Kingsmead rain garden before and after
Greening the grey: introducing rain gardens
25th October, 2012
In order to reduce the flow rate of urban rainfall - and clear the often polluted water from buildings and streets - cities must learn to mimic the natural 'bioretention' properties of undeveloped land, says Laura Laker
Once people - and councils - start thinking about sustainable water management the possibilities are almost endless
Water management is a major issue in large urban areas, where after heavy rainfall, rooftops, streets and pavements act as funnels. This sends huge volumes of water very quickly into drainage systems, putting pressure on rivers and increasing the risk of flooding.
In contrast, undeveloped land absorbs and utilises water, thus slowing its progress to rivers. It is this natural bioretention that our towns and cities must learn to mimic.
Rain gardens do just that. In its most basic form a rain garden is a planted depression in the ground, providing porous and absorbent materials into which water can soak, with plants that can withstand occasional temporary flooding.
Portland in the USA is a city that has utilised rain gardens since the early '90s, and Philadelphia, Pennsylvania, has embraced green infrastructure and water management extensively as part of its Green City, Clear Water plan. In Seattle, municipal funding - alongside the efforts of local NGOs -has helped produce neighbourhood champions who maintain their own local rain gardens (www.12000raingardens.org).
Dusty Gedge, famous for his pioneering work with green roofs, is a strong proponent of rain gardens. He says the UK lags behind by 'at least a decade'.
In South London, Owen Davies, an engineer working for Lambeth council, is starting to change all that. An inner city borough, most of Lambeth is paved, leaving very little green open space, Victorian sewer systems and only the Thames as an open water channel.
Davies, Lambeth's Sustainability Engineer for Environmental Services & Highways, says: '15% of [Lambeth's] land use is public highway; it is a public open space and I think it should be looked at not just as a place to drive through; it should also be enjoyed.'
Existing traffic calming measures are often simply concrete structures, he points out, but councils could be much more creative with the way they manage roads.
Following a surface flooding assessment Davies started work to introduce rain gardens onto the borough's road network and also started encouraging residents to de-pave their gardens.
One traffic-calming scheme in Norwood was met with enthusiasm by residents when the introduction of rain gardens was proposed. Davies said: 'It started off as traffic calming with rain gardens and it became rain gardens with traffic calming. People like them so much they want them in their front gardens.'
On a town - or city - wide scale rain gardens can also help support and even improve biodiversity. They will mitigate the urban heat island effect and help to create more pleasant places to live. Measures like bioretention strips and tree pits use absorbent materials to slow and absorb water run-off from roads and pavements, while Bioswales - those long depressions often seen running parallel to roads - can be used to manage water runoff or to collect and channel water across a garden or park.
In this way sustainable drainage systems - or SuDS- mimic the natural landscape, with depressions and further strategic planting helping water soak away into the ground. This is particularly helpful after a drought where large areas of parched, compacted soil in parks and gardens often experience high water runoff.
But for as much as local authorities have responsibilities for water management, Dusty Gedge says we all have a stake in water management and that there is no reason why we shouldn't all be creating our own rain gardens.
"Sustainable urban drainage is demanded by the engineering world and we think that the public can make a difference, too," Gedge says.
What he refers to as a manifesto, a guide co-written by Gedge (downloadable at raingardens.info) gives practical tips on how anyone can, in most situations, create rain gardens where they live, whether outside a block of flats or in their back gardens.
'The idea about a rain garden is not about where is best [to put one], it is wherever you can intervene to stop water going down the drain,' says Gedge.
He suggests anything - from planted beds beside blocks of social housing or offices with water redirected from drainpipes, to planters intercepting water from individual roofs. He adds that these measures can even incorporate food growing. As with all rain gardens, if there is too much rainfall the excess water must still be able to reach the drains.
Once people - and local councils - start thinking about sustainable water management the possibilities are almost endless, says Gedge: 'I was out with one of the [London] boroughs the other day, it was raining. We identified nine or ten sites for rain garden creation.'
As with Davies' experience, Gedge says wherever he goes his ideas are not only met positively, but they really seem to capture people's imaginations."Every time I put a picture up of a rain garden in one of my talks everybody says "I want to make one",'"he adds.
Another urban area now actively looking at SuDs is Hackney in East London. The nearby River Lea's (or River Lee) water quality is adversely affected by urban pollution while around 3% of the borough's households are at risk of flooding
Kingsmead Estate, within 500 metres of the river, is currently part of a rain gardens pilot managed by the charity Groundwork London, as part of a wider Hackney SuDs programme and the Environment Agency's local flood mitigation plan. Groundwork (www.groundwork.org.uk) is consulting with estate residents, following the introduction of smaller rain gardens in June, to potentially introduce a substantial rain garden in a grass square between two large blocks of flats.
Alex Forrester, Groundwork Community Project Officer, said there is cautious support from residents:"There is a history of antisocial behaviour in this area so residents were concerned about what it would be used for. They put forward the idea of using it as a [fenced in] wildflower meadow."
He added that even if it remains simply a grassy area, rain garden technology could be installed under the turf to absorb water from the surrounding roofs.
As Owen Davies puts it, we should be making our green spaces mean more than simply inert grass: "If we are going to green the grey, let's make it useful, too," he said.
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