Experimental research on the stability of the armour and secondary layer in a single layered tetrapod breakwater

Preface This thesis was submitted in conformity with the requirements for the degree of Master of Science (MSc) in Civil Engineering. The preparation and evaluation of the experiments were performed at Royal Boskalis Westminster nv. The experiments have been carried out at the Fluid Mechan-I want to thank him for guiding me through the total process of this research. During this graduation work, Jelle Olthof guided me most extensively. I want to thank him for his valuable comments on the experiments and their results and for his undiminishing enthusiasm. I thank Marcel van Gent for his knowledgable insights and many detailed comments on my report. I also thank Harry Fontijn and Henk-Jan Verhagen for their practical advice on the setup of the experiments and their useful comments during this research. Furthermore I thank Royal Boskalis Westminster nv for their financial support during this complete research. WL|Delft Hydraulics provided me the miniature tetrapod units. Finally I thank all the members of the support staff of the Fluid Mechanics Laboratory for their help and advice and of course for giving me a good time during the experiments. single armour layer breakwaters exposed to irregular waves, demonstrated a positive relationship between an increase in the packing density of the armour layer and the stability of the structure. This relation particularly held if the single armour layer consisted of tetrapod units. A higher packing density increased interlocking, resulting in lesser displaced units. Within a packing density range of n v = 0.25 − 0.4 a fairly stable top layer was achieved. However, the single top layer with its slenderly shaped units caused the secondary layer to be rather exposed to wave attack, which led to wash-out of the secondary layer rock material, thereby seriously undermining the stability of the structure. Bhageloe and Van den Bosch both used rules of thumb recommended by Van der Meer (1993) to determine the dimensions of the secondary material used in their scale models. Therefore, these recommendations seem to lead to underlayer rock material that is too small for use in a single layered tetrapod armour layer. This research focused on both the stability of the single tetrapod armour layer and the secondary layer. By means of several wave flume experiments, testing various combinations of secondary material , wave steepness and armour packing density, an attempt was made to establish the influence of these governing parameters on the stability …