Impacts of Tailwater on the Design of Several Stilling Basins in the USA

Abstract

The dissipation of excess energy from flows exiting a spillway is often needed to prevent or reduce to acceptable levels conceivable negative impacts to the downstream channel, spillway, and dam (e.g., erosion, undermining). The optimization of a hydraulic jump type stilling basin using general-purpose, published design methodologies (i.e., USBR, SAF, etc.) for a project can be challenging, as these methodologies may not account for all site specific conditions and structure formulations by designers. Also, it is not often clear to a designer as to which flow rates (and corresponding flood events) will have the greatest influence on the basin geometry and features (i.e., jump formation location and stability). This can be further obscured for projects where a high tailwater condition is predicted during flood events. This paper presents several recent dam rehabilitation projects where a high tailwater impacted the designs of the stilling basins. Each project features a different spillway, chute, and basin configuration. An overview of each site and summary of key challenges encountered during the design of the spillways and stilling basins is included. In addition, a discussion of which design methods were selected, why they were selected, and additional measures that were taken to address the uncertainties at the site is included. This documentation of unique site conditions and design methodologies for stilling basins is intended to show the importance of collaboration between the designer and the owner in selecting a design approach for a specific situation.