FOR more than 60 years, the Mulwala canal, Australia’s largest irrigation channel, has carried water to the farmland and towns of southern New South Wales (NSW) and Victoria. The canal diverts up to 11,000M litres/day from the Murray river, running for over 120km at grades as low as 1 in 10,000. It is also used as a bypass for water for South Australia.

Work began in 2002 on a channel-based hydro power plant on the Mulwala canal. Nicknamed ‘The Drop,’ after the irrigation control structure of the same name, this hydro plant will produce some 2.3MW, enough to power about 1600 homes.

The Drop is located halfway between the small towns of Berrigan and Barooga in NSW and it passes over 6000M litres/day during the irrigation season.

Water passing through the control gates falls 4m at that point. In August 2001, Pacific Hydro Limited, one of Australia’s leading independent renewable power producers, with the backing of the NSW Sustainable Energy Development Authority and the Australian Greenhouse Office, initiated a US$3.8M project to capture the energy potential generated by this waterfall.

Pacific Hydro contracted MWH Constructors Australia, a subsidiary of MWH Inc, to design and construct the facility. A key design criterion was that the facility be able to generate without affecting the irrigation or mitigation potential of the canal system.

The horizontal propeller-type turbine selected is sized for the design flow of 6000M litres/day. With a runner diameter of 3.5m, the turbine/generator weighs 110t. The flow requirements into and out of the station required significant civil and structural works with complex structural geometry. Flow modelling was carried out to optimise the off-take from the canal. Deep excavation immediately alongside the old canal and a road parallel to the canal presented additional challenges and the design effort included mitigating risks to exiting structures and water flows. Excavation was up to 17m deep and approximately 80,000m3 of clay and weathered siltstone soils was removed.

  The Drop is located about 150 km from the nearest large city, Albury, and over 300km from Melbourne. This posed some difficulties with scheduling and sourcing materials. However the local Cobram and Shepparton region in Victoria was able to serve the project well in terms of services and materials. Another major engineering challenge was integrating of the station (specifically during start up and shutdown of generation) with the existing infrastructure and SCADA system already operated by Murray Irrigation.

Under the contract the plant was constructed over ten months, with a canal dry period from June to August. Due to the risks involved in co-ordinating with this dewatered period, a temporary dam was left on the inlet and outlet channels.

The civil works include 80,000m3 of excavation, 2200m3 of concrete and 320t of reinforcing steel together with some very substantial fabricated steel structures and complex concrete shapes in the inlet and outlet area.

Electricity generated at The Drop is supplied to the local electricity grid via a 220kV line that runs close to the site. The Drop power station was handed over to Pacific Hydro in early December 2003 and is now in operation.