Ehsan Jafari Nodoushan

The study of sediment transport phenomena, including the processes of erosion and sedimentation, in rivers is of great importance. Human interventions, such as the construction of hydraulic structures in river cross-sections, disrupt the natural sediment transport cycle. For example, the construction of upstream dams results in sediment accumulation in the reservoir and a significant reduction in suspended load downstream. During flood events, the increased flow velocity caused by the narrowing of the river cross-section at bridge piers intensifies bed erosion, which can threaten the stability and functionality of the bridge structure. This research was conducted with three main objectives: first, to analyze the impact of hydraulic structures on the river’s cross-sectional hydraulic parameters such as water surface elevation, flow velocity, and bed erosion potential during floods; second, to investigate changes in hydraulic parameters such as sediment concentration in the flow, bed alterations, and the effect of water temperature on bed erosion in the Saqqez River due to the presence of an upstream dam; and third, to estimate the maximum scour depth at the piers of the Kordmall commercial complex bridge and accurately locate this phenomenon. he study was conducted using hydrodynamic models in the HEC-RAS software to simulate steady and quasi-unsteady daily flow discharges, along with the Yang total sediment load equation. Additionally, the FLOW-3D sediment model was employed to analyze scour at the bridge piers. The results show that dam construction upstream leads to increased erosion in river cross-sections, whereas sedimentation occurred in these sections in the absence of the dam. The peak flow sediment discharge entering the study area for the no-dam and dam scenarios was 30640.83 tons/day and 9964.063 tons/day, respectively, indicating a 67.48% reduction in sediment discharge due to the upstream dam. The outgoing sediment discharge from the study area was 12333.93 tons/day without the dam and 7,723.62 tons/day with the dam, showing a reduction of approximately 59.75% and 22.48% compared to the upstream sediment discharge. The altered flow pattern caused by the river’s interaction with the abutments intensifies the scour depth at the side piers. The maximum scour depth at the side piers reached 1.96 meters, while the scour depth at the central piers was less, reaching a maximum of 1.09 meters. The difference in maximum scour depth between the side and central piers was 79.82%.