Scour Caused by a Propeller Jet

The scour hole induced by a propeller wash is experimentally investigated with noncohesive sediments. The associated scour profile at the asymptotic condition, which can be expressed by a combination of three polynomials, comprises (1)a small scour hole beneath the propeller, (2)a primary scour hole downstream of the small scour hole, and (3)a deposition mound farther downstream of the primary scour hole. The time-dependent maximum scour depth induced by propeller jets is closely related to the densimetric Froude number, a reference time scale, offset height relative to the propeller diameter, and sediment size. A semiempirical formula, which shows good agreement with the experimental data, is proposed to describe evolution of the maximum scour depth. An equation for the determination of the time to initiate scour is also proposed. Moreover, empirical equations for the maximum asymptotic scour depth induced by a propeller and three-dimensional submerged offset jets agree well with the experimental data. The critical condition for the initiation of scour caused by propeller and offset jets is also presented.