Axial velocity decay and scour mechanisms of propeller jets under near-bed conditions in restricted channels

This study investigates the velocity evolution and scour characteristics of propeller jets under near-bed conditions in restricted channels using physical model experiments and dimensional analysis. The effects of propeller diameter, bed clearance, and rotational speed on centerline axial velocity, maximum axial velocity, and maximum scour depth are examined. The measured centerline axial velocity generally increased downstream of the first measurement section and then decayed, although the measured peak occurred at 0.1 D p under a few conditions. All measured peak locations were within 0.1 D p ∼0.4 D p . Increasing propeller diameter and rotational speed enhances the maximum axial velocity and bed scour, whereas increasing bed clearance weakens the near-bed jet effect and reduces scour intensity. The resulting bed morphology consists of a main scour zone, smaller local scour zones, and surrounding deposition zones. Based on these findings, equations are developed for post-peak velocity decay, maximum axial velocity, and maximum scour depth. The maximum scour depth equation is further validated using independent external datasets under comparable conditions. These results improve the quantitative understanding of propeller-jet-induced velocity decay and scour response under near-bed confinement, and provide guidance for bed-protection design in restricted channels and confined shallow-water areas.