Retrofitting Sustainable Urban Water Solutions
September 2006

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 context.  

The main objectives of this research were:

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 major driver.

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. CSO spills.

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 Scotland.

Key words: [Sustainable Urban Drainage Systems, SUDS, retrofit SUDS, diffuse pollution]

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