Three years ago Network Rail engineers inspecting the embankment that carries the Spalding to Doncaster line alongside the Foss Dyke canal, west of Lincoln, spotted something seriously wrong.
Where an existing sheet pile wall ended, close to a recently opened pleasure boat marina, the canal bank was suffering significant scour.
Erosion was getting close to our safety limits for embankments, ' reports Network Rail scheme project manager Nigel Lea. 'We began regular monitoring and programmed remedial works for the 2007-8 maintenance period.
'But the monitoring revealed the bank had been scoured back up to 2.5m in a year over a length of more than 2km, so we had to take action much faster.' Originally dug nearly 2,000 years ago, (see box), the 18km Foss Dyke was one of the first UK canals to see a boom in pleasure boating after the disappearance of commercial traffic in the 1950s.
Being wider than most canals - 18m on average - it can take larger boats, which routinely exceed the 6km/h speed limit.
The wake they generate is much more destructive than anything the canal's Roman engineers ever dreamed of.
Protecting this stretch of the canal was not going to be easy. Logistic and environmental concerns meant that the hard engineering option - extending the sheet piling along the area under threat - was far from attractive.
There were kingfishers and water voles to consider.
And relatively shallow water near the banks ruled out heavy piling barges. Access for piling rigs from land would only be possible during railway possessions, and the cost estimates were high, ' Lea explains.
A second feasibility study, however, uncovered an option already being considered for another part of the canal network.
'The basic idea was to install treated softwood stakes in the canal close to the bank to support a protective 'wall' made up of dead willow faggots topped with coir rolls planted up in advance with water plants, ' explains main design and build contractor May Gurney Rail environment manager Craig O'Brien.
'Behind the wall would be dredged material or imported washed stone backfill supporting preplanted coir mattresses.' May Gurney won the contract with a £650,000 tender. But when it teamed up with specialist subcontractor Salix River & Wetland Services the basic scheme underwent some subtle modifi ations. Salix sales manager Richard Evans explains.
'We did a value engineering exercise and decided we didn't really need stone or gravel simply as a void filler. The coir mattresses needed very little support in practice. The sides of the embankment and the bank of the canal were being cleared of bush and scrub as part of the contract, and the original plan was to chip the brushwood and leave it on site.
Instead we decided to recycle it into the void behind the faggots (see diagram).' Although the coir will rot away in four or five years, the faggots and brushwood will last indefinitely in their anaerobic environment. Sedges, rushes and reeds will root down from the coir before it disintegrates, while the treated softwood stakes should last at least 50 years, Evans adds.
Salix made up the rolls and mattresses from free trade Sri Lankan coir last year and planted them up in its nursery in Wales. By the time they arrived on site in February the water plants were well established and breaking into new growth.
Out on the water, May Gurney has a mini-armada led by a bank clearing barge, which also retrieves and recycles the brushwood. Behind it comes the stake driving barge bearing a Hitachi long reach excavator fitted with a hammer.
Stakes, up to 1.8m long, are driven to refusal then trimmed.
Salix operatives wade into the canal to install the faggots and other components - in the deeper areas up to five layers of faggots are needed. 'Since we started on 6 February we've had temperatures down to ?5 oC, ' reports May Gurney site agent Mark Anderson.
'Nevertheless we expect to complete the job well inside the original 22 week programme - maybe in as little as 14 weeks.'