Experiments to classify in-ground stilling basins and determine the factors affecting the optimization of flood mitigation dams

• Hydraulic jumps within in-ground stilling basins were classified into five types. • Steady submerged hydraulic jump is the most effective jump type. • The flow pattern depends more on the cross-wall height than the hydraulic conditions. A "stilling basin" is a structure used downstream of a dam to reduce energy flow and prevent damage to the dam structure or erosion downstream. The literature on energy dissipation in stilling basins has identified mechanisms such as abrupt drops, sudden enlargements, steps, and cross walls. However, the combined effects of these mechanisms in a comprehensive experimental approach have not been thoroughly studied. This experimental study explores flow behavior within an in-ground stilling basin (ISB). In contrast to previous studies, this study systematically investigated the interaction between geometric parameters and flow behavior. Five types of hydraulic jumps were identified based on their surface patterns and water flow profiles. Among these, the steady submerged hydraulic jump is the most effective for reducing the flow velocity, maintaining stability, and ensuring symmetry. This study also highlights that the height of the cross-wall influences the flow pattern in the ISB more than variations in upstream conditions, such as the Froude number. Increasing the length of the ISB reduces the flow stability. Furthermore, incorporating free spaces in the ISB design improves flow stability and symmetry while offering additional benefits, such as better flushing efficiency, fish passage, and improved downstream flow conditions, which have not been fully addressed in prior studies. The results of this study provide the foundation for practical recommendations and design guidelines for ISBs in flood mitigation systems.