2024 – Optimising Stilling Basins: Lessons from the Somerset Dam Physical Model

Ben Modra, Laura Montano, Anna Hams, Stefan Felder, Brett Miller

Hydraulic jump stilling basins are a commonly used and effective energy dissipation structure for dams. A hydraulic jump is a complex hydraulic phenomenon which can be altered by dissipator elements such as baffles, chute blocks and end sills. While there are long standing guidelines for the design of stilling basins, physical hydraulic modelling is recommended for safety critical dams or for challenging design conditions. Optimising the arrangement of dissipator elements can improve flow behaviour and dissipation, especially during extreme flood events. Somerset Dam, like many aging dams, requires significantly higher flood conveyance capacity over the original design due to additional hydrologic data available since construction, modern hydrological methods and anticipated climate change impacts on flooding. This will require the dam to achieve significantly greater dissipation in the stilling basin over the original design conditions. With existing physical and constructability constraints at this dam, the conditions are challenging for ensuring a safe and effective dissipator design. An extensive physical hydraulic modelling campaign has been used to guide the dam designers through the preliminary and detailed design phases of the project. This paper presents learnings from the modelling program, providing insights into the design of a complex stilling basin to ensure dam safety for the long term and for all design flow conditions. The study demonstrates the effectiveness of physical hydraulic modelling in assessing designs for the Somerset Dam Improvement Project, with a focus on the development of the dissipator design to meet the design objectives.