All involved in urban drainage recognise that conventional piped systems can give rise to problems of excess quantity, ie. flooding and poor quality due to diffuse pollution. They also appreciate that, if properly designed, sustainable urban drainage systems (SUDS) can be a useful tool in alleviating these problems and improving local amenity and biodiversity.

Unfortunately the absence of guidance on design and construction, alongside water industry requirements for definitive answers to questions of ownership, and responsibility for ongoing maintenance, have meant that SUDS have been taken up relatively slowly in England and Wales in comparison to Scotland. In Scotland the legislative framework has provided more clarity on responsibilities and as a result large developments have incorporated sustainable drainage.

The issue of appropriateness is also one that concerns the water industry. Southern Water’s Chris Stewart says: “We want to ensure SUDS are installed in appropriate locations to appropriate standards, with the right provisions for long-term maintenance. We are concerned that if they do not meet these criteria we will swap one set of problems for a different one.”

Thanks to a number of initiatives this situation is changing. For instance, the government’s planning guidance, PPG 25, Development and Flood Risk, encourages the use of SUDS. A working group has been set up with the Environment Agency (EA) taking the lead role, and involving the water industry and both local and central government. This group aims to produce a set of design standards and clear definitions of responsibility that all parties should feel able to sign up to. Publication is expected early in 2003.

The Construction Industry Research and Information Association (CIRIA) has produced its own design and best practice manuals on SUDS and has just completed a study on source control using pervious surfaces, with the production of a technical note – Source Control Using Constructed Pervious Pavements, Hydraulic, Structural and Water Quality Issues. Pervious pavements can enable control of the rate and quality of runoff at or near source and can be used as stand-alone systems or in a mix-and-match fashion alongside other SUDS techniques to reduce downstream flooding and pollution risks.

construction errors

One of the biggest obstacles until now has been lack of contracting know-how leading to errors in construction, for instance, the use of regular sub-base materials (such as Type 1 aggregate) rather than high-void porous sub-base materials, which have far superior storage properties. This is an indication of the lack of guidance on the construction of such surfaces and the fact the approach turns traditional pavement design on its head – normally the aim is to minimise infiltration into underlying layers.

The CIRIA technical note tells contractors how to design and install pervious systems successfully, notably giving useful guidance on designing porous pavements to ensure they can carry the required traffic loads without failing. Further CIRIA research is looking into the development of model agreements for the operation and maintenance of SUDS. Maintenance and monitoring regimes will be the subject of further technical guidance notes. Another industry research body, HR Wallingford, is looking at

the application of SUDS to high-density sites.

Emerging proprietary technologies include:

  • sub-garage storage systems, that collect and hold water from roofs and drives during storms,
  • polypropylene matrixes that clip together to form an infiltration basin which can be wrapped in a permeable geotextile to allow controlled release of water or in an impermeable membrane to provide a sealed storage unit.

One promising new technology is being developed by SUDS expert Professor Chris Pratt, Dean of Coventry University’s School of Science and the Environment. The oil retention system is used in conjunction with under-sealed porous paving, which can retain both standing water and oil. If it proves successful, this will help to counter concerns about spills and pollution of groundwater.

The oil separator adds to the university’s existing work, which has produced porous paving with a honeycomb structure that retains oil. A further development means that by adding bacteria the trapped oil can actually be biodegraded. Professor Pratt says: “Many car parks just have the odd car with a dripping sump, which the other systems can deal with. But if there are concerns about massive leakage, the new system can cope.”

An EA project is examining the entire catchment of the Bourne stream, in the Bournemouth area, in partnership with local authorities, the local university and water company. It aims to identify opportunities for using SUDS, in this heavily urbanised environment, in both new development sites and for retrofitting within existing development. The intention is to help prevent diffuse pollution from the stream, which affects Bournemouth Pier beach. Phil Chatfield, the EA’s diffuse pollution project manager, says: “The project will ask what sort of SUDS could be used and might be appropriate.”

Consultants Binnie Black & Veatch, the University of Abertay and HR Wallingford are undertaking research in conjunction with UK Water Industry Research (UKWIR) and the Water Environment Research Foundation (WERF) in the US, monitoring SUDS demonstration projects to determine their whole-life costs. This work should also provide lessons for design and maintenance.

The problems of flooding and diffuse pollution from existing drainage systems have been recognised and the initiatives being undertaken illustrate the many efforts underway to facilitate the development of appropriate and effective SUDS systems to help relieve those problems

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