Taking stock of Norway’s dams15 May 2001
Thomas Konow, secretary of the Norwegian National Committee on Large Dams, traces the development of water power and dam construction in Norway
Historically, Norway’s topography and climate has made the country an ideal environment for the exploitation of hydro development at a reasonable cost. Hydro power initiated the country’s first larger industrial developments and soon became important for further development where power intensive industries played a vital role. Today, average annual hydro power capacity is estimated at 118TWh, which represents about 99% of the Norwegian electricity supply.
In order to secure a reliable power supply the construction of dams and reservoirs gradually became essential. Until the early 1920s the dams being constructed were mostly masonry and gravity types. However, as the highest hydro power potential was located in remote areas with poorly developed communication links, the disadvantages of gravity dams became apparent: they required large volumes of construction material that needed to be transported to the site.
New solutions were called for and reinforced concrete began to play a more important role. However, until the middle of the 1920s, engineers lacked confidence about designing reinforced concrete dams.
The Norwegian engineer Ambjørnsen introduced the slab buttress dam in the US. This type of buttress dam was named after him and is often referred to as the Ambursen dam.
Norway’s first buttress dam was built in 1914. It was an intake to Osfallet hydro power station but the dam breached after only two years in service. However, this represented a new technology of thin reinforced concrete structures and by 1916 the 12m high Fjergen dam was completed, which was in service until 1992 when it was replaced by a rockfill dam.
Throughout the 1920s and 1930s the slab buttress dam became more and more accepted. The engineer Chr.F.Grøner and his company were pioneers in construction of these structures. Grøner (now Statkraft Grøner) has designed about 50 of Norway’s buttress dams, among them northern Europe’s highest, the 31m high Langevann on the Tysso river.
Concrete dams, particularly buttress dams, are labour intensive and require a large amount of formwork and reinforcement. Since 1970 only three buttress dams have been built in Norway.
The first arch dam built with reinforced concrete was the 29m high dam Storlivatn in 1931. Construction of arch dams was limited until the beginning of World War II but as this dam type became more recognised as a cheap and safe design, it rapidly increased in numbers. From 1950 to 1970, 30 arch dams were built; the highest being the 91m high Zakariasvatn dam built in 1969. Since then only six arch dams have been built, among them the 144m high Virdnejavri dam which is part of the Alta hydro power scheme.
Most of the large Norwegian dams were built in the period from 1955-85. During 1959-75 about ten large dams were completed each year. Concrete dams were a dominating force until the middle of the 1950s but thereafter the embankment dam gradually increased its share of the market. From 1970 to 1995, 85% of all large dams constructed in Norway were embankment dams; almost all of them rockfill dams. The Oddatjørn dam, at a height of 140m, is the highest.
Out of a total of 179 embankment dams 166 are rockfill dams, with 124 having a central moraine core. Some 25 of the embankment dams are concrete-faced, nine have a concrete core and ten an asphaltic concrete core. Only 13 of the embankment dams are earthfill dams.
The Norwegian Geotechnical Institute (NGI) has been the leading engineering company designing embankment dams in Norway since the middle of the 1960s. NGI has designed 65% of Norwegian embankment dams and the majority of the remaining dams were designed either by Ing. Chr.F.Grøner or by Ing. A.B.Berdal (now Norconsult).
Embankment dams became popular mainly for economic reasons. Improvements in earth moving equipment and blasting techniques made the embankment dam cheaper and more attractive. The dam sites were often better suited for embankment dams and moraine deposits were generally available throughout the country, thanks to the ice age 10, 000 years ago. For large reservoirs, embankment dams were the preferred option since filling of the reservoir could start during the construction.
The zoned rockfill dam, with a moraine core, dominated the market until 1980. After that about 40% of rockfill dams were built with a bituminous core, and ten of these have a central asphaltic concrete core. The asphaltic concrete core has given excellent results and has increasingly been selected for designs. The solution has several advantages. The placement and compaction is much less susceptible to adverse weather conditions – compared to an earth core – which in turn enables the contractor to extend the working season to keep construction on schedule. Furthermore the asphaltic concrete core is virtually:
• Resistant to erosion and ageing.
• Workable and compactable.
• And offers jointless core construction.
When properly designed, the ductile properties provide a self-healing ability if cracks should develop. It is therefore also suitable for seismically active regions.
In Norway, the asphaltic concrete core has often shown to be economically competitive even when moraine material is available locally. Today, the 125m high Storglomvatn dam is world’s highest dam with a asphaltic concrete core, while the 90m high Storvatn dam is the world’s largest asphaltic concrete core dam, with a total design volume of 9.5M m3.
Performance of the Storvatn dam
The Storvatn dam forms part of the Ulla-Førre scheme, with a net production capacity of 4500GWh in an average year. The dam was completed in 1987 with a maximum height of 90m and a crest length of 1472m. The reservoir is situated at a mountain plateau with a maximum storage capacity of 3100M m3 at a reservoir level of 1055m asl.
After 15 years of service the dam has an excellent performance record. During this period, the maximum settlement of the top core has been 0.29m which represents 0.375% of the dam height at this point.
The total registered seepage has been below 10 litre/sec with a slowly decreasing tendency since initial filling of the reservoir. The leakage through the core is even smaller, when seepage through the foundation and abutments are withdrawn.
During planning and construction all Norwegian dams are now subjected to approval by the government, represented by the Norwegian Water Resources and Energy Directorate – NVE. However, until 1981 dam regulations were not enforced and prior to this the leading engineering companies often set design requirements and construction practice.
During the construction period in Norway, operation and maintenance has not been seen as a priority. However, there is increasing recognition that resources must be allocated for maintenance and to improve dam safety during operation. This is reflected in the new Norwegian dam regulations which came into force in 2001, where requirements on the operational phase are given more attention and are covered in more detail. Some of the key elements in these regulations are:
• Qualification requirements for personnel involved with the operation and evaluation of dams.
• Requirements for internal control systems, including a programme for inspection.
• Emergency action planning and dam break analysis with inundation mappings.
Among the elements of the internal quality control system are the requirements for a reassessment of dams at least every 18 years. This is almost a redesign of the dam, where every element is evaluated and assessed following the latest design requirements. The reassessment includes an estimate of the original load conditions and an evaluation of the strength and stability of the dam and appurtenant structures.
Furthermore, the quality and any deterioration of the dam material is monitored and compared with test results from construction, while flood calculations are assessed to include possible long term changes in an unpredictable and changing climate. An inspection of the upstream face of the dam is also required.The reassessment often concludes that rehabilitation is required, particularly for dams built before 1981 when the first dam regulations came into force. Rehabilitation is often needed to improve stability or to increase flood capacity. Rockfill dams often require repair or upgrading of the upstream slope protection and strengthening of the downstream toe to withstand large leakage. Raising the crest height may be required in order to withstand new and increased flood estimates and to fulfil the regulatory requirements.
TablesData on Norwegian dams