Pressurized Sediment Flushing at a Dam Hydropower Intake: A Physical Model Investigation of Flow and Bed-Load Mobilization

Reservoir sedimentation reduces storage in hydropower facilities and threatens turbine integrity through sediment entrainment at turbine intakes. Pressurized sediment passages are utilized as countermeasures; however, their performance in reservoirs with irregular topographies is not well documented. This study reports the flow dynamics and sediment flushing in a 1/30 scale physical model of a dam. Three-dimensional velocity components were recorded using a Nortek Acoustic Doppler Velocimeter at 240 points across five elevations upstream of the sediment passage entrance. Reynolds stresses, bed shear stresses, and Shields parameters were derived from the data. The highest turbulent shear stress, over 12 Pa, was observed in front of the passage inlet. The Shields parameter values exceeded the 0.045–0.06 critical range, consistent with bed-load motion during flushing. Approach velocities above 1 m/s were confined to approximately 30 m upstream of the passage entrance, defining a finite influence radius. The measured flushing cone length and volume agreed within 4% with prior experimental data on flushing cones, although the scoured area geometry was asymmetric due to the passage offset. Lowering the reservoir from 1147 to 1133 m at the spillway-crest-level increased the flushed volume by only approximately 6%, whereas the near-inlet velocity field changed little between the two cases; the local velocity gradient at the passage entrance, rather than the reservoir level itself, governed the flushing efficiency.