Conon Hydro Scheme

The Conon Hydro 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 (or million units per year).^[1] 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.

v t e River Conon system
Legend
Abhainn a' Chadha Bhuidhe River Bran Loch Fannich Loch Achanalt Fannich tunnel and dam Kyle of Lochalsh line Grudie Bridge Power Station Loch a' Chuilinn  A832  Bridge of Grudie Achanalt Power Station River Grudie River Bran Loch Vaich Kyle of Lochalsh line Loch Glascarnoch Loch Luichart Mossford Power Station Loch Meig Luichart dam and tunnels River Conon Glascarnoch River Allt a Ghlinne Little Scatwell bridge River Vaich River Meig Luichart Power Station River Orrin Loch Garve Orrin Reservoir Loch Achonachie Orrin Power Station Kyle of Lochalsh line Torr Achilty Power Station Black Water  A832  Orrin Bridge  A832  Moy Bridge Moy Island River Orrin Dunglass Island Dunglass power stn Far North Line  A862  Conon Bridge Garrie Island  A835  Conon Bridge Cromarty Firth

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.^[2]

The reservoirs, with elevations above ordnance datum (AOD) for the six power stations are as follows (listed roughly north to south, upstream to downstream):^[3]

• Loch Droma (270 m) and Loch Vaich (256 m) both feed via tunnels into Loch Glascarnoch (252 m) which is then diverted through a tunnel to Mossford Power Station on the northern bank of Loch Luichart.
• Loch Fannich (256 m) 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 (111 m) feed Achanalt Power Station, which discharges into the River Bran, a tributary of Loch Luichart.
• Water from Loch Meig (87 m) is diverted through a tunnel to Loch Luichart (85 m)^[4] 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 (256 m) is transferred by tunnel to the Orrin Power Station on the southern bank of Loch Achonachie.
• Finally, Loch Achonachie (30 m) is impounded by Torr Achilty Dam which includes the power station, discharging into the River Conon around 14 m above sea level and 12.5 km (7.8 mi) from the coast.^[5]

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.^[6] 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.^[7] 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.^[8] 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.^[9]

The project involved the construction of several aqueducts and tunnels, to divert additional flow into Loch Fannich. The surface of the loch was around 820 feet (250 m) AOD, while the surface of Loch Luichart is 289 feet (88 m) AOD.^[10] A sloping tunnel was constructed from a point above the site of the power station to a point around 90 feet (27 m) below Loch Fannich. From there it was driven beneath the loch until there was only 25 feet (7.6 m) of rock between the excavation and the water. Rock was carefully removed from the top of the tunnel to reduce the thickness to 15 feet (4.6 m), 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 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.^[11]

The main tunnel was around 3 miles (4.8 km) long.^[12] No dam was initially built at Loch Fannich, as the water level could be drawn down by up to 50 feet (15 m), but five years later a dam was constructed, raising the surface level to 840 feet (260 m) 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 8 feet (2.4 m) 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.^[13]

The power station building is clad in red Tarradale sandstone and was designed by the architect James Shearer of Dunfermline.^[14] 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,^[15] 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.^[16]

The second phase of the scheme involved 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 1,753 feet (534 m) long, with a maximum height of 141 feet (43 m). The Vaich dam was smaller, at 843 feet (257 m) long and 123 feet (37 m) high, and was built by the same contractor. The flow into both lochs was increased by building aqueducts and tunnels to capture water from other watercourses, and a tunnel fed water from Loch Vaich into Loch Glascarnoch. The surface level of Loch Vaich is 840 feet (260 m) AOD, while that of Loch Glascarnoch is 827 feet (252 m) AOD. A 4.5-mile (7.2 km) tunnel from just above the Glascarnoch dam feeds water to Mossford power station, on the bank of Loch Luichart.^[17]

The waters of the River Bran, which flows into Loch Luichart, were exploited by building a small dam across the river below Loch Achanalt and Loch a' Chuilinn, to supply the Achanalt power station close to the Grudie Bridge station. This arrangement provides a head of 66 feet (20 m) and the station has a capacity of 2.4MW.^[18] A mass gravity dam was built at the eastern end of Loch Luichart, with a tunnel to supply water to Luichart power station at the western end of Loch Achonachie, which can provide 34MW from a head of 184 feet (56 m). Additional water was obtained by building a concrete gravity and earthfill dam across the River Meig, to form Loch Meig. The dam was built by Duncan Logan (Contractors), and was 585 feet (178 m) long with a height of 86 feet (26 m). A tunnel transfers water from Loch Meig to Loch Luichart. The final part of this stage was to construct a dam across the River Conon at Torr Achilty, to create Loch Achonachie. The mass gravity dam is 808 feet (246 m) long and 76 feet (23 m) high. The water level only varies by 2 feet (0.6 m), as the main function of the dam is to regulate compensation water in the river below, but discharges are fed through Torr Achilty power station, which can generate up to 15MW.^[19]

The third stage of the project was to modify the River Orrin, which drops by some 700 feet (210 m) in the 6 miles (9.7 km) above its junction with the Conon. A mass gravity dam was constructed 8 miles (13 km) from the junction, which was 1,025 feet (312 m) long and 167 feet (51 m) high. The resulting reservoir is nearly 5 miles (8 km) long. Because the water level is around 135 feet (41 m) higher than it was when it was just a river, a second dam had to be built to prevent water flowing down a branch in the valley. This is an earth dam with a concrete core, of a similar length to the main dam. Over 3 miles (4.8 km) of tunnel and a steel pipeline deliver the water to Orrin power station on the southern shore of Loch Achonachie. This phase cost £2.8 million, of which a substantial amount was needed to ensure that fishing could continue in the valley.^[20]

Because of the large changes in the surface level of the reservoir, the only type of fish pass that was acceptable to the fishing interests was a Borland fish pass. This is similar to a navigation lock, with an upper and lower pool, connected by a sloping shaft. Water flowing out of the lower pool attracts migrating fish such as salmon, and a sluice gate then closes so that the shaft fills, enabling the fish to reach the upper pool and to proceed upstream. Juvenile smolts can enter the upper pool and slither down the shaft when leaving the spawning grounds. A Borland fish pass can accommodate changes in the upper surface level of 20 feet (6.1 m), but because the level of the Orrin Reservoir varies by much more than this, four fish passes were built. Similar fish passes were also installed at Torr Achilty, Luichart, Meig and Achanalt dams, and whereas fish had previously been unable to access the Conon above Conon Falls, the fish passes opened up some 20 miles (32 km) of river to them, as far upstream as Achnasheen. As part of the project, 200,000 salmon fry were released into the Upper River Bran.^[21]