EXPERIMENTAL STUDY OF FLOW PATTERN IN MULTISLOPE STEPPED SPILLWAYS

Abstract

Stepped Spillways are those types of chute spillways whose face is provided with a series of steps, from near the crest to the toe. The main function of a spillway is to dispose of excess water from a reservoir safely and in addition to it the steps of a stepped spillway dissipate the falling energy of flow and reduces the size of dissipater generally provided at the toe of spillways. Stepped spillways have been used since more than 3000 years. These are used in different types of dams (masonry dams, concrete dams, earth dams etc). Stepped channel are used for handling flood releases in storm water channels in river training works, water treatment plants etc. Stepped fountains have been constructed in different cities (HongKong, Taipei, Tokyo etc) for the aesthetical applications. Check dams are usually constructed as a succession of drop structures to reduce the steep gradient in mountain areas. It has been used in irrigation and power channels also. The stepped channels are mainly of three types: flat steps, pooled steps and inclined steps channels. Flat stepped. are horizontal, pooled steps are those where a sill height is provided at the end of steps. The inclined steps are inclined upward and downwards. A stepped chute consists of an open channel with a series of steps or drops. The flow over stepped chute can be divided into three regimes: nappe flow regime,transition flow regime and skimming flow regime. In nappe flow regime the total fall is divided into a number of smaller free falls. The water proceeds in a series of plunges from one step to another. The energy dissipation occurs by jet break-up in air, jet mixing in the step and formation of fully or partially developed hydraulic jump on the step. In transition flow regimes the flow is neither nappe nor skimming flow regime. The flow is characterized by significant air entrainment and flow instabilities.In the skimming flow regime the water flows down the stepped face as a cushioned by the recirculating fluid trapped in between them. The .external edges on the steps .form a pseudo-bottom over which the flow passes. Beneath this recirculating vortices are developed...