Stone stability in breakwater toes based on local hydraulic conditions

Abstract

This study investigates the stability of rocks in breakwater toes. Previous studies have coupled the toe stability directly to the wave height, which is not physically accurate. In this study the local hydraulic conditions at the point of incipient motion are considered. First the forces acting on a single stone are identified. Thereafter the moment of force about the rotation point of the stone is computed. When this moment becomes positive, the stone is expected to move. This is called the 'moment criterion'. To verify the moment criterion, a wave flume experiment was designed. In this experiment all the outfacing stones were glued together, so that no movement could occur. Seven cavities were left open on the toe, in which seven 'target stones' were place. Pressure sensors were place underneath these stones and a velocity sensor and a wave gauge above the toe. Using this arrangement the local hydraulic properties at the point of incipient motion were determined. It was found that the pressure difference over a stone is the most important local hydraulic parameter to determine the point of incipient motion. The forces and moments on the stones are computed using the measured local hydraulic conditions, thereafter the performance of the moment criterion is determined. It was found that the moment criterion does have predictive capabilities, but it cannot pinpoint the point of incipient motion. The moment criterion can be improved by including turbulent forces and incorporating a damage parameter to increase the applicability of the criterion.