THOMSON RIVER DIVERSION TUNNEL PERMIT –24 FEB 2016
The Executive Director Heritage Victoria has determined to issue a Permit for works associated with a fishway around the tunnel.
The following provides background and information which informed the decision making.
Background
Thomson River Diversion Tunnel (H1990) is located about 4km south-west of Walhalla in Gippsland, 1.5km below where Stringers Creek joins the Thomson River. The diversion tunnel was driven through a ridge of land called Stockriders Spur, around which the Thomson River forms a horseshoe bend. Diversion tunnels facilitated alluvial gold mining in dry river beds.
Q. When and why was the diversion tunnel added to the Victoria Heritage Register?
A. The Thomson River Diversion Tunnel was added to the Victorian Heritage Register in 2002 due to itshistorical and archaeological significance in demonstrating the dying stages of an alluvial gold mining industry that had dominated the river systems of Gippsland for more than 50 years.
Q. What area is covered by the Registration?
A. The actual registered area is the tunnel and 5m curtilage around the tunnel; at ground level including the entrance and outlet. The upstream end of the fishway will be 70 metres from the registered area for the tunnel’s inlet, the downstream end of the fishway will be approx. 200m from the tunnel’s outlet.
Permit
Q. Why was there a Permit application by West Gippsland Management Catchment Authority?
A. From the time of its construction in 1912, the tunnel has diverted water flow resulting in the bypassed reach (a 1.2km section known as Horseshoe Bend) of the Thomson River remaining more-or-less dry. West Gippsland Management Authority considers that there is conclusive evidence from the state government centre for fishwater ecology (Arthur Rylah Institute) that the diverted water flow prevents the passage of most migratory native fish. There is also strong evidence that these species will repopulate the upper reaches of the Thomson River if a more permanent and suitable flow is re-stablished.
Q. What are there responsibilities of the Executive Director for places listed on the Victorian Heritage Register?
A. The Executive Director of Heritage Victoria can only consider works or activities (via Permit applications) in relation to registered land— in this case, the tunnel and 5m curtilage around the tunnel – at ground level including the entrance and outlet.
Q. What was covered by West Gippsland Catchment Authority’s Permit application?
A. As all the works to do with the fishway occur outside the registered area for the tunnel, the Permit application was for management of potential indirect impacts on the tunnel from blasting and reduced waterflow as a by-product of the construction of the Thomson River fishway
Impacts
Q. What were the indirect impacts to the tunnel identified and addressed by West Gippsland Catchment Management Authority?
A. Indirect impacts included:
- Collapsing due to blast vibrations: the Geotechnical Report by SMEC Australia found the tunnel to be stable and unlikely to be affected by blast vibrations. As a safety precaution, and a condition imposed by the Executive Director, monitoring of the tunnel during construction is required to ensure the vibration limit set by SMEC Australia is not exceeded, and if there is signs of collapsing within the tunnel, blasting is to cease until mitigation measures, to the satisfaction of the Executive Director, are introduced to reduce vibration to an acceptable level.
- The tunnel drying out causing deterioration of its walls and ceiling: the geotechnical assessment found that historically the tunnel has dried out completely during droughts and yet it remains virtually as constructed. The proposed project results in 40% less flow through the tunnel and so does not mean complete drying out.
- Visual impact of a rock alignment opposite tunnel’s outlet: In order for fish to migrate past the tunnel outlet it is necessary to create an alternative flow path through the turbulent water. What has been designed is a placement of two parallel rows of one metre diameter rocks, south side of the river bed, opposite the tunnel outlet. The alignment will be approx. 30m long and less than 3m wide. The installation will involve one metre diameter rocks. The top of the rocks will be at or below low flow water levels.
- Stockpiles: The placement of excavated rock will have no visual impact on the tunnel’s inlet and exit and the stockpiles have been located and designed so as not impact on identified archaeological sites and areas.
- New walking track: will improve access and allow more people to visit and view the tunnel.
Q. What were the potential impacts to the registered area identified in the 30 submissions received by Heritage Victoria and that the Executive Director could take into consideration?
A. Potential impacts included:
- Vibrations: blasting/construction vibrations may impact on the integrity of the tunnel, that is, cause it to collapse.
- Reducing the water flow through the tunnel by 40% meansthe tunnel’s walls and roof may dry out, leading to collapse. Reduced water flow would also mean more people going into the tunnel, resulting in an increased vandalism.
- Stone alignment at the outlet involved the placement of huge boulders in front, or around, the outlet.
- The visual impact of the stockpiles upon the tunnel.
- Reduced flow having an adverse impact on cultural significance because the tunnel will no longer be ‘a fully functional historical exhibit’. Linked to this was the view that returning river flow around the bend would reduce understanding of the purpose of the tunnel; that there will no longer be a dry bend demonstrating the original purpose of the tunnel.
Options and consultation
Q. Was this the only option put forward to return river flow?
A. In the past, WGCMA has explored various options which more directly impacted upon the tunnel, including blowing up the tunnel entrance, installing a steel and concrete drop bar structure to seal the inlet, and constructing a concrete coffer dam across the inlet.
The coffer dam option was subject to a permit appeal (2005) and refused by the Heritage Council on the grounds of its physical and visual intrusive impact on the entrance of the tunnel and the elimination of water flow through the tunnel.
The Friends of Horseshoe Tunnel in its submission to the Executive Director offered a ‘low impact’ alternative to the proposed fishway; the recommissioning an ‘original water channel that the old timers used’. West Gippsland Catchment Authority assessed this alternative proposal, andfound that it was not practicable, but did accommodateaspects of the proposal into the final design.
Q. Was the community consulted on the design of this option?
A. After the failed permit application in 2005 to build the coffer dam across the inlet, discussions did take place which eventually led to the decision that the key point was flow split – how much should go through the tunnel?; and how much should flow around the bend to allow effective fish migration?. The issue of the flow split was discussed at a meeting in June 2011 between the Friends of the Horseshoe Bend Tunnel, West Gippsland Catchment Authority and Department of Sustainability and Environment (now Department of Environment, Land Water and Planning). The resultant design of the fishway thus balances key requirements including to:
- continuity of flow through the tunnel (60%)
- accommodating the lack of gradient of the river bed immediately down stream of the tunnel’s inlet – its relatively flat for the first 260m
- ensuring the successful operation of the fishway in respect to flow split (40%), seasonal fluctuations in flow, and the flow requirements for the passage of native fish (eg, depths, water velocity and resting pools).
Q. How did the Executive Director assess the indirect impacts?
A. The Executive Director took into consideration documents submitted by West Gippsland Catchment including Geotechnical Report by SMEC Australia; as well as the matters raised in the 30 submissions received in response to the Permit application being advertised.
Q. Why did the Executive Director approve the Permit application?
A. In assessing the documents submitted by West Gippsland Catchment Management Authority and the 30 submissions lodged, the Executive Director determined that the approval of the Permit would not affect the cultural heritage significance of the registered place. The tunnel will still demonstrate its purpose, there will be no visual impact on the inlet and outlet, and the recommended blasting levels/monitoring of tunnel and reduced water flow will not negatively impact on the heritage values of the tunnel.
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