This article will make only brief reference to the railway. It is mainly concerned with otters, bats, fish, birds, trees, farmers, fishermen and river engineering. One of Devon’s major rivers, the Taw, flows roughly north-west from the northern flanks of Dartmoor and out to sea at Barnstaple. Along its route it collects many incoming tributaries, some flowing in from the southern edge of Exmoor.
It is a river with a dramatic personality. From a gently flowing stream passing serenely through a pastoral valley and broad flood-plain, it can change its mood in a matter of hours to a raging torrent, surging through the channel, pushing debris downstream and subtly changing course with every flood event.
The single track, but well-used, passenger branch line between Exeter and Barnstaple is one of the country’s scenic routes. Although it passes through attractive farmland for the whole journey, it also has the dubious privilege of sharing the River Taw’s valley, interacting with the river at no less than 11 underbridges and 12 embankment locations.
This railway route is known as “The Tarka Line”, named after the well-known modern classic, “Tarka the Otter” by Henry Williamson, published in 1927 and set in this area of North Devon. Here is a quotation from the book, which clearly illustrates the river/railway relationship, though it was still a double track line in those days:
Twin burnished lines were set by the river, touching its banks, straitly leaving it to its windings, and crossing it on stone bridges topped by tarred iron girders. Under the girders, jackdaws were building their nests of sticks and sheep’s wool and paper picked up in the early mornings from cottage gardens. The rolling thunder over their heads did not bother them, for, like the otters, they had grown to the noise of the trains in the valley.
A critical element of the inspection and maintenance regime for this route is the early warning of, and rapid response to, potential underwater scour and lineside erosion threats.
At Colleton Mills, halfway along the route, the River Taw was causing a new concern with regard to the integrity of the railway support zone. In this area, the railway is carried on a shallow embankment. When originally constructed by the North Devon Railway in 1854, the river flowed innocuously past the site some 100 metres away at its closest point.
This is a dynamic river, however, and, over the passage of time since the opening of the railway, a large meander has formed, taking the main river channel alongside the railway embankment over a length of approximately 45 metres. Aerial photography of the site clearly shows the remains of the old watercourse abandoned by the river.
Because of this, partway through the life of the railway, a mass concrete wall was provided to prevent erosion of the foot of the embankment as the meander developed and approached.
Recently, inspections had noted the progressive undermining and structural failure of this wall to the point where urgent remedial work became essential.
In May 2017 Network Rail let a contract to Construction Marine Ltd (CML) to carry out remedial work before the next winter. Will Johnson, contracts manager for CML, told Rail Engineer that the proposal was originally to demolish the old wall and install a new protective wall over a 30-metre extent using “Redi-Rock”, a proprietary retaining wall system using a menu of precast units.
Because of environmental constraints, the work had to be completed within a very restricted eight-week period. This had to be after both the end of the lamprey spawning season at the end of July and the completion of sand martin nesting, and before the upstream migration of salmon and other fish species from the beginning of October.
It was apparent that this was going to be a challenging piece of work. Initial plans were to install a temporary cofferdam and then remove and replace the retaining wall. However, pre-start surveys identified that the scour-related problem was deeper than expected, and affected an increased length of the railway embankment.
As a result of this latest information, CML proposed to Network Rail that a temporary diversion of the river away from its natural watercourse would give much improved access for construction of the replacement wall.
Network Rail called in its consultant, WSP, to review the geomorphological and environmental implications of this proposal. This is where the story starts in earnest.
WSP and CML get the credit for the innovative nature of the scheme that was developed. Hamish Hall of WSP said that, following evaluation of the CML proposal to temporarily divert the river and also the evidence of the river’s original course, the question that emerged was: “If we are going to the trouble and expense of creating a temporary channel, then why not instead put the river back where it was two hundred years ago?”
By this time, there were only a few weeks remaining before the start of the available eight-week period during which interventions to the river regime were permitted. Between the end of June and the beginning of August, if the new proposal was to go ahead, then a full scheme had to be designed. Environmental and geomorphological implications needed assessment and approval by all the relevant authorities.
Hamish reckoned that, under normal circumstances, an 18-month timescale would be realistic to carry out scheme design, environmentally assess and gain approval. Waiting this long would have resulted in a full collapse of the wall and closure of the line.
At this point in time then, there existed neither site survey nor environmental assessment. The Environment Agency (EA) was rapidly brought up to speed with the new proposals. Four separate departments within the EA had to be involved in developing the scheme: Environment Officer, Flood Risk Officer, Geomorphological Officer and Fisheries Officer. For its part, WSP assembled a team that, at times, involved 39 staff.
Design and environmental assessment
In order to design the new channel, its physical layout, cross-sectional properties, longitudinal gradient profile and its surfacing materials needed to be understood. A topographical survey was needed, but also a good understanding of the geomorphology of the river bed upstream and downstream of the site of the new channel that was to be constructed.
Environmental assessments had to include otter surveys, bat surveys, and surveys of reptiles and freshwater pearl mussels. Kingfishers, fish species and sandmartins nesting in the river banks also had to be considered. Of particular importance amongst the many issues of ecological balance is the quality of the river bed gravels. These have to be of the correct grading and free enough of silt to ensure an acceptable and attractive habitat for lamprey, and other fish spawning. To gain approval, all parties had to be confident that, after the work, the wildlife mix and habitat diversity available would be no worse than before disturbance and preferably enhanced.
To be confident that the geomorphology of the river would be correct and in particular that it would enable the overall stability of the river, a specialist geomorphological consultant, CBEC, carried out numerical modelling of the proposed new section along with adequate lengths of the existing upstream and downstream sections remaining unaltered by the works. Working in partnership with WSP, a channel geometry was designed to be morphologically interesting.
Simon Dart, area flood risk officer for the EA, explained that, by cutting across normal timescales and by tremendous co-operation between all parties, the EA was able to issue an Environmental Permit, enabling construction work to commence at the beginning of August.
Specifications and work programme
The new section of river channel has a complicated geometry, necessitated by the geomorphological requirements, and therefore the setting-out for the earthworks was also, in turn, very complex. Cross-sections were detailed and drawn for 45 locations along the new section, roughly every five metres throughout the design length, to give close control and high accuracy for excavating and profiling the new channel.
The new reach includes two gentle meanders, each with a ‘pool’ area, in order to replicate as much as possible the flow and habitat conditions being replaced. Also, the most downstream end of the old river meander, which would now be abandoned, was left as a “backwater” area, would form another important element in the overall compensatory and mitigating environmental provision. The remainder of the old river channel would be filled in after diversion of the flow to the new channel.
Before any earthworks could commence, temporary ‘Ecological Exclusion Zones’ were identified and shown on a location plan. Also clearly identified to the contractor were all the instances of environmental mitigation being incorporated into the overall works. For example, in the residual backwater, a clean cut face is provided to replicate the lost sandmartin and kingfisher habitat.
Where it was necessary to remove certain areas favoured by reptiles, three new hibernacula (rotting log piles and stones, loosely covered with topsoil and turf) were constructed to provide an alternative habitat. Areas of the river bank thought to be used by otters were carefully monitored throughout the works.
Very few trees were removed or affected, and replacement tree planting was specified for two major areas at the top of the new channel adjacent to the two new bends. An integral part of the design of the new channel was the provision of live willow and hazel faggots, staked in to the channel sides around the normal water level. Within fourteen days of planting, these were already showing new green shoots.
The volumes of cut for the new channel and the fill required for the old channel were designed to be in balance. The only major importation of material required was limestone and granite rock armouring for the new channel bed and sides. The channel was lined with a geotextile before the placement of the rock armouring. At the top of the channel side slopes and onto the horizontal part of the banks, coir matting was lapped and pegged in.
Perhaps the most critical stage of the works was the transfer of the river flow from the old to the new channel. At each end, sheet piling had been installed to prevent inflow until the new channel was ready. There was also elaborate provision for the management of silt, with silt traps adjacent to the sheet piling at each end and a collecting area for silt in the “island” area between the old and new channels.
To avoid fish becoming stranded in the abandoned stretch of old channel, they had to be captured and relocated. This was done by ‘electro fishing’, a technique employed by a specialist company, Fishtek, to temporarily stun the fish for netting and transfer. 4,000 fish of all sizes and various species were caught over the three day period leading up to the river diversion, including two flounder – a long way upstream from their normal marine environment and in flagrant breach of all the wildlife surveys and appropriate provision being meticulously carried out!
A minor tributary stream also had to be accommodated. Because of relative levels across the site, this has had to be culverted through the ‘island’ area to rejoin the new channel at the downstream end of the new work.
A praiseworthy success
The new channel was fully opened on 27 September, which was a tremendous achievement and in time for the salmon to migrate, with or without maps of the new layout! As the works have passed the completion phase, otters have been regularly observed moving through the ‘new’ river.
The Environment Agency told Rail Engineer that it was really pleased with the way that this project had been delivered. It was particularly impressed with the manner in which CML involved the local community in an understanding of the need for and the nature of the works by holding open days for the public, consulting with the local community and organising school visits to the site.
There was a real risk that, due to the severity of the scour identified, Network Rail would have needed to consider emergency works to temporarily stabilise the wall as further deterioration could have had a significant impact on the train service. Richard Edlington, programme manager for Network Rail, said that he could not praise WSP and CML enough for achieving the impossible and delivering this scheme in such a short time period. This project has not only removed a significant infrastructure risk but has also ensured the railway will not be susceptible to scour at this location in the future.
The Environment Agency also recognised the risk and enabled the project team to expedite the permitting process in order to enable the works to be completed before winter. The number of stakeholders and third parties that worked together to make this project happen against all odds was quite outstanding.
Rivers and railways – looking to the future
This story has been about rivers, and railways. However, it is possibly a foretaste of, and relevant to, things to come. The infrastructure maintenance community is now facing several challenges. Firstly, with climate change and changes in rainfall patterns, faster flash flooding events are becoming more prevalent. Secondly, there are large scale changes in morphology over shorter timescales.
Both of these lead to more challenging inspection, maintenance and renewal regimes. This is especially relevant to the railway environment. Many railways have, for very practical reasons, been built in river valleys or alongside available level coastal strips. They are therefore in prime position to be affected by any changes to the ‘traditional’ weather pattern.
The project at Colleton Mills has been a great example of an imaginative and innovative response to the rapidly evolving infrastructure environment. It also demonstrates the value of delivering more than one benefit. The original project was simply to protect the railway line from the threat of undermining erosion by the river. What was finally achieved is the creation of some ecological improvements and the reduction of long-term risk to the railway.
This article was written by Mark Phillips.