Medieval watercourses, a flat landscape and green belt restrictions are key constraints for a new drainage scheme in Cambridge reports Adrian Greeman.
Cambridge is an exception in the faltering UK economy. Its ancient university colleges and modern hi-tech industries are still prosperous. Demand for housing in the city continues apace.
To cope, the city has sanctioned major new developments, one of which is on its southern edge within the agricultural green belt just under 4km from the centre. Its 2,300 houses will sit adjacent to a new medical research facility for genetics and embryology at the world renowned Addenbrookes Hospital. Up market buyers seem likely.
The quid pro quo for the developer, Countryside Properties, is that the scheme must contain significant natural space and make no impact on the natural runoff regime. Some 49ha of its overall 109ha site must be “green corridor” and about 40% of the houses must be “affordable” units.
Sustainable drainage techniques (Suds), which the city council has long favoured, are the solution to most of the drainage issues on the site. They involve a cascade of techniques but are mainly focused on a series of retention ponds that will become attractive community and ecological resources in themselves. In conjunction with inlet flow “defenders”- which remove silt - and outlet hydrobrakes, the drainage aims to mimic existing conditions.
That is a reasonable challenge for the drainage engineering and its designer URS.
The site, like most of East Anglia, is quite flat. “Its former existence as Clay Farm gives away the nature of the ground,” says Countryside senior technical manager Nigel Borrell, “and the water table is pretty high, just 1m down.”
The Environment Agency insists that runoff from the new development is no different from that of the greenfield site which was calculated at 2l/s per hectare. But on top of that, the runoff must be particularly clean. The main route out for water is Hobson’s Brook, a narrow watercourse running through the site.
“It is a man-made construction dating back to 1571,” says Borrell. “It was designed to feed pure water into the city from the Nine Wells, an area of springs nearby.”
The watercourse used to supply colleges and other institutions before eventually running into the river Cam. It, and some other freshwater channels and boreholes of the time, are credited with preventing the outbreaks of cholera that hit cities like London. The brook has its own board of trustees, who are charged with its management, and who have to be consulted about changes.
Hobsons Brook runs more or less through the middle of the site and will be a boundary between the green parts of the development and the network of roads and driveways that will eventually make up the estate.
“The brook is a man made excavation dating back to 1571. It was designed to feed pure water into the city”
Nigel Borrell, Countryside
That is being developed by several housebuilders because the whole scheme is too large for one single investor. Countryside is the strategic land developer though, and its own housing division will be one of the three firms building the houses. Skanska and Bovis are the other two involved.
The first phase of Suds surface drainage will occur within the housing areas, using roof drainage to water butts, permeable parking areas, swales and other local infiltration methods. A strategic planning framework developed by Countryside for the whole development sets out the design principles the housebuilders must follow which is to use as little piped drainage as possible.
But there are limits to what is achievable, particularly with the high water table, says URS principal drainage engineer Tracy Neal. He says an assumption of 60% impermeable ground is used.
The main method for controlling water therefore is to drain it through a pipe network to storage buffer areas from which it will slowly drain out to the brook at controlled rates.
The flat landscape ruled out a single water retention area as any large scale pipe network would have gone too deep for proper discharge, says Neal. So four pond areas have been created, allowing the drainage to be in four very local networks.
The split tied in with existing treelines and other dividing features on the site, says Borrell, including the well-known Cambridge guided busway, which runs through the site.
As a result, four ponds are provided, three of which are clay lined and will be permanent water features, and a fourth which is currently a storage area and soakaway in flood conditions, though it may be rethought as a wet area too. Each has capacity for a 1 in 100 year flood plus 30%, with the overflow stored in shallows around each central pond.
The largest of the ponds will also service a second adjacent housing development on next door Glebe Farm, with runoff from 300 more houses.
“Each pond will have a different character,” says Borrell. The big one will be a nature reserve for local birdlife, with limited public access while the others will have community uses.
The ponds will be settling areas for any particulate matter in the water but another earlier line of defence is also provided. Each pond gets a “defender” unit from Hydro International, which is a large chamber in which induced vortexes will separate out silts and any heavy metal particles before they enter the ponds. The flows also force oils and other light items like plastic bags to rise to a top chamber.
The design of the defenders includes various baffles that ensure silt and sediment are not stirred up again when new flows rush in. Instead silt will be removed at intervals if maintenance inspections indicate a need, says Hydro International sales manager Phil Collins.
Series of syphons
The defender units are installed on the housing side of the brook and are connected to a series of siphons which take the drains under the watercourse. They are quite large, some 5m deep and required some well supported excavations to be made by civils contractor Tamdown when they were installed last October.
Despite their 3m diameter, however, the defencers were not too cumbersome, since each is made from polypropylene. The units come in two pieces which are connected to pre-installed pipework and then backfilled.
For the largest pond, a small entry area is also provided after the inlet, with a weir allowing further sediment trapping.
Finally, exits from the pond into the brook are safeguarded with stainless steel hydrobrakes. These units, also supplied by Hydro International, are much smaller than the defenders and easily manhandled. They also use vortex induction, this time to throttle back the water flow to the calculated maximum and ensure the brook is not overloaded.
Infrastructure for the site is currently nearing completion after work began in 2009. The first areas of housebuilding are underway although it is likely to be another decade before everything is finished.
“Timescales will depend on market conditions,” says Borrell.