The Nepean Dam is a heritage-listed dam split across Avon in the Wingecarribee Shire and Bargo in the Wollondilly Shire, both in New South Wales, Australia. The reservoir created by the dam spreads across Avon, Bargo and also Yerrinbool in Wingecarribee Shire. The Nepean Dam is one of four dams and weirs in the catchment of the Upper Nepean Scheme, in New South Wales, Australia, and provides water to the Macarthur and Illawarra regions, the Wollondilly Shire, and metropolitan Sydney. Completed in 1935 under the supervision of Ernest Macartney de Burgh, the dam is currently managed by the WaterNSW and is listed on the New South Wales State Heritage Register.
History The Upper Nepean Scheme was commenced in 1880 after it was realised that the Botany Swamps scheme was insufficient to meet Sydney's water supply needs. The Nepean project consisted of the construction of a weir across the Nepean River to divert of the rivers Cataract, Cordeaux, Avon and Nepean, to the Prospect Reservoir.
By 1902, however, the water supply was found to be inadequate and a Royal Commission was held which recommended impounding the waters of the Cataract River by means of a major storage dam. The Cataract Dam was completed in 1907. In November 1918, a Board of Experts was appointed by the Government to draw up recommendations for the amplification of the Sydney water supply. It was upon the recommendations of this body, as approved by the Public Works Committee of the NSW Parliament, that not only Cordeaux, but also Avon and Nepean Dams were built. The Cordeaux Dam was completed in 1926 and the Avon Dam in 1927, on the Cordeaux and Avon Rivers respectively. As Cordeaux and Avon Dams were approaching completion, a fourth dam, Nepean Dam, was commenced in 1925.
The site chosen was on the Nepean River, about above the point where the Cordeaux River joins the main stream at Pheasants Nest Pass and about by road southeast of Bargo railway station. The dam was designed by engineers of the Public Works Department under the direction of the Chief Engineer for Water Supply and Sewerage, EM de Burgh.
Construction of the dam was undertaken by the Public Works Department until 1 September 1928, at which time the department was re-organised and the majority of the staff associated with water supply were transferred to the Board of Water Supply and Sewerage, who continued the work on the dam. At that date, excavation for the foundations of the wall was complete and placing of concrete about to begin. Most of the plant and equipment used for the construction of Avon Dam was dismantled and re-erected at the Nepean Dam site, including the cableways and timber towers. A suspension bridge was constructed across the Nepean River which was later used at Warragamba Dam.
Railway sidings were established on the Main Southern railway line at a point between Bargo and Yerrinbool. Transport to the dam site was again by light railway, on this occasion of standard gauge. This avoided transhipment from the Government vehicles bringing materials from large commercial quarries, effectively making the line an extended privately owned siding. The line was long through gentle countryside. Trains were worked by a variety of locomotives, including a former Sydney Steam Tram Motor. Additionally, there was a system of narrow () gauge lines in use at the dam construction site.
A township for workers was constructed of timber obtained from the clearing of the catchment for the Avon Dam. This township provided four and five-roomed cottages for the married men and had metalled roads with concrete kerbing. A typical working man's accommodation consisted of a simple timber hut with a galvanised roof. Later, a barracks was constructed for single men. A Public School and recreation halls were also provided, as were full-time medical staff and facilities.
Shortly after construction of the dam was transferred from the Public Works Department to the Water Board, it was decided upon the recommendation of the Chief Engineer of the Board, Gerald Haskins, that the height of the dam wall be increased by . This added nearly 50% to the storage capacity of the dam.
Work on the dam continued until March 1930, when the depression compelled the Board first to curtail and then suspend work completely. The work recommenced on 12 June 1933 and continued without interruption until completion of the dam in 1935. Although the dam was ready to store water by September, 1935, there was little rainfall on its catchment during the ensuing three years; however, by October 1938 the dam was overflowing. At this time it was realised that scouring at the toe of the foundations of the dam required an alteration to the direction of the flow of floodwaters discharged over the spillway weir. This was done by constructing a concrete channel approximately long, wide, widening to at the discharge weir. Construction of the channel commenced in July 1943, and was completed in November 1947.
In the late 1960s, as part of a plan to interconnect the various water supply systems for Sydney Wollongong and Sutherland, a tunnel was constructed between the Nepean Dam and the Avon Dam (which now supplies the Wollongong area). This allows water to be transferred in either direction, as required.
In the 1980s, work was undertaken on Nepean Dam to alleviate uplift pressure on, and leakage through, the foundations of the dam wall, in accordance with modern design criteria. The downstream face of the wall was supported with an embankment of quarried sandstone blocks and sandstone fill, the latter being compacted by grid and vibratory rollers. It was similar to the work undertaken at the same time for Avon Dam.
Description
The Nepean Dam is a large cyclopean masonry dam. This consists of large blocks of sandstone quarried at the site, embedded in and surrounded by sandstone concrete. To ensure water-tightness, all construction joints are grouted. The upstream face consists of blue metal concrete thick while the downstream face is of similar material thick. Both of the facing concrete sections are merged progressively into the central sandstone masonry concrete. All the latest improvements in dam design and construction of the day were embodied in the work, building upon the experience gained from the construction of the previous three Upper Nepean Scheme dams. Inspection galleries are provided, with pressure and drainage pipes from the foundations, and rubble drains lead into them. Provision is made for any leakage of water past the copper seals in the contraction joints to be conducted into the galleries and thence drained away. To facilitate the work of maintenance staff, an electric lift was provided in the wall. The water is discharged through two diameter outlet pipes controlled by Ring Falkner gate valves and FCD valves.
After construction was complete and the dam had filled, the spillway weir was altered by the construction on the western side of the stream of a concrete channel approximately long and wide, widening to at the discharge weir.
The Nepean Reservoir has a small storage capacity in relation to its large catchment area. While its capacity is much the same as that of Cataract and Cordeaux Reservoirs, its catchment basin is two and a half times as extensive as Cataract and three and a half times as extensive as Cordeaux. The Nepean Reservoir therefore fills more rapidly and, under normal conditions, is drawn upon more freely than the other reservoirs.
Specifications of Nepean Dam: Total capacity: 17,898 million gallons (81,400 million litres). Years of construction: 1926–1935. Cost: £2,062,000 ($4,124,000). Crest level above sea level: . Amount of concrete in the wall: 300,218 cu yards (229,600 cu metres). Length of crest including pylons: . Length of spillway weir: . Width at crest: . Width at base: . Full supply level above sea level: . Greatest depth of water: . Area of lake: . Area of catchment: 123 sq miles (319km2). Average annual rainfall: . Average annual yield: 23,515 million gallons (106,993 million litres). Average annual evaporation: .
The Nepean Dam continues to supply Sydney with water. It is discharged as required into the river bed below the dam and flows to Pheasant's Nest where it is diverted by a weir to the Nepean Tunnel, long, discharging into the Cataract Tunnel, long, emerging into the Upper Canal, by which it is conveyed to Prospect Reservoir. The Nepean Dam is now also cross-connected to the Avon Dam, which supplies the Wollongong area, to provide additional water supply to that district. If necessary, it can also receive additional water from the Shoalhaven Scheme. Water is supplied to the Picton and Bargo districts from Nepean Dam, with this water treated and filtered in the Nepean Water Filtration Plant, a treatment facility added to the dam to improve the water quality to this district, which is now supplied directly from Nepean Dam.
The Nepean Dam includes within its area three brick residential staff cottages, two apparently dating from the 1960s and one from the 1980s, and two older weatherboard cottages, one of which, utilised by the Filtration Plant staff, is the original Resident Engineer's Cottage. Just north of the entrance to the Dam picnic area are the former Catchment workshops, a series of small corrugated iron and weatherboard utilitarian buildings which were formerly the central maintenance facility for the Upper Nepean Catchment area. They remain in use at a low scale by the Sydney Catchment Authority and include a weatherboard residential cottage similar to the second of the weatherboard residences at the Dam. The area of the former construction village for the Dam remains undeveloped and heavily overgrown, with remnant road borders and evidence of buildings visible on the ground.
The public area has been maintained by the Water Board and its successors, with picnic grounds, landscaped gardens and a children's playground. Remnants of the interwar period landscape at the Nepean Dam site are mainly concentrated in two areas - around a small park, with cement faux rockwork bedding edges, to the east of the dam wall, including a long line of Roman Cypress (Cupressus sempervirens) along the spillway alignment; and in a park-like picnic area along a terrace above the dam. Features of the latter area include pleasant plantings of mixed deciduous and evergreen exotic trees and Australian trees enclosing a long raised shelf, which is the remnant railway platform from the temporary construction railway, with a series of small timber picnic shelters with a somewhat log cabin-style character. Nearby a considerable line of dry-packed sandstone walling has been used and drystone walling is used generally to control access. The main entry road includes the remains of an old Cypress avenue and an ornamental gum avenue.
Heritage listing Nepean Dam was the fourth of the four water supply dams built as part of the development of the Upper Nepean Water Supply Scheme, one of the most important engineering works and items of public infrastructure in Australia, and had the highest wall of all the NSW water supply dams at the time of its construction. It was designed by the NSW Public Works Department under the direction of one of Australia's leading water supply engineers, E.M. de Burgh.
The completion of the Nepean Dam was a significant step in the continuing process of providing a reliable water supply for Sydney and surrounding areas as part of the Upper Nepean Scheme. Even by the international standards of the time, Nepean was a large dam and was a significant work of engineering in its day. It continues to play an important role as a major source of water supply for the Sydney region. Additionally, the Nepean Dam is a handsome, well proportioned structure with strong Stripped Classical style architectural character which complements the monumental nature of the structure and its attractive natural surroundings.