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Market value

by Ruth Slavid
The water strategy at Rathbone Market, a high-density development in east London, provides precious high-quality open space as well as cutting down on the need for stormwater interception.

Rathbone Market is an intensive development in a tough urban area. Photo ©: Tim Crocker

If you moved into an apartment at Rathbone Market in east London, you probably would not immediately be aware of its SuDS strategy. But you could not fail to notice and enjoy one aspectof that strategy – a pond at the centre of the scheme that provides visual interest.

Rathbone Market is in a built-up area of London, sandwiched between a flyover on the A13 and the Barking Road. Outdoor space is at a premium, so what could be nicer than a pond, planted with marginal plants? But in fact it is part of an intelligently considered drainage strategy on all parts of the three-phase development. By retaining water in green, brown and blue roofs, and by allowing the level of the pond to change to accommodate run-off from storms, the project cuts back severely on the volume of attenuation tanks that are needed to hold back stormwater before releasing it to the sewage system. In the second phase of the project, tree pits are also used for water storage.

This is an ambitious project on which the design team – landscape architect Churchman working with architect CZWG – has worked with a sympathetic and enthusiastic client in English Cities Fund (ECf) to create the most water-sensitive design possible. The fact that it was still not able to avoid the use of attenuation tanks is an indication not of lack of imagination or of willpower, but of the fact that such high-density schemes offer limited space for alternative measures.

An elegant rill recirculates the water. Photo ©: Tim Crocker

The development is in three phases, and centres around the revitalisation of what was once a thriving market space, with the intention that it will be so again.

In total it provides 650 new homes and 20,000m2 of commercial space. The three-phase construction was unavoidable, since there had to be a degree of decanting. The project mixes social housing and housing for sale in a manner that is indistinguishable. The aim is to build a community, and having a social focus such as the pond helps with this.

This pond is surrounded on three sides by tall housing blocks and on the third by an acoustic living wall, designed to absorb the road noise from the A13. All but one of the blocks have biodiverse roofs, with a planting substrate and plug plants. These roofs will in any case reduce the rate of run-off of rain, but water that passes through them is channelled down to the podium level.

The only roof that is not planted is the lowest, on top of a block that is entirely social housing. On that roof the design team has created a series of allotment beds. It is tempting to call them urban allotments, but of course many allotments are urban. What distinguishes these is that they form part of the building rather than being on a piece of waste ground close by – a clever move since space for allotments is increasingly under pressure.

The allotment ‘plots’ are cleverly detailed, with integral spaces for keeping tools. Irrigation is carried out manually, using water from a butt that collects rainwater from an adjacent, slightly higher roof. This can be topped up with mains water in dry periods.

There are allotment boxes on top of one of the roofs. Photo ©: Tim Crocker

The intention is to run the allotments in association with Capital Growth, an organisation set up in London to increase the number of community food- growing spaces. This is the only roof that will be publicly accessible, and can evidently also be used for leisure. There is some shading to protect from sun and wind, but enough open area to allow plants to thrive. The paving around the beds is raised on pedestals, and rain water runs through the gaps between the slabs, again to be collected and directed to the pond.

This pond itself is an elegant and sophisticated piece of design, of irregular shape. It has water plants around its margins and has been designed to be relatively inaccessible – the idea is that residents should enjoy being around it, but not wade into it. The clever part of the design is that the water level will be allowed to rise and fall by around 200mm. This effectively creates a storage facility of 40m3.

The water will, says Andrew Thornhill of Churchman, be ‘clear but not crystal clear’. It is recirculated through a ‘spring’, effectively a small fountain, that opens into a rill that then runs into the pond. A perforated pipe runs through the gravel beneath the planting at the edge of the pond to draw water through the filter bed. There is also a silt trap. One of the advantages of this arrangement,
in addition to the visual excitement of moving water, is that it creates some ‘white noise’ which distracts from the sound of the nearby road.

This open, planted space serves the residents of the surrounding development. (it is not accessible to the general public). This drainage strategy  only deals with the water in phase one. The second phase, as well as including more residences, also incorporates a library and a substantial retail element. The layout meant that there wasn’t adequate space for another amenity pond.

The pond is at the heart of the drainage strategy. Photo ©: Tim Crocker

Instead, storage happens within the roofs. The two tallest buildings, which will not be overlooked, will have straightforward ‘blue’ roofs – flat roofs with parapets and the ability to collect water. Two of the lower roofs will be green amenity roofs – but will also be ‘blue’, while the roof to the community hub, which will not be accessible but will be overlooked, will be a brown biodiverse roof – and again will be ‘blue’.

These dual roles are made possible by placing a storage medium – effectively an open cellular structure, beneath the planting substrate and typical drainage board.

As on the pure blue roofs, this can be used as a storage capacity to hold back water after a heavy storm. In total the capacity of these roofs is around 110m3.

Stored water of course adds a structural load, in the case of these roofs around 80kg per m2 but, said Thornhill, it did not actually lead to any increase in the structural frame, since the additional loading was absorbed in the design factors required for snow loading. 

A living wall provides sound attenuation from the adjacent major road. Photo ©: Tim Crocker

The other additional water storage comes within the root areas of the trees. The trees share a root zone which when combined reduces the volume of soil that each individual tree needs. But the clever move here is to use a geotextile material for this dedicated root zone. Because the geotextile prevent soil compaction, it can take up water very easily but also drain very easily. This both enhances the irrigation and hence the health of the trees, and provides another degree of attenuation. A conservative estimate allows for 20% of the soil volume in water retention.

All these measures put together have allowed the number of attenuation tanks on the site to be reduced, with a cut in capacity of around half.

At Rathbone Market the below-ground drainage is simplified considerably, which would be a positive result. But what is truly admirable about the project is that it has been reached with such an enormous increase in amenity – biodiverse roofs, a lovely pond, healthy trees and space for growing vegetables.

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