Site Records

Deanie Power Station - Monar Dam

Monar dam provides the storage for Deanie power station, and is a fascinating structure. You might recognise it from its appearance a few years ago in a TV advert for DIY products. It's the only large arch dam in the UK.

As the name suggests, an arch dam resists the pressure of the water behind it due to the strength of its arched shape, rather than by the sheer mass of its concrete or rockfill structure as most dams do. Monar is a double-curvature thin-arch dam - it curves against both the valley floor and sides, and this allows a very thin wall to do the job - the arch of Monar dam is as little as 12ft thick in some areas!

The reason arch dams are very rare in the UK is that they can only be built in steep-sided rocky valleys or gorges, and only then where the rock is very good. Such sites are uncommon in the UK. In the Alps, where deep V-shaped valleys are common, arch dams are plentiful, and on a very large scale - some can be up to 700ft high, but only perhaps 200ft long.

The curve is also very evident from behind...

A couple of engineering drawings:

A cross-section...

The left abutment. Abutments are clearly very important to an arch dam: these were most carefully investigated. Note the tunnel entrance in the lower right of the image:

This adit was driven before construction started, to explore the rock in the left abutment and ensure that it would bear the forces involved.

The right abutment - and another exploratory adit.

Adjacent to the dam is the intake tower for the Deanie tunnel. The intake itself is deep underwater, and is controlled by a single roller gate. The four sloping concrete columns in front of the intake tower carry the rails for the screen rakes; there are screen to prevent debris from entering the tunnel, the screens have to be raked clean from time to time.

The same tower, during a period of exceptionally low water many years ago. Still well short of exposing the tunnel intake; shows how deep it is!

The rear of the dam during the same low-water period. The curve of the structure in the vertical as well as the horizontal plane is very evident here - you can *feel* the dam arching against the water pressure.

There's normally no flow in the river immediately downstream of Monar dam; compensation water is released instead into the Uisge Misgeach a couple of miles away. On occasion the valves at Monar are used to release freshets, however:

Words and images © 2003 Michael J. Ross