The river's edge - a journey of transformation
The team working on the rivers and wetlands of the Olympic Park, one of Europe's biggest greening projects, faced numerous challenges, explains Ruth Boyle of engineering consultancy Atkins
When Mike Vaughan, who heads up an Atkins multidisciplinary design team of river engineers, geomorphologists and ecologists, was approached to be the river edge engineer for the planned London 2012 Olympic Park there was no doubt about the scale of the opportunity. Rivers and wetlands are at the heart of the Olympic Park, which lies in east London's Lower Lea Valley.
Atkins is used to large-scale projects and to working alongside a number of other teams, but the Olympic Park project is at the far end of the scale. The 246ha of formerly derelict land is one of Europe's biggest ever greening projects.
The objective was to provide new river edges to the park, the frog ponds and the wetland areas. The official remit for the team was to manage everything below the 4m contour line; the work above this line was owned by the landscape architects. Vaughan and his team had a clear vision - to play a significant role in the transformation of the Park for the London 2012 Olympic and Paralympic Games, and for the legacy that would follow. The transformation has been unprecedented.
Over 8km of riverbanks have been restored as part of the project, as well as the creation 2ha of reed beds and ponds, along with 9,000m2 of rare wet woodland. The challenge was to achieve visual and physical access to the river and to make the rivers more open, and therefore an integral part of the Park.
"The idea was to open up the river corridor by making the steep slopes that line the river flatter," explains Vaughan. "By dropping the slopes, we've brought the river into the Park and made it much more accessible - people can now get close to the river and see what's going on there."
Getting the riverbank geometry right was a delicate task. Too steep and the banks would need costly artificial reinforcement; too shallow and they would start to eat into valuable land at the site. An optimum slope of 1:2.5 - about 22° - was chosen. The riverbank area was limited by the need to convey floodwater and the location of terrestrial landscape and infrastructure. The team used two approaches to steepen the riverbanks. Firstly, where possible, the riverbanks were terraced using coir rolls and timber stakes. In other locations, where only a 70° bank was possible, a reinforced detail was used, providing layers of geo-grid and steel mesh cages, faced with a riverside turf.
Now, following an intense transformation, the revitalised waterways continue to be vital during the games as well as into the legacy to follow. The features ensure flood resilience, enhanced biodiversity and visual appeal.
The Olympic Delivery Authority's vision for the Park was always to achieve a natural habitat and Vaughan and his team have played a vital role in making this a reality. What now exists is a natural habitat of greenery and accessible waterways, quite a difference from the previous site of complex weeds and neglected riverbanks.
For Atkins, the process began by developing a detailed understanding of the maze of waterways and channels that twist and turn their way through the Park. 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.
Atkins undertook analysis of the risk of flooding caused by frequent rainfall, taking into account the tidal impact. 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.
Vaughan had concluded that the tidal effects were of great importance as they would directly affect the flow of water out of the River Lea and fluctuations were on average 400mm/day. The findings impacted on the landscaping profile, highlighting the need for raised riverside paths, the absence of which could have impacted negatively on the Park today, particularly in adverse weather. Maintaining the correct water levels was critical for survival of the landscape.
Sustainable drainage techniques were also used across the Park. In the landscaped areas, porous strips were used, feeding into bioswales, which drain down into the riverside ponds. Surface conveyance, underground pipes and other storage features have also been used.
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 10m in places. Atkins' team was also responsible for the final look of the riverbanks and wetlands, considering the ecological and aesthetic demands, including seasonal planting to ensure that the site would be in bloom for the Games. Choosing the right species with the right root systems was also 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 create issues for the new plantation. The Atkins team conducted a trial along a 50m stretch of the Lea in the Olympic Park, a unique activity to ascertain the right plantation. In addition to the River Lea were other bodies of water.
To the north of the Park lie three new triangular ponds. Two of these were designed to dry up in the summer, forming moist grassy hollows. The third pond was created to retain water; however preventing it from drying out was complex. The team created a connection between the third pond and the river to act as an overflow and a feed, which could also be regulated. When the pond level rose too high, water could be drained back into the river; when it started to dry out, a valve could be opened to release river water back into the pond. This is believed to be the first of its kind on this scale.
In addition to the waterways, the team was involved in wet woodlands. These areas required some shallow water in order to support the natural habitat in a moist environment. Control of water flow in the two wet woodlands areas was achieved by enclosing them in low earth embankments, which are overtopped by the river during higher flow.
The wetlands project was extensive and the detailed flood risk assessments enabled a multifunctional and resilient landscape to be created. The carefully designed waterways emerged and enabled plantation of what we can now see in full bloom on the dramatically transformed Park. The design of the wetlands area has given the park a sustainable habitat, breathing natural life into the London 2012 Olympic and Paralympic Games.