|Historical notes: ||History of the Upper Nepean Scheme.
In 1867, the growth of Sydney coupled with recurring dry seasons, brought into sharp focus the pressing need for a water supply, which was larger and more reliable than the existing Botany Swamps source. This lead the Governor (Sir John Young) to appoint a special Commission to investigate how an adequate long term supply might be achieved.
The Commission reported in 1869 and recommended the Upper Nepean Scheme whereby water from the head waters of the Upper Nepean River and its tributaries, the Avon, Cordeaux and Cataract Rivers, would be conveyed by canal, tunnel, pipe and aqueduct to a storage reservoir to be built at Prospect. From there another canal would carry the water to a basin at Guildford from where it would be piped to a smaller service reservoir at Potts Hill for distribution to Sydney.
After a lapse of six years during which no decision was made, and a number of alternative proposals were circulated, the government decided to engage an eminent English civil engineer, W. Clark, M.I.C.E., to review the various proposals. Clark arrived in November 1876 and, in May 1877, after reviewing eight schemes, strongly endorsed the Upper Nepean Scheme.
An Appropriation Act was passed in July and work commenced in 1880. Construction was carried out by contractors under the direction of the Harbours and Rivers Branch of the Public Works Department. The head of that branch was Edward Orpen Moriarty, who contributed a great deal to public engineering works of the time. He was responsible for the both the design and execution of the works and had a number of site engineers under him. His signature appears on most of the plans of the Scheme.
When the Scheme had initially been proposed in 1869, Moriarty had then drawn up plans for parts of the system, such as weirs and aqueducts. Between 1869 and when work actually commenced in the 1880s, alterations had been made to the initial plans. In 1869, Moriarty had proposed to bridge the creeks, which the Upper Canal would cross by means of aqueducts with approaches built on top of dry rubble stone walls. When the Scheme was finally under construction in 1884, he drew up new plans to cross these creeks with wrought iron inverted syphons.
Work proceeded as rapidly as possible once contracts were let, but by June 1885, because of continued dry seasons, there were only about ten days supply remaining in the Botany Swamps. In response to urgent demands for relief, the government of the day accepted an offer made by the Sydney engineering firm of Hudson Brothers (later to be incorporated as Clyde Engineering) to provide a temporary supply by bridging the gaps in the Upper Canal where creeks remained to be crossed, and, also, carrying the water from Pipe Head by elevated temporary fluming to the Botany Swamps.
Matters proceeded rapidly and, when a bond to commence the work was signed on 3 September 1885, work was already underway. Maximum use was made of the firm's workshops at Redfern and Granville to fabricate the various components, including the manufacture of 1200 cast iron pipes. Many of these 36 inch diameter pipes were laid to operate as inverted syphons, supported on timber trestles above flood level, over the creeks intersecting the route of the Upper Canal.
Hudson's Emergency Scheme delivered its first water in January 1886 and functioned until the Upper Nepean Scheme was commissioned in 1888, after which it was dismantled and sold.
Construction and Operation of the System.
The great merit of the Upper Nepean Scheme is that it was, and still is, a gravity one. Water harvested in the Southern Highlands, when diverted by the Pheasants Nest and Broughtons Pass weirs, flowed all the way down the Upper Canal into Prospect Reservoir, thence along the Lower Canal to Pipe Head, then by pipe to Potts Hill Reservoir and again by pipe to Crown Street Reservoir from where it was reticulated to the major portion of the city and suburbs of the time, all by gravity.
The Upper Canal was built of a variety of materials with section profiles depending upon the nature of the country through which it was passing. Where the ground was soft, the Canal was V shaped and the sides were pitched with shale or sandstone slabs. In other sections, a U shape was utilised and here the sides were walled with sandstone masonry, or, if cut into solid rock left unlined. Where the canal had to go under a hill, tunnels were excavated. These were left unlined if cut through in solid rock, or lined with brick or stone, if cut through softer material. Where the canal crossed creeks or large depressions, such as Elladale, Simpson's, Ousedale, Mullaly, Woodhouse, Nepean and Leaf Creeks, the water was carried across in wrought iron inverted syphons resting upon stone piers.
To supply water for towns along the route, such as Camden, Campbelltown, Ingleburn and Liverpool, offtakes were built at suitable points along the Canal, where stop logs were used to divert water. At the Liverpool Offtake, a small storage dam was built in the 1890s, for use when the canal was emptied for cleaning or repairs.
In addition, stopboards or bulk-heads permitted the closing of sections of the Canal for cleaning and repairs. So that stormwater did not find its way into the Canal, and pollute Sydney's drinking water, a series of flumes carried stormwater over the canal. At first many of these flumes were built of timber, but gradually they were replaced by wrought iron and even later, by concrete flumes. Bridges carried major roads such as Camden Road over the canal. In addition, "occupation bridges" allowed property owners access between their holdings.
After travelling a total of 39 3/4 miles (64 kilometres) from Pheasant's Nest, the water entered the Trafalgar Tunnel, where it passed over a measuring or gauging weir and then along the inlet race into Prospect Reservoir. Prospect Reservoir was built in the 1880s as the major storage dam for Sydney's water supply. It is an earth dam with a crest length of 7300 feet (approximately 1.37 miles or 2.2 kilometres) consisting essentially of a puddled clay core with shoulders of selected earth placed in layers 12 inches thick and compacted by rolling. The upstream face of the dam wall is pitched with locally quarried diorite blocks 18 inches thick as protection against wave action. The maximum height of the dam is 86 feet (26.2 metres).
The Lower Canal was constructed in similar fashion to the Upper Canal although most of it was built as a V section open cutting lined with stone pitchers. Below Prospect Hill, it entered what was called the "covered way". In 1903, the covered way collapsed when the Canal was emptied and it was rebuilt in concrete.
The Lower Canal terminated at Pipe Head Basin, initially built simply as the point at which the water was let into a 72 inch pipe feeding the Reservoir at Potts Hill. At Potts Hill, the water was screened to remove dirt, vegetable matter and other unwanted debris. From Potts Hill, the water was then piped under gravity pressure to consumers in the various parts of Sydney.
The water supply was managed by a Resident Engineer, housed at Veteran Hall at Prospect Reservoir until 1912, when the construction of an additional Reservoir at Potts Hill meant that he needed to be housed at that site for better supervision. In 1933, the position of Resident Engineer to control Head Works, was created, and the incumbent was housed at Pipe Head.
By 1898, a telephone line was in operation along the whole length of Upper Canal. In that year, the existing line was duplicated. That line was an integral element in controlling the System. Maintenance men were positioned along the Upper Canal, at Prospect, and at Pipe Head. At the weirs and at Prospect Reservoir, there were valve controllers responsible for the discharge of water along the system.
Care and maintenance of the Upper Canal in particular, was in the hands of Inspector's and maintenance men. They were housed along the Canal in cottages, owned and maintained by the Board. Initially, the men walked or used horses to patrol the length of Canal assigned to them. By the late 1890s, a gradual process of adding roadways along the Canals was under way. The larger creeks were not finally bridged until 1935-6, although one of the early photographs shows a stone embanked roadway across the bed and up the sides of one of the gullies crossed by the Upper Canal.
During the cooler months when the demand for water was lower and requirements could be supplied from water impounded at Prospect, repairs and maintenance were carried out on the Upper Canal. The sides were regularly cleaned, and, by the 1900s, some lengths were being relined. More extensive work, was either contracted out, or completed by the Board, utilising day labour.
Progressive Development of the Upper Nepean Scheme after 1888.
An outstanding feature of the Upper Nepean Scheme as originally envisaged and constructed was its potential for progressive development. Initially, it was a "run of rivers" scheme, because there was virtually no storage behind the Pheasants Nest and Broughtons Pass weirs. Immediately after its completion in 1888, drought and population growth necessitated its further development and this was implemented over a period of nearly fifty years by the construction of major storage dams on the Cataract, Cordeaux, Avon and Nepean Rivers as follows:-
- Cataract Dam. Built 1907. First large cyclopean masonry dam in Australia.
- Cordeaux Dam. Built 1926. Curved concrete faced cyclopean sandstone.
- Avon Dam. Built 1927. Curved, concrete faced, cyclopean sandstone.
- Nepean Dam. Built 1935. Curved, concrete faced cyclopean sandstone.
The provision of these major storage dams changed the role of Prospect Reservoir from being Sydney's first storage reservoir to that of being a vital service reservoir to cover the daily fluctuations of demand in the distribution system.
The Upper and Lower Canals continued their role as the main arteries of the system, but upgrading was necessary. The Upper Canal needed only minor work to bring its capacity up to 150 million gallons per day but more extensive works were needed to improve the Lower Canal structure and increase its capacity. The capacity of the System downstream of Pipe Head was amplified by the progressive provision of additional 72 inch diameter steel mains, and, in more recent years, by their boosting with electric and diesel pumping stations. An additional major service reservoir was built at Potts Hill between 1913 and 1923.
The work on the Upper Canal consisted mainly of improving its flow characteristics by concreting rough spots on the bottom and sides, and replacing some stone pitching by concrete. By-passes were also provided around the wrought iron inverted syphons crossing the creeks to enable their internal maintenance when demand conditions permitted. Work to improve the capacity of the Lower Canal commenced in 1902. Initially, a length of 1909 lineal feet was reconstructed in concrete, and 646 lineal feet in Monier plates, i.e. pre-cast concrete slabs. An inquiry judged the Monier plates to be the better solution for upgrading the canal, and by 1912, the remaining walls of its whole 5 mile length had been raised 2 feet and lined by this method. The Lower Canal, as reconstructed, had its capacity increased from 50 to 93 million gallons per day whilst subsequent minor improvements and operating procedures have lead to its maximum current day capacity being 100 million gallons per day.
A feature of the Lower Canal was the Boothtown aqueduct of 22 brick arches, each 30 feet span, which carried the canal over a valley. From 1892 onwards, it suffered a series of structural failures to the brick sides of the water channel, until, in 1907, it had to be replaced by a reinforced concrete inverted syphon, 10 feet 6 inches in diameter, located in an earth bank beside the old aqueduct. This was the largest continuous concrete work of is kind constructed in Australia up to that time. It was fitted with the more modern "stoney gates", which were also used to replace the earlier "butterfly" gates to Broughton's Pass in 1912.
As previously mentioned, Prospect Reservoir was completed in 1888, but in 1898, its storage level was raised by 1 foot 8 inches to give it greater operating capacity. The Prospect earthen bank, with its clay core, suffered a series of slumps in 1893, 1898, 1899, 1902. Various remedial measures were carried out, and these included:-
i) The driving of tunnels into the downstream toe to relieve soakage water, and their later conversion to permanent rubble drains.
ii) The placing of 12,000 cubic yards of blue metal spawls on the upstream slope at the slump areas to stabilise the toe of the bank
iii) Renewal of parts of the leaking clay puddle core
iv) Later extensive re-making of the puddle core down to a depth of 40 feet and further weighting of the toe of the embankment.
By 1905, the situation was stabilised, and the technique developed of keeping the clay puddle core suitably moist by means of surface drains to stop its alternate shrinking and expanding with consequential leakage and earth movements.
Although no serious trouble was experienced from then on, in 1980, the Board completed a major strengthening of the dam by greatly increasing the volume of the downstream side of the embankment and providing improved drainage facilities in the light of modern knowledge of the stability of earth dams. This work did not alter the length or height of the wall, or the top water level, but only the volume and slope of the downstream side.
Since 1930, two major electric pumping stations have been built on the eastern shore of the Prospect Reservoir:-
i) One to pump water to the adjacent service reservoirs (one an elevated structure) on Prospect Hill to serve the Blacktown and adjoining areas to the north.
Ii) One to pump water to a major service reservoir at Thornleigh for the Upper North Shore, thus supplementing the Ryde Pumping Station which was commissioned in 1892, and has received its suction water from Pipe Head since 1903 (prior to that from Potts Hill).
As the ever increasing demand for water was met by the construction of the major storage dams previously mentioned, the provision of additional conduits to carry it to the city was also necessary. Particularly deficient was the system between Prospect Reservoir and Pipe Head where the amplified Lower Canal could carry only 100 million gallons per day as compared with the 150 million gallons per day the Upper Canal could carry to Prospect from the dams.
In 1926, a scheme was considered whereby a pressure tunnel would be built between Cecil Hills on the Upper Canal to link up with another pressure tunnel then under construction between Potts Hill and Sydney, thus by-passing Prospect, the Lower Canal, Pipe Head and Potts Hill. This would have been extremely expensive, and, in the event, a 54 inch diameter woodstave main was constructed from the Upper Canal not far from where it entered Prospect Reservoir to the Pipe Head basin and then on to Potts Hill. It was completed in 1927 and could deliver 50 million gallons per day to Pipe Head and Potts Hill or 33 million gallons per day to Potts Hill alone. Later in 1937, it was replaced by a 72 inch (1,800 mm) diameter steel main laid between the Upper Canal, from just before its discharge into Prospect Reservoir, and Pipe Head. This main could also be fed directly from Prospect Reservoir. It had a capacity of 84 million gallons per day under Upper Canal head and 45 million gallons per day (later 60 million gallons per day) under Prospect head. Still later, in 1958, when Warragamba water became progressively available to Prospect Reservoir, an 84 inch (2,100 mm) diameter steel pipeline was commissioned between Prospect and Pipe Head with a capacity of 90 million gallons per day.
In more recent years, pumping stations have been constructed to boost the flow through these conduits.
Until 1913, screening of the water was carried out in a large circular screening chamber at Potts Hill, so that, with the changeover of the Ryde Pumping Station suction offtake to Pipe Head in 1903, screens had to be provided there also.
Between 1913 and 1928, three screening basins each 250 feet long by 40 feet wide were constructed at Pipe Head and became a key installation in the System. The entry of water to each chamber was controlled by a "stoney gate".
Each screen was approximately 14 feet (4.3 metres) by 3 feet 3 inches (99 centimetres) and consisted of copper mesh. Initially, a mesh of 625 per square inch was used, then this was later changed to 36 per square inch, but later again to a fine mesh. Of the individual screens, some 600 all told, about 400 were individually lifted by small mobile cranes for regular cleaning, whilst the remainder were cleaned in situ. The cranes were originally powered by crude oil engines, but were converted to electricity in 1917, and two are still in regular use.
In the 1970s, two of the main screening basins were dismantled and replaced by a set of four modern rotary drum screens, known as microstrainers. These have a mesh of 120 X 120, i.e. 14,400 minute holes per square inch.
The scheme has continued to be modified and expanded to meet Sydney's water supply needs. This includes the construction and operation of a water filtration plant (not owned by the SCA) and the Raw Water Pumping Station completed in 2007.
A chlorination plant was installed at Broughtons Pass in 1948 for disinfecting the flow into the Upper Canal after periods of heavy rain. In June 1960, following the installation of a more modern plant, continuous chlorination was implemented to operate under all flow conditions.