Improving the Safety of an old Austrian Dam

Updating of the design flood for the 90-year old Gosau dam in the Austrian
Salzkammergut region to meet present-day safety standards made it necessary for the
owner and operator, Energie AG Oberoesterreich, to raise the dam’s discharge capacity
from approximately 40m³/s to 70m³/s by providing an overflow spillway section at the
dam crest. Two energy dissipation alternatives were considered, one with a stilling basin and one with a ski-jump. The complexity of the situation led the owner to resort to hydraulic scale-model tests for the design studies. The results proved both alternatives to be technically feasible. In the end, the ski-jump variant was chosen and the design submitted to the authorities. In addition, a bottom outlet connecting to the upstream end of the existing spillway tunnel was provided to enable water-level drawdown in the case of emergency. This was studied in a further model test. The results have shown that both the bottom outlet and the old spillway can be expected to work perfectly well. Following approval by the Water Right Authority, construction was commenced in the autumn of 2003 and completed in the summer of 2004.