Physical and numerical modeling of the spillways and plunge pools of Koman Dam in Albania

The 115 m high Koman Dam is located in the northern part of Albania and is part of the Drin River hydroelectric cascade. The dam is equipped with two tunnel spillways with flip buckets at their downstream limits. Two large scour holes were created in the downstream alluvium. The hydraulic behavior of the two spillways and the formation of the scour holes were studied on a physical scale model and by numerical simulation with Flow-3D. The flow in the tunnels and over the ski jumps, as well as the jet trajectories and the impact zones could be determined. The evolution of scour holes and their extent were studied. The physical model tests as well as the numerical simulations showed that, for the spillways, the flip buckets can deflect the jets suitably only up to 50 % of the nominal capacity. Beyond that discharge, the flip buckets cannot guide the flow according to theory, and the jet length decreases, approaching the impact zone to the downstream dam toe. The results from the physical and numerical modeling for the current state could be validated against in situ observations. The physical model and the numerical simulation are able to reproduce accurately the plunge pool scour in an alluvial bed due to the plunging jet impact.