| Laggan Dam | |
|---|---|
 Laggan Dam | |
| Official name | Laggan Dam |
| Location | Scotland |
| Coordinates | 56°53′23″N4°40′24″W / 56.88972°N 4.67333°W |
| Opening date | 1934 |
| Dam and spillways | |
| Impounds | River Spean |
| Height | 48 m (157 ft) |
| Length | 700 ft (210 m) |
Laggan Dam is a mass concrete gravity dam located on the River Spean south west of Loch Laggan in the Scottish Highlands. It forms the second reservoir for the Lochaber hydroelectric scheme.
The structure was built as part of the Lochaber hydroelectric scheme by Balfour Beatty for the British Aluminium Company and construction was finished in 1934. The supervising engineers were the firm of C S Meik and William Halcrow, now known as the Halcrow Group. [1]
The dam was designated a Category B listed building in 1985. It was upgraded to Category A listing in 2011, following a review as part of Hydroelectric Power Thematic Survey 2010. [1] [2]
The dam is about 700 feet (210 metres) long, [3] and 48 m (157 ft) high between the level of the foundations and the crest of the spillway. [4] It is curved upstream like an arch dam with a radius of curvature of 2,000 ft (610 m), but works purely on the principle of a gravity dam. [5]
The dam impounds Laggan Reservoir and Loch Laggan, which are connected via a short section of the River Spean. This was dredged and straightened over a length of approximately 1.3 miles (2 km) to the confluence of the Amhainn Ghuilbinn. [6] The reservoir has a capacity of 40,000,000 cubic metres (1.4 billion cubic feet), [4] between a top water level of 820 feet (250 m) OD and maximum drawdown level of 804 feet (245 m) OD, giving an operational range of 16 feet (4.9 m). [6]
The whole crest of the dam, except for a section in the middle that houses equipment, is a spillway broken into 29 bays by piers that support a roadway across the dam. [4] As well as the spillway, there are six self-priming siphons embedded into the centre of the dam, controlled automatically by system of air valves. [4] The siphons are set to operate in pairs, priming at 820.5, 821.0, and 821.25 feet OD (250.1, 250.2, 250.3 m OD), and discharge through 4′2″ (1.3 m) diameter Glenfield-Kennedy jet disperser pipes. Laggan Dam was the first large siphon spillway used in the UK, . [6]
The foundations are built on granite, and the dam was built in seven sections, with copper strip and hot poured asphalt water stops in the joints. [4]
Water from the dam is conveyed to Loch Treig through three miles (5 kilometres) of tunnel. From there, the waters travel through a further 15 miles (24 kilometres) of tunnel, 15 feet (4.6 metres) in diameter, before descending the hillside to a power house at Fort William through five steel pipes.
The dam can be found next to the A86 road from Fort William, however there is no public access over it. The catchment area of the dam was increased by an aqueduct which diverts flow from the River Mashie into the River Pattack. This can be seen at the side of the A86 road in Strath Mashie.
Between 1941 and 1943, the catchment was further expanded by constructing another dam across the headwaters of the River Spey, and diverting flow through a tunnel to Loch Laggan. This was constructed by the 1st Tunnelling Company of the Canadian Army. [7]
Sir William Halcrow was one of the most notable English civil engineers of the 20th century, particularly renowned for his expertise in the design of tunnels and for projects during the Second World War.
Loch Laggan is a freshwater loch situated approximately 6+1⁄2 mi (10.5 km) west of Dalwhinnie in the Scottish Highlands. The loch has an irregular shape, runs nearly northeast to southwest and is approximately 7 mi (11 km) in length. It has an average depth of 68 ft (21 m) and is 174 ft (53 m) at its deepest. The eastern end of the loch features the largest freshwater beach in Britain. Since 1934 Loch Laggan has been part of the Lochaber hydro-electric scheme. At the northeast end of the loch is the hamlet of Kinloch Laggan.
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The River Spean flows from Loch Laggan in a westerly direction to join the River Lochy at Gairlochy in the Great Glen in the West Highlands of Scotland. Major tributaries of the Spean include the left-bank Abhainn Ghuilbinn and River Treig, the right-bank River Roy and the left-bank river known as The Cour. The river is accompanied by the A86 road for almost its entire length, running from (upper) Loch Laggan west to Spean Bridge. The river is spanned by a bridge carrying the A82 road near its junction with the A86 at Spean Bridge. A minor road bridges the Spean just above the falls at Inverlair. Two further road crossings exist - a private estate road across the short stretch of river between upper Loch Laggan and the Laggan reservoir and a road traversing the top of Laggan Dam. The West Highland Line crosses the river near Tulloch Station and follows its north bank before re-crossing one mile east of Spean Bridge. A branch of the railway formerly continued west beside the river from Spean Bridge, crossing it once again to the west of the village.
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