Kárahnjúkar Dam spillway, Iceland: Swiss contribution to reduce dynamic plunge pool pressures generated by a high-velocity jet

Spillways often include a flib bucket as terminal energy dissipator, combined with a plunge pool. For large jet fall heights, the residual energy may provoke scour at the river bed. Additional measures are then required, such as chute widening, increase of the plunge pool depth, or insertion of baffles at the chute end. The effect of these measures was investigated in hydraulic modelling. The dynamic plunge pool bottom pressures in the jet footprint area were systematically recorded. Both the time-averaged and the fluctuating dynamic pressure heads are considered as references for the jet scour potential, beside the related pressure coefficientes. The investigated measures were proven to be effective in terms of reduced pressures, especially in combination. This research relates to the Kárahnjúkar spillway model investigation, in which the principal working conditions as canyon topography, jet fall head and discharge spectrum were determined. The relevant parameters of the herein presented measures were systematically varied in a wider range, independent of the project.