Mitigating cavitation on high head orifice spillways

Abstract

ABSTRACT A large hydropower potential lies in the Himalayan region due to perennial rivers. North east India, especially Arunachal Pradesh, is blessed with Brahmaputra and its tributaries which makes it a rich hydropower potential state. Heavy sediment laden rivers are the main challenge to tap hydropower in this region. Orifice spillways are the obvious choice as they serve the dual function of flood disposal and flushing of sediment through the reservoir. The crest of the spillway is provided as near to the river bed as possible with high head over spillway crest. This arrangement, due to high head over crest, is susceptible to cavitation damage due to very high velocity flows. Cavitation damage and its prevention are of increasing concern in designing and operating high head spillways. Proper design of spillway leading to acceptable pressures for all operating conditions, improvement of spillway surface, use of cavitation resistant materials and aeration of the flow are few techniques to prevent the cavitation erosion on the spillway surface. Three high head spillway case studies where the head over spillways is more than 50 m and spillway surface is prone to cavitation damage due to high velocity of the order of 30 to 50 m/s are discussed. This paper describes the choice of cavitation prevention method depending upon hydraulic and functional constraints of particular case.