Conon Hydro Scheme

The Conon Hydro Scheme, or Conon Valley Scheme, is a series of hydroelectric power stations located on the River Conon and tributaries in Scottish Highlands, between Inverness and Ullapool. The scheme was developed and built by the North of Scotland Hydro-Electric Board in three phases between 1946 and 1961. It is now operated by SSE Renewables.
The scheme contains six power stations at Achanalt, Grudie Bridge, Mossford, Luichart, Orrin, and Torr Achilty. They have a total capacity of 107.2 MW and an annual output of around 470 GWh. Water is impounded in eight reservoirs by nine large dams. There is also a complex series of nine tunnels and many aqueducts to divert the water to the power stations, some coming from other river basin catchments.
The first phase harnessed the catchment of Loch Fannich, diverting it through a tunnel to Grudie Bridge Power Station, which was constructed at the confluence of the River Guidie and River Bran, just upstream of Loch Luichart. Grudie Bridge started generating in 1950. Originally built without a dam on Loch Fannich, one was added five years later increasing the head and storage of the scheme.
The second phase comprised four power stations. Mossford, which takes water via tunnels from Loch Glascarnoch and Loch Vaich, both of which were created by damming tributaries of the Black Water. Achnalt, on the River Bran downstream of Loch Achnalt and Loch a' Chuillin. Luichart, downstream of Loch Luichart which is supplemented by water from Loch Meig. Torr Achilty, the lowest power station in the scheme located in a dam across the River Conon forming Loch Achonachie.
The third and final phase was to construct the Orrin Reservoir with two dams, plus a tunnel through the hillside to the power station on the shore of Loch Achonachie.

Scheme description

The Conon Hydro Scheme six power stations and six major dams, plus several other lesser dams and weirs. These operate in a cascade, with almost the entire Conon catchment passing through the lowest scheme at Torr Achilty.
The reservoirs, with elevations above ordnance datum for the six power stations are as follows :

Loch Droma and Loch Vaich both feed via tunnels into Loch Glascarnoch which is then diverted through a tunnel to Mossford Power Station on the northern bank of Loch Luichart.
Loch Fannich feeds Grudie Bridge Power Station, which discharges into the River Grudie just upstream of its confluence with the River Bran.
Loch Achnalt/Loch a' Chuillin feed Achanalt Power Station, which discharges into the River Bran, a tributary of Loch Luichart.
Water from Loch Meig is diverted through a tunnel to Loch Luichart and, together with water from the three stations above, this flows through Luichart Power Station, discharging back into the River Conon upstream of Loch Achonachie.
Water from the Orrin Reservoir is transferred by tunnel to the Orrin Power Station on the southern bank of Loch Achonachie.
Finally, Loch Achonachie is impounded by Torr Achilty Dam which includes the power station, discharging into the River Conon around 14 m above sea level and from the coast.

Station name	Capacity	Gross head	Average annual output	Year completed
Achanalt	3	20	7	1956
Grudie Bridge	18.6	168	87	1950
Mossford	18.6	161	121	1957
Luichart	34	56	135	1954
Orrin	18	222	80	1959
Torr Achilty	15	16	42	1954

Scheme development and construction

The Conon Valley was one of the early hydroelectric schemes implemented by the North of Scotland Hydro-Electric Board, although it was constructed in three phases between 1946 and 1961. After the board was created in 1943, Edward MacColl the chief executive produced a list of 102 projects which he thought could be built, from small ones to huge ones involving several neighbouring glens. The initial scheme for the Conon Valley involved using water from Loch Fannich, which would flow through a tunnel to Grudie Bridge power station on the banks of the River Bran just before it entered the western end of Loch Luichart. When the board promoted their second scheme, that at Tummel-Garry, members in the House of Commons attempted to get the findings of the tribunal which had considered objections to it overturned, but this was defeated. In the House of Lords, Lord Kinnaird wanted to introduce a similar motion, but was persuaded not to. However, he introduced a debate to consider the future of the board, at which various members of the house suggested that hydroelectric power was not needed in Scotland, as it would soon be replaced by nuclear power. Lord Kinnaird did not succeed, and during the debate Lord Westwood, the leader of the house, announced that all objections to the Fannich scheme had been withdrawn following discussions between the board and the objectors. It thus became the board's third project when it was authorised in late 1945.

First phase: Gruidie Bridge power station and Loch Fannich

The project involved the construction of several aqueducts and tunnels, to divert additional flow into Loch Fannich. To the west of Loch Fannich, the Allt a' Choin Idhir and Allt Dearg are diverted via a low-pressure concrete pipeline approximately long that discharges into the reservoir to the north of the dam. The burns and river on northern slopes of Fionn Bheinn, including Allt a' Chlaiginn are also diverted via a similar pipeline long that discharges into the Allt na Mòine a short distance from the southern shore. This water would otherwise have flowed to the River Ewe on the west coast.
The main tunnel is around long. This sloping tunnel was constructed from a point above the site of the power station to a point around below Loch Fannich. From there it was driven beneath the loch until there was only of rock between the excavation and the water. Rock was carefully removed from the top of the tunnel to reduce the thickness to, and a large sump was excavated, into which the final plug of rock would settle when it was blasted away. Two temporary concrete bulkheads and one steel bulkhead were constructed in the tunnel, to protect the tunnel and to ensure that debris from the rock plug would not be washed downstream. The final blast, which became known as "operation bathplug", was successful, and was the second time that such a technique had been used in Britain. Balfour Beatty carried out the work, and had also been responsible for the first use of this technique, at Loch Treig for the Lochaber hydroelectric scheme.
The surface of the loch was around AOD, while the surface of Loch Luichart is AOD. No dam was initially built at Loch Fannich, as the water level could be drawn down by up to, but five years later a dam was constructed, raising the surface level to AOD. At the lower end of the tunnel, water is conveyed to Grudie Bridge power station by a steel pipeline which is above ground. It is in diameter, but as it progresses downhill, the internal diameter reduces, and the walls get thicker. Close to the power station, it splits into two feeds, each of which supplies a vertical-shaft Francis turbine, originally rated at 12 MW. This was one of the last projects to use a surface pipeline, as the board were keen to reduce the visual impact of their schemes subsequently.
The power station building is clad in red Tarradale sandstone and was designed by the architect James Shearer of Dunfermline. Shearer had previously worked on other schemes, including the first to be commissioned at Nostie Bridge and had approached MacColl with the suggestion that the power station should be built of stone, to blend in with the surrounding architecture. MacColl readily agreed, but when it came to designing the structure at Grudie Bridge, he felt that construction in stone would be too slow and cost too much, and so prepared sketches for a concrete, steel and glass building. MacColl asked if Shearer had seen concrete buildings after they had been exposed to the Scottish weather for 20 years, and when Shearer admitted that he had not, MacColl organised a weekend visiting buildings to see how concrete weathered. As a result, the use of local stone to face large concrete buildings became a policy within the Board, and Grudie Bridge power station was redesigned as a consequence.

Second phase

The second phase of the scheme involved building four power stations and six reservoirs, with preliminary work commencing in 1951.

Mossford power station, Lochs Vaich and Gascarnoch

To provide water for Mossford Power Station required damming the River Vaich to form Loch Vaich and the Glascarnoch River to form Loch Glascarnoch, above the point at which the two rivers became the Black Water. To reduce the amount of cement used, the Glascarnoch dam consisted of a concrete gravity dam with earth fill. It was constructed by Reed and Mallik and was long, with a maximum height of. The Vaich dam was smaller, at long and high, and was built by the same contractor.
The Vaich dam is of unusual construction for Scotland, with a rubble construction on both sides of a vertical concrete core wall. The upstream face is coursed random rubble and the downstream face is covered with turf. As this facing would erode if the dam overtopped, two spillway towers were constructed in the reservoir. These connect to a pipe through the base of the dam, safely discharging water downstream when the reservoir level is high. The rubble for the dam came from spoil from nearby tunnelling work. This innovative construction, designed by Williamson and Partners, minimised the need for concrete and significantly reduced the time and effort to build the dam.
A tunnel feeds water from Loch Vaich into Loch Glascarnoch. The surface level of Loch Vaich is AOD, while that of Loch Glascarnoch is AOD. Initially, there was no power generated at the outflow, but a 320 kW turbine was added later. A second tunnel, long from just above the Glascarnoch dam feeds water to Mossford power station, on the bank of Loch Luichart.
The catchments of these reservoirs are again augmented by several small dams and aqueducts. To the east of Loch Vaich, four tributaries of the Abhainn Srath Rainich which would flow into the Black Water downstream are diverted via a pipeline and tunnel to Loch Vaich. To the north, the Abhainn a' Ghlinne Bhig is diverted south through a tunnel to the Allt Toll nam Muc, and the western slopes of Carn Crom-loch are captured by an aqueduct feeding the Allt nam Fiadh. South of the Glascarnoch dam, another aqueduct captures water from the Allt Giubhais Beag, which would have otherwise joined the Glascarnoch River below the dam.
A low dam was constructed across the western end of Loch Droma, and a penstock diverts water from the eastern end into Loch Glascarnoch. A compensation flow is provided to the Abhainn Droma
, the original outflow of the loch, which drains westward into the River Broom and Loch Broom. Several intakes and pipelines divert tributaries of the Abhainn Droma
into Loch Droma to provide additional water for the hydro scheme.