A huge sustainable drainage project in Leeds aims to revolutionise efforts to improve urban flood resilience.  Margo Cole reports.

Engineers for a £250M car-free housing development in Leeds have developed a sustainable drainage system (SuDS) which uses space normally given over to vehicles. The development is one of the most comprehensive in the UK combining a range of different methods for dealing with surface water run off. Consultant Civic Engineers came up with the plan for developer Citu, which wanted to prevent surface water run off from the development from reaching the city’s drains. 

The development in the city’s Climate Innovation District straddles the River Aire south east of the city centre. The drainage design aims to mimic nature incorporating permeable footpaths and thick vegetation, holding back run off and channelling it into the river.

In a city where the memory of the devastating floods of Boxing Day 2015 is still raw, preventing surface water from entering the city’s drainage network is key.

The first phase of what will total 800 terraced houses and apartments is currently under construction on the north bank, using timber framed panels fabricated in Citu’s own factory on the other side of the river. In October a new footbridge was installed to link the two halves of the development and to give people already living in the area a means to cross the river and access the riverside path and cycleway, which go into the city centre.

The site benefits from Leeds City Council’s £50M flood prevention scheme, which opened in late 2017 and uses moveable weirs to massively reduce flood risk in Leeds City Centre. But the development’s riverside location has also provided a great starting point for innovation in SuDS design, as all the surface water collected on site can be discharged into the river in a controlled way.

“[Rainwater] is constantly slowed and treated before it ends up in the river,” explains Civic Engineers director Paul Morris whose firm designed the SuDS. 

“The ‘route one’ solution would be to provide an outfall and a big tank underneath, but as a practice we want to bury as little infrastructure as possible. Instead, the idea is to catch the water where it lands, so we have designed the landscape to be permeable – like a big sponge. Then water trickles out into the river through outfalls in the gabions.”

Phase 1 of the development features SuDS elements such as permeable street surfaces and gabion walls

Although parking is provided, it is in an undercroft car park, enabling all the areas around the houses to be car-free, including the central street, Solar Avenue. As a result, over half the land within the Climate Innovation District development is open green space, creating plenty of opportunities to incorporate different SuDS features that add amenity value for residents and the wider community. 

Houses are accessed by permeable gravel paths. There are wildflower meadows and beds of edible plants and older trees have been retained on the site, as well as new trees and shrubs, which have been planted to help slow down surface water filtration.

The houses themselves are built sufficiently high above the river to withstand a 1 in 200 year flood. Every house on the development has its own small “rain garden” outside its front door – a planted area that stores rainwater and lets it slowly infiltrate the ground. The houses on the river frontage, in the area known as the Secret Garden, open out onto a communal meadow leading to a deck alongside the riverbank; those on Solar Avenue open onto a car-free street punctuated by rain gardens and seating areas. 

But this soft appearance belies a hard-working landscape. Although the development is car-free, it had to been designed so that emergency vehicles can get to the front door of every home if required, including those in the grassy meadow of the Secret Garden, so the grassed areas and the gravel footpaths have to be capable of being trafficked.

“We were desperately keen to avoid a hard road. We wanted to landscape everything,” says Morris.

This was achieved by providing a stone sub-base beneath the whole area, and using a strong interlocking recycled plastic grid system to reinforce the grass and the gravel paths. The grid system is open enough for surface water infiltration, so is ideal as part of a SuDS solution.

“Any water that lands on the “street” element infiltrates through into the reduced fines sub-base,” explains Morris. “From there it goes through the voids in the sub-base and out through the gabion outlet into the river.  “One of the big parts of our brief was to make all the infrastructure work, which is what you have here, because we are using the sub-base as part of the drainage system. In extreme events that fills with water which filters and drains back into the river.”

The sub-base also runs under the rain gardens, so the same mechanism operates here, with rainfall eventually finding its way into the sub-base.

Roof run-off goes into a downpipe that discharges into coarse stone beneath the rain garden and from there into the sub-base. However, every house also has a water butt that intercepts this downpipe, so it is only if the butt is full that roof run-off will go into the sustainable drainage system.

We were desperately keen to avoid a hard road. We wanted to landscape everything

The rain gardens can store large amounts of rainwater and host a variety of plants. Residents can also add their own planting to personalise their homes and help shape the look and feel of the street. Morris says in normal rainfall conditions water is collected within the rain gardens and gets used up by the plants as they grow, so will never make its way into the lower levels. It is only during bigger rainfall events that the sub-base will be called into action for storage.

“The strategy is that the plants will take up the first 5mm of rainfall,” he explains, adding that the extensive tree planting across the site will also play a major part in reducing the volume of water reaching the river.

The houses in the Secret Garden also have “green roofs”, created using the perennial herb Sedum Acre. This layer of vegetation retains rainwater and promotes evapotranspiration, providing a level of attenuation that will prevent large volumes of water pouring into the downpipes during sudden downpours.

All these measures not only slow down and reduce the volume of water getting to the river, they also ensure  that the water that does go into the river is clean. “The filtration process treats the run-off, as well as providing amenity and biodiversity benefits,” says Morris.

Towards the back of the site, the area between the terraces of houses and the apartments will be made up of pathways crossing a large dry basin that acts as a series of attenuation ponds. Most of the time, these planted areas will provide amenity space, enhanced by natural play equipment. But after periods of heavy rain, they fill up to become ponds, and the water slowly seeps into the ground.

“In the absolutely worst case of a 100 year storm, [the basin] should fill up,” says Morris.

We would have had to provide sub-base anyway. We’re just making it work harder in terms of surface water drainage

The only road on the development is a short section of highway at the entrance. Any run-off from this bit of road will drain to a central rain garden, which will reduce flow in the same way as the gardens do on the rest of the site, and also filter the run-off so it is clean when it reaches the river.

This road leads to the undercroft car park, which sits beneath the Solar Avenue. Developer Citu wants residents to be less dependent on cars, as part of its overall carbon reduction strategy, but the planners insisted on the provision of quite a lot of parking in this first phase. 

There will be no drainage at all in the undercroft car park, says Morris: “The strategy being tested is to rely on evaporation for any water that makes its way into the car park. That avoids an additional pipe network and interceptors; and everyone’s accepted that, in rainfall events, there will be water in there.”

The development is not entirely free of pipes. A network of perforated pipes runs below ground throughout the district to take surface water run-off from hard surfaces – such as the paving in Solar Avenue – and slowly sequesters it into the ground.

Morris is looking forward to pushing the SuDS  strategy even further on future phases of the development. These will include more homes, offices and a multi-generational building housing a primary school and elderly persons’ care home. Across the river, Citu is building an extension to its timber panel factory. It includes more sustainable features, such as rainwater harvesting and a reed bed filtration system.

“We need to push it a bit harder,” he says.

Morris is convinced the SuDS strategy makes financial – as well as environmental – sense. “This is more cost effective, I’m absolutely certain of it,” he says. “There are no oversized pipes, and no attenuation tanks; the dry basin is part of the earthworks balance, so that’s effectively free attenuation; and with the need to make [the Secret Garden] capable of being trafficked, we would have had to provide sub-base anyway. We’re just making it work harder in terms of surface water drainage.”

He adds: “It’s essentially a functional landscape.” 

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Tagged with: Flood Protection SuDS

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