Abstract
Overflow structures are among the most important hydraulic structures used for measuring flow, controlling floods in reservoirs, and regulating water levels in open channels. Alternative options, such as combined structures like spillway-gates, are preferred due to their compatibility with natural and ecological needs. This study investigates the impact of different soil gradations downstream on the scouring profile of combined spillway-gate structures. Scouring and sedimentation downstream of the spillway-gate were examined under various particle sizes of 0.0008, 0.001, and 0.0014 m, with a constant density in both free and submerged flow conditions using the FLOW-3D software. In this study, the k-ε, k-ω, LES, and RNG turbulence models were evaluated, and the RNG turbulence model was selected among that group. In free flow conditions, the highest sediment deposition occurred with the smallest particle diameter. For larger particle diameters, the void spaces between the particles reduce friction and increase the movement threshold, leading to increased scouring and decreased sediment height. In submerged flow conditions, the changes in scouring for different particle sizes were minor, with results being closely aligned. In submerged flow conditions, increasing the particle diameter resulted in a decrease in sediment deposition in the post-scouring area.