A branch of the Great Western Railway, connecting Exeter to Barnstaple in Devon, is fondly known as the ‘Tarka Line’. Named after Henry Williamson’s Tarka the Otter, the railway runs along the banks of the River Taw for much of its route, precariously shadowing its many twists and turns.
Beautiful though it is, the river has a determined tendency to flood. At a particularly aggressive meander, near Colleton Mill in North Devon, the power of the flood had caused fatal damage to a retaining wall which supported the railway line. The wall had succumbed to the effects of scour, affecting the integrity of both the river bed and the banks around it. Having already partially failed when we were called in last year, the remaining structure was so seriously undermined that there was a risk of further collapse, and ultimately, closure of the railway.
When we started work in June 2017, winter was just a few months away and it was not possible to guarantee the wall's survival for another season; consequently, we had to act fast to find a solution.
Number of months to design and deliver a project that would normally take 18
3,741 Number of fish rescued from the existing channel and released downstream of the diversion
Length of the River Taw’s new channel at Colleton Mill
Against the clock
There was also the matter of fish, breeding birds, bats and otters - to name but a few local inhabitants. The River Taw is renowned for salmon and sea trout, with a protected spawning season widely regarded as starting from around 01 October. In partnership with Network Rail, the Environment Agency and the wider team, we committed to ensuring the necessary permits were in place to complete the works in just four months, rather than the 18 months that a project of this scale and sensitivity might otherwise take to plan, design and construct. In doing so, the new river would be diverted, in time for the salmon spawning season.
Ready for the future
The radical solution involved working with the Environment Agency, to develop a route which would permanently move the river away from the railway. Rather than trying to construct a new retaining structure along the existing alignment, our vision was to develop a design for the future. This dramatic plan would provide long-term security for this locally important rail infrastructure, increasing its resilience to the effects of future climate change and allowing us to reinstate habitats to cater for the resident wildlife. Importantly, it would also provide a safer exit route for livestock previously at risk of being stranded in floods.
The new 185m alignment was designed to mimic the sinuosity of the existing upstream reach and follow one of the routes the river had, at least in part, originally taken some 150 years ago. Now constrained by a listed road bridge and the railway, the river had migrated downstream over time until it could move no more. The new alignment sees the river diverted away from the railway entirely, and includes specially designed pool and riffle features, maintaining the overall character of the river, albeit some 100m from its most recent path.
This dramatic plan would provide long-term security for this locally important rail infrastructure, increasing its resilience to the effects of future climate change and allowing us to reinstate habitats to cater for the resident wildlife.
We used soft engineering approaches, such as coir matting, willow spilling and tree planting, along with rock armour, as a means to manage the river bed and bank stability. The needs of multiple stakeholders were foremost in our minds; not only did we have to consider the requirements of our client, but also the safety of railway passengers, the landowner and also local residents, whose interest and enthusiasm we are still grateful for. We did, of course, also have to consider the environmental constraints associated with the sensitive nature of the site. Mitigation measures were adopted, including the creation of a backwater channel as a new habitat for wildlife and purpose-built areas for reptiles to populate. Once diverted, the landowner’s access and egress route was reinstated, allowing him to safely move his cattle from the floodplain during high flows. This was something, which until now, had not been possible since the partial failure of the wall and localised collapse of the embankment.
Re-homing the inhabitants
When the time came to re-route the river along its new alignment, each end of the existing meander was bunded and water was quickly diverted along its new path. This of course almost instantly left a stretch of water with no flow. We had to consider fish trapped behind the bunds, along the meander of the old channel - in particular, those residing in the deep pool beside the failed retaining wall. This involved a two-day fish rescue with our specialist partners and allowed an astounding 3,741 fish to be recovered. They were safely removed and released back into the channel, downstream of the diversion.
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. By adopting a holistic approach and by designing for the future, we were able to provide a new river environment that caters for the many flora and fauna that delight in it.
The new channel was opened on 27 September, ahead of time, and ready for the salmon spawning season. Otters have, since then, regularly been observed along our new length of river. We think Tarka would have been pleased.