Memorandum submitted by British Water
(FL 157)
SUSTAINABLE DRAINAGE SYSTEMS AND MANAGING
SURFACE WATER FLOOD RISK
A review of sustainable drainage systems for
surface water control and their potential impact on flooding.
SUSTAINABLE DRAINAGE
SYSTEMS AND
FLOODING
British Water and its Sustainable Drainage Systems
Focus Group has prepared this paper to provide the Environment,
Food and Rural Affairs Committee with information on the impact
of sustainable drainage systems on the control of surface water
drainage and mitigation of the potential consequences and impacts
of excessive rainfall, including flooding.
It should be clearly stated at the outset that
drainage systems are designed to manage defined maximum volumes
of water; these defined maxima are based on historical rainfall
records. If design maxima are exceeded then drainage systems will
be over-whelmed and flooding will occur, inadequate design and
lack of maintenance may also be contributory factors.
Climate Change predicts increases to the incidence
of excessive rainfall events which will challenge the adequacy
of existing drainage systems and design criteria for new projects.
Professional judgement will influence design selection for a project
but this will be predicated on accurate data providing sound modelling
of future climatic patterns on which to base design specifications
for effective sustainable drainage system.
There are many factors that contribute to flooding
and the construction of an adequate drainage system is but one.
Historical urban and rural developments and plans for future developments
will, in combination with the vicissitudes of climate change,
continue to challenge the effectiveness of drainage systems.
The principles embedded in the philosophy of
sustainable drainage systems will maximise their effectiveness
and minimise the future impact of excessive rainfall events in
urban and rural situations. However, the siting of future developments
will have a crucial impact on the ability of sustainable drainage
systems to minimise the incidence and consequences of flooding.
The management of stormwater is a challenging
problem which is exacerbated by uncertainty about future drivers
such as climate change. Historically in the UK and most of the
developed world, stormwater problems have been addressed using
systems (drains, sewers and watercourses) to dispose of excess
stormwater runoff as quickly as possible away from urban areas
in particular, to points where it is no longer believed to be
a problem.
The realisation over the last century that stormwater
disposed of in this way can lead to problems downstream due to
high flows, flooding, watercourse erosion, pollution and consequential
ecological impacts, has led to the development of alternatives
to piped and channelled drainage systems that try to more realistically
replicate the natural physical, chemical and biological processes
of evapotranspiration, filtration, detention, and dispersion.
A major facet of these new "natural"
stormwater management approaches is the need for greater engagement
of those involved in development planning, maintenance, operation
and in general sustaining the performance of these systems.
Extracts from Executive Summary of the Global
Watch Mission Report Sustainable drainage systems: a mission to
the USA. March 2006. This DTI funded mission was initiated and
managed by British Water, the report is available via the British
Water website
http://www.britishwater.co.uk/document/Default.aspx?uid=822a4662-711d-437c-bd6b-e94138d98ab2
1. INTRODUCTION
This paper has been prepared to outline and
encourage the implementation of contemporary surface water management
techniques (sustainable drainage systems) that authentically contribute
towards sustainable development.
With respect to Gro Harlem Brundtland "Sustainable
drainage is drainage that meets the needs of the
present without compromising the ability of future generations
to meet their own needs. (Adapted from "Our Common Future"
Brundtland 1987).
The widespread implementation of sustainable
drainage systems (also known as "SUDS" or best management
practices "BMPs") should be integral to the surface
water management strategy in any and all developments. This will
provide the platform to mimic the response of the existing catchment
and its surfaces, ultimately with some betterment, negating any
increased off-site flood risk that development could cause.
2. TERMINOLOGY
The term SUDS came into use to describe
a drainage philosophy primarily for urban environments (Sustainable
Urban Drainage Systems) and in general accentuated
the use of natural features eg swales, balancing ponds. The philosophy
has evolved and is now applied to all drainage systems whether
urban or rural and including natural and engineered structures,
thus the derivation of SUDS became SUstainable Drainage
Systems. However it retains a strong emphasis on "natural"
systems which also have an obvious amenity value.
The term SUDS is UK terminology, the philosophy
it describes amounts to best practice and this is reflected in
a term that is used in many other countries Best Management Practice
(BMP): other terms are WSUD—Water Sensitive Urban
Design (Australia) and LID—Low Impact Development (USA).
Although BMP does not include an obvious reference to drainage
it encourages a more holistic approach which includes a recognition
of the potential for urban and rural systems to interact and influence
the effectiveness of linked drainage systems. The phrase sustainable
drainage systems is increasingly used so as to be inclusive
of solutions (BMPs) in urban and rural situations and including
natural and engineered structures.
An integrated water management system leads to
benefits elsewhere and Sustainable Drainage helps in the management
of flood risk, improve water quality in the environment and can
contribute to increased biological and ecological diversity. (Drainage
Assessment—A Guide for Scotland 2007)
3. OVERVIEW OF
SUSTAINABLE DRAINAGE
SYSTEM DESIGN
REQUIREMENTS
A contemporary sustainable drainage methodology
for managing surface water runoff should use Best Management Practices
to focus on three key areas:
— controlling surface water quantity
(reducing off-site flow rates);
— improving surface water quality;
and
— providing added development amenity
value.
Provision of all three in equal measures will
not always be possible or necessary.
It is anticipated that contemporary sustainable
drainage techniques shall be used throughout any and all developments
to control surface water runoff and help manage residual flood
risk in line with current planning requirements.
Traditional (pipe and chamber) systems can be
designed and implemented as sustainable drainage components, if
dictated by development constraints, but a contemporary methodology
must replace conventional thinking (ie a flow controlled contemporary
system replacing a freely discharging conventional system).
The limited experience of sustainable drainage
systems in the UK means that better arrangements need to be in
place to ensure good design and construction. This requires a
whole life performance perspective and the education and training
of all stakeholders, especially planners and building control
officers. In addition schemes ... in which up-front bonds have
to be lodged prior to construction should be considered in order
to ensure that these systems are properly constructed. This may
necessitate the establishment of specialist drainage inspectors
... who may also be trained in other stormwater management aspects
such as local flood risk management advice to householders and
property managets. (Global Watch Mission Report: Sustainable
drainage systems: a mission to the USA 2006)
Three key tenets should be developed as part
of an integrated and sustainable surface water management strategy:
1. Maximise a reduction in natural runoff
by the use of infiltration techniques wherever feasible.
2. Manage the residual flood risk as well
as reducing the total volume of surface water runoff discharged.
3. Maximise quality improvements in surface
water runoff.
All three tenets can be satisfied using natural
or proprietary techniques alone or in combination.
In order to ensure that the sustainable drainage
system is designed to mimic the natural characteristics of the
catchment and attain the key tenets above, the surface water management
train should ensure that the application of runoff control is
considered for three key stages during the conveyance of surface
water to the regional fluvial or tidal outfall. These stages are:
4. DRIVERS AND
BARRIERS
There are both drivers and barriers to the design
and uptake of truly sustainable drainage systems, the impact of
which can vary depending on site and size of a development.
Drivers for Sustainable Drainage
There are many contemporary drivers for the
implementation of sustainable drainage techniques, including:
— Flood Risk Assessment (Planning Policy
Statement 25 Development and Flood Risk and accompanying Practice
Guide).
— The Water Framework Directive.
— Making Space for Water.
— CIRIA design guidance C697.
— The Building Regulations.
— Interim Code of Practice.
— Code for Sustainable Homes.
— Designing for Exceedence.
— Approving authority and stakeholder
requirements.
— Contemporary thinking and the aspiration
for sustainable development.
New surface water drainage systems are typically
designed to cope with an event likely to happen, on average once
every 30 years. The Government should review flood risk protection
standards for inland, coastal and surface water flooding to ensure
that they are still appropriate in view of climate change. (Environment
Agency: Review of summer floods—2007)
Barriers to implementation of sustainable drainage
systems
Currently, for every driver there is often a
barrier preventing implementation of some sustainable drainage
techniques:
— Concerns regarding adoption of many
natural techniques.
— Longevity and whole life issues,
including maintenance.
— Misunderstanding of contemporary
sustainable drainage techniques.
— The required land-take for some natural
techniques.
— Too many surface water stakeholders—Environment
Agency, Water Companies, Highways Agency/Authorities, Internal
Drainage Boards, Land Drainage Authorities and riparian owners.
— Too many surface water design standards
with contradicting requirements.
— Shortfalls in the planning process.
— Ambitious housing densities.
— Unfulfilled retrofit potential; green
roof technology.
— Future floodplain development and
not brownfield reclamation.
It is apparent from US practice that there are
considerable benefits from providing greater incentives for the
use of innovative stormwater management techniques. These are
most effective where the stormwater costs are clearly identifiable
within charging schemes.
... Clearly identifiable costs and discount or
rebate opportunities can aid in engaging each of the stakeholders
...
In many areas of the USA separate stormwater
utilities (municipal or private) deliver a service associated
with a defined income stream as above.
... There are clear advantages of such utilities;
however, they need to be properly positioned within an integrated
water management and planning system. (Global Watch Mission
Report: Sustainable drainage systems: a mission to the USA 2006)
5. NATURAL AND
PROPRIETARY (MANUFACTURED)
BEST MANAGEMENT
PRACTICES
Contemporary sustainable drainage should be
achieved by designing and implementing a blend of natural and
proprietary BMPs, complemented by traditional drainage techniques
where required. Some techniques can be very simple; for example,
downpipe disconnection as a BMP technique, either as new build
or retrofit, has been available as a drainage option for quite
a while with unfulfilled potential.
US experience has shown that the incremental
and localised small-scale management of stormwater ... can collectively
provide significant benefits to managing local and downstream
water quality and quantity ... In the UK it is likely that stormwater
disconnections (retrofit) as part of a protfolio of approaches
will become increasingly important (if not essential) to meet
the requirements of the Water Framework Directive, as they could
potentially reduce both discharge volumes and remove significant
pollutants from discharges into natural water bodies. (Global
Watch Mission Report: Sustainable drainage systems: a mission
to the USA 2006)
Seamless integration into the infrastructure
of a development is required to create an authentic and contemporary
sustainable drainage system. Such integrations may include, although
not be limited to the following.
A. Natural Sustainable Drainage
These structures will be focused around natural
techniques and materials and integrated into the landscape, which
may include utilising engineered features. Without doubt, where
development constraints permit, the maximum sustainable drainage
return against the key tenets of quantity, quality and amenity
will be best achieved through the implementation of natural techniques.
Such techniques include green roof technology,
surface water planter boxes (in footways or as kerb extensions
into the highway), rainwater gardens (located in larger landscaped
areas), grass or planted swales, infiltration trenches and basins,
unlined open channels, detention basins (dry features), balancing
ponds (wet features) and temporary floodable areas (where deemed
acceptable in terms of residual flood risk).
They can be designed to operate with or without
infiltration and will all generally afford excellent attenuation
and bioremediation properties. It is usual to integrate proprietary
flow control devices in many of these BMPs to restrict discharge
into the receiving downstream system. Health and safety considerations
may need to be reviewed.
B. Proprietary Sustainable Drainage
These BMPs are a range of manufactured techniques
that include porous or pervious surfacing, retrofit green roof
technology, geocellular storage systems, on or off-line detention
tanks (constructed from a range of materials), flow control devices,
hydrocarbon separators, advanced material filtration and absorption
technologies, treatment train systems containing modules of specific
processes and traditional piping.
They will all be integrated into the surface
water drainage infrastructure and generally be hidden below ground.
Proprietary systems are providing solutions for
dealing with particular water quality or quantity problems (in
the USA) ... Although these systems are effective when applied
appropriately and can provide some valuable solutions for removing
contaminants, there is no evidence that there is a "magic
bullet" device that can provide all of the treatment needs
in a single unit. (Global Watch Mission Report: Sustainable
drainage systems: a mission to the USA 2006)
C. Bioremediation
Bioremediation is a natural surface water cleansing
method utilising the biological and biochemical processes available
from the natural flora (micro-organisms, plants, trees, etc) to
remove levels of contamination associated with urban pollution,
diffuse or otherwise.
Certain plants have the ability and tolerance
to take up high concentrations of toxic chemicals, hydrocarbons
and metals included, and to process some of them to less toxic
derivatives. The same natural cleansing principles can be applied
as source control treatment to the pollutant load where the surface
water runoff originates.
Harnessing the bioremediation potential available
in natural sustainable drainage may become commonplace as the
improved surface water quality targets set by the Water Framework
Directive become actively enforced by the Environment Agency.
Aside from their natural filtration properties,
the importance of plants as a sustainable drainage BMP should
not be overlooked. For example, the value and contribution of
mature trees within a catchment is a valuable technique to provide
a natural treatment train for rainfall surface water interception
(including flow rate reduction), evapotranspiration, infiltration
and bioremediation.
6. THE TOOLBOX
OF SUSTAINABLE
DRAINAGE TECHNIQUES
If misused, the term "SUDS" can be
confusing. It sometimes conjures up a misplaced belief that all
SUDS should contain only natural BMPs and this simply isn't correct.
The successful balance for a contemporary surface water management
train must lie in a toolbox of sustainable drainage techniques
that includes natural, proprietary, traditional and lateral (soft
techniques, such as public engagement) BMPs.
For example; if some desired natural techniques
are not technically feasible on a project, for which there may
be a variety of reasons, eg adoption issues, then a development's
surface water system can still be designed as authentic sustainable
drainage by satisfying the residual quantity and quality targets
with proprietary techniques.
It is also clear that the selection of contemporary
sustainable drainage techniques goes beyond the defined key tenets
of quality, quantity and amenity, although all three are the prime
considerations for authentic and valid sustainable systems.
Novel and more flexible approaches to stormwater
management are becoming increasingly important for addressing
the drivers ... The technologies to do this ... are emerging ...
(to) assist with particular applications in the UK, such as high
density housing, retrofitting to resolve existing problems and
to meet the requirements of the Water Framework Directive. Compared
with the USA, the UK has greeater challenges as to how stormwater
can be managed due to limited space in urban environments. (Global
Watch Mission Report: Sustainable drainage systems: a mission
to the USA 2006)
Recent experience has underscored the importance
of considering construction and maintenance towards successful
sustainable drainage implementation for the long-term; and there
is an overriding need to understand the whole life issues of sustainable
drainage BMP selection for the UK water industry.
Regardless of the techniques implemented, it
is essential that both visible and invisible sustainable drainage
structures are seamlessly integrated into the proposed landscape
architecture. This is essential for the long term success of contemporary
sustainable drainage on any development site.
Technical guidance giving a brief outline of
the proprietary systems and components available was published
by British Water in partnership with the Environment Agency in
2005. It can be downloaded from the British Water website http://www.britishwater.co.uk/document/Default.aspx?uid=a05d6e5b-51ab-445f-a3dd-eaa01d216f1b
There is a lack of general public awareness of
collection, transmission and tratment of sewerage and drainage.
The mindset of the public is that of "out of sight out of
mind". There is a lack of understanding of how to minimise
environmental impacts of personal activities, or appreciation
of the need to pay for drainage costs. (Environment Agency—2003)
7. SUSTAINABLE
DRAINAGE DESIGN
The toolbox of techniques available for contemporary
sustainable drainage implementation is many and varied. Consequently,
so are the design methods, of which there are too many to detail
here.
It is necessary to stress the importance of
a robust design process, using the correct guidance and tools
for each bespoke project.
Design guidance may be sought from, although
not necessarily limited to, the following:
— CIRIA design guidance C697.
— The Building Regulations.
— British Water Technical Guidance
to proprietary sustainable drainage systems and components—SUDS.
— Industry developed "Source Control"
software.
8. OTHER RELATED
ISSUES
The significance of sustainable drainage systems
and the selection of Best Management Practices in this context
is affected by related factors.
A. Flood Risk Management
Artificial drainage systems designed to manage
surface water runoff can pose a flood risk if the system is overwhelmed.
This may occur if the amount of surface water
runoff exceeds the system's capacity or if the system becomes
blocked or surcharged by the receiving watercourse.
It is wrong to raise public expectations that
you can prevent floods. No matter how much engineering we put
in, there will always be the possibility of a flow in excess of
its design capacity. We should be talking, therefore, in terms
of flood risk management, not prevention. By acting wisely, we
might at least reduce the frequency with which the system fails
and the severity of the disaster when it does.
We might not be able to reverse the shortcomings
of the past—there is no real possibility of retro-fitting
every town and city with separate storm drains—but we can
avoic repeating them in future. (Jean Venables, ADA in response
to summer floors 2007)
Current planning policy guidance considers surface
water management a key flood risk issue and sustainable drainage
techniques should be employed to manage residual flood risk wherever
feasible. The majority of development planning applications requires
a Flood Risk Assessment (FRA) to be carried out. The FRA must
clearly demonstrate that sustainable drainage techniques have
been used to mitigate against any residual surface water flood
risk.
B. Climate Change
Increasing global temperatures and changing
weather patterns confirm that climate change is a reality. Therefore,
an allowance for the impact of climate change is a critical part
of any assessment of flood risk and inclusion of design mitigation
measures, including sustainable drainage implementation. Current
planning policy guidance requires an FRA to demonstrate that sustainable
drainage design peak rainfall intensity has been increased in
line with the development's design life. There is still a debate
as to the size of the allowance that needs to be factored in to
provide for potential consequences of climate change on rainfall
and the consequences for surface water and other drainage systems.
LIST OF
REFERENCES FOR
TECHNICAL AND
REGULATORY INFORMATION
1. Global Watch Mission Report Sustainable drainage
systems: a mission to the USA March 2006.
2. Technical Guidance. Guidance to proprietary
sustainable drainage systems and components—SUDS BW: 2/05
2005.
3. Flood Risk Assessment (Planning Policy Statement
25 Development and Flood Risk and accompanying Practice Guide).
4. The Water Framework Directive.
5. Making Space for Water.
6. CIRIA design guidance.
7. Sewers for Adoption.
8. The Building Regulations.
9. Interim Code of Practice.
10. Code for Sustainable Homes.
11. Designing for Exceedence.
12. Industry developed "Source Control"
software.
Dr Ian H Pallett
Technical Director British Water
BRITISH WATER
Overview
British Water is the lead trade association
for the UK water and wastewater industry representing all areas
of the supply chain including contractors, consultants, manufacturers,
equipment suppliers, and many specialist service providers such
as leading law firms, academic departments and environmental publications.
British Water is recognised as the voice for
the water industry supply chain. It has unrivalled access to and
is consulted by government, regulators and customers. It provides
information on home and overseas water and wastewater markets
and provides access to prospective customers, partners and suppliers.
It promotes the UK water industry expertise
in the international market place. It manages international trade
visits and seminars with government support and assists with programmes
for international government and industry figures visiting the
UK.
As well as actively promoting best technical
and commercial practice, British Water represents the interests
of the UK water and wastewater industry in the development of
technical standards, both in the UK and Europe. It provides the
Acting Secretary-General and secretariat for Aqua Europa, the
European federation of supply chain trade associations which is
recognised by the European Commission as the representative body
for the supply chain.
An active commercial programme of conferences,
seminars, training and accreditation schemes and publications
provides additional value for members and the industry.
Structure
British Water has three major divisions the
UK Forum, the International Forum and the Technical Forum. Each
Forum has subgroups to facilitate discussion and promotion of
focussed interests.
The Technical Forum has a range of Focus
Groups covering technical issues of commercial interest to members.
One, the SUDS Focus Group, promotes best practice in the
development of sustainable drainage solutions and provides a forum
for discussion between industry, regulators and government. It
initiated the Global Watch mission to the USA 2006.
The British Water SUDS Focus Group comprises
40 representatives from 24 companies across the spectrum of British
Water membership—consulting engineers, contractors, equipment
manufacturers and academia. A numbers of actively participating
members meet regularly and they are joined by representatives
from the Environment Agency and Department for Communities and
Local Government. This paper has been prepared by a sub-group
representing the diverse expertise of the Focus Group:
Alex Stephenson (Convenor) Hydro International Ltd
David Schofield, Arup
Alan Corner, Halcrow Water
Gareth Samuel, Hepworth Building Products
Andy Thompson, Klargester Environmental
Alan Rafelt, Environment Agency
Mike Johnson, Communities and Local Government
Dr Ian H Pallett, Technical Director, British Water
British Water
January 2008
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