Mechanism-Based Assessment of Upstream Scour and Flow-Redistribution Countermeasures at a Multi-Bay Regulator

Upstream scour at multi bay regulators represented a critical sediment related problems that can threaten structural stability. it remains far less studied than downstream scour associated with hydraulic jumps. This study investigated flow sediment interaction mechanisms governing upstream scour at Samarra Barrage–Tharthar Regulator system Iraq using an integrated framework combining prototype scale field measurements GIS based bathymetric analysis and 2D numerical modeling. Bed morphology was surveyed using echo sounding and flow velocities were measured with an Acoustic Doppler Current Profiler under representative operational discharges. A calibrated HEC-RAS 2D model employed to simulate flow redistribution and evaluate shear stress related scour indicators. Field observations and numerical results demonstrate that upstream scour is primarily driven by lateral flow contraction and asymmetric discharge distribution between bays which generate strong transverse velocity gradients and secondary vortical structures. These flow features locally increased bed shear stress beyond the critical threshold for the non cohesive sand bed leading to sediment entrainment and initiation of localized scour upstream of the central bays. Three countermeasure configurations spur dikes semi circular middle wall and an elliptical middle wall were numerically evaluated. The elliptical middle wall get the most effective reduction in transverse velocity gradients and inferred upstream scour potential (60%) relative to the existing condition while also minimized hydraulic energy loss. The results shown that effective mitigation of upstream scour requires control of approach flow redistribution rather than localized bed protection alone. The proposed mechanism based framework provides transferable insight into upstream scour processes and sediment response in large multi bay hydraulic structures.