Grand Western Canal
Above footage courtesy of ITV News
During a period of exceptionally high rainfall, the Northern side of an 18m high section of earth embankment was substantially damaged when the canal overtopped. Dramatic amateur footage of the breach and embankment collapse made the national news. The resulting rush of water opened a 23m wide chasm in the embankment and caused significant erosion within a 400m length of the channel of the canal and much of the towpath to the Southern side was compromised.
Devon County Council contacted SWH, who mobilised resources to work through the night to minimise the damage by installing temporary dams to staunch the flows. In excess of 16 million litres of water were lost from the canal and formed a lake up to 5m deep in adjacent farmland. At one point it was feared that the village of Halberton may have needed to be evacuated as the accumulated water threatened to cause further collapse of the embankment and flood the village.
The contract to repair the damage caused and prevent any recurrence comprised the reinstatement of the breached embankment, remedial works to the canal bed either side of the breach where it had been eroded by the outrushing water, installation of an in situ Reinforced Concrete channel to provide support to a masonry overbridge and the reinstatement and upgrading of towpaths.
The works started with site clearance, badger fencing, construction of temporary access roads and a site compound. Excavation of acceptable material from the drained section of the canal was removed and stockpiled for reuse. We then excavated the sides of the breach, forming benches in preparation for reinforced earth reconstruction of the embankment. Once the embankment was rebuilt, we reconstructed the canal channel, stabilising 5000m3 of wet inert subsoil with lime.
Throughout the project, SWH and Jacobs worked collaboratively to re-evaluate designs in order to reduce costs and environmental impacts. The initially planned draft of the canal was reduced and cross-sectional profile re-assessed to allow a further 2000m3 of reclaimed material to be incorporated into the permanent works. Overall, innovative measures to avoid disposal and replacement of fill material saved in excess of 100,000 miles of lorry movements and reduced potential scheme costs.
As different stages of work were being undertaken, it was necessary to drain additional sections of the canal. When this occurred, netting was undertaken so that more than 3000 fish could be rescued and released.
With the canal reshaped, a specialist subcontractor was employed to install a watertight HDPE liner along a 400m length of the canal. More reclaimed silty material was spread over the liner to form the new canal bed.
New stoned towpaths with timber edgings were constructed along the tops of the new embankment and works were completed by placing topsoil and planting with sympathetic plants and trees.
During the contract, we liaised with the local archaeological society and afforded them access to inspect and catalogue a previously lost engineering Brindle swing lock gate. A number of other artefacts were recovered from the dewatered sections of the canal and these were removed for investigation and cataloguing.
In addition, railway artefacts, buried and abandoned along the length of the site, were recovered and reinstated as the project progressed.
Despite some extremely inclement weather conditions over the winter months, the scheme was completed in time for the canal’s Bicentenary celebrations in May of 2014. The reinstated section of canal was formally reopened to the public on 19th March 2014 at a ceremony attended by County and Parish councillors, local schoolchildren, landowners and canal enthusiasts from the whole of the UK. A flotilla of narrow boats travelled through in a convoy headed up by a boat owned by one of the farmers, across whose land the construction haul routes had passed.
“The catastrophic failure of the largest embankment on the Grand Western Canal in November 2012 will go down as the most significant event in the history of the 200 year old waterway. South West Highways Ltd played a major part in limiting the damage on the day of the breach and in restoring the canal to its former glory.
"Steve Guilbert, SWH Structures Manager, was on hand on that awful day in November to help Devon County Council engineers install makeshift dams to prevent complete loss of the canal’s water through a 20m high x 20m wide breach. Steve’s knowledge of the local supply chain was invaluable in obtaining emergency plant and materials and he worked tirelessly throughout the day and night to prevent the catastrophe from turning into a disaster.
"The commencement of construction work to rebuild the embankment progressed swiftly thanks to the close working relationship of SWH and Devon CC; whilst good weather in the summer of 2013 certainly helped with progress the can-do attitude shown by the SWH construction team, led by Jerry Bricknell, ensured the scheme was completed in time for a major landmark, the canal’s 200th anniversary.”
Kevin Dentith, Chief Engineer (Bridges and Structures), Devon County Council
“The Grand Western Canal is a treasured part of Devon’s natural heritage and an important asset to the region. With several thousands of visitors every year, it plays an important economic role as well. My thanks go to the hard work of all the partners, the friends of the canal, the officers and members who all did so much during the severe weather which damaged this stretch of the canal.”
Councillor Bernard Hughes OBE, Chairman of Devon County Council