Hydraulic Design of Plunge Pool Downstream of Ski-Jump Bucket of Orifice Spillways

The Himalayan region is blessed with perennial rivers and heads required for hydropower generation. This blessing comes with challenges like flash floods, cloudbursts, high sediment load, etc. Low-level spillways, in the form of orifice spillways, are an oblivious choice in this region considering their capacity to flush the sediments during floods. Special considerations for the design of energy dissipators are required for orifice spillways. As an energy dissipator, ski-jump buckets are used in most spillways where tailwater depths are shallow for hydraulic jumps to form downstream of spillways. Most commonly, scour holes occur downstream from the point of impingement of the ski-jump jet. These retrograding scour holes may become a threat to the spillway structure itself and hence need to be assessed carefully. To prevent an uncontrolled erosion of riverbanks and the river bed, a preformed plunge pool is provided. Taking into consideration the geological features at the site and the water jet impact location, the size, location, bed level and the slopes on both flanks of the plunge pool are decided. The physical model studies have traditionally been used to predict equilibrium scour depth and its location and extent. Despite the qualitative nature of these studies, past experiences suggest that the ultimate scour is realized in the prototype after a long period of operation when the rock fragments and behaves as non-cohesive material. Dimensions of the plunge pool including location, width, length, depth and upstream and downstream slopes can be worked out using the model observations. Two such case studies are discussed in this paper where the effect of hydraulically optimized plunge pool in substantial dissipation of kinetic energy of incoming flow jet and reduction in water level fluctuations in the powerhouse tail pool located just downstream of the spillway was observed.