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Laggan-Treig Tunnel [Outfall]

Loch Trieg Dam, SE of Spean Bridge, Inverness-shire.

Tunnel, Hydro-electric scheme.

A tunnel driven through bedrock takes water from an intake at the SE end of Laggan Dam as far as a concreted tunnel mouth set into the bed of the Loch Treig reservoir.

"As with the main pressure tunnel downstream of Loch Treig, the tunnel between Laggan Reservoir was driven through rock and lined with concrete. Again, it was constructed from multiple faces with three intermediate adits, which were then used to collect water from streams. The tunnel is approximately 14 feet (4.3 m) in diameter with a flattened invert, and has a fall of 15 feet (4.6 m) over the 2.75 miles (4.4 km) length."

Alternative Names: Laggan Tunnel

Notes: Part of the Lochaber Hydro Electric Scheme.

"Laggan Dam was constructed at a constriction in the valley approximately 4.5 miles (7 km) downstream of Loch Laggan creating Laggan Reservoir. This is linked to Loch Laggan by a short section of the River Spean, which was dredged to make use of the storage capacity in the loch. While the downstream dam site required a larger dam than an alternative location upstream it had several advantages: i) A shorter tunnel to Loch Treig, with a smaller diameter for an equivalent capacity, ii) Increased storage capacity in Laggan Reservoir, including dead storage below the tunnel invert level for the accumulation of sediment, and iii) Removing the need for an aqueduct to divert the Roughburn." [Wiki]

Outfall structure well-visible on OS website satellite imagery taken at time of low water level (used for given 10 fig. NGR).

Given length / VR is approx & based on straight line between intake & outfall.

Hydrographic Feeds: Laggan Reservoir

Hydrographic Resurgences: Loch Treig