Groundwater problems in moberg and lava formations are frequently encountered in connection with development of hydroelectric power resources in the volcanic zones in Iceland. The paper gives account of experience by VST from design work and evaluation of performance related to groundwater and seepage at some of Landsvirkjun's projects in Southwest Iceland. The major problems encountered are: leakage under dams on lava, seepage from reservoirs on lava and moberg formations, seepage from canals and dewatering of excavations. The most relevant problems are associated with dams founded on lava and seepage from reservoirs on lava. The discussion is concentrated on three such dams, at Thorisos, Hrauneyjafoss and Sultartangi. Brief description is given of the methods used for estimating seepage and comparisons between forecasts and observations after construction. In general the reservoirs and structures have performed satisfactorily.

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