North East Water (NEW) is decommissioning Bakers Gully dams in Bright, Victoria.
The dams impound two reservoirs in series on Bakers Gully Creek (Lower and Upper reservoirs). By‐wash spillways for both dams are located on the right abutments. Both these spillways consist of channels dug into rock which cascade back to the creek downstream of the toe. The spillway for the lower reservoir has a concrete gravity weir while the upper reservoir does not have a formal crest structure. The original outlet works include conduits extending under the embankments at each dam.
Both dams were built more than 100 years ago and were taken out of service several decades ago. The original capacity of the lower dam was approximately 18 ML and the upper dam 21.5ML. These capacities have since reduced to 8.1 ML and 3 ML respectively due to siltation.
The consequence category of both dams prior to decommissioning was High C, in accordance with ANCOLD (2012).
The dams possessed the highest risk in North East Water’s dam portfolio. The main components of the risk were insufficient spillway capacity, piping through the foundation (lower dam) and slope instability (upper dam).
The main engineering consideration in the concept design was to lower the consequence categories of the dams from High C to Low/ Very Low in accordance with ANCOLD (2012).
The first concept design proposed dewatering the upper dam and lowering the water in the lower dam. This concept design was rejected by the community in a community meeting convened by NEW in February 2014.
Following this meeting and advice from the community, NEW engaged an ecologist to conduct a Platypus habitat survey in Bakers Gully dams’ site.
A second concept design was prepared taking into consideration the environmental and social values of the site, derived from the ecology report as well as community consultation sessions. This concept design maintained the water level in the upper dam, lowering the water level in the lower dam and widening spillways in both dam by approximately 40m.
After making further changes derived from further community consultation, the concept design was finalised in September 2014.
The detailed design phase commenced in November 2014 with a geotechnical investigation and an additional feature survey. The first detailed design was undertaken in parallel with Flora & Fauna and Cultural Heritage Studies. The capacity of spillway was recommended to be 1 in 500 AEP to satisfy ANCOLD’s requirement for Low/ Very Low consequence category. Also, the detailed design suggested lowering the spillway and embankment levels in both dams and lowering the water level in the lower dam.
The cost of construction based on this design exceeded NEW’s budget.
The alternative design in April 2015, suggested 1 in 100 AEP for spillway capacity (lowest range level according to ANCOLD). The spillways’ widths were designed to be 9.3m and 9.6m in the lower and upper dams respectively.
NEW organised a community information session in Bright to present the outcomes of Detailed design, Flora & Fauna and Heritage studies.
Further revisions to the proposed works were required following an independent review of the design in June 2015. The independent review emphasised that the spillway was required to pass the 1 in 1000 AEP flood. The design was revised to incorporate flattened embankments and the inclusion of geotextile on the embankment crest and bigger rip rap on the downstream face. The designs are shown in figures 2 and 3.
Construction commenced in mid- April 2016, after receiving approvals for removal of native vegetation, working on waterways and major decommissioning works. The work could only proceed outside of the Platypus breeding season which was the prime construction period.
During excavation for the embankment and spillway channel and crest, weathered and silt materials were found in both upper and lower reservoirs which had not been identified in the geotechnical investigation prior to detailed design and a further design change was necessary.
For the lower reservoir, a key consideration was significant wave action in the reservoir during a flood event which would erode the exposed embankment crest. The upstream edge of the crest is the most vulnerable location during overtopping. To address this issue, it was suggested that:
- The upstream side of the embankment be covered with crest capping and rip rap
- The left bank of the spillway channel be protected by Gabions
- The spillway channel bed be protected with Reno Mattress
- Loose material be removed by pressure cleaning of the spillway channel bed and dental concrete with anchors be placed to fill up the area
The construction works were scheduled to be complete by the end of July 2016, however due to the extra works and very wet season (327mm rain from April to July 2016) construction was re-scheduled to be completed by the end of October 2016.
As October is in Platypus breeding season, NEW engaged an ecologist to assess the risk of remaining construction works on Platypus’ borrows. It was concluded that all bulk earthworks along the crest of the upper dam, lowering of water levels in both dams, spillway works on both dams and grouting the outlet pipes should be completed by the end of September, The remaining works will need to be undertaken after Platypus breeding season (in March 2017).
Landscaping and re-vegetation of the site will be implemented at the completion of civil works.
Author: Aida Baharestani- North East Water- Victoria