TAS – Trevallyn Dam Project

Author: Andrew Rumsby, Project Manager.

Trevallyn Dam is located in the upper reaches of the Cataract Gorge on the outskirts of Launceston in northern Tasmania. The 33 metre high concrete gravity dam was constructed in the 1950s and includes 3 large diameter conduits, each fitted with Bopp and Reuther flow regulating valves. Each valve weighs around 19 tonnes and performs the primary function of controlling the lake level to allow maintenance of the Trevallyn Power Station intake and to reduce loading on the dam should a dam safety issue occur.  Flooding in the catchment passes over the Dam’s central ogee spillway crest and over the roof and downstream wall of the valve chamber at the downstream toe of the dam, below the spillway flip-bucket.
The valves and conduits were found to be significantly corroded and a project was initiated to replace 3 of the valves and grit blast and paint each conduit.  Contractors MSD and The Engineering Company (TEC) commenced construction in January 2016.  The methodology for removal of the existing valves was to skate each valve through a 3 metre x 3 metre penetration cut in the downstream toe of the valve chamber.  Bulkheads were fitted at the entrance to each conduit at the upstream face of the dam to prevent water ingress into each conduit.

The location of the valve chamber underneath the Flip-Bucket of the dam’s central spillway necessitated the development of detailed flood modelling and innovative construction solutions to prevent water ingress into the valve chamber and dam gallery in a spill event during the valve replacement project.

Up to 450mm of rainfall fell in some parts of Trevallyn Dam’s catchment in late January 2016.  At this time, one of three penetrations had been made into the downstream toe of the valve chamber and the flood forecasting model predicted a spill of 300 cumecs.  Three dual purpose work steel platforms/sealing drawbridges had been installed to seal the penetrations in the downstream wall of the valve house.  The drawbridges were raised and the 1.3 metre depth, 300 cumecs spill was managed admirably with no leakage through the drawbridges and no noticeable damage to the riparian bypass pipework, the majority of which was left in place during the spill.

The project is due to be completed in late March 2016.

Skating the valve chamber wall out to expose the regulating valve

Skating the valve chamber wall out to expose the regulating valve

Skating a valve out of the opened valve chamber

Skating a valve out of the opened valve chamber

Drawbridges covering penetrations and flooding site to minimize the impact of turbulence

Drawbridges covering penetrations and flooding site to minimize the impact of turbulence

Worksite prior to spill

Worksite prior to spill

Spill of approximately 300 cumecs

Spill of approximately 300 cumecs