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Susanne Charlesworth: we must use sustainable drainage systems to green our cities

Dr Susanne Charlesworth

7th February, 2011

Our ageing drainage systems need to be reinvented. Instead of trying to rush water into our sewer systems we should be making use of it to reduce flooding and encourage green space

In general, climate change is due to industrialisation and urbanisation leading to increased fossil fuel use for heating/cooling homes, increased concrete or other impermeable surfaces, decreasing numbers of natural water bodies such as canals and ponds, loss of vegetation on the whole and replacement of some street trees with small, non-native species. This means that, contrary to popular belief, it is not transport that is responsible for the greatest percentage of greenhouse gas emissions, but buildings. Globally, homes, workplaces, public buildings etc, are said to emit up to 48 per cent of greenhouses gases.

A changing climate leads to changes in rainfall regime and thus flooding becomes a major problem. The devastation caused by the summer storms of 2007 and 2008 in the UK highlighted that its ageing sewerage infrastructure cannot cope with the situation as it is now, let alone if some of the more conservative predictions of the possible impacts of global climate change become reality.

Sustainable drainage systems (SUDS) are an alternative to the conventional approach which goes far beyond simply mitigating flooding, but has a role in improving water quality, greening and cooling urban areas, reducing the urban heat island effect and having a positive impact on human health. Conventional hard drainage tends to concentrate on managing water quantity by gathering all the runoff water from impervious streets and pavements into storm sewer systems which pass via gullypots, pipes and water treatment facilities into the receiving watercourse. SUDS encourages infiltration and detention of surface water on site. It is a different way of managing water. Instead of treating it as an embarrassment, to be hidden from sight and forgotten, water is seen as an asset.

Cooling through vegetation


The ‘urban heat island effect’ was first noted in 1819 in London and is peculiar to cities where, even in winter, urban areas can be several degrees warmer than the surrounding countryside. The causes of heat ‘islands’, include: lack of vegetation, transport, heating, cooling and the thermal properties of the fabric of urban structures which store and then release heat. Studies have found that vegetation in urban areas can mitigate urban heat islands, creating what is called an ‘oasis effect’ whereby temperatures are reduced at the local level near planted areas, whether for buildings surrounding a park, or with vegetation planted around the individual construction. SUDS devices which have been used extensively for this purpose are green roofs, which have the added advantage that they can be retrofitted to suitable buildings without the need for extra space. New York’s Heat Island Reduction Initiative centres on increasing vegetation, in particular green roofs. Should 50 per cent of New York’s flat roofs be greened, the heat island effect could be reduced by up to 0.88C. Green walls have been used less than green roofs, but they, too, have the same benefits in terms of heat reduction, storm peak attenuation and insulation, both for maintaining heat inside buildings in the winter and cooling the building during the summer.

In the UK, with the exception of Scotland, there has been a general lack of uptake of SUDS among local authorities, civil engineers and construction firms. The barriers to SUDS include:

Land take: developers make less money out of a detention pond than an extra house, but in housing developments there is a premium on houses fronting SUDS devices such as ponds and wetlands which can go some way to making up for the loss of land for construction.

Maintenance:
SUD devices are for water treatment and like any such construction will need maintenance to ensure that they carry out their function efficiently. Unless ownership is sorted out, no-one will be responsible for carrying out and paying for essential maintenance to be carried out.

Lack of long term R & D:
an argument is often given that there is insufficient evidence to support the claims made of the efficacy of the SUDS approach. There is a lot of evidence from other countries, but there is a lack of England and Wales-specific studies.

SUDS are expensive:
When PCs first came out they were beyond the pockets of most. However, as production costs declined and more were sold, the price decreased. The same will be true of SUDS. At the moment, hard engineered solutions are familiar and components are mass-produced, hence they are cheaper. In the future the same will be true of SUDS devices when their use becomes commonplace. Those considering incorporating SUDS into urban infrastructure need to think in the longer term than the life of a Council or Government term. Whilst SUDS devices need maintenance and maintenance costs money so do hard engineered solutions. Gully pots need emptying, flood damage needs repairing, surface water disposal costs money. It is easy to undertake a cost-benefit analysis in monetary terms, but far more difficult to incorporate other benefits which SUDS provide in terms of the environment, amenity provision and increased biodiversity.

One of the other reasons is the need for SUDS devices to be retrofitted - rather than the more straightforward opportunity of including the principles in new developments. However, systems of small-scale retrofitting can be undertaken in densely occupied urban centres, such as green roofs and walls, areas of porous paving and rainwater harvesting, so that SUDS acts as a supporting mechanism to relieve the pressure on conventional systems. Street trees can be retrofitted where existing service infrastructure such as cables and pipes allow and provide shade for buildings, both from the sun and also from prevailing winds. Rain gardens or street planters can also be retrofitted at street level; made of stone, they integrate well into the built environment. Suburban areas can support larger devices such as roadside swales, ponds incorporated into roundabouts and larger areas of porous paving on supermarket and industrial estate car parks. Private gardens can be used as rain gardens, encouraging water to infiltrate and dissipate slowly. The largest devices, such as constructed wetlands and ponds, can be used in suitable areas on the urban periphery.

There are many strands to the reasons for climate change and therefore many and varied ways of dealing with it. There will therefore not be a single technique which can be used to solve the problem as a whole, but rather a suite of approaches, tailor-made depending on the situation (geographical location, local climate, city structure, etc.), which can be applied. However, even relatively small initiatives can have a major impact. A study in Greater Manchester calculated that if towns and cities increased their green cover by as little as 10 per cent, surface temperatures would remain the same in spite of climate change.

Dr Susanne Charlesworth is Leader of the Sustainable Drainage Applied Research Group at Coventry University

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