Tummel hydro-electric power scheme

The Tummel hydro-electric power scheme is an interconnected network of dams, power stations, aqueducts and electric power transmission in the Grampian Mountains of Scotland. Roughly bounded by Dalwhinnie in the north, Rannoch Moor in the west and Pitlochry in the east it comprises a water catchment area of around 1,800 square kilometres (690 square miles) and primary water storage at Loch Ericht, Loch Errochty, Loch Rannoch and Loch Tummel, in Perth and Kinross. Water, depending on where it originates and the path it takes, may pass through as many as five of the schemes nine power stations as it progresses from north-west to south-east. The scheme was constructed in the 1940s and 50s incorporating some earlier sites. It is managed by SSE plc.

The idea of Loch Ericht as a source for hydro-electric power was first anticipated in 1899, when the Highland Water Power Bill was put before Parliament. The plan was to generate electricity for industrial purposes, but the bill did not receive Parliamentary approval. The next attempt was the Loch Ericht Water and Electricity Power Act, which received approval in 1912, but it included a clause that prohibited alteration of the water level of the loch, making the scheme uneconomic. Dundee Corporation sought to use Loch Ericht, Loch Rannoch and Loch Tummel in a scheme proposed in 1919, but there was strong opposition to it, and the plans did not come to fruition.

The potential for hydro-electric power in the Highlands of Scotland was recognised by the Snell Committee, who published reports in 1919 and 1920. Against this background, the Grampian Electricity Supply Bill was laid before Parliament. The promotors were aware of the "fundamental principles" set out by the Snell Committee, and ensured that these formed part of the bill. Consequently, the scheme would treat a single catchment area comprehensively, and would ensure that some of the power generated would be made available to residents who lived within the catchment of the scheme. The promotors included the Duke of Atholl and the chairman of Lloyds Bank, John William Beaumont Pease, both men who were known to be honest and trustworthy, and who were held in high regard locally. The bill became an Act of Parliament in 1922, and allowed the promotors to use the waters of Loch Ericht, Loch Rannoch and Loch Tummel. Loch Ericht would be augmented by water diverted from Loch Seilich and Loch Garry, increasing the catchment area to 418 square miles (1,080 square kilometres). The power generated would supply an area of over 5,000 square miles (13,000 square kilometres), covering the counties of Perth, Kinross and Forfar, together with parts of Inverness-shire, Argyllshire and Stirlingshire. Some of the power would be sold in bulk to the Scottish Central Electric Power Company and the Fife Electric Power Co.

The newly formed Grampian Electricity Supply Company failed to raise the £1.75 million of authorised capital, and asked George Balfour for help. Balfour, with Andrew Beatty, had formed the engineering company Balfour Beatty in 1907, and in October 1922 this had been restructured so that Power Securities Corporation Ltd bought the share capital of Balfour Beatty and a number of utility companies for which they had provided engineering or management expertise. Power Securities was created specifically to provide funding for capital intensive schemes, and despite the deep depression of late 1922, were able to help. They bought the Grampian company, which was passed to the Scottish Power Company Limited, the Scottish arm of the new organisation, and Balfour Beatty became its engineers and managers. They realised that the scheme could only be economic if some of the power generated was exported to the industrial heartland of central Scotland, as their own area was too sparsely populated. Indeed, the newly-formed Central Electricity Board rejected the idea of extending the national grid into the Grampian area, because of its low demand for power. However, in June 1927 they had agreed to buy bulk electricity from the Grampian company via a link to the national grid at Abernethy and Balfour was ready to proceed with the Grampian scheme.

The 1922 Act of Parliament allowed the Grampian company to raise the maximum level of Loch Ericht by 53 feet (16 m) and to vary the level by 77 feet (23 m) when the scheme was operating. There were much stricter limits on Loch Rannoch, where the level could only vary by 8 feet (2.4 m). Some ingenuity was required to make the scheme work efficiently under all conditions. The solution adopted was to use a power station on the River Tummel below Loch Rannoch to provide most of the power under flood conditions, and to store water in Loch Ericht. When there was less water available, most of the power would come from a power station on the banks of Loch Rannoch, fed with water from Loch Ericht, and Loch Rannoch would be used to store water. Accordingly, Rannoch power station had a capacity of 48 MW and Tummel power station had a capacity of 34 MW.

The scheme was split into two phases, with work on the first beginning in 1928. A concrete gravity dam, 1,400 ft (430 m) long was built at the southern end of Loch Ericht, from where a tunnel cut through solid rock carried the water to a point above the north bank of Loch Rannoch. The tunnel was nearly 3 miles (4.8 km) long, and a steel pipeline then ran down the hillside to Rannoch power station. Several smaller projects supplemented the flow into Loch Ericht by diverting the headwaters of the River Truim, the Allt Ghlas, and other streams. This phase was completed in November 1930. A dam was also constructed at the northern end of Loch Ericht close to the village of Dalwhinnie to contain the raised water level, and prevent it from discharging into the River Truim and flooding Dalwhinnie. The dam has a concrete core with earth embankments on the upstream and downstream faces. It is around 1,150 ft (350 m) long and has a maximum height of 15 ft (4.5 m) above ground level.

The growing need for electricity in the central belt of Scotland meant that the scheme was able to supply 12 MW to the Central Electricity Board, and when the sale of bulk electricity was doubled to 24 MW in late 1930, the Grampian Board had the resources to extend their network to some of the more remote parts of their authorised area, and to press on with the second phase of the scheme. This was the Tummel development, on which work started in Spring 1931. A control weir was built at the outlet to Loch Rannoch, and the river channel below that was made deeper. A dam was built at Dunalastair, 4 miles (6.4 km) below the control weir, creating a new loch called Dunalastair Water. Three miles (4.8 km) of open aqueduct were constructed, running from the dam along the south side of the valley, at the end of which the water descended 170 ft (52 m) through two steel pipes to reach the Tummel power station located on the banks of the Tummel at Tummel Bridge. Rannoch and Tummel power stations were notable for being the first storage high-head stations, where water is held in a reservoir above the power station, rather than simply relying on water passing as part of the run of the river as had previously been done in Scotland.

Water from the surrounding hills collects in Loch Ericht. At its southern end, the dam was built on the River Ericht some distance downstream of the loch's outlet, which raised the water level by 14 feet (4.3 m). In order to drain the loch down to 5 feet (1.5 m) below its original level, a channel was cut from the dam for 1.5 miles (2.4 km) at this lower level. This resulted in approximately 78,050 acre feet (96,277,000 m³⁾ of water storage. Water then passes through tunnels and increasingly narrow pipes, reducing from 12 ft 4 in to 7 ft 10.5 in (3.7 m to 2.4 m), as it drops the 485 feet (148 m) to Rannoch power station on the northern bank of Loch Rannoch. At the time Rannoch was fitted with two 22,000 hp (16,500 kW) water turbines with provision for a third. Steel towers, some 97 feet (30 m) high, carried 132 kV power lines east along the bank of the Loch, continuing for 58 miles (93 km) to connect to the National Grid at Abernethy, to the south-east of Perth. A separate 33 kV transmission line ran for 74 miles (119 km) to Arbroath.