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15 Sep 2010
How do you plant along a river’s edge, knowing that millions of people could be passing through the site in the near future? How do you design, create and maintain the surrounding wetlands, knowing that man-made wet woodland is very rare and transitional by nature? How do you ensure that the habitat being created remains viable and sustainable in the long-term? The engineers of the wetlands and river edges for the London 2012 Olympic Park have been tasked with finding answers to all of these questions.
Covering more than 100 hectares of formerly derelict industrial land, London’s new Olympic Park for the 2012 Olympic and Paralympic Games is one of Europe’s biggest-ever urban greening projects. Rivers and wetlands are at the heart of the vision for the new park, which lies in east London’s Lower Lea Valley. The landscape that’s now emerging will provide a backdrop for the main action of the 2012 Games.
As river edge and wetland engineers for the project, Atkins is playing a critical role in turning the vision into reality. Atkins’ remit includes design of the soft river edges and wetlands, including riverbank restoration and bioengineering.
The transformation is unprecedented. More than 8km of riverbanks are being restored as part of the project; in tandem with this, two hectares of reed beds and ponds are being created, along with 9,000 square metres of rare wet woodland.
The masterplan for the park is the brainchild of two companies, LDA Design and landscape architects Hargreaves Associates.
“Rivers are generally neglected in our urban areas, but the great thing about them is that they bring activity and life into an area,” says David Thompson, director of Oxford-based LDA Design.
“The challenge from a landscape point of view is about getting people both visual and physical access down to the river. This was the main change that the design team brought into the project – to actually make the rivers more accessible and more open, and therefore the centrepiece of the park,” says Thompson.
Mike Vaughan heads Atkins’ multidisciplinary design team, which includes river engineers, geomorphologists and ecologists: “The idea is to open up the river corridor by making the steep slopes that line the river flatter,” he explains. “By dropping the slopes, we’ve brought the river into the park and made it much more accessible – people can get close to the river and see what’s going on there.”
Getting the riverbank geometry just right was a delicate balancing act. Too steep, and the banks would need costly artificial reinforcement; too shallow, and they would start to eat into valuable space on the site. An optimum slope of 1 in 2.5 – about 22 degrees – was chosen.
Today, with the new landscape rapidly taking shape, it’s easy to forget that one of the Lea Valley’s best-known landmarks, until recently, was the notorious Hackney fridge mountain. And until the Olympic Delivery Authority (ODA) took possession of the site in 2006, many of the river channels that criss-cross the site were clogged with invasive weeds, along with the predictable detritus of urban decay: abandoned shopping trolleys and car tyres.
The Lea Valley’s neglected river network wasn’t only an eyesore, but also an obstacle – a gulf separating Hackney and Tower Hamlets in the west from Waltham Forest and Newham in the east.
“Before we started work on the site, there were really only three crossings,” says John Hopkins, ODA project sponsor, Parklands and Public Realm. “But we’ll be leaving behind more than 30 bridge connections across the Lea Valley and Lea Navigation.” These new crossings will be vital not only during the Games, but also after 2012: they’re an integral part of the legacy solution, stitching the new Park and its waterways into the wider fabric of east London.
Making the most of the site’s rivers and natural features to create sustainable habitats is a key part of the Olympic Delivery Authority’s vision for the Olympic Park. But the process of transforming the Park’s rivers from weed and rubbish infested gulches into pristine watercourses has been long and tough.
For Atkins, that process started with developing an intimate understanding of the labyrinth of waterways and channels that wend their way through the site. Flows and velocities were measured at different points over a period of time, with data used to construct a detailed hydraulic model to predict flood risk. That’s of critical importance, because Atkins has responsibility for everything below the four metres AOD (above ordnance datum) contour on the site.
The modelling exercise was made considerably more complicated by the impoundment of the river system during the course of 2008; in effect, this eliminated the direct tidal influence of the Thames. But its indirect influence is still felt.
“When the tide comes in on the Thames, it stops water flowing out of the river Lea,” explains Vaughan. “So the river levels fluctuate by 400mm a day.”
Atkins’ modelling calculations correctly predicted this phenomenon, and also the increased risk of flooding.
“These discoveries led to some changes in the landscaping profile,” says Vaughan. “The riverside paths have been raised by up to a metre and the profile of the wetlands was also raised, as maintaining correct water levels is critical to their survival.”
The first step in the river restoration process was to “lay back” the banks, many of which were precipitously steep. This re-profiling was necessary because much of the surrounding land was “made” ground, the result of centuries of tipping that had raised the ground level by as much as 10 metres in places. The cocktail of materials on the banks included rubble, glass, animal bones and, more recently, war time demolition materials from London’s east end.
Another challenge facing the Atkins team was the prevalence of invasive weeds. These included Himalayan balsam, Japanese knotweed and giant hogweed. All are fast-growing non-native plants introduced to Britain in the 19th century as garden curiosities; all have prospered on the wrong side of the garden wall.
Invasives are bad news for riverbanks. They reproduce and grow with prodigious speed, driving out native plant species. And they’re highly resilient. Knotweed can force its way through solid concrete, while giant hogweed contains furocoumarins, sun-activated toxins that can cause skin ulceration. Elimination was a priority – soil was treated throughout the site and the banks stripped of all remaining vegetation.
Atkins is responsible for the final look of the riverbanks and wetlands – and deciding what to re-plant presented a challenge. With banks now bare, new planting would have to fulfil not only ecological and aesthetic demands – they’d be expected to be in bloom for the Olympic Games – but engineering imperatives too.
The Atkins design team chose bioengineering techniques, rather than culverting and hard engineering, for the project. That means protecting and consolidating riverbanks by using vegetation and natural products instead of concrete. Choosing the right species with the right root systems would be critical to protect the banks from erosion.
An added challenge was that the river network is semi-tidal. The twice-daily rise and fall of around 400mm had the potential to play havoc with new planting, and the river’s high sediment loads threatened to smother anything planted from seed or plugs.
“We don’t actually have a natural river system,” notes Vaughan. “Plants don’t cope well in those conditions.”
To find out which plants would fare best – and to establish the most effective planting methods – Atkins conducted a unique riverbank planting trial along a 50-metre section of the Lea in the Olympic Park.
“We trialled plants of different elevations and different installation techniques. These were monitored over a year,” says Ian Morrissey, senior environmental scientist with Atkins. “That’s really helped to inform exactly what species we should plant and where.”
The trial revealed that plug plants would be just too vulnerable. But plants pre-grown in coir – coconut fibre matting – resisted being washed away or swamped. Coir has other benefits too – it’s easy and quick to install in rolls and pallets two metres long and a metre wide.
“The mat itself acts like a mulch, so you prevent any weeds growing up through it that might already be within the bank material. But more importantly, when the banks become inundated, you get fine sediment trapped within the coir. That helps to bind the roots and feed the plants,” says Morrissey.
Creating a sustainable riverbank ecosystem means using native species. So before the banks were scraped back, seed was collected from suitable native aquatic species – a process managed by Atkins – and stored in a seed bank. Some of this seed was then used by bioengineering and nursery specialists, Salix, who were appointed by the Olympic Delivery Authority to cultivate plants offsite in what’s believed to be one of Britain’s biggest-ever nursery contracts.
The offsite growing operation is huge and sowing for the project commenced in June 2009 – plants must be a year old and well established in coir pallets before encountering the tough riverbank environment.
Plants for the wet woodlands, including sedges, are being raised in more than 7,000 pots at Salix’s nursery on the Gower peninsula, near Swansea. And in Norfolk, the company created a new 16-acre nursery dedicated to the 2012 project. Here, more than 300,000 plants representing some 28 different species, including sedges, common reed, marsh marigolds, water mint and yellow flag irises, are being grown on more than a thousand coir pallets, ready to be transported to London over the coming months.
“The riverbanks are a landscape feature as well as an ecological feature,” says Salix’s David Holland. “We’ve grown certain species that would flower during the Games, such as purple loosestrife. To have the habitats and the plants established in time for the Olympic Games, 2010 is the year to plant out on site.”
Planting and earthworks are being managed by construction engineers BAM Nuttall. Over the coming months, the plants on their coir mats will arrive at the Olympic Park, 300 lorry loads of them. Co-ordinating delivery and installation of the pallets is like setting out a giant jigsaw puzzle. And the ability to correctly identify each planting pallet is essential.
“This is a massive job in terms of river edge refurbishment and it’s a logistical challenge,” says BAM Nuttall’s Isayas Tecleberhan. “To make it easier, each of the 1,000 pallets and rolls is tagged. It’s important each one goes in exactly the right space because you don’t want to be cutting and trimming the roots and rhizomes of the plants – we’re laying them out in blocks, to make sure this doesn’t happen.”
Installing the large plant pallets and rolls at the very edge of the river presents a challenge – the ground is soft, wet and difficult to work on. To get round this, contractors will be using a specially-adapted pontoon – a floating platform that provides easy access to the banks from the river itself.
As well as improved riverbanks, new wet woodlands will be a notable feature of the Olympic Park. They’re now a rare habitat in the UK, and the ones in the park are being created from scratch.
“It was quite a novel thing to be asked to do,” recalls Atkins’ Morrissey. “The challenge was to make sure we had the right water levels within the wet woodland areas. Atkins was responsible for working out the topographies and the channels, and how they would interact with the river.”
Wetlands have a tendency to become dry land eventually, a process that can be slowed down by selecting the right vegetation, careful water level management and maintenance.
“The sedge species we selected were chosen because they are quite vigorous so are able to compete well with terrestrial species,” says Morrissey.
Tree species for the wet woodland include willow, alder, birch and the now rare black poplar, points out Atkins’ Vaughan: “It’s fantastic for wildlife. You get a lot of invertebrates in there, as well as nesting birds.”
Birds, though, can present a challenge, particularly on the freshly planted riverbanks.
“There’s a risk of wildfowl grazing our plants when they get on site,” says Vaughan. To prevent that happening, hundreds of metres of deterrent fencing is being erected around new vegetation. “That will stay there until spring 2012.”
The transformation of the Lower Lea Valley and the creation of the new park, now nearing completion, is remarkable by any standards. Visitors to the Olympic Park – up to 250,000 every day at the peak of the Games – will encounter one of the greenest and most environmentally friendly parks ever to be created for the Olympics.
The benefits will be felt long after 2012. “We’re pulling that really difficult trick of putting in infrastructure that’s good for the Games, but will work in legacy,” says Hopkins. “This will be a great place to live and work, with rivers and parklands at the heart. Socially, economically and environmentally, there will be a terrific legacy – it’s a new landscape powering a new piece of city.”
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