Thomson River Fishway 2017-2019

Horseshoe Bend Tunnel is four kilometres from Walhalla on the Thomson River. It was built in 1911 and 1912 to drain water from the Thomson at Horseshoe Bend to allow for alluvial mining of the river bed. It gained heritage status in 2002. If you’re interested in the history of the tunnel, please download a copy of the Heritage Report part one and part two.

Migration of native fish, such as the vulnerable Australian Grayling, have been stopped by the tunnel since its construction. The lack of river flow around Horseshoe Bend and the turbulent water from the tunnel provides a physical barrier for fish to swim upstream. The new fishway will see flows return to the original river course, with natural eddies and pools constructed to provide respite for fish as they journey upstream as part of their breeding cycle.

Fish migration is extremely important in the lifecycle of many of our native fish, including the protected Australian Grayling. The Grayling is listed as vulnerable under the Environment Protection and Biodiversity Conservation Act 1999.

About the Australian Grayling

The Australian Grayling is a small to medium-sized, slender, silvery fish with soft-rayed fins. It lives in south-eastern Australia, including Victoria, Tasmania and New South Wales. It is a nationally vulnerable species listed under the federal EPBC Act.

The Grayling has a very short life span of 2-3 years and migrates throughout its life cycle between fresh and marine waters. Adult Grayling live in freshwater river systems. They move up and down the river looking for food and during autumn migrate to near the estuary to spawn. Its eggs are swept downstream to the sea and the adults return upstream. The young fish remain in the sea for about six months before returning to freshwater.

In our region it can be found in the Thomson, Macalister, Avon, Latrobe, and Tarwin rivers.

Extensive consultation has been carried out to ensure the fishway balances the significant heritage and cultural values of the site with environmental improvement objectives.

A project consultative committee of community, government agencies, WGCMA representatives and other stakeholders commissioned several designs and investigations before settling on a solution that shares the flows between the original river course and the tunnel.

We are grateful to have received considerable input from the project consultative committee, including members of the Hannaford family and the Friends of Horseshoe Bend Tunnel. Their suggestions strongly influenced the final design and the extensive research we completed will help ensure the project is done with as little impact to the heritage values as possible.

The fishway will be 200 metres long and ranging in depth from a maximum of 2.7m deep upstream down to no excavation downstream. The average depth is 1.4m. The width also varies to a maximum of seven metres wide. In context, the valley floor at Horseshoe Bend varies from 20m to 50m wide. Throughout the consultation period we have explored every option to minimise the size of the fishway. The result of this work is a low flow and a visually low impact fishway that will present itself as a meandering waterway.

There are four main elements to allowing fish passage around Horseshoe Bend.

• Technical experts, fish experts and engineers have helped us create a design that delivers just enough water for fish passage while still maintaining flows through the tunnel. A 60% to 40% split is proposed
• The design of a fishway entrance, at the tunnel outlet, to attract fish past the outlet
• Passage over existing rock riffles and;
• A constructed fishway through the old river channel near the tunnel inlet.

Construction of the fishway and walking tracks began in February 2019 and were completed in August 2019.

Existing walking tracks were widened to allow machinery to the riverbed to construct the fishway, on completion these tracks were rehabilitated into narrow walking tracks.

The fishway was constructed using an excavator with a rock-breaker attachment. Approximately 2200m³ of bedrock was removed to create the fishway. Due to the type of rock and construction method, high levels of accuracy between design and construction was achieved, ensuring the desired flow splits and fishway hydraulics were obtained.

During construction heritage areas were protected by creating exclusion zones and vibration monitoring was undertaken to ensure rock-breaking didn’t impact on the structural integrity of the tunnel.

Minor adjustments to the fishway were made during construction to utilise natural features in the riverbed. This reduced the volume of rock excavated and subsequently the overall impact to the site.

Large rocks were required at a number of locations for the project: stepping-stones, tunnel outlet and rock-blockage. These were sourced from the site itself, ensuring they fit in with the surrounding area. This also reduced the impacts associated with external materials such as increased traffic and soil contamination.

Throughout the works, a stakeho9lder group was established and overseen by an independent Chair. The group was provided with regular updates on project milestones, issues and developments.

it is expected to take 2-5 years for the vegetation at the site to fully regenerate and complete the rehabilitation phase of the project.

During the creation of the fishway, the structural integrity of the tunnel was closely monitored.

Interested community groups and stakeholders were kept fully informed of construction impacts, and their input sought where required.

Works were completed in August, 2019. Rehabilitation is expected to take between three to five years.

The walking track which had been widened for machine access, have been rehabilitated and will be monitored over the next few years to ensure natural regeneration and to control weeds. It is expected to take 2-5 years for the site to be completely regenerated.

The flow through the fishway has been measured to test if the flow split is correct and if the flow conditions will allow for fish passage. More than 60% of the river still passes through the tunnel under all flow conditions. Water depths and velocities through the re-engaged river course and the constructed fishway will allow for the easy passage of fish.

Flows out of the tunnel still provide a visually compelling sight and interpretation of it’s historic function. Large rocks placed near the exit have been placed to best fit in with the natural surrounds and provide a protection area for fish to safely navigate past flows from the tunnel outlet.

Fish monitoring will continue over the next few years to track the success of the fishway in allowing migratory fish species access to the upper reaches of the Thomson and Aberfeldy river.

The images below show the site in June 2019.