Monar Dam And Deanie Tunnel Intake Tower, Glen Affric Hydro Electric Scheme, Aird and Loch Ness, Highland

Description

Probably James Williamson and Partners (engineers to North of Scotland Hydro Electric Board technical panel), W M Halcrow and Partners (principal contractors), 1963. Prominent horseshoe shaped double arch dam set in steep gorge with walkway oversailing spillway gates on concrete piers (battered to upstream face). Concrete construction with painted steel flap gates to spillway and stepped concrete abutments flanking downstream face. Slightly flared downstream face with slightly advanced angled lip beneath gates. Needle valves at base of downstream face with round arched housing above. Arched upstream profile with deep lip beneath spillway gates. Small single bay gantry and instrument house to far left (W) with large teak doors.

INTAKE TOWER: single bay flat roofed gatehouse tower with large sloping concrete buttress composed of linked individual piers to rear; all set on square pedestal housing tunnel intake normally below water level. Plain surrounds to large teak vehicular door and bipartite metal windows.

Statement of Interest

Monar dam is a highly unusual example of a double arch dam set within a spectacular setting in a steep gorge in the upper section of the Strathfarrar valley. It is a key component of the Glen Affric hydro electric scheme, one of the major post-war hydro electric developments by the North of Scotland Hydro electric Board (NoSHEB). The scheme played a key role in the realisation of the social agenda of NoSHEB by providing power to remote north highland communities and stimulating economic regeneration. The dam is the only example of its kind in Britain. Arch dams require specific conditions to be successful, with a requirement for very solid abutment to either side (in the form of a gorge in this case) to withstand the force of the water behind it. The technology was first used in 1911 for the construction of the Roosevelt Dam in Arizona and for the Hoover dam in Colorado in 1931. The architectural design is largely functional with the elegant curves of the dam walls expressing their function and contrasting with the fractured rock of the gorge walls. The inlet tower continues this theme with a striking modernist functional design which echoes the dynamism of the dam walls with the sweeping buttress of concrete piers which are angled down into the water from the rear of the structure.

The dam is composed of two continuous concrete arches which dissipate the thrust of the water against the sides of the valley, making the abutment into solid rock on the sides of the gorge a key factor in the choice of the design for this location. This construction method represented a 9% saving in materials over a conventional gravity dam, with the dam wall a mere 12 feet thick in some places. The upstream face of the dam is arched in both horizontal and vertical planes asymmetrically to the shallower doming of the front of the dam, the tension this creates is the key factor in counterbalancing the thrust of the water on the upstream face. The stepped concrete abutment to the walls of the gorge on the downstream face is to help prevent excessive erosion which would threaten the structural integrity of the dam.

Monar dam formed a key part of the Strathfarrar section of the Affric Cannich hydro electric scheme. The dam plays a key role in providing storage capacity and in regulating the flow of water down the scheme. From this point the scheme acts as a cascade with water flowing through four power stations sequentially until it is discharged into the Beauly Firth below Kilmorack station. The proposal for the construction of the dam was controversial and the expansion in Loch Monar as a consequence of its construction drowned 60 houses and a large area of fertile arable land.

The Affric / Cannich hydro electric scheme was one of a large number of schemes developed in Scotland by the North of Scotland Hydro Electric Board (NoSHEB), formed after 1943 as a nationalised body to oversee the development of Scotland's resources for water power. The scheme played a key role in the realisation of the social agenda of NoSHEB by providing power to a remote community. Power generated on schemes in the southern Highlands, such as Tummel (see separate listings) was exported via the grid to the central belt, the profit from which subsidised the provision of power to remote north Highland communities and stimulated economic regeneration. Under the leadership of eminent chairman Sir Tom Johnston the board undertook developments throughout Highland Scotland and his aspirations saw the development of schemes in locations such as Loch Dubh near Ullapool and Storr Lochs on Skye. Johnstone's social aspirations and wider wishes to reinvigorate the economy of the Highlands ensured that schemes in remote areas formed a key part of the NoSHEB development plan.

All of the developments carried out by NoSHEB were subject to parliamentary approval and objections on the grounds of scenic amenity were common. In order to meet these objections the board appointed a panel of architectural advisers which included Reginald Fairlie (1883-1952), James Shearer (1881-1962) and Harold Ogle Tarbolton (1869-1947), appointed in 1943. Initially the role of the panel was to adjudicate on competition entries for designs, but by 1947 it had become one of designers. The panel had little control over the functional form of the buildings, as they left this to engineers, but they did influence the appearance and the style of the designs. The rigid views on the roles of engineers and architects during the design process resulted in the development of a style which can be characterised as vernacular modernism. This style is characteristic of many NoSHEB buildings and is a direct product of the strict role which engineers and architects played in the design process and of the increasing desire to harmonise buildings with the landscape.

The design of Monar Dam is typical of Williamson's approach, with an innovative solution designed specifically to suit the requirements of a particular site. In the case of Monar Dam the double arch plan is unique for Britain and allowed for significant savings in costs and materials over a more conventional design. Williamson was a prominent engineer who specialised in the design of dams following his work on the Galloway Hydro Electric scheme (see separate listings) in the 1930s. He acted as one of the chief engineering advisors to NoSHEB and was the lead engineer for a number of schemes until his death in 1953 after which the role passed to Williamson and Partners Ltd.

(Listed 2011 as part of Hydroelectric Power Thematic Survey)

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