Baleh Hydroelectric Project (Baleh HEP) features a high-head spillway where hydraulic performance during extreme floods is critical to structural safety. Physical model tests were conducted to evaluate and refine the original spillway design, focusing on the guide wall, aeration system, and flip bucket. Initial testing revealed several concerns: water-wing formation at the chute entrance; insufficient aeration at Aerator 1 under high discharges; excessive backflow at Aerators 2 and 3; and strong atomisation at the chute end. Plunge-pool energy dissipation was confined to surface layers, resulting in an uneven velocity field. Corrective measures were developed and verified through testing. A 20-metre gradually descending guide wall was introduced to stabilise inflow. Aerators were reshaped by adjusting ridge geometry, lowering vent elevations, and adding 1-metre ventilation openings to improve air entrainment and cavity stability. A 10° continuous flip bucket replaced the original to ensure consistent jet landing and reduce atomisation. The right-bank plunge-pool slope was shifted upstream to minimise local scour. The optimised design achieved smoother chute flow, improved pressure stability, aeration levels above 2.5%, and more effective energy dissipation, with jet impact on the plunge-pool bottom avoided and downstream flow more uniform. These refinements enhance spillway safety through stable flow behaviour, improved cavitation protection,