Supplementary memorandum submitted by
Yorkshire Water (FL 122a)
BRIEFING NOTE
ON SUSTAINABLE
DRAINING SYSTEMS
(SUDS)
1. SCOPE AND
SCALE OF
SUDS IMPLEMENTATION
1.1 Yorkshire Water follow the guidance
given in the `Interim Code of Practice for Sustainable Drainage
Systems (SUDS)' (July 2004, National SUDS Working Group, Published
by CIRIA). Our experience is in the range of SUDS solutions that
can be defined as those which relate to sewerage assets.
1.2 These assets are generally at the "harder
engineering installation" end of the SUDS spectrum, with
control of surface water run-off for a new development most commonly
achieved by use of "over-sized" sewers with flow control
devices.
1.3 The design and construction of these
installations is carefully controlled and we have not experienced
any serious difficulties to date in the operation and maintenance
of these assets.
1.4 It is however relatively early in the
asset life of these installations so further work is required
to understand any longer term performance and potential failure
scenarios. The extent to which the different SUDS solutions has
been employed in Yorkshire is set out below. It is difficult
to quantify the number of SUDS we maintain, since each component
is simply a sewer or sewer ancillary and is, therefore, shown
on our records as sewerage assets.
1.4.1 Pipes All systems installed
to manage surface water are now designed with due regard to the
principles of sustainable drainage, so all recent surface water
sewers could be considered as SUDS. The principles of sustainable
drainage have certainly been widely applied since the publication
of the Interim Code of Practice and, in many cases, have been
applied prior to this date.
1.4.2 Swales A small number
of adoptable SUDS systems have incorporated swales (a wide, shallow,
vegetated channel) which floods to relieve intense rainfall events.
The design of these features is carefully controlled to eliminate
future maintenance risks. The significant footprint of these
features has meant that limited use of these methods has been
taken up, and again it is too early in the asset life to judge
the success of these assets.
1.4.3 Ponds Ponds are not
recommended for adoption by sewerage undertakers in the interim
code of practice. Where these have been proposed we have made
it clear that adoption by the sewerage undertaker is not covered
by our duties as sewerage undertaker.
Ponds present significant difficulties in achieving
the balance between the operational tasks required to ensure that
the pond performs hydraulically, against the safety, pollution
control and amenity aspects associated with them.
Local authorities and highway authorities, who the
interim code of practice indicates can consider ponds, have with
few exceptions been reluctant to date to adopt ponds, citing similar
concerns about the practical operation of ponds as an engineering
solution.
1.4.4 Modular storage systems
A number of modular storage structures are available
on the market for the storage of surface water. These are not
currently accepted since they are unproven as far as the ongoing
maintenance is concerned. We do not currently own or maintain
any such arrangements.
1.4.5 Soakaways Source control solutions,
of which soakaways are the most common example, are always recommended
by YW in accordance with Part H3 of the building regulations when
we are consulted by local authorities and developers on surface
water control at the planning stage. However they are rarely
proposed by developers as the burden of proof that they will operate
effectively is generally too onerous.
1.5 We adopted the Interim Code of Practice
following it's publication in July 2004, and have continued to
develop the technical approach set out above to reflect the latest
best practice recommendations from the working group and other
relevant sources.
2. GENERAL CONCLUSIONS
FROM OUR
EXPERIENCE OF
SUDS
2.1 As indicated above the construction
industry has been cautious in embracing many of the SUDS types
available to date.
2.2 Nonetheless, the following observations
can be drawn from the sites where SUDS have been employed:
— It is crucial that ownership and
maintenance agreements/strategies are in place before construction.
— There is little understanding of
longer term performance.
— Currently there is insufficient understanding
of performance to enable accurate forecasting for capital investment
planning or development of operational maintenance schedules.
— In terms of maintenance there is
a need to more fully understand and propose strategies for vegetation
management and sediment removal.
— The same underlying principles of
hydrology and hydraulics as used for conventional systems should
be applied.
— Wider benefits of amenity and environmental
enhancement need to part of the undertaking for the responsible
body.
— There is no single "correct"
solution.
— Establishing the right balance of
stormwater management, water resources, impact on society and
global energy concerns is difficult to achieve
— Separation and source control are
required to ensure we maintain levels of service using current
practice
3. SUMMARY
3.1 A review of relevant SUDS documentation,
together with discussions with the appropriate technical experts
and stakeholders, provides strong support for the principles of
SUDS and there is evidence that SUDS can and do work.
3.2 However, whilst supporting the principles
of SUDS, it is also important that we acknowledge and seek to
address the barriers to the consistent application of SUDS which
have yet to be overcome. In summary, the most significant outstanding
issues are:
— As the sewerage undertaker is not
able to adopt all SUDS components, the design of drainage systems
that best meet the SUDS aim of control at source may be compromised.
— Issues relating to future liability
for pollution from SUDS features have not been clarified.
— The mechanisms for funding future
maintenance of SUDS components and the scope and scale of the
maintenance are not fully defined
Yorkshire Water
January 2008
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