Dam good celebrations

AUSTRIA – Finstertal dam

The Sellrain-Silz hydro power scheme is situated in the Austrian province of Tyrol, 40km west of the capital Innsbruck. It was constructed in 1977-1981 for Tiroler Wasserkraft AG (TIWAG), who designed the scheme, produced the plans and supervised the works.

The main structure of the complex is the 150m high Finstertal embankment dam, located at an altitude of 2325m asl, which creates a seasonal water reservoir with an active storage capacity of 60Mm³. The rockfill dam was built from 1977-80. Difficult climatic conditions, at an altitude of over 2300m, meant that construction work was only possible on around 100 days in the year.

Of the dam types investigated for the site, including concrete wall designs, a rockfill dam with an asphaltic concrete core membrane and steep slopes (1:1.5 on the water side and 1:1.3 on the air side) proved to be the most cost-effective solution. Such a design also blends in well with the mountain scenery and with the nearby winter tourism centre of Kühtai.

More than two-thirds of the foundation contact area of the dam was only thin covered bedrock, while the remaining area (especially the left upstream abutment) was covered with an overburden of moraine material up to 20m thickness. On the basis of large-scale laboratory tests, conducted to improve the key between the rockfill and the smooth bedrock polished by the abrasion of the glaciers, notches were blasted into the rock with an average depth of 70cm over an area of 20,000m³.

Thanks to its favourable situation on the projecting rock, the core membrane only needs to be 96m high. The thickness of the core membrane that inclines to the air side is 50-70cm. The design moreover ensures that the core membrane receives additional support from the shoulders, so that there is little risk of undesirable deformation occurring.

The total dam volume amounts to 4.6Mm³. Seven fill zones of the dam mainly consist of blasted material (granodiorite gneiss); only 20% of the dam volume is moraine material from the overburden of the quarry area. In order to optimise the compaction of the material deposited, the height of the layers was limited to 75cm, the material was kept moist and vibrating roller compactors were used.

The dam surveillance system comprises a total of 793 measuring points for monitoring external and internal deformations, pore water pressure, earth pressure, temperature, response acceleration and seepage water losses. An unusual feature for an embankment dam is the construction of an accessible shaft, which permits monitoring of the core and surrounding areas. It was constructed of precast concrete rings, embedded independently of each other in the fill in two staggered sections to take account of the inclination of the core membrane. This shaft accommodates two precision plumb-lines, permitting continuous telemetering of crest and centre zone deflections and the orderly routing and arrangement of all lines and cables to the monitoring chamber. Besides typical instruments (fluid level settlement devices, extensometers etc.), a new type of magnetic deformation gauge for measuring the thickness of the asphaltic core membrane was also installed in the shaft.

The Finstertal dam has operated without a hitch over 28 years – an obvious confirmation of the soundness of the principles on which it has been designed. Surface deformations, with maximum settlements of 38cm and maximum horizontal deflections of 19cm, are of a magnitude that would hardly have been considered possible with earlier rockfill dams of such a height. Total seepage water losses through the 37,000m³ core membrane surface were only 9l/sec on first filling, and declined to 1.5l/sec by the end of 2008.

Austrian National Committee on Large Dams

WALES – Brianne dam

Owned by Dwr Cymru/Welsh Water and completed in 1972, the 91m high Brianne dam is the highest dam in the UK. It has a rugged appearance that blends well with the rocky valleys in this part of Wales and the true beauty of the dam lies in the simplicity of its design – a symmetrical central rolled clay core supported by shoulders of compacted rockfill.

The Towy Scheme was selected to provide a major water supply for Swansea and the rapidly expanding industrial area of South Wales. Direct supply reservoirs were then out of favour and a large river regulating reservoir was required in the headwaters of the River Towy to support the abstraction for the scheme at the tidal limit of the river downstream. The site of Brianne dam was a rock gorge but the left abutment was intersected by a major fault plane and a rockfill embankment dam was selected as the most appropriate structure. The dam has a number of unique features.

The rock in the area is highly cleaved slatey mudstone of Silurian Age. Such a rock type had not previously been used in high rockfill embankments. The core was to be formed of boulder clay which would be placed in wet conditions and would be highly deformable within the stiff rockfill shoulders of the embankment.

In l968, when the dam was being designed, embankment dams were being analysed by the semi-empirical method of limit state design. Essentially this assumed failure surfaces through the embankment, assigned shear strength to the materials, assumed equal strain and sought to establish factors of safety against failure. With widely differing materials, usually having non-linear stress/strain relationships, it was not possible to predict the embankment deformation when fully loaded by the reservoir water. It was found empirically that, with factors of safety usually in excess of 1.4, the deformation would be acceptable.

The designer, Bill Carlyle of Binnie & Partners, felt that it was unsatisfactory that such a large and potentially dangerous structure should be constructed when its deformation under fully applied load could not be predicted. Thus Brianne dam was equipped with a unique array of instruments to measure stress and strain within the embankment during construction and subsequent reservoir filling. The data from these instruments has been published and widely used in the development of finite element analysis methods for embankment dams and is now routinely used in design.

The contract for construction provided for the design engineers to retain full control of the embankment fill operation, specifying the compaction control and fill moisture content and operating the on-site laboratory to do so.

The dam has proved to be entirely satisfactory in operation for its first 37 years of life. The design and construction of Brianne reservoir demonstrated that a major dam can be engineered without complex solutions to provide a durable and reliable structure that will benefit the people of Wales for many decades by providing water, power and a popular amenity.

British Dam Association

MOROCCO – Al Wahda dam

The Al Wahda dam is in many respects the pride of the Kingdom of Morocco in the mobilisation of water resources. This 88m high, 2600m long dam has a storage volume of 3800Mm³.

Since swinging into full operation in 1997 the dam has fulfilled its functions of:

• Protecting the rich and vast plain of the Gharb against floods which occur on average every two years.

• Producing 400GWh of hydro power, especially during peak hours.

• Supplying drinking water for the surrounding cities.

• Contributing towards national solidarity by transferring water to the deficit southern basins.

The Al Wahda dam symbolises the efforts of the late King Hassan II to mobilise water and aid agricultural development within the country. It represents the culmination of the country’s dam policy undertaken since the mid 1960s. It must also be remembered that the dam was completed successfully during a period characterised by an unfavourable attitude to the financing and construction of international dams.

Morroccan Committee on Large Dams

ITALY – Menta dam

The Menta dam is located on the river Menta in the Calabria region of Italy. The 90m high, zoned earthfill dam has a bituminous concrete multilayer membrane on the upstream face.

Constructed in 1998 its main purpose is to supply drinking water to the town of Reggio Calabria. It also has a hydro power capacity of 22.5MW. The hydro production is connected to the high available head between the reservoir, the treatment plant and the supply zones.

Menta dam was built within the slopes of the Aspromonte of the Cabride Complex inside the Aspromonte National Park, in order to assuage the chronic shortage of water of the Calabrian peninsula.

The works for the dam were completed in spite of many technical and economic difficulties. The ancillary works, namely all the diversion and distribution works, were delayed due to political and economic difficulties. Now, at last, all those are nearing the completion, and in a few months it will be able to supply Reggio Calabria with a first phase of 250 litre/sec of drinking water, and shortly after, all the scheme will be fully operational.

Comitato Italiano Grandi Dighe

SOUTH AFRICA – Wolwedans dam

Wolwedans dam is a 70m high RCC arch/gravity dam situated on the Great Brak river near Mossel Bay in the Southern Cape, South Africa. The dam, owned by the Department of Water Affairs and Forestry (DWAF), was designed and constructed in-house by DWAF and completed in 1989. The dam provides water for industrial use.

This was the world’s first arch-gravity RCC dam. Other unique features include groutable joints and a stepped spillway with a relative short apron (the latter was confirmed with model studies at the time). The dam was instrumented quite extensively to monitor temperatures and deformation pattern across all the joints.

With a downstream slope of 0.5H:1V the dam has to rely on arch-action for stability. Shrinkage of the RCC and the associated cracks would therefore pose serious problems especially with respect to arch-action if they are not grouted. A groutable system of induced crack joints was developed by engineers of DWAF to overcome the problem.

South African National Committee on Large Dams

SWITZERLAND – Grande Dixence

Switzerland participated in the foundation of the international-commission-on-large-dams in 1928 in Paris. It was represented by the Swiss Commission on Dams, a board of five famous Swiss engineers; pioneers in the design and construction of large concrete dams.

Contractors, dam owners and operators joined the dam experts of the Commission in order to found the Swiss National Committee on Large Dams in 1948. The Committee was constituted as a professional association, which is still its actual status. In 2000, the Committee changed its name to Swiss Committee on Dams, expressing its interest to not only focus on large dams, but on all aspects related to all size of dams and their reservoirs.

There are 205 large dams in operation in Switzerland according to ICOLD criteria. Forty-eight of them exceed 60m height and 25 are more that 100m high.

Grande Dixence dam was completed in 1961. It is the highest dam in Europe and the highest concrete gravity dam in the world. This dam, with a crest elevation of 2365m asl in the Alps, is the key structure of a huge hydro power scheme including more than 100km of water transfer tunnels, 75 intakes on mountain streams, four other dams for water transfer, four underground powerhouses with a total installed capacity of 2100MW and five pumping stations.

Mauvoisin, Luzzone and Contra arch dams are more than 200m high. Mauvoisin and Luzzone arch dam were both heightened in the 1990s in order to increase their reservoir volume.

The Swiss Committee on Dams is strongly represented in ICOLD Technical Committees with highly skilled experts in all domains of dam engineering and management. It has also established several working groups with the aim to summarise and to describe the state of the art of the Swiss experience on the behaviour, surveillance and maintenance of dams.

Presently, dam construction activity is limited in Switzerland as in most Alpine countries. However, several projects of rehabilitation, upgrading and heightening of existing dams are currently being studied and will be implemented in the future. The evolution of the European Interconnected Electricity Market and the fast growth of wind energy may also lead to a new age of hydropower construction in Switzerland with a focus on pumped storage for the coming decades.

Swiss Committee on Dams