flooding and flood
risk issues for 60 years both as a Government institution to 1982
and then as an Independent Research Organisation[34].
Recent and ongoing research includes the development of a range
of tools and methods to underpin the evolving flood risk management
practice in the Environment Agency—tools and methods that
were used to support the OST Foresight Future Flooding report
and provide the Agency and Defra with much of its information
on present day risks and impacts. HR Wallingford has acted to
provide detailed forensic examination of events (for example the
Boscastle flash flood) for EA.
1.2 HR Wallingford has been actively engaged
in flood risk management research and practice in the UK for most
of its 60 years. Major recent advances include:
— new Conveyance Estimation System
(CES);
— guidance for local authorities and
other stakeholders on development in flood-risk areas;
— work on Flood Resilience of new buildings;
— performance of existing flood defence
systems;
— deterioration of flood defences;
— performance of coastal / sea defences;
— use of Temporary / Demountable protection
systems
1.3 We continue to extend these and other
techniques and guidance through a number of research projects,
including leading and coordinating the largest ever EC research
project on flood risk management (FLOODsite), and leading roles
in national flagship research programmes sponsored by EPSRC (FRMRC[35]
) and NERC (FREE[36]
).
1.4 Our commentary is limited to specific
remarks on scientific and engineering issues that contribute to
impacts, lessons that may be learnt from experience of flood risk
management planning in UK and Europe. Our main concerns are over
delays in up-take of techniques or guidance to reduce flood risk;
inefficiencies in the flood protection system and the degree of
public engagement in developing and meeting realistic expectations
for defence or resilience.
2. THE WRONG
TYPE OF
STORM—WINTER
V SUMMER
EVENTS; USE
OF EXTREMES
ANALYSIS IN
PLANNING AND
DESIGN OF
FRM
2.1 Floods take place throughout the year.
Generally accepted views are that rivers flood in winter due to
saturation of the land from extended periods of rainfall. In contrast,
it has long been understood that urban drainage systems are most
tested in summer due to short duration high intensity thunderstorms
which are limited in extent. Analysis shows that summer floods
in July are not particularly unusual (in that there have been
many instances of serious flooding events in summer), including
Lynmouth in August 1952, Boscastle in August 2004 and the Yorkshire
floods in the summer of 2003.
2.2 The flooding in Gloucester, however,
shared many characteristics of a typical winter flood (slow rise
of water level, plenty of warning, velocities of flow that were
not extraordinary). These complications demonstrate that flood
risk scenario modelling must consider storm "events"
in the context of preceding rainfall, and must consider appropriate
roughnesses for a wide range of possibilities. Current single
event statistical techniques used by many practitioners miss these
critical interactions between rainfall events and fail to predict
the true risk at a given location. Simplified methods also fail
to capture the likelihood of multiple flood events occurring at
different locations simultaneously and the heightened probability
of a secondary or tertiary flood occurring at one location.
2.3 It is probable therefore that many models
may have used the "wrong type of rain" to underpin designs
and plans, without reflecting fundamental differences between
winter and summer events, and without capturing interactions between
rainfall events within the design statistics. This is compounded
if estimates of river resistance are based on "winter"
conditions (representing conditions in historic calibration events
used by the models), but these floods were in the summer with
much more vegetation, and hence greater resistance to flow. Flooding
in Abingdon arose as intensive flow into the Ock was blocked by
summer vegetation.
2.4 Improvements to flow modelling through
the CES, and in continuous simulation of the flood system through
links to weather radar, provide ways forward in our ability to
understand the behaviour of a system and how best to manage it,
but only when advanced capabilities are used in practice.
3. RISK IS
MORE THAN
PROBABILITY TIMES
CONSEQUENCE—BUILDING
IN RESILIENCE
TO DEAL
WITH EXTREME
EXTREMES
3.1 Standards for drainage provision are
very variable. Highway drainage may use 1:1 to 1:5 year return
periods. Sewerage Undertakers design for a standard of protection
of 1:30 years against sewer flooding (rarely achieved), but are
measured against a 2:10 year standard (DG5) for internal flooding
of domestic properties. New developments are required to meet
1:100 (fluvial) or 1:200 years (coastal / estuarial), standard
to which the EA aspire. Reservoirs have a range of higher return
periods associated with the risk to people downstream.
3.2 Different standards may be justified
where there are implicit cost / benefit balances behind the standards.
An alternative way of looking at this is that flooding is on a
scale ranging from nuisance through to catastrophic. The fact
that standards have been selected that are cost effective and
affordable does not mean that catastrophic events are accepted
by society. Expectations are deeply ingrained that society should
be able to deal with (mitigate) all situations. A result of Hurricane
Katrina is that blame has been apportioned to a range of organisations
for not being able to deal with the disaster.
3.3 Current research is focusing on a risk
based methodology (looking at all probabilities and consequences),
but it still has a fiscal/damage cost approach to identify optimal
management of drainage assets. This tends to minimise rare events
due to their low frequency of occurrence. What is evident from
the recent event is that above a certain threshold, disasters
result in potentially extreme social consequences. Power grids
and water supply infrastructure can be lost leading to potential
short term social breakdown. Simulations of flood risk therefore
needs to include the occurrence of potentially extraordinary events
and the wider impacts that these may have, not just relating to
damage/cost functions. This may apply not only to the direct impact
of flooding, but also the consequential impacts due to loss of
electricity, water supply, essential transport, and other high
social dependency requirements.
3.4 It is widely noted that responsibility
for drainage is distributed across many organisations, each with
different legal remits, which does not encourage looking at flooding
holistically. There are events where positive drainage provision
cannot be made, so planning must take account of these extreme
events and ensure that consequences are minimised. Thus hospitals,
bungalows, water and sewerage works, critical infrastructure,
or toxic / reactive industrial processes must be considered in
terms of vulnerability and impacts that flooding can cause. For
example, the Thames Barrier arose from the recognition that the
consequences of flooding London are very high and the required
standard of protection was set appropriately high. The polder
system in Holland are required to give 1:4000-1:10000 year standards
of protection.
3.5 The lack of resilience in public infrastructure
was clearly demonstrated this summer. Lessons from extreme events
world wide (the Asian Tsunami, Katrina, etc.) have already shown
that it is imperative that critical infrastructure and emergency
services continue to function when they are needed most. The lack
of clarity on the location and vulnerability of critical infrastructure
compromised abilities to manage flood risk, to minimise disruption,
and to recover quickly. Registers of critical infrastructure should
be accessible and should show their vulnerabilities to flooding.
There is little point in seeking to promote flood resilience for
private houses (see output from HRW studies on FD2320—http://www.hydres.co.uk/—and
work on Flood Protection products) yet fail to assess and improve
flood resilience for national infrastructure.
4. TOWARDS INTEGRATED
FLOOD RISK
MANAGEMENT
4.1 Making Space for Water advocates an
integrated approach, but current funding methods, the lack of
analysis tools that support an integrated understanding of flood
risk (all sources and pathways), and lack of innovative thinking
in developing integrated strategies all tend to lead to single
issue solutions. Perhaps most importantly, Making Space for Water
is inherently a policy aimed at allowing more floods not less
(but less damaging) yet it is unclear whether the public recognise
or accept their critical role in achieving this, particularly
in the absence of compensation.
4.2 Better awareness and building inherent
resilience into the community (private and public) can really
help reduce flood impact, but from media reports it appears that
few of those affected were well prepared (even if informed).
4.3 It can therefore be seen that Making
Space for Water is about managing flood risk, but
— Has "the public" understood
or accepted this when the policy impacts on their own protection?
— Do some communities expect to be
defended at all costs?
— What is the forum in which the public
trade-off is done?
5. SPATIAL PLANNING
5.1 The new DCLG policy statement PPS25
covers development and flood risk, providing a framework for considering
flood risk in spatial planning decisions at scales from regional
planning to development control. Spatial planning decisions are
tiered, developing policies at lower levels that are consistent
with policies and principles at a broader scale. Planning officers
are not normally specialists in flood risk and there is a need
to improve the appreciation of the full spectrum of integrated
flood risk management. It is important that the necessary physical
and organisational infrastructure are available to manage flood
risks from all events including extreme events.
5.2 Some development policies may make matters
worse. Development of high density housing has potentially serious
consequences for flooding. High densities result in intensive
coverage by buildings and/or paving for which positive drainage
systems will be unable to drain extraordinary rainfall. The inevitable
surplus will flow over the ground seeking flood pathways. Current
and planned housing densities will therefore cause greater damage
as these flood flow away, or simply pond as in Hull.
6. EUROPEAN DIMENSION
6.1 On 25 April 2007, the European Parliament
agreed with the Council on the text of the Directive on "the
assessment and management of flood risks". Formal adoption
by the Council is expected later in 2007 (see http://ec.europa.eu/environment/water/flood risk/index.htm).
The Directive adopted by Council and Parliament will require that
Member States take a long-term planning approach to reducing flood
risks in three stages:
— Member States will by 2011 undertake
a preliminary flood risk assessment of their river basins and
associated coastal zones.
— Where real risks of flood damage
exist, they must by 2013 develop flood hazard maps and flood risk
maps.
— Finally, by 2015 flood risk management
plans must be drawn up for these zones. These plans are to include
measures to reduce the probability of flooding and its potential
consequences. They will address all phases of the flood risk management
cycle but focus particularly on prevention (ie preventing damage
caused by floods by avoiding construction of houses and industries
in present and future flood-prone areas or by adapting future
developments to the risk of flooding), protection (by taking measures
to reduce the likelihood of floods and/or the impact of floods
in a specific location such as restoring flood plains and wetlands)
and preparedness (eg providing instructions to the public on what
to do in the event of flooding).
6.2 All stakeholders must be given the opportunity
to participate actively in the development and updating of the
flood risk management plans. Risk assessments, maps and plans
must furthermore be made available to the public. These three
steps are to be repeated in a six-year cycle to ensure that long-term
developments are taken into account. The Directive creates an
EU framework for flood risk management that builds on and is closely
coordinated and synchronised with the 2000 Water Framework Directive,
the cornerstone of EU water protection policy. There are other
European level actions which will support the work in Member States
in implementing the Directive including the Integrated Project
FLOODsite (led by HR Wallingford in the UK), the research funding
network CRUE (led by Defra) and EFAS, the European Flood Alert
System, being developed at the Commission's Joint Research Centre
(Ispra, Italy). The future management of flood risk within the
UK will be set within the context of the Directive and its reporting
requirements and should make use of the knowledge and evidence-base
arising from such international collaborations as those above.
In the context of the EU stakeholder group for the EU Action Plan,
the UK approach to understanding flood risk is seen to represent
best practice whilst the Dutch probably have the most rigorous
management of flood risks. Any actions that are developed as a
result of lessons learnt will have to be developed within this
context.
7. REFERENCES
Thorne, CR, Evans, EP and Penning-Rowsell, E
(2007) (Eds), Future Flood and Coastal Erosion Risks, Thomas Telford,
London.
Evans EP, Ashley R, Hall JW, Penning-Rowsell
E, Saul A, Sayers P, Thorne CR, Watkinson A, 2004a, Scientific
Summary: Volume 1, Future Risks and Drivers, A Foresight Flood
and Coastal Defence Project: Office of Science and Technology,
London, 1-366.
Evans EP, Ashley R, Hall JW, Penning-Rowsell
E, Saul A, Sayers P, Thorne CR, Watkinson A, 2004b, Scientific
Summary: Volume 2, Managing future risks, A Foresight Flood and
Coastal Defence Project: Office of Science and Technology, London,
1-416.
Institution of Civil Engineers (2001), Learning
to Live with Rivers, Institution of Civil Engineers, Thomas Telford,
London.
http://www.crue-eranet.net/
Http://www.floodsite.net/
Various Environment Agency Reports provide useful
background, particularly previous lessons learnt reports, including:
— Bye report of Easter 1998
Paul B Sayers
Group Manager Floods
Ian H Townend
Research Director
HR Wallingford
September 2007
34 HR Wallingford is a company limited by guarantee
with no share capital and Section 508 status, whose objects are
to promote, carry out and disseminate research in hydraulics and
related fields for the study of the natural and built environment. Back
35
Engineering and Physical Sciences Research Council: Flood Risk
Management Research Consortium. Back
36
Natural Environment Research Council: Flood Risk from Extreme
Events. Back
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