1911 - Design and Construction

The major initial design components and years of construction of the O’Shannassy System were:

The Aqueduct 1911-1914

The O’Shannassy Weir 1911-1914

The O’Shannassy Reservoir 1922-1927

The Silvan Dam 1927-1931

By 1915, The O'Shannassy Aqueduct System carried water some 82 km to the Surrey Hills Reservoir. 

In 1931, the large Silvan Holding Reservoir was completed.

The System was a key component in Melbourne's water supply system (especially supplying the fast-growing eastern suburbs of Melbourne) and was managed first by the Melbourne and Metropolitan Board of Works (MMBW) and later by Melbourne Water.  Whilst most of the Aqueduct remains physically in place today (in a much deteriorated condition), it has been disused for some time and has fallen into considerable disrepair (some parts of it have disappeared altogether.

The O'Shannassy River, and subsequently the Aqueduct, was named after Sir John O'Shannassy, three times Premier of Victoria. The O'Shannassy River rises in the slopes of Mounts Strickland, Arnold and Grant in the ranges east of Melbourne.

Design Concept
The O'Shannassy Aqueduct was a brilliant piece of engineering design and construction. It was to carry water from a diversion weir on the O'Shannassy River via an open concrete channel along the slopes of Mount Donna Buang and several other steep mountainsides for over 80 km to supply clean, fresh drinking water to the eastern suburbs of Melbourne.

The Aqueduct was designed to have a constant gradient of 2 feet to the mile (1:2500) and was to operate completely by gravity, without the need for any pumps or other mechanical devices. 

Water would be carried up and down steep slopes, across roads, creeks and rivers and through both farmland and residential areas by means of siphons, pipes and weirs.  Some of the pipes were timber and some steel.

Water flow into and out of the Aqueduct (both from the weir at its source and from numerous creeks and rivulets along its course) was to be carefully controlled via a series of manually operated locks and gates.  Caretakers would be stationed along the route to maintain the aqueduct, keep it clean and ensure the water always flowed.

The Route
Plans were drawn up and construction work commenced in the second half of 1911. Defining the route and clearing the forest trees was the first step. A series of wooden marker posts were then built along the full length of the aqueduct to help workers locate particular points and maintain prescribed levels. 

Where the aqueduct crossed the lower lying areas around Wandin and approaching Mount Evelyn, it was in steel or wood-stave piping, converting to open concrete-lined channel when the higher levels were reached. It went through the township of Mount Evelyn in open channel (except for a short distance adjacent to the railway line) winding around the local contours to just above what is now Johns Crescent, then part of the ‘Pine Mont Estate.’

From this point, the aqueduct was again transferred to steel pipes to go under the Olinda Creek and Swansea Road to feed the Olinda Reservoir, in the Edinburgh Road area. This reservoir, now roofed over, was open to the skies.

The pipes crossing the Olinda Creek valley cut across the Lilydale Water Race, so water was fed from the Aqueduct to the Race, the pipe creating the Race at the Cascades was cut away, and the section of the Race upstream fell into disuse. The part of the Race downstream to the small reservoir on Birmingham Road was lined with concrete.

Mount Evelyn residents adjacent to the open channel were permitted to syphon water out of the channel for their own use. Those on the high side of the channel were only allowed to pump water where there was no possibility of drainage back into the open channel. One was the Pine Mont Guest House, where a windmill pumped water up to the house on top of a small hill, and the drainage went down the other side. This first aqueduct system, before later enlargement, had a capacity of 20,000,000 gallons (75 megalitres) per day.

Construction Techniques
The Aqueduct channel was principally dug by hand (aided by horse-drawn scoops). It comprised an open channel 9 feet 3 inches (2.81 metres) wide at the top and 3 feet 4 inches (1.026 metres) deep and was founded on a solid clay base. 

Next to the channel, a flat access track was constructed from the excavated material and fitted with rail lines to enable workers access to the site and to facilitate the delivery of heavy construction materials including timber, stone, pipes, steel and cement. 

Many of these were delivered by horse-drawn or manual trolley. Stones and crushed rock came from Blacks Quarry in Coldstream.

Horse-drawn flat-top wagons carried a mortar board on which to mix cement and this was hinged to enable the cement to be tipped into the channel to be hand-troweled into place by the men standing in the channel below.  It was important that the cement was mixed at exactly the right consistency so that it would "stand" in place on the slope and not slump to the bottom. If it was too dry, it would crumble and break up.  Cement mixers were used in some locations to keep up a steady flow of concrete to the workers. 

Construction was hard work and labour-intensive and continued for just over three years (39 months) in sweltering hot summers and freezing cold, wet winters. Falling trees and limbs, landslips and wild storms were constant hazards. Construction took a relatively short time however, considering the degree of difficulty and the exacting nature of the work and the fact that it involved hundreds of workers and almost 100 horses.  The aqueduct consisted of 23 miles (37km) of open or covered channels, three tunnels totalling just over half a mile, over 25 miles (40km) of steel pipes (up to 36 inches in diameter) and a water supply capacity of 20 million gallons (75.7 million litres) per day.

The new system was completed in October 1914, just in time to help Melbourne avert a major drought which affected most of Australia. 

The total cost of construction (as at 30 April 1915) was 426,890 Pounds ($853,780).

 

In 1928, the weir was replaced by the new larger O’Shannassy Reservoir, upstream.