River Avon Gunville

Project Brief

A 1.2km length of the River Avon, to the north of Salisbury, displayed all the classic habitat degradation features associated with post war river engineering – heavily impounded by a downstream relic sluice structure, over-wide, heavily silted and with a sand-impacted bed. All of which had led to a loss of habitat and species biodiversity along this internationally important chalk river.

Upstream end of the degraded reach – note the overwide channel and lack of flow variability

This stretch of the River Avon has been immortalised by the author Frank Sawyer, and is still to this day managed as a fishery for the Services Dry Fly Fishing Association (SDFFA) as it was during Frank’s time when he was the riverkeeper.

Gunville hatches – located at the downstream end of the reach

Unfortunately, the degradation of chalk river habitat continues to be a problem within this catchment, as it was during Frank’s time. Degradation that in part can be associated with the presence and on-going use of historic water meadow structures (i.e. referred to as sluices or hatches in this part of the world).

Hoping to restore a once popular wild trout fishery, SDFFA commissioned us to investigate the reasons for deterioration along the 1.2km reach and to design and construct a solution to re-energise the river and restore the degraded ecosystem. In doing so it was also hoped the work would also help to deliver Natural England’s target of achieving ‘favourable condition’ for the river, which is designated as a Special Area of Conservation (SAC) and Site of Special Scientific Interest (SSSI).

Work/Solution

Before

During a site visit with the client in November (2011) we identified a number of problems along the reach associated with degraded channel morphology, including:

  • Over-wide channel – lateral erosion of the banks of the Avon had over-widened the overall channel cross-section, causing flow velocities to be reduced resulting in the deposition of fine sand and silt on the bed, obscuring the gravels beneath.
  • Poor flow variability – lack of flow variability and hydraulic scour had led to a very uniform river bed.
  • Impounded flow conditions – a section of deep, slow-flowing water was observed a short distance upstream of a sluice structure (Gunville Hatches). The sluices had been used historically to maintain artificially high water levels for stocked trout and, in more recent years, to facilitate the easy passage of cut Ranunculus weed downstream from the upper channel. The effects of impoundment are clearly deleterious to the health of the river in the form of very slow flows and heavy sediment accretions.

Sandy bed substrate at the top end of the reach

Over-wide, sandy, shallow channel within the middle of the reach

Heavily silted channel and impounded flow conditions

Deep, impounded flow caused by downstream Gunville Hatches

Greensand deposits upstream of Gunville Hatches

During

Previous river restoration works have been undertaken along this stretch and while initially successful in delivering a re-energised river system for a few years, they often rapidly revert to their original status unless the appropriate restoration techniques are employed. The client was therefore keen to ensure the scheme would be durable enough to deliver a long-term solution.

Through our work on the nearby successfully restored reach at Chisenbury. We were able to demonstrate to the client that degraded channels can be significantly improved using a combination of tried and tested techniques.

We developed a design to deliver the following hydro-morphological and habitat improvements along the reach:

  • re-sizing the channel
  • re-introducing sinuosity and channel meander features
  • optimising channel depth-to-width ratio
  • optimising flow velocity
  • removal of impediments to free flow
  • introduction of complex flow patterns and sub-surface cover via the extensive use of mature site-won trees such as Alder and Willow.
  • maximising durability, especially during high flow periods (winter floods and pre-June weed cuts)

Marking out during installation

Hardwood posts installed to create the new deflectors

Soil filled deflector shortly before completion

Completed berm - halving the width of the channel

Woody material installed downstream to encourage silt deposition

After

The completed work has had a very positive and dramatic impact on the river. The narrowing berms re-energised flow which has remobilised deep silt deposits and exposed the clean gravel bed beneath. Woody debris and hinged trees have scoured the bed creating deep pools and kicking up gravels to form new gravel riffles. This improved morphology has provided the right conditions for Ranunculus to regrow and flourish within the reach.

Initially the very downstream end of the reach was still impounded, due to the impact of the now significantly reduced structure at Gunville, as the stanchions had to be retained, which was still causing some constriction to flow. These were finally removed in 2015 during an Environment Agency funded project.

After - narrowing berms and newly created wet woodland habitat

Hinged live willow trees and narrowing berms/deflectors

Increased flow velocities around the deflectors

Immediately after the works were completed in November 2011

1 year later (November 2012)

2 years later (September 2013). Note the aquatic weed growth

Testimonial

“…we are absolutely delighted with the Gunville Project – it stands head and shoulders above the other work on our fishery, and although we were concerned about the effect on the weed clear down, this has not really been a problem either last year (high water) or this. It has vastly improved the habitat, both in-channel and along the river margins and it is clear that all wildlife has benefited. It makes a fascinating contrast with the much softer approach conducted by Aquascience downstream of Gunville, and with the minimalist (LWD only) approach undertaken by the Wildlife Trust and Wild Trout Trust at the top of our water.” Brig C. Anthony G. Wells – SDFFA

Awards:

Contributed towards winning the River Restoration Centres 2017 UK River Prize