Regulations Our rapidly changing climate means efficient urban drainage is more important than ever. But if we are to prevent a repeat of last month’s floods, systems need to manage water flow rather than simply sucking it away quickly.
There used to be a few things you could count on from a British summer – the early exit of our contenders at Wimbledon, bank holiday travel chaos and the inevitable hose-pipe ban. Although the first two may still be true, this year it has been the abundance of water that has made the headlines. The floods at the end of June can be partially attributed to the changing climate, which is bringing us warmer and wetter weather. Some of the devastating effects can also be partially blamed on poor water management.
“Our existing drainage simply washes pollutants into water courses and when there’s a downpour it has a flushing effect,” says Phil Chatfield, policy officer at the Environment Agency. “We need to go back to something closer to the natural state, or our urban areas will be overwhelmed.”
This is why the Environment Agency, the government and local authorities are keen to promote the use of sustainable urban drainage systems (Suds) on new developments. Rather than removing water from a site as quickly as possible, Suds have three main aims: to reduce the amount of water flowing from a site, to manage its flow so that sudden influxes are avoided and, where possible, to clean it along the way.
The use of such schemes is recommended in planning guidance. PPS25 directs planners to encourage the use of Suds and PPS23 shows how they can help to control pollution. Then there is the Code for Sustainable Homes and Part H of the Building Regulations, both of which are concerned with water management.
But despite this recognition of the value of Suds, they are still not being included in many new developments. In the past, this has been put down to ignorance about how they work and the industry’s reluctance to adopt new practices. Chatfield points out that this is partly because there is no one-size-fits-all solution: “Using Suds means designing something bespoke – the way sustainable drainage will work on each site is different.”
However, these barriers do seem to be breaking down. “In my experience, most consulting engineers are aware of Suds and of how to design them. If they’re not, they know where to get that information,” says John Howe, general manager of precast concrete paving and kerb association Interpave. “They will also be aware – as will architects and developers – that the government’s planning policy is saying we must have Suds.”
So if the information is getting through, what’s the problem? Rupert Lodge, partner at engineer Max Fordham, suggests that developers are baulking at the price. “Clients hate it because sustainable drainage is expensive but you have to do it – why should somebody downstream suffer because your scheme is well drained?”
However, the overall picture seems more complex. Howe and Chatfield point out that Suds are being increasingly used in developments that remain in private ownership. This is because, in these cases, there is a single point of responsibility.
But this is not always the case. “Housing is a different kettle of fish because developers build houses and their infrastructure and then pass them over to the local authority,” says Howe. “That’s where Suds are falling down. Who is going to take on their maintenance and liability?”
This is the main barrier to the adoption of Suds. The management of water is split between utilities companies, the Environment Agency and councils. Once you throw a developer into the mix, it is not clear who is required to maintain the systems.
Defra is aware of this problem, and has set up urban drainage pilot schemes. The aim is to get stakeholders together to see whether they can manage the water in a holistic way.
Some councils are taking the initiative themselves – Oxfordshire is cited by many as one of the most progressive councils in its approach to drainage. It is using permeable paving (see right) in some of its developments and, importantly, it is designing schemes so that these are isolated from the services to avoid maintenance problems. Chatfield says that Bristol and Edinburgh are considering similar schemes.
But for many the pilot schemes do not go far enough. The impetus for real change could come in the form of the EU’s water framework directive, which requires a reduction in the pollutants entering rivers. In order to comply, the country’s rivers need to achieve a “good” status by 2015 – currently, one in seven rivers in urban areas is of poor or bad quality. Suds are seen as vital to achieving this and Defra is expected to launch a consultation soon on measures to reduce pollution. These could include rules on which organisations will be responsible for Suds’ long-term maintenance.
If this happens, the last barrier to the widespread use of Suds would have been removed. As the recent floods show, such systems are more necessary than ever. As Chatfield says: “In lots of places we’ve buried our rivers and lost contact with the natural environment and then we’re very surprised when it comes back to haunt us.”
All about Suds
Sustainable urban drainage systems come in various forms. The techniques below can be used on their own or in combination with each other. There are five main categories:
This allows rainwater to filter through a concrete surface, such as a road or pavement, for temporary storage before it infiltrates the ground. The water is reused
or released into a water course or another drainage system.
Soakaways and infiltration trenches
These are underground features filled with crushed rock, which aid the dispersal of surface water run-off. The main problem is that they tend to become clogged with silt so their performance deteriorates over time.
Swales and infiltration basins
These are ground-level ponds or depressions that use vegetation to filter surface water. Swales disperse water and provide temporary storage during storms; infiltration basins are the final point in Suds.
Land drainage systems
These include perforated pipes that direct surface water to a drainage dispersal point, such as a soakaway.
Onsite attenuation and storage
These systems are used where it is not possible to disperse surface water through infiltration (for example, because the water might include pollutants) or where the peak flow rate needs to be restricted. The water is stored on site and its dispersal restricted by a flow control device.
Specifier 20 July 2007
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