Supplementary memorandum submitted by
Water UK (FL 95a)
SUSTAINABLE URBAN DRAINAGE SYSTEMS (SUDS)
Currently in the UK the Environment Agencies
and water companies are encouraging the inclusion of sustainable
urban drainage systems (SUDS) in the planning and design of new
developments.
SUDS are not designed to provide flood protection
or act as prevention systems. Their primary purpose is to provide
surface water treatment and slow down surface water flow into
the drainage system. They can be used to ensure there is no increase
in downstream flood risk and can help manage surface water run-off,
reducing the impact of urbanisation on flooding and pollution.
They are part of the solution to urban flooding but are not sufficient
on their own.
SUDS are commonly used in other countries, notably
USA and Scandinavia.
The water industry supports sustainable drainage
initiatives and is involved in all 15 Defra Integrated Urban Drainage
pilots. The SUDS initiative is seen as a valuable tool to improve
the manner in which the sector deals with surface water. SUDS
are not however flood defence measures.
SURFACE WATER
DRAINAGE AND
FLOODING
Impervious concrete and tarmac surfaces in urban
areas stops infiltration of rain water into the ground. As a result,
built-up areas need to be drained to collect and convey the resulting
surface water into a watercourse or public sewer. Traditionally
this has been done using underground pipe systems designed to
prevent local flooding by conveying the water away as quickly
as possible.
The underground pipe systems in urban areas
may be either provided specifically for surface water or provided
to carry both foul and surface water drainage (combined sewer
systems). Combined sewer systems are owned by water companies[11]
whereas surface water drains may be the responsibility of the
water company, the local authority or the highways agency.
Since the 1980s separate foul and surface water
systems have been provided for new developments, however, in larger
urban areas these invariably connect into older combined systems
further downstream.
If the combined system is overwhelmed or blocked
this will result in foul flooding (ie flooding with dilute sewage).
Localised flooding may also be caused by blocked road gullies,
rather than a problem with the design capacity of the sewer.
Appendix 1 presents a more detailed legal perspective
on surface water urban drainage.
SURFACE WATER
DRAINAGE AND
POLLUTION
Surface water run-off can become highly polluted
in urban areas due to the uptake of contaminants deposited on
the ground. Direct discharges via a separate surface water system
may therefore have an adverse effect on the receiving watercourses.
Combined sewer systems convey surface water to treatment works
but in storm conditions these are designed to overflow to watercourses.
This is a "safety valve" to prevent overloading the
system and localised flooding of property. These overflows contain
a mixture of sewage and surface water and as such will affect
the quality of the watercourse they discharge to.
HOW DO
SUDS HELP REDUCE
FLOODING?
The most important way in which SUDS help to
minimise the risk of flooding is to slow down the rate of surface
water flow thus reducing the load on the drainage system. This
is done by combinations of the options listed below:
— Encouraging householders to collect
water from roofs in rain butts or even underground rainwater harvesting
systems.
— Use of "green roofs" (ie
roofs that are partially or completely covered with soil and vegetation).
— Allowing more water to soak into
the ground. This includes simple soakaway systems and filter drains.
— Providing more permeable and porous
surfaces eg roads, pavings and car parks to allow water to soak
into the ground.
— Temporary storage of water in a normally
part-filled pond with an overflow, to attenuate flow (Figure 1).
— Temporary storage of water in a grassed
detention basin which is normally kept dry (Figure 2).[12]
— Temporary storage of water in an
area which can be allowed to flood with no damage to property
eg a car park, sports ground.
— Temporary storage of water in a constructed
wetland area.
— Collecting water into a roadside
swale or filter strip. These are grassed surface features that
drain water evenly off impermeable areas (Figure 3).[13]
Swales are long shallow channels whilst filter strips are gently
sloping areas of ground.
SUDS AND THE
ENVIRONMENT
Properly designed SUDS can have many environmental
benefits:
— They are able to reduce pollution
of watercourses by settlement or filtration.
— They even out peaks in run-off flow
to reduce the surface water load on combined sewers and thus reduce
the risk of sewer flooding of downstream properties. They may
also reduce the quantity of storm water overflows.
— They provide attractive wildlife
habitats—particularly wetland areas and ponds.
— They provide attractive landscape
features.
— They encourage natural ground water
recharge by allowing water to soak into the ground.
LIMITATIONS OF
SUDS
SUDS cannot be effective if the ground water
reaches the surface or they are inundated by river water. SUDS
can only be a part of sustainable flood management system which
uses natural features to reduce the risk of flooding rather than
constructed flood prevention schemes which often shift the problem
from one area to another. Sustainable flood prevention systems
include the managed use of floodplains and constructed wetland
areas to store water and attenuate flow in river catchments.
MANAGEMENT OF
SUDS
The water industry was instrumental in developing
an interim Code of Practice[14]
with the National SUDS Working Group[15].
A number of model agreements have also been drawn up to be used
either through the planning process or for use between two or
more parties outside the requirements for planning permission.
There is a lack of provision for the implementation
of SUDS technologies within existing legislation and this is a
barrier to their implementation. There are issues of:
— The right of water companies to refuse
a design that is not appropriate.
— Restoration of the right of Sewerage
Undertakers to discharge to watercourses.
— Removal of the automatic right to
connect surface water discharges to the sewerage system.
— Health and safety issues associated
with open water features that would be owned by private companies
in residential areas.
— Many techniques are relatively new
and the long term costs of maintenance are not well understood.
— Water industry responsibility for
SUDS should be limited to those SUDS features that have connectivity
with surface water sewerage systems (porous pavements, isolated
infiltration systems, soakaways and swales should remain in private/local
authority ownership).
These issues will need to be addressed by changes
to legislation to enable the adoption of SUDS by water companies.
Under Scottish legislation Scottish Water will assume responsibility
for certain types of new shared SUDS so long as they are constructed
according to the Sewers for Scotland technical manual[16].
The water industry is keen to resolve issues
preventing the implementation of sustainable drainage systems
and, through Water UK, is lobbying Government to adopt legislation
which reflects the existing Code of Practice, developed by the
national SUDS working group. In this it is particularly important
that the following are addressed by legislation:
— Sewerage undertakers being involved
in the planning process.
— Ownership and maintenance and funding
of SUDS.
— Definition of works capable of constituting
SUDS.
— Restrictions on rights of drainage
to SUDS and connection to surface water drainage.
— Rights of discharge from SUDS to
watercourses.
— Protection of groundwater and surface
water quality in accordance with the Water Resources Act 1991
and EC Directives.
SEWERS FOR
SEWERAGE
Central to the issue of maintaining adequate
sewer capacity is ensuring that they are used for the purpose
and only the product for which they are designed. Misuse of the
sewer network by disposing of inappropriate materials such as
cooking oils, sanitary products, food waste and other waste materials
can drastically reduce sewer capacity or cause blockages that
lead to backing up and flooding.
Figures: Typical SUDS arrangements
Figure 1: Retention Pond—from
http://www.ciria.org/suds/images/pond.gif
11 There are also a large number of private sewers
(foul, surface and combined) that discharge into company owned
public sewers. Government has recently consulted on the transfer
of these private sewers to Sewerage Undertakers Back
12
Not printed. Back
13
Not printed. Back
14
"Interim Code of Practice for Sustainable Drainage Systems",
National SUDS Working Group, 2004-www.ciria.org/suds Back
15
National SUDS Working Group includes-CLG, Defra, DoT, Welsh Assembly
Government, Ofwat, CIRIA, HBF, LGA, Natural England, EA, Planning
Officers' Society, County Surveyors' Society Back
16
"Sewers for Scotland", 2001-www.wrcplc.co.uk/sfs Back
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