The flood discharge tunnel of the Ogee type free-flow spillway is widely used in hydropower projects. For a spillway tunnel with high head and large discharge, the conventional aerators can not fully prevent the cavitation damage to the tunnel. Recently, a tunnel of this kind in China was seriously damaged. The destroyed section was about 400m in length and about 6000m~3 of the reinforced concrete lining of the tunnel and basement rock had been eroded away.The designed water head and maximum discharge of the tunnel were 180m and 3800m~3/s respectively. Five conventional aerators were installed along the bottom of the tunnel. Model experiments have been conducted to investigate the reasons of the damage. It is concluded that the main reasons are that the flow speed is too high and there exist non-aerated flow regions on the sidewalls of the tunnel.In order to prevent the cavitation damage, a composite lateral aerator was proposed to be installed on the sidewalls downstream the arc bucket. After the reconstruction of the tunnel, a comprehensive examination of the tunnel function was performed. The results show that the composite lateral aerator work very well and the cavitation damage has been fully eliminated.
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