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Present design tools, as found in the Rock Manual or Coastal Engineering Manual, for the determination of toe rock size for rubble mound breakwaters are based on test data with a large spread: data is relatively dispersed around the centre and descriptive equations have limited applicability ranges. New research has been undertaken to contribute to a more accurate description of toe rock stability. Flume tests have lead to an empirical design criterion for toe bunds in very shallow water based on the Hudson-type stability number. Herein the foreshore slope turns out to have an important influence. An approach with theoretical background has been used for toe bunds in surging wave conditions. The resulting stability description, based on local flow velocities, has been verified with existing data sets. Additional flume tests were performed to measure flow velocities at the toe bund. Results are used to calibrate the velocity-approach, providing an improved design criterion., Hydraulic Engineering, Civil Engineering and Geosciences

This thesis investigates the stability of rocks in toe bunds for rubble mound breakwaters. Presently recommended design tools lack a reasonable degree of accuracy to assess required toe element dimensions. The goal of this research is to improve the insight in the physical process related to stability of toe bund elements under wave load. Analysis of presently recommended methods led to the following conclusions: a. The formula of Van der Meer is no improvement of the formula of Gerding. b. Both methods use an inappropriate relation between stability and damage. c. The method of Van der Meer may often underestimate the required toe rock size. A new hypothesis is formulated to describe toe rock stability. The concept for the model of this study is based on two steps: Step 1: Assessment of the local water motions at the toe bund. The amplitude of local velocity is calculated by summation of the contributions of the incoming wave and down rush, taking a phase difference into account. Step 2: Description of the critical load on a toe rock. The Rance/Warren stability criterion is used with a theoretical adaptation that accounts for porous outflow. Coupling these two steps implies that a rock will move if the occurring velocity exceeds the critical velocity. An evaluation of the hypothesis is performed and the model is fitted to available test data. The accuracy of stability assessment is increased with respect to previous methods. The model of this study has more resemblance to the test data and can be used to predict the required toe rock size in design. This is verified for the applicability range of the data set of Gerding., Civil Engineering and Geosciences