Author's Note
The names "Silvan Dam" and "Silvan Reservoir" refer to the same thing. Early documents and images show "Dam"
- later works show "Reservoir.
Planning
The
final major phase in the O'Shannassy project was to be the construction of the Silvan Dam, near Mt Evelyn. This had been
planned from 1916 as a storage (holding) reservoir for the O'Shannassy system, particularly to hold water from the aqueduct
and provide a back up storage in times of extreme drought, such as that in 1914. It would also act as a settling basin
to further ensure the quality of Melbourne's drinking water.
The site was selected in 1919 and the 442 hectares
of Crown Land plus 449 hectares of freehold land was vested in the MMBW. The site was conveniently close to the O'Shannassy
aqueduct so that both inlet and outlet channels would be as short as possible. Pipelines from Silvan would carry water
to Surrey Hills reservoir and from there be distributed to the higher eastern suburbs of Melbourne.
It was decided
to construct Silvan using day-labour and "modern" machinery, building on the success of this approach used at the earlier
O'Shannassy Dam construction.
Works began in 1927 and the project was finally completed in 1930 at a total
cost of just over 500,000 Pounds ($1 million).
In 1917, following a severe drought, a site in Silvan was selected across the Stoneyford Creek
(a tributary of Olinda Creek) for extra water storage for Melbourne.
Components
The four components of the ‘Silvan Dam’ were completed by 1933,
and comprised
-
The Silvan Reservoir (a
major dam across Stoneyford Creek adjacent to Stoneyford Road and two small dams adjacent to McCarthy Road and adjacent to
Monbulk Road) with a storage capacity of 40,000 megalitres, completed in 1932. An aerial ropeway was constructed from a special
siding at Mount Evelyn railway station to the southwest end of the reservoir wall site, a distance of about 5 km, to convey
the sand and cement in 100 buckets needed for the wall construction. The railway siding was at the back of what is now Bowen’s
timber yard.
-
The Diversion Aqueduct, mainly open channel leading from the original O’Shannassy aqueduct near
the Wandin Yallock Primary School to a tunnel under Silvan township, to feed water into the newly constructed Reservoir.
Note: The
DIversion Aqueduct remains operational to the present day, as a secondary inlet to the Dam, delivering some 8 megalitres/day
from the Corronderk Aqueduct, Don Rd, Launching Place.
-
The Outlet Aqueduct from the Silvan Reservoir
to feed water back into the existing system. Mainly open channel, now known as the Mt Evelyn Aqueduct, becoming a concrete
tunnel under Monbulk Road, surfacing near Ormeau Road and the (now) Melba Centre, there joining into the original Aqueduct.
-
An increase in capacity of the existing system
downstream of the point of entry from Silvan Dam. Flow of water was maintained during the widening by using temporary pipeline
which was mostly above ground and took short cuts instead of following surface contours. Generally, one side and the floor
of the original concrete channel were removed, the channel was deepened and widened and then re-concreted. Where the original
aqueduct was in pipe, an extra pipe was laid.
The health concerns accompanying increases in population caused the
open channels to be replaced by pipes to and from the Silvan Reservoir by the late 1960s. Enclosed pipes had no need to follow
surface contours, and new shorter routes were adopted.
Commissioning
The dam was officially opened on 7 July 1931 (by which time it was
almost three-quarters full). Total capacity was 40,000 megalitres.
The main dam wall was 644 metres long at its
crest, 219 metres wide at its base and 43 metres high.
The wall required 46,000 cubic metres of concrete and 1.3 million
cubic metres of earthfill. The cellular core wall principle was almost entirely new and represented a notable advance
in the construction of large earthen dams. The foundations of the core wall extended up to 140 feet (42 metres) below natural
ground level.
Innovations
A number of innovations were used on the project, including:
-Electric
power was brought 11 kilometres from Lilydale to provide power for the works (including lighting to enable operations
to continue at night)
- a 5 kilometre
long aerial ropeway (using an electric powered motor) was built from Mt Evelyn station to carry sand, cement and
other materials to the construction site: there were 200 buckets on the line, carrying an equivalent of 5 bags of cement each
and each bucket took about 52 minutes to reach the site - providing around 16 tons of cement per hour - and skips dumped
their loads directly into the formwork on the dam wall
-
an aerial cableway carried material from the crushing plant to the dam wall
-
a nearby quarry provided rock for the dam wall
-
compressed air crushers were used to crush the rock
-
a 3 foot 6 inch rail transport line was built and steam locomotives carried rock from the quarry to the on-site
crushing plant
-
use of the largest steam shovel in Australia at the time
-
use of steam powered excavators, locomotives, rollers and cranes
-
workers were accommodated for free in huts on site (all quarters were provided with water, sewerage and garbage
services)
-
the camp and works had its own telephone exchange with 30 lines
All
of this was absolutely "best practice" at the time.
The scheme provided a model for future Board projects involving the use of the latest technology
(replacing much manual labour and horse-power), efficient use of day labour and the purchase of its own plant and equipment. The
project actually came in several hundred thousand Pounds under budget and was hailed a great success!
Silvan was fed
from both the O'Shannassy and Maroondah aqueducts (from O'Shannassy Weir and Graceburn Weir respectively).
In 1933
parts of the Mt Evelyn section of the O'Shannassy aqueduct were widened to increase its capacity. One side of the concrete
channel was removed, the channel was deepened and widened, then re-concreted. Piped sections of the aqueduct were also
duplicated to further increase flows and to maximise the benefits now available from Silvan.
With the completion and commissioning of Silvan, all three principal components of the O'Shannassy aqueduct system were
now complete: O'Shannassy Reservoir, the aqueduct and Silvan Dam.
The
system was later supplemented by the construction of the Upper Yarra aqueduct, which was commenced in 1929.
In 1936, after more drought conditions, the Board reported that Silvan
"has been a tower of strength in maintaining the supply of water under these unusual conditions" and without it Melbourne
would have been almost without a water supply.
