Toe stability of rubble-mound breakwaters

This Master Thesis contains a study on the stability of the toe structure of rubble-mound breakwaters based on small scale model tests performed in the large wave flume of the Laboratory of Fluid Mechanics of the Faculty of Civil Engineering of Delft University of Technology. In this report the study of Gerding (1993) on the stability of toe structures was continued. This has been done according to the design relation suggested by Gerding. In his tests the density of the stone was not varied, although it is a parameter in the formula. The main purpose of the study at hand was to check the validity of the density in the Gerding-relation: therefore the stone mass density ?s in this research was varied: namely 1900, 2550 and 2850 kg/m3, respectively the materials brick, porphyry and basalt. Only one cross section of a rubble mound breakwater with various toe structures was investigated. The existing knowledge and the influence of all governing parameters is reviewed. The main variables were toe height, Zt, water depth in front of the toe hm, water depth above the toe, ht, wave height, Hs, nominal stone diameters, Dn50 and stone mass density ?s. Variation of the wave steepness wasnt of influence on the toe stability, because the range of values of the surf similarity parameter was such that the reflection coefficient was almost constant. Existing knowledge is presented from which an analysis of the influence of the parameters involved followed. The tests are described and then the result of the measurements is analyzed. The data showed too much scatter and therefore elaborated to one damage level Nod=0.5 (start of damage). The influence of the relative density of the stone, Δ, was reproduced in the Gerding-relation in the right way, because different stone mass densities gave similar results for Hs/ΔDn50 as a function of ht/Dn50. The design curve by Gerding be used for the design of toe structures, albeit that the designer must not forget the influence of hm which is not mentioned in this relation. In the present study the same trend is found but the values from the present study show a higher stability of the structure than with the suggested relation of Gerding, which is more conservative. Computations of toe stability were performed according to Shields and to Rance & Warren to get an analysis on analytical grounds, because empirically found results dont necessarily have to be right. Only the computations according to Rance & Warren could fit the results of the measurements with addition of a small amplification factor. The parameter of the local water depth in front of the toe structure, hm, had still an influence on the relation found between Hs/ΔDn50 and ht/Dn50; this followed both from the tests and the computations. So it is risky to use the design curve for all hm. The working-out of the computations must be improved and more experiments are needed to enlarge the knowledge of the influence of hm and to simulate the measurements more accurate. Differences have been found between this study and the research of Gerding. The damage estimated was on the average two times smaller than the damage Gerding had determined. The reason why this happened is still unknown. It is therefore recommended to perform further research to explain why this difference occurs. More insight into toe stability may be found when the velocity distribution over the water depth in front of the toe structure is better defined and when the influence of the shape of the foreshore and of the reflection on the structure on toe stability would be taken into account.Civil Engineering and Geoscience