River Wylye – highway bank protection, near Salisbury

Project Brief

Severe bank erosion along the River Wylye near Salisbury was threatening the integrity of a major highway and farm access bridge. The eroding bank was as little as 6m from the A36 trunk road in the worst affected areas, so further erosion or slumping would have had serious implications for the stability and safety of the highway.

To further complicate matters the River Wylye is a Site of Special Scientific Interest and located within an Area of Outstanding Natural Beauty, so a traditional hard engineered solution was not an option for such an environmentally sensitive location.

We were engaged to investigate and survey the site to design/build a long-term, environmentally sensitive solution to protect both the bank and farm access bridge from further erosion.

Work / Solution

A 45m stretch of bank erosion along the River Wylye was threatening the stability and safety of the A36 road bridge and adjacent highway.

Before

Eroded bank looking downstream from Southington farm bridge (the A36 is just visible on the left of the photo)

Erosion cavity formed within the footing for the roadbridge

Erosion cavity formed within the footing for the roadbridge

During

The solution that we developed for the client consisted of a bio-engineered structure constructed from geotextile, gravel and live willow planting and tree hinging to stabilise and protect the bank.

The structure consisted of a 3-tier protective revetment installed along the 45m of left bank downstream of the farm access bridge. This bio-engineered structure is constructed from geotextile, which was retained with sweet chestnut stakes installed from a floating works platform. Gravel was then placed behind the geotextiles, using an innovative, low impact pumping system, and finally the back-fill area was planted with live willow stakes.

Construction of a three-tiered revetment downstream of the bridge

Gravel is pumped behind the new bank line formed from hardwood posts and geotextiles

Completed backfilled revetment

After

The completed works successfully reinstated the eroded bank margin near the major trunk road and around the farm access bridge and in doing so delivered an aesthetically pleasing, environmentally sensitive solution; for approximately a third of the cost of a more traditional hard-engineered solution.

Bank protection works immediately after completion – view looking upstream towards the bridge

Bank protection works immediately after completion

10 months after completion showing willow sprouting through the revetment

2 years later!

A natural ‘pinch-point’ was formed d/s of the main bank works to improve flow variability and habitat biodiversity

Hinged live willow used to protect the bank and to provide valuable riparian habitat

Testimonial

“A traditional repair involving sheet piling, gabion walling and granular backfill would have been very disruptive and incongruous with AONB, SSSI and Special Area of Conservation status, harming the water quality and causing concern to our client and local stakeholders. It became clear that we needed a radical approach to deal with such a delicate issue. CBE, led by Simon Cain, devised a solution which would meet all of the challenges faced by the crumbling river bank”.

Geotechnical engineer at Balfour Beatty Mott MacDonald