Influence of Toe Clearance on Propeller Scour around an Open-Type Quay

Abstract

This study experimentally investigates the temporal and spatial development of the scour hole induced by a confined propeller jet in the presence of an open type quay. The longitudinal distance between the propeller face and toe of the quay slope, i.e., toe clearance, was systematically varied to examine its influence on the scour hole development under different test conditions. The scour depth along the centerline of the propeller axis was measured at predetermined time intervals and the three-dimensional scour profile was obtained at the end of each test when the asymptotic state was reached. The asymptotic centerline profiles at varying toe clearances, which were compared with published unconfined propeller scour data, show that evolution of the scour hole is significantly influenced by the toe clearance. The asymptotic scour profiles that formed at different toe clearances were classified into three types with reference to the toe clearance field in the vicinity of the slope toe, namely the near toe clearance, intermediate toe clearance and far toe clearance fields. Two scouring mechanisms, namely jet diffusion and quay obstruction, were found to govern the scouring process and dictate whether the scour hole forms in any one of these three fields. The influence of these two mechanisms on the development of the scour characteristic dimensions in each field were also investigated based on which quantitative demarcations among the fields were obtained.