How we moved a river: re-routing a waterway and protecting the environment
A year ago, we completed work moving a river to save a railway and preserve the surrounding are.
The radical solution has reduced the long-term risks of erosion, protected biodiversity and provided better land access. All in just four months, with the new channel opening on 27 September.
Between Exeter and Barnstaple in Devon lies a branch of the Great Western Mainline known as the Tarka Line, named after writer and naturalist Henry Williamson's Tarka the Otter.
Though beautiful, the River Taw next to the railway tends to flood. At one meander, near Colleton Mill in North Devon, the flood caused substantial damage to a retaining wall supporting the railway line. It compromised the integrity of the river bed and the banks around it, with a risk of further collapse and closure of the line.
In June last year, Network Rail partnered with consultancy WSP to secure the wall for another winter – but we had to act fast.
18 months in four
The approaching winter, and the protection of the river's inhabitants, meant we needed to complete 18 months of work in just four. As well as breeding birds, bats and otters, the River Taw is home to salmon and sea trout, with a protected spawning season that starts in about October.
Network Rail, WSP, Construction Marine and the Environment Agency committed to completing the works much faster than a project of this size and sensitivity might otherwise take.
The new route would permanently move the river away from the railway line and protect the environment and rail infrastructure for the future. It would enable us to reinstate habitats for local wildlife. It would also provide a safer exit route for livestock, which had previously been at risk of becoming stranded in floods.
The new 185m alignment would mimic the river's existing direction and follow one of its original routes. A listed road bridge and the railway had constrained the river, leading it to migrate downstream until it could move no further.
The realignment would include a specially designed pool and rocky and shallow parts of the stream to maintain the river's overall character – 100m from its most recent footprint.
Environmental constraints and soft engineering
The project accounted for the needs of multiple parties, including the land owner, residents and railway passengers, whose safety was paramount.
We also considered certain environmental constraints. We created a backwater channel to provide a new habitat for wildlife and purpose-built areas for reptiles. The land owner's access route was reinstated following the river's diversion, allowing him to move his cattle safely from the floodplain – something that was previously impossible due to the wall's partial failure and a collapse of the embankment.
We used soft engineering approaches to manage the river bed and stability of the bank. This included materials such as coir matting, traditional willow spilling and rock armour.
Click on the gallery to see more images
Re-homing the inhabitants
To re-route the river, we built retaining walls at each end of the existing meander, diverting water along its new path. This left a stretch of water with no flow. Fish would become trapped behind the newly built walls – or bunds – along the old channel's meander. We carried out a two-day rescue to recover 3,741 fish and released them downstream of the diversion.
WSP said: "Our project demonstrates that with imaginative, future-ready thinking, one investment can deliver more benefits than the original aim. In this case, the target had been to stabilise the railway. The result is a safe rail route protected from current and future erosion, ensuring that the needs of all those touched by the railway, in one way or another, were appropriately considered."
The new channel opened ahead of time, on 27 September – ready for the salmon spawning season.
Realigning the River Frome
It's just one example of how we have carefully redirected a waterway to benefit the landscape, railway infrastructure and passengers.
Between autumn 2016 and spring last year, we replaced an aqueduct carrying the River Frome above the railway whilst taking in to consideration local biodiversity. It was part of the Greater West Programme to electrify the railway from London to Cardiff – a plan to enable greener and more reliable journeys, and a quieter, cleaner environment for lineside neighbours.
We needed to raise many structures such as road bridges to provide enough clearance for overhead line equipment. One of these structures is the aqueduct carrying the River Frome over the railway near Chipping Sodbury in South Gloucestershire.
The project included the realignment of a 200m section of watercourse upstream of the aqueduct and the provision of a pond to alleviate the effects of flooding.
Flooding can cause huge problems for the railway
Previously, the ecological status of the watercourse upstream of the aqueduct was poor due to a sluggish flow rate and heavy shading by vegetation. Careful analysis ensured we preserved and improved the characteristics of the new channel.
The ambitious scheme involved the removal of the existing aqueduct and the installation of a new structure while improving local biodiversity.
The railway and our pollinators