Roxburgh Dam machine hall
Location of Roxburgh Dam in New Zealand
|Location||Central Otago, New Zealand|
|Dam and spillways|
|Type of dam||Concrete gravity dam|
|Impounds||Clutha River / Mata-Au|
|Height||76 m (249 ft)|
|Length||358 m (1,175 ft)|
|Width (crest)||10.7 m (35 ft)|
|Width (base)||61 m (200 ft)|
|Surface area||6 km2 (2.3 sq mi)|
|Commission date||1956 - 1962|
|Installed capacity||320 MW (430,000 hp)|
|Annual generation||1,650 GWh (5,900 TJ)|
The Roxburgh Dam is the earliest of the large hydroelectric projects in the southern South Island of New Zealand. It lies across the Clutha River / Mata-Au, some 160 kilometres (99 mi) from Dunedin, some 9 kilometres (5.6 mi) to the north of the town of Roxburgh. The settlement of Lake Roxburgh Village is close to the western edge of the dam.
- 1 History
- 2 Design
- 3 Lake Roxburgh
- 4 Gallery
- 5 See also
- 6 References
- 7 Bibliography
In 1944 the State Hydro Department estimated that even with the power stations currently under construction they would only be able to meet projected South Island load up until 1950 or 1951 and that a new large power station was required. Detailed investigations by the Public Works Department identified two alternatives, Black Jack's Point on the Waitaki River (where eventually Benmore Power Station would be built) and Roxburgh Gorge on the Clutha River. The Roxburgh location had the advantage of being less remote, requiring less geological investigation, half the materials for the same power output and a better climate in which to undertake construction work, which were important considerations at a time of serious shortages of labour and cement. Historical records showed that the long term flow of the river was 17,650 cusecs (499.8 m3/s) and that a controlled flow of 15,000 cusecs (424.7 m3/s) would be possible through the power station. To avoid exceeding the height of the 1878 flood at Alexandra, the maximum retention level of the lake which would be impounded behind the dam was fixed at 430 ft (131.1 m) above sea level. The designers estimated that with a overall efficiency of 85% the mean output would be 160 MW and assuming a annual power factor of 50% the station could deliver a maximum output of 320 MW.
The Clutha River is fed from lakes Hawea, Wakatipu and Wanaka. There were already existing control gates on the Kawarau River at the outlet of Lake Wakatipu and it was decided to control the flows from the remaining lakes. After investigation found that the soil conditions were unsuitable at Wanaka a control structure was only built at Lake Hawera. This was commissioned in 1958 and consists of four radial gates housed in an earth dam. The dam raised the existing lake level and currently provides approximately 290 GWh of storage.
In 1947 the Labour Government approved plans to build a hydro power station on the Clutha River. The Clutha River between Alexandra and Roxburgh runs through a deep gorge which offered a number of locations for a power station. Detailed investigations found that Tamblyn's orchard where the river exited the Roxburgh Gorge near the town of Roxburgh, offered the most power output, better tailwater conditions, the best access and would be closest to suitable locations for both construction and permanent villages. A concrete design was selected as due to the geology of the site this was a better option than a earth dam at the wider Pleasant Valley location. The Ministry of Works also had limited experience with earth dam construction and its only engineers with the necessary experience where engaged on the Cobb Power Station. Many of the design decisions were based upon results from studies undertaken from 1949 to 1954 on a 1:80 scale model of the dam at the DSIR's Hydraulics Laboratory at Gracefield, Lower Hutt.
In March 1949 that the government committed to building at Tamblyn's orchard. The Ministry of Works began work in July 1949 on the excavation of a diversion channel at the site. This would eventually be 2000 ft (185.8 m) long, 100 ft (9.3 m) wide and 70 ft (6.5 m) deep which required the removal of 255,000 cubic yards (194,961 m3) of material. By the end of 1950 720 workers were being employed on site.
The Ministry of Works which would be the government department that designed and constructed the power station had identified that it had a shortage of the engineering and drafting staff to undertake the large amount of power station construction that the government had committed to in the North and South islands. An unsolicited bid was received from a British consortium consisting of civil engineering contractors Richard Costain, electrical manufacturers and contractor English Electric and Insulated Callender Cables to design and build the power station using imported workers. The Ministry of Works had reservations about the lack of a guaranteed completion date, difficulties with divided responsibility if the consortium undertook both the design and construction, the potential for the cost to be higher than if competitive tenders were called and that it could give the consortium a monopoly over future projects of a similar nature. The State Hydro-Electric Department didn't want to be restricted to one electrical equipment manufacturer and also saw the offer as a threat to their transmission line construction staff. Taking these concerns into account and wishing to avoid using up precious overseas funds the offer was formally rejected in September 1949 by the Minister of Finance in the Labour government. Meanwhile work continued on site on building a construction village and creating the diversion channel 200 ft long, 100 ft wide and 70 ft deep. However progress was slow, with completion of the diversion now instead not expected until 1953 instead of the planned 1951.
Call for tenders
In 1949 the newly elected national government, which ideologically favoured private enterprise, appointed "Stan Goosman" as both Minister of Works and Minister of the State Hydro-electric Department. By 1951 the projected project delays were serious enough to draw criticism from the Electrical Supply Authority. By now aware of the projected energy shortfalls and the shortage of government resources to complete six other hydroelectric projects that were underway as well as complete Roxburgh, Goosman's response was to announce on 25 September 1951 that tenders would be called from interested parties to undertake the civil aspects of the project. This required the rapid production of tender documents and specifications by a short staffed government design staff. Bidders had the choice of offering on either a bill of quantities basis or by nominating a 'target estimate' plus a 4% fee. In this type of contract the Government met all costs and the contractor received a fee of 4% of the total cost up to the target estimate. If the cost varied from the estimate, then 25% of the change was added to or subtracted from the fee. A 'no loss clause' meant that if the cost overruns where high enough the contractor could lose their entire fee but would not suffer any further loss, other than those for not meeting agree completion dates. Eight tenders were received. Three were fixed price with a bill of quantities and the remainder were target estimates. The Ministry of Works had estimated that the work would cost £10,198,000 and the average for seven of the bidders was £10,068,838. The lowest bid was £7,4412,419 from Holland, Hannen & Cubitts of England. The government engaged Sir Alexander Gibb & Partners of London to assess the ability of the bidders to undertake the work.
After negotiations with Hannen, Holland & Cubitts of England who were joined by S A Conrad Zschokke a revised bid was received and on that basis a contract with a target estimate of £8,289,148 and a 4% fee of £331,566 was awarded on 25 July 1952. The contract had provision for a bonus of £350,000 for early completion. There was a penalty for late division of the river and a £1000 penalty for each day past July 1955 that the power station was not ready for service. The target completion date was 1 June 1955.
By late August 1952 the Ministry of Works had completed the two cableways that were to be used to carry concrete to the workface. To manufacture concrete on site the Ministry of Works purchased a Johnson concrete batching plant that had been used by the United States Navy in the reconstruction of Pearl Harbour after the Japanese attack in 1941. This came into operation in early April 1953. Upon completion of Roxburgh the plant was transported first to Benmore power station and then later to Aviemore power station and the Pukaki dam to mix aggregate for the penstocks, spillways and other concrete structures.
The consortium bought from overseas 82 engineers, supervisors and administration staff and 322 workmen to the project and took over the civil aspects from the Ministry of Works on 29 Sept 1952. By this stage the Ministry of Works had completed the diversion channel and the consortium also took over these workers.
Prior to their involvement with the Roxburgh project Hannen, Holland & Cubitts experience had been limited to commercial and residential buildings. Zschokke who had expertise in the construction of hydraulic structures were limited to only providing engineering services while Cubitts personnel filled all the management roles.
By March 1953 the Ministry of Works became concerned at the progress being made by the consortium and that their management team lacked the experience to construct a hydro power station, which was highlighted by the large amount of rework being undertaken. Progress was not helped by the Government directing the employment of a large number of assisted immigrants many of whom had little construction experience and limited English. In early 1953 at the government's expense the consortium flew out 309 workers from Great Britain. By October 1953 it was clear that the consortium would not meet the contracted July 1955 date for generation of the first power. In an attempt to improve progress the contractor replaced a number of senior project staff. Labour relations were also deteriorating due to uncertainty over the management changes, reduction in working hours to 40 per week and the impact of cost overruns on the workers pay. In November 200 British workers demanded either a 70 hour working week or their tickets back to Great Britain.
With it having been necessary in 1953 to introduce power rationing in the South Island due to a shortage of generation the government decided the slow progress couldn't continue and requested two directors of Downer & Co, a major New Zealand construction company to attend in two days time a meeting at the Prime Minister's summer cottage on 24 April 1954. At this meeting which was attended by representatives of the consortium, Arnold Downer and Arch McLean from Downers were requested by the government to enter the project as the managing partner with a 25% interest. After spending £4 million the existing contract was cancelled and a schedule of rates contract was agreed upon with the renamed Cubitts Zschokke Downer with a planned completion date of late 1956.
As a result of the forming of this new consortium Arnold Downer was put in charge of all site activities.
The preliminary works for the diversion of the river got off to a bad start when the explosive charge used in mid-June to remove the upstream dumpling damaged the steel sheet pile cofferdam downstream of it. This cofferdam has been constructed to ensure that water didn't carrying any blast debris from the upper dumpling into the sluice channel. Eventually the debris and the cofferdam were removed, allowing unrestricted flow down the diversion channel. In a 12 hour operation on 30 June 1954 the river was successfully diverted using 12 bulldozers into the diversion channel despite the flow down the river having increased to a far higher level than studies had indicated were optimum. If this diversion hadn't been able to have been undertaken before the peak winter flows the project would have incurred a 9 to 12 month delay.
Once the river was diverted over the next three months cofferdams were constructed upstream and downstream of the dam, the water pumped out between them and excavation for placing of the foundations commenced.
In July 1954 Downer replaced 20 senior contractor staff that he had inherited with people of this choosing, many from Morrison-Knudson Co. A significant appointment was that of A. I. Smithies, a very experienced hydro construction engineer from Morrison-Knudson Co as construction superintendent. Under Downer's management the pace of construction increased with the weekly concrete pour rapidly improving. The improved management allowed the workfare to be reduced from the 1107 when Downers took over, to 850.
The dam was constructed in 50 ft wide concrete blocks with 5 ft wide slots between them constructed in two profiles, those associated with the penstocks had an additional section containing intakes and screens as well as a downstream slope to support the penstock while the other profile had a flatter slope and were only wide enough at the top to house the road across the top of the dam. In conjunction with the block sizes, different concrete mixes and cooling coils through which were passed cold water were used to maintain the block temperature at 10˚C (50˚F) and thus cracking of the concrete. Cracking can allow water into the body of the dam which can lead to allow uplift and instability during earthquakes. Once the blocks had reached its final stable temperature the slots were filled with concrete.
Once the concrete in a block was stable the coils were filled with grout. A 20 ft (1.86 m) deep low pressure consolidation grout curtain was installed on the upstream side of the dame and extending into both abutments to improve the strength of the rock under the dam and prevent leaks. Drainage holes were constructed just downstream of the grout curtain as well as under the power house with 40 pressure gauges installed to record the up pressure on the structure.
During construction a large gravel filled hole was discovered in the centre channel or "gullet" of the river bed. This gullet which was 50 ft (4.6 m) deep and varied in width from 50 to 100 ft (9.3 m) was dug out and filled with a mix of pozzolana (fly ash) and cement under the dam while under the powerhouse Prepakt concrete was used as this reduced demand on the batching plant which was fully occupied supplying concrete for the dam blocks. A total of 700,000 cubic yards (535,188 m3) of concrete were used in the construction of the dam and spillway consuming 600,000 cubic yards (458,733 m3). The cement was sourced from Millburn's Bankside plant at Dunedin while the aggregate was obtained from the Clutha River at Commissioner's Flat. Water came from the river.
The penstocks were manufactured by Stevenson & Cook of Dunedin. After rolling the plates in their factory in Dunedin, they were transported by truck to the site and fabricated into sections on site in a purpose built workshop. All welds were X-rayed during fabrication and radiographed after installation as well as pressure tested except for the concrete encased section at the intake.
In November the joints in the stator windings of the generators were discovered to be faulty. Fortunately sufficient time became available to re-make all the joints when from 24 November 1955 for 23 work days up until the Christmas break the members of the New Zealand Workers Union were on strike in support of a union crane driver who had refused to lower a load being carried by his crane when the siren went for a tea break which the contractors estimated would delay the commencement of lake filling by two months.
With power cuts being applied across the South lsland by June 1956 the Minister of Works requested the contractors to concentrate all resources on work that would bring forward lake filling as far as possible. To encourage to the workforce the government offered a bonus of ₤2 per week plus ₤1 per day that the lake was filled before 19 August. At midnight on the 21 July 1956 lake filling began and the lake level commenced rising at an average of 3 ft an hour.
As the lake began to fill increasing levels of water began to flow from the drainage channels behind the grout curtain in the right abutment, which indicated that the grout curtain was faulty. Investigations concluded that further grouting would have to be performed (which took about a fortnight) before the lake could be raised it its final level. The decision was made to allow the lake to fill to no further than the crest of the spillway while the contractors began drilling and inserting more grout.
By 11:20 am on 23 July 1956 the lake had filled to the crest of the spillway water. With a desperate shortage of electricity effecting the South Island, commissioning of the No 1 machine immediately commenced. Once the engineers were satisfied that the machine was fit for service it was connected at 6 pm to the national grid. Due to the reduced head the machine's output was limited to 30 MW. By the end of the next day No.2 machine had completed commissioning and was also connected to the system. This allowed the 220 kV line to Islington to be bought into service as two machines were needed to provide sufficient reactive power to charge the long length of line. The third machine was commissioned on 18 August 1956 and the fourth machine on 11 December 1956. The power station was officially opened on 3 November 1956 by Stan Goosman.
Delivery of machines 5 to 8 began late in 1959.
The commissioning of Roxburgh removed the need for power restrictions in the South Island and ensure a surplus of power for many years.
The State Hydro-electric Department undertook the design, purchase and installation and commissioning of the electrical equipment. Tenders for supply of the major electrical plant were issued in October 1949 with contracts awarded in May 1950 at a cost of £1,000,000 for the first four machines. Where possible equipment was shipped on the Dunedin-Roxburgh Railway Line to Roxburgh and from there transported by road to the power station. Because of the line larger tunnels on the Waikaka Branch railway line the turbine runners and generator bearing support were transported on this line. From the line's terminus at Waikaka, they were transported by road to the power station using a specialized transporter.
The State Hydro Department established itself on site in June 1953. Access to undertake their activities was first provided in August 1954 and erection of the first machine began with the first scroll case concreted in by March 1955.
To connect the new power station to the major load centres a 52 mile long new 110 kV wood pole line was first built to Gore. The linemen then commenced constructing a 89 mile long double circuit 110 kV overhead transmission line using lattice steel towers to the Halfway Bush substation at Dunedin which was completed in July 1955 at a cost of approximately £500,000.
The principal connection however was a new 266 mile (428 km) long 220 kV single circuit overhead transmission line built using lattice steel towers from Roxburgh to a new substation at Islington on the outskirts of Christchurch. By 1949 the surveys for this line were well under way with by 1951 the construction camps established and the material was on order. By 1954 the first section of the line had been completed which allowed it to carry power from Tekapo A to Christchurch. A second section as far south as the Waitaki Valley helped improve supply conditions during winter. The Roxburgh-Islington line cost approximately £1,000,000 and was completed by the winter of 1956.
To house the workforce the Ministry of Works first built in 1947 a single men's camp and cookhouse on the west bank of the river. In 1949 a single lane Bailey bridge was installed to connect it to a new temporary village being constructed on the east bank of the river. Eventually the village grew to 724 houses complete with a 90 bed hostel, a 600 child primary school, a cinema, a social hall, 17 shops, three churches, a fire brigade and ambulance building, four tennis courts, a swimming pool and a piped sewage scheme. In addition there were four single men's camps (two on the east and two on the west bank) containing a total of 1000 huts. These facilities cost a total of £2,241,925.
As the Otago Central Electric Power Board's network could not provide sufficient power to the village and the project, the government built a temporary power station containing two 1 MW and one 0.4 MW diesel generators, to supplement the supply.
In December 1947 the government expected the project to cost a total of £11,500,000. By September 1949 when the final location and type of dam had been chosen the cost had increased to £17,000,000.
A contract of £8,620,074 was awarded to Hannen, Holland & Cubitts in association with Conrad Zschokke. This was a target estimate contract with a 'no loss clause'. In May 1954 the contract was re-negotiated to include Downer & Co as the principal. The new contract was based upon 'a schedule of rates' at a value of £10,120,000.
The final total cost of the project was £24,102, 800 of which £19,151,700 was for civil engineering, £445,000 on the bulkhead caissons and stage 2 civil works, £4,506,100 on the purchase and installation and commissioning of the eight generators and outdoor switchyard. Included in the civil engineering cost was £900,000 for the early completion bonus and £35,900 on expediting the programme.
A total of 3,500 drawings were produced between the Ministry of Works, the State Hydro-Electric Department and contractors to build the power station.
In December 1965 a generator coil failed on unit 2, followed by a series of further failures between 1971 and 73, which in an effort to correct, the windings were reversed. Units 1, 3 and 4 had their stators rewound in 1975 to 76. The sluice gates 3 in 1996 and gate 2 in 2001 were modified to enable the power station to pass an increased maximum design flood of 200,000 cusec (5,663 m3/s). Gate 1 was also plugged with concrete. To improve the structures seismic withstand ability the original heavy chain and counterweight spillway gate operating system was replaced with a hydraulic system while the dam's top bridge was strengthened and the gantry towers lowered. In the 1990s the power station's control systems were automated with new control and protection systems which allowed it to be demanned. Control of the power station is now undertaken from a control centre at Clyde Power Station. On 1 April 1996 ownership of Roxburgh was transferred to Contact Energy, a State Owned Enterprise which subsequently passed into private ownership in 1999. With the separation of Transpower a new control room was constructed on the former carpark to house the Transpower equipment needed to operate the transmission equipment. The original air blast circuit breakers were replaced with Sprecher & Schuh SF6 circuit breakers in the late 1980s.
In 2012 the original 50 MVA 220/110 kV interconnecting transformer T10 was replaced with a new 150 MVA unit which removed a significant restriction on operating of the Southland 110 kV network. This also removed the station's previous restriction of the 110 kV generation to 90 MW and hence the total station output to 290 MW.
With the commissioning of Roxburgh the sediment which had previously flowed down the Clutha river became trapped behind the dam. Regular surveys commenced in 1961 to monitor this sediment. By 1979 the average river bed level downstream of the Alexandra bridge has increased by 3.6 metres since the lake was created in 1956. The completion of the Clyde Power Station in 1992 reduced the sediment inflows from the Clutha River, leaving the Manuherikia River as the principal source. Floods in 1979, 1987, 1994 and 1995 have led many residents of Alexandra to put pressure on the owners of Roxburgh power Station to better manage the sediment build-up. A major flood in 1999 caused flooding of large parts of the main business area of Alexandra. This led to Contact Energy and the government purchasing flood affected properties and flood easements over others as well as constructing a flood bank. Contact Energy has also introduced a program of drawing down the lake level during floods in an attempt to move flush sediment downstream.
The power station consists of an 1170 ft (108.7 m) long, 185 ft (17.2 m) high concrete gravity dam from which eight steel penstock supply water to a powerhouse containing the turbines. The penstocks change from an 18 ft (1.7 m) square intake section to 18 ft in diameter before tapering to 15 ft (1.4 m) where they enter the scroll case. Spillway gates are located on the West (right) side of the dam. The designers anticipated a 500 year flood of 120,000 cusecs (3398 m3/s). As a result the spillway was designed with a capacity of 150,000 cusecs (4247 m3/s).
At the base of the spillway are sluice gates designed to pass 80,000 cusecs (2265 m3/s). During construction these sluice gates were used to divert the river via a diversion channel. The upstream section of the diversion channel was unlined and followed an old natural channel of the river before reaching the spillway and sluice gate block which is curved at the exit to direct water away from the outdoor switchyard. The surfaces were finished to a high standard to ensure a smooth flow of the water during medium and high flows.
The superstructure of the powerhouse is constructed of welded steel framed clad in precast concrete panels.
Each spillway drives a Francis turbine supplied by Dominion Engineering of Canada. The turbines have a nominal speed of 136.4 rpm with a guaranteed maximum runaway speed of 252 rpm. The turbines have a rated output of 56,000 hp at a net head of 148 ft (13.7 m) , which consumed 3575 cusec (101 m3/s).of water. The runners weigh 28 tons and have a diameter of 12 ft 10 inches (1.2 m). The speed of the each turbine is controlled by a Woodward supplied governor located on the generator floor. Each turbine is directly connected to a dedicated to a 44 pole 11 kV synchronous generator supplied by British Thomson-Houston (BTH). Each generator has an output of 44.44 MVA at a power factor of 0.9 and a total weight of 362 tons with the rotor weighing185 tons. The generators are air cooled by fans on the top and bottom of the rotor circulating air, while water cooled radiators located each corner of the generator pit removed heat from the air. Monitoring and local control of each generator is provided from control cabinets located on the downstream side of the generator floor.
The output of the each generator is connected to three single phase transformers supplied by Ferranti which had two equal secondary windings which allowed them to be configured to provide either 110 kV or 220 kV. Generators 1 to 5 are connected to the 220 kV system and 6 to 8 to the 110 kV system. The transformers are located on a platform above the draft tube. Each transformer weighs 59 tons when fill of oil. From the transformers overhead conductors carried the power across the tailrace to an outdoor switchyard.
The machines were delivered with guaranteed efficiencies of 92.2% at three quarters load turbines, 97.36% at three quarters load and 97.67% at full load with a combined efficiency of 89.77% at three quarters load.
The 110 kV and 220 kV systems were connected by a 50 MVA 220/110 kV interconnecting transformer supplied by Brown Boveri. The outdoor 220 kV and 110 kV circuit breakers were also supplied by Brown Boveri and were of the air blast type.
Auxiliary power supply
To ensure a reliable auxiliary to the power station two auxiliary turbine-generator sets were installed below the unloading bay and supplied from a shared 3 ft (0.27 m) diameter 243 ft (22.6 m) long penstock which ran from the top of the dam. Each unit has a horizontal Francis 765 hp turbine supplied by Drees & Co of West Germany which drove via flywheel a 625 kVA 400 V generator supplied by General Electric. At full load each unit consumes 5.82 cusec (0.164 m3/s) of water.
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