Retrofitting Sustainable Urban Water Solutions
Background to research
Sustainable Urban Drainage Systems (SUDS) have become widely accepted
as drainage features for surface water runoff within new
developments. However, the UK has a legacy of existing urban
and industrial areas where diffuse pollution and flooding remain an
intractable problem. Consequently, questions have been raised
as to whether sustainable drainage systems can be a) retrofitted to
existing urban areas in order to improve water quality and/or the flow
regime; and b) undertaken on such a scale as to address the full extent
of flooding and water quality problems in many existing urban areas.
There are a number of international examples of successful retrofit
SUDS schemes that have achieved significant benefits. The
most notable of these are Malmö in Sweden and a number of
North American city initiatives. However, retrofitting SUDS
is a comparatively new concept in the UK and there is much to be
considered and learned.
Objectives of research
In February 2006 SNIFFER commissioned SISTech and The University of
Sheffield to conduct research on retrofitting SUDS in a Scottish
The main objectives of this research were:
to consider and evaluate the opportunities for retrofit SUDS at a case
study site in Scotland; and
to use this experience to develop a generic decision-making methodology
for selecting the preferred SUDS retrofit option (with the clear
intention that this approach would be tested and debated by others).
Key findings and recommendations
The Houston Industrial Estate was selected as the case study
site. Surface water from much of the industrial estate,
together with some of the surrounding residential areas, forms the
headwaters of the Caw Burn. The Caw Burn has a history of
water quality problems resulting from urban diffuse pollution.
A set of six design options was put forward for the Houston Industrial
Estate. Two of these (design options 3 & 4)
concentrated on the retrofitting of SUDS in the catchment.
Design option 3 represents a solution whereby SUDS serving
sub-catchments are sited in land in public ownership. Design
option 4 considered the implementation of SUDS within the curtilage of
individual properties in the industrial estate.
It was concluded that design option 3 would be unlikely to provide
significant improvements to water quality in the Caw Burn.
This was due to the fact that very little of the greenspace within the
Industrial Estate lies within public ownership. However,
design option 4 - the use of retrofit SUDS by a large number of
properties in the industrial estate - would be likely to have positive
benefits on the downstream water quality in the Caw Burn.
However, it was concluded that at present no specific driver existed
for these properties to implement retrofit SUDS. The project
team made a set of recommendations for changes in policy and drainage
service charging structure that would provide greater economic
incentives to property owners to retrofit SUDS. Key to this
was the alteration of the current charging structure to take into
account the use of storage SUDS as opposed to complete disconnection
from Scottish Water assets.
Green roofs offer considerable potential to manage stormwater at
source. It is unfortunate that the prevalence of low budget
pitched metal roofs on Houston's industrial buildings essentially ruled
out the widespread implementation of green roof retrofits.
However, even here there remains a significant opportunity to insist on
the installation of green roofs on any new/replacement buildings that
are constructed on the estate.
The report highlights a number of lessons learned during the case
study. Most importantly, the need to clearly define targets
and issues concerning data constraints. To address the latter
problem a number of preliminary appraisal tools were utilised,
developed and adapted as part of the scoping study for the Houston
Industrial Estate. The three most significant
‘tools’ were: the development of an updated version
of the Swan & Stovin hierarchical approach aimed at the
identification and prioritisation of SUDS retrofitting opportunities;
the utilisation of GIS data to characterise land-uses and to associate
these with potential opportunities for SUDS retrofit; and the use of
treatment volume (Vt) as a simple indicator of scheme
performance. This latter tool may be of particular value when
a validated hydraulic model does not exist and/or for the assessment of
large catchments. It is envisaged that these tools will be
tested and verified against ongoing case studies.
The report proposes a generic decision-making methodology for
retrofitting SUDS. It has been created with the aim of
addressing some of the gaps identified in the literature review, using
the experience gained from the Houston Industrial Estate. The
approach is applicable to areas where diffuse pollution is the major
driver. However, it is expected that some of the framework
will generalise to other situations e.g. where CSOs or flooding is the
The decision-making methodology has been sub-divided into two distinct
phases. Phase I is intended as a scoping stage, focusing on
determining whether or not SUDS retrofit is a viable option within a
specific problem context. Phase II focuses on detailed design
and construction. Multi Criteria Decision Making (MCDM)
techniques provide a means of assessing design alternatives in both
phases. It is intended to test the methodology on future case
studies to establish how well it generalises to other situations e.g.
A series of recommendations were made for future work areas, these were
focussed on further work to develop the preliminary appraisal tools,
investigation into the setting of system performance targets to achieve
water quality, and the establishment of a demonstration project in
Key words: [Sustainable Urban Drainage Systems, SUDS, retrofit SUDS,
Copies of this report are available from the Foundation, in electronic
format on CDRom at £20.00 + VAT or hard copy at
£35.00, less 20% to FWR members.
N.B. The report is available for download from the SNIFFER Website