VIC – Loombah Dam – Spillway Upgrade utilising a Piano Key Weir in a confined site

Author: Peter Buchanan, GHD. 

Loombah Dam is 13.5 m high central concrete core wall rock fill dam located on Ryan’s creek, approximately 25 km south east of Benalla, Victoria.  The dam is one of two dams on Ryan;’ creek forming the storages for the Benalla Water Supply. The Storage is operated by North East Water.  The original spillway consisted of unlined excavated channel through the right abutment of the embankment.  The right hand side of the spillway is a near vertical face of rock, approximately 15 m high.

Following major flooding in 1993, when the dam came within a few hundred millimetres of overtopping, the embankment was raised with a parapet wall, to provide additional flood storage.  In the early 2000s the flood capacity was again reviewed with updated hydrology and consequence assessments and the spillway capacity was found to be inadequate.  An options study was commissioned to identify the preferred spillway upgrade, which identified an auxiliary spillway option through the left abutment, given the difficulty of widening the existing spillway.  This option was selected based on very limited geotechnical data.

During the detailed design phase when additional geotechnical investigations were undertaken it was found that there was no rock at a reasonable depth on the left abutment, making the left abutment option very expensive.  The design then looked at the option of increasing capacity of the existing spillway.  A craeger type labyrinth spillway was selected as the preferred option, and taken through to detailed design and tendered for construction.  This arrangement could be constructed without any significant excavation of the vertical rock face, but did require lowering the rock in the original alignment by about 3m.

During the excavation works it was found that the rock was extremely hard, and with no blasting allowed adjacent to the dam, excavation proved to be very slow and expensive.  Following discussions with the contractor and designer, alternate spillway arrangements were considered which required a reduced quantity of excavation.  The final arrangement adopted and constructed was a Piano Key Spillway designed for a 1 in 10,000 (years) Average Recurrence Interval.

The construction started in early 2012 and is now nearing completion. Construction was delayed  by 6 months due to high rainfall in the catchment in the winter of 2012. The scope of the works not only included the spillway but also widening of the spillway downstream and the construction of a new access bridge.

Please see photos below

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