The brief
To design and deliver a series of works to prevent further erosion of the canal banks in order to stop the erosion and flooding of the adjacent footpath, as well as improve habitat east of the work site for watervoles as mitigation for the bank works.
The canal supports high watervole populations so the design was advised by a watervole specialist with input from the Environment Agency and Natural England, delivered under strict class licence conditions.
The Solution
The project was delivered across three phases:
Phase 1: Path surfacing works.
Phase 2: Bankside revetment works.
Phase 3: Habitat mitigation works.
Phase 1: Path Surfacing Works.
Due to the canal overtopping its bank, the adjacent Right of Way had been closed on safety grounds as a deep gully had formed by water eroding the path when overtopping the canal following high rainfall events.
To match the existing path, we resurfaced 150 meters of footpath along the flood-prone area with Neogem Path Gravel, which leaves a smooth, comfortable walking surface.
To prevent overtopping of the path surface in high water events, the level of the path was raised at key points along the 150 meter stretch up above the height of the bank, as well as finishing the surfacing with a parabolic camber to help shed water from rainfall.
Phase 2: Bankside revetment works.
To rebuild the bank, two rows of recycled plastic sheet piles were installed.
The works were carefully timed to minimise their impact on watervoles and thorough surveys were conducted under class license along the banks of and around the worksite for their presence, with vegetation cut back to temporarily discourage their use of the area.
The revetment was designed to create a stepped bank profile featuring a marginal vegetation shelf, blending both hard and soft engineering.
The back row of piles were installed at the average bank height and the outer row installed 500mm out - in line with the original bank - and just above the average water level.
The piles were driven in to the canal bed with an excavator and strengthened with 100mm king posts. The void between the eroded bank and the back row of piles, and the space between the rows of piling, was then back filled with gravel.
Timber walers were then attached to each side of the sheet piles and secured to the king posts, before timber capping was installed. The planting shelf was then topped with soil scraped back during the path surfacing works to aid in the establishment of marginal vegetation.
Finally the stretch was fenced off to prevent people and dogs entering the canal, and to allow the area to re-establish after the works.
Phase 3: Habitat mitigation works.
The final phase of the project was improving the habitat along the adjacent ditch, doubling the available suitable watervole habitat as mitigation for the bank revetments.
80 meters of over stood scrub - primarily willow - along the ditch banks was cut back and the brash burned, with larger logs stacked as habitat piles, allowing more light to reach the water body and banks, increasing the marginal vegetation along both sides of the ditch.
Bare areas of the ditch banks and the marginal shelf of the bank revetments were then planted up with Branched bur-reed plugs - a source of food and protection for watervoles.
The outcome
This stretch of bank regularly overtops and erodes the adjacent footpath surface and there have been numerous attempts to repair this section of bank using natural materials which not been successful in preventing significant erosion of the adjacent footpath.
Finding a suitable solution balancing the environmental requirements set by Natural England and the Environment Agency, and the aims of the client was no small feat but our blended 'hard' and 'soft' engineering approach provides a solution which is both permanent and aesthetically pleasing.
The location is a fantastic wildlife spot and is heavily used by walkers and bird watchers - M:P staff included.
With the installation of our sheet piling revetment and the increased path height, the path remains solid and usable all year round no matter what happens up stream.
