17.4.7. Cascade Spillway In case of very high dams the kinetic energy at the toe of the dam will be very high and the tail-water depth in the river may not be adequate for a single-fall hydraulic jump or roller bucket stilling basin. Narrow and curved canyons consisting of fractured rock would not be suitable for trajectory buckets. In such situations, especially for high earth and rockfill dams for which spillway is a major structure, possibility of providing a cascade of falls with a stilling basin at each fall (Fig. 17.19) must be considered. The cascade spillway (2, 8) is likely to be an ideal choice for a high rockfill dam for which the material has been obtained from a quarry located downstream of the dam so that the flood waters may be discharged over the quarry face. As the quarry would usually be excavated in benches, they may as well form the steps in the cascade. A cascade spillway has been planned at the proposed 218 m high Tehri dam on the Bhagirathi river in the Ganga valley of the central Himalayas. At Darmouth dam in Australia, the spillway to pass 2700 m3/s of flood discharge is an unlined cascade in granite. The benches of the cascade will be 5 m high and of varying widths to suit the topography of the site. Although, a cascade spillway would be an attraction during floods, it may not be always acceptable for environmental reasons which demand that the quarries be always located upstream of the dam and below the normal water surface level of the reservoir so as to cause minimum disfigurement of the land.
