REVIEW OF HYDROLOGY, HYDRAULIC DESIGN OF STILLING BASIN AND ENERGY DISSIPATION ARRANGEMENT OF ERACH DAM

Abstract

A case study of Erach Multipurpose Project (EMPP) has been taken to review the hydraulic design of stilling basin, energy dissipation arrangement and hydrological studies (Flood Frequency Analysis). As during construction of this project, the Tail Water Level (TWL) changed so in this study an attempt has been made to optimize the Energy Dissipaters without changing the design parameters. Conventional methods are used for Flood frequency analysis of the Project. HEC-RAS Model is being used to analyse the hydraulics of the structure. Based on the results of hydraulic model, the calculations are made in different conditions to find out basin parameters and its optimization. The design of stilling basin should consider three interrelated parameters that are jump position, jump type and TWL. The hydraulic jump can be controlled by adding structure in the stilling basin or by stilling basin modification. The hydraulic jump is used as an energy dissipation device for many spillways, outlet works, and canal structures. Performance of hydraulic jump in a basin is related to tail rating curve of the river. In situations where the tail depth is higher than the jump, no clear jump is expected and entering jet oscillates from bottom to surface resulting in pulsating action. In such cases the wavy action last in long reach downstream of spillway. Computation of basin parameters require setting of basins at very high almost infeasible level but after isolating the basin width by providing divide walls in basin the discharge intensity in the separated compartment is increased and good jump can take place even at comparatively lower seated basin floor. Thus, a good and stable flow regime of the hydraulic jump may be achieved in the divided stilling basin for a large range of frequently occurring low flow rates. The height of divide walls is decided by water depth in the basin for lower range of discharge up to 30% of the peak flood. The present thesis describes an arrangement in stilling basin with divide walls which can specially be applied for optimization of energy moderators, where tail water depth is quite high than the required jump depth