Physical Modelling of the Bogong Headpond

Client: Sinclair Knight Merz
Year: 2006
Project Reference: 06013
WRL Technical Report: Bogong Headpond Physical Model (2006/20)

WRL was commissioned by SKM Pty Ltd on behalf of AGL Southern Hydro to construct and test a physical model of the (then) proposed Bogong headpond. Before the headpond was constructed, water would leave the McKay tailrace before heading over a spillway and returning to the river. The purpose of the headpond is to divert this water by turning it through 90° and letting it fall through a 5 m diameter dropshaft to another hydro-power station.

(Clockwise from left) Testing showing variable position vanes; Flow from the tailrace; As constructed - photo courtesy of AGL Southern hydro.

The objective of the physical modelling was to optimise the size of the headpond, position steering vanes, ensure reasonable flow behaviour in the headpond and to avoid swirls and air entrainment at the dropshaft.

The hydraulic model was scaled at 17:1 using Froude Scaling. WRL observed the flow patterns by visual observations such as ‘tell tale’ strings, dye and paper tracers. An Ott meter (OTT-C31) and an Acoustic Doppler Velocimeter (ADV) were used to provide quantitative velocity measurements throughout the model.

Without any steering vane in place, a large eddy formed in the headpond and velocities approaching the dropshaft were not uniform. By adjustment of the steering vane position, a reasonably uniform flow down the headpond was achieved. This testing was undertaken with SKM and AGL Southern Hydro staff present to iterate on the optimum location. The position of the vane was determined such that maintenance vehicle access into the tailrace would not be restricted, while also providing a good spread of the flows to either side of the headpond.

After successfully locating the steering vane, the length of the headpond was reduced in length by 7.75 m which allowed for significant cost savings.

Further a vortex suppression baffle was tested to restrict any air entrainment into the drop shaft.