Site Records

Deanie Power Station - Surge shaft

At the end of the 29,000ft low-pressure tunnel from Loch Monar, the water drops down a 260ft deep, 16ft diameter, vertical high-pressure shaft, from which a short length of high-pressure tunnel conveys the water to the turbines. Above the high-pressure shaft is the surge chamber, 170ft high. At Deanie this is of differential type; there is a main shaft, 50ft in diameter, and a separate riser shaft, 15ft in diameter. When a surge occurs, it is dissipated as water rises up the riser shaft and overspills into the main shaft. The conventional arrangement for a differential shaft is to have it concentric with the main shaft, as at Invergarry. At Deanie (and Culligran), an innovative design was used, with the riser shaft placed upstream of the main shaft.

There were several reasons for this:

1. Since the riser shaft is upstream from the control gate, it can relieve minor water hammer when this gate is closed.

2. It allowed the main shaft to be so designed that the top of the high-pressure shaft was contained within its diameter, considerably facilitating construction.

3. The riser shaft also functions as an intake shaft for the North Strathfarrar aqueduct system.

This drawing clearly shows the entire structure:

Approaching the surge shaft. Uniquely amongst Scottish hydro schemes, the surge shaft at Deanie is roofed over, not open to the sky. The outline of the shaft is just discernible as a difference in the colour of the surface. The reason is clear, from the steepness of the hillside and the extent of concrete buttressing required to the right of the shaft; if it were open to the sky, there would be a considerable risk of rockfall entering the shaft, with rocks conceivably entering the high-pressure system and damaging the guide vanes. The air movements which accompany surges are accommodated by the large grey mushroom ventilator.

The building houses the operating machinery for the control gate; the riser shaft is locate behind the gatehouse.

Behind the gatehouse, the top of the riser shaft. Again the outline of the shaft can be discerned. This is a noisy place - there's a significant roar from the water from the side-stream intake system falling down the shaft.

There's a grand view from the surge shaft. Loch Beannachran below, with the entrance to Deanie power station, and the adjacent substation, immediately below.

Looking up from below, the surge shaft is prominent on the hillside.

Words and images © 2003 Michael J. Ross