Author(s): Mogens Jensen
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Abstract: Vittjarv Power Plant in the lower part of Lule River in the north of Sweden has a head of 6 m. It was built during the years 1971-1975. There are three bulb units utilizing a maximum total flow of 690 m3/s. Two units were taken in operation at the end of 1974. In early 1975, serious problems arose at the plant because of a mess of ice floes, frazil slush and also timber logs blocking the flow through the trash racks. To a great extent water had to be discharged through the spillways, of which the aprons are equipped with jet deflectors to improve on energy dissipation. Additional frazil was produced in the spillways, enhanced by the free jets, and in open water downstream from the plant. The downstream water level rose 2.5 m due to ice – 40 % of the plant's head was lost. The phenomenon of ice production in the jet flow was examined and reported to the IAHR Symposium on Ice Problems at Lulea, 1978, by L. Billfalk and R. M. Desmond. In the paper, some measures are described - mainly excavations - carried out during the years 1975 - 1978 in order to eliminate the problems. Serious ice problems were not expected at that plant, so when they occurred decisions had to be taken about preventive measures without sufficient field studies. The design could therefore not be supported by theoretical calculations or records from the ice regime of the river, but had to be based mainly on general knowledge and judgements. The following winters some features such as air and water temperatures, development of ice covers, and head losses were recorded. During the intermediate summers, excavation works were carried out. Until this year it seemed as if the chosen proportions of the works had given acceptable results.
Year: 1981