Physical and Numerical Model Studies for Hydraulic Design of Stilling Basin as an Energy Dissipator of a Spillway - A Case Study

The design of the stilling basin for a dam spillway is site specific and hydraulic model studies are required to be carried out for evolving the best possible design. A properly designed hydraulic jump stilling basin can ensure 60-70% dissipation of energy in the basin itself. Supported by hydraulic model studies and prototype experience, a suitable stilling basin caters to a variety of conditions covering discharge intensity, head, tail water variation, and topographical conditions. The most serious problem with hydraulic jump type dissipator is more structural strength rather than hydraulic efficiency. Experience in recent years gives many examples of stilling basins having damages due to uplift, vibration, cavitation and abrasion, all having their origin in the internal structure of hydraulic jump. Model studies play an important role in optimizing the length of the stilling basin, elevation of the stilling basin floor level for a high head spillway and height of the end sill and hydraulic jump characteristics. This paper discusses hydraulic model studies conducted at Central water and Power Research Station (CWPRS), Pune, India, for the hydraulic jump stilling basin as energy dissipator for Kotlibhel dam spillway stage 1-B, Uttarakhand, India, at the conceptual stage, which played an important role in enhancing the overall performance of spillway and energy dissipator by incorporating modifications such as lowering of stilling basin floor level and modifying the height of the end sill. Through recent advances in computing power and modeling software capabilities, it is now feasible to undertake complex three-dimensional analysis using Numerical/Computational Fluid Dynamics (CFD) modeling techniques. Numerical model studies were carried out to assess the performance of the stilling basin using Computational fluid dynamics software, FLOW-3D. The major benefit of the CFD modeling is that it allows early identification of problematic flow features and modifications to the design/layout could be tried rapidly and cost-effectively. The results obtained from the numerical model were compared with the results of the physical model studies. Comparison of the results from the two modeling approaches has provided a very good insight into the design process to optimize the stilling basin design. This use of physical and Numerical modeling (CFD modeling) techniques has provided invaluable insight and greater confidence for future use of standalone CFD analysis in spillway/energy dissipator design as complementary tool for physical model studies. After studying the performance of the stilling basin for Kotlibhel stage 1-B Hydro electric project, both through the physical and numerical techniques, it is found that CFD modeling can be used as a valuable tool to test the preliminary empirical design and in the optimization phase of the stilling basin design prior to conducting physical model studies. The details of physical and numerical model studies conducted for improving the overall performance of stilling basin are described in the paper