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Hydraulic Stability of Cubipod Armour Units in Breakingconditions Lien Hydraulic stability of Cubipod armour units in Breaking conditions Lien Vanhoutte Promotor: Prof. Josep Medina (UPV Valencia) Co-Promotor: . Prof. dr. ir. Julien De Rouck Masterthesis to obtain the degree: Master of Science in Civil Engineering Laboratory of Ports and Coasts, Polytechnic University of Valencia Departement of Civil Engineering, Ghent University Academic year 2008-1009 i Preface I would like to thank my tutor of this project Prof. Medina for giving me the great opportu- nity to make my nal year project at the Laboratory for Ports and Coasts of the Polytechnic University of Valencia, and for his guidance throughout the project. Special thanks also to Prof. De Rouck as my Erasmus-coordinator and co-tutor of this thesis for providing the possibility of this abroad experience. Deep gratitude goes to Guille, for his guidance throughout the project, for sharing his ex- perience, for helping me with every single doubt, for encouraging me and helping me out in the stressful moments. A warm thanks as well to Jorge, Vicente, Kike, Mireille, Steven, César and Pepe, for providing a very nice working space in the laboratory. Finally I want to thank my parents, my sisters, friends, and at mates in particular, for their support and many hours of listening during this thesis. COPYRIGHTS The author grants the permission for making this thesis available for consultation and for copying parts of this thesis for personal use. Any other use is subject to the limitations of the copyright, specically with regards to the obligation of referencing explicitly to this thesis when quoting obtained results. 1st of June 2009, Lien Vanhoutte ii Overview Hydraulic stability of Cubipod armour units in breaking conditions Author: Lien Vanhoutte Master thesis to obtain the degree of Master of Civil Engineering Academic year 2008-2009 Tutors: Prof. Josep R. Medina, Laboratory of Ports and Coasts, Polytechnic University of Valencia Prof. Julien De Rouck, Department of Civil Engineering, Ghent University Summary In this report, the study of the new armour unit, Cubipod, designed by the Laboratory of Ports and Coastas of the Politecnic University of Valencia, is described. The general stability of mound breakwaters are discussed and an overview of dierent existing armour elements is given. Further, the wave height distribution in shallow water is analysed theoretically and compared with the obtained results. An experimental study of the Cubipod armour unit is carried out on a physical scaled mound breakwater model in breaking conditions. Results on reection and damage progression are presented and compared with previous similar tests in deepwater conditions. A rst estimation of the hydraulic stability coecient of the Cubipod in breaking conditions is proposed. The results show that the Cubipod has low reection and high hydraulic stability. Keywords: Cubipod - armour unit - mound breakwater - hydraulic stability - breaking conditions HYDRAULIC STABILITY OF CUBIPOD ARMOUR UNITS IN BREAKING CONDITIONS L. Vanhoutte1 Supervisor(s): J.R. Medina2, J. De Rouck3 1 Masterthesis student, Faculty of Engineering, Ghent University, Belgium 2 Professor, Lab. of Ports and Coasts, Polytechnic University of Valencia, Spain 3 Professor, Faculty of Engineering, Ghent University, Belgium Abstract—In this Masterthesis an experimental study of the Cubipod ar- The Cubipod armour unit is designed to benefit from the ad- mour unit was carried out on a physical model breakwater in shallow water. vantages of the traditional cube, but to correct the drawbacks. The Cubipod is a new armour unit, designed by the Laboratory of Ports and Coasts of the Universidad Politcnica de Valencia. As the wave height is an Therefore, the design of the unit is based on the cube in order to important value when designing mound breakwaters, theories estimating obtain his robustness. The protuberances of the Cubipod avoid the maximum wave height in breaking conditions were studied and com- face-to-face settlement and increase the friction with the filter pared with the measured results in the Laboratory. Results on reflection layer as can be seen in figure 1. They avoid sliding of the ar- and damage progression were presented and compared with previous sim- ilar tests in deepwater conditions. An estimation of the hydraulic stability mour elements and thus, Heterogeneous Packing and loss of el- KD coefficient of the Cubipod in breaking conditions was proposed using ements above the still water level is reduced. All this indicates a the Virtual Net Method[2]. The results show that the Cubipod has low re- higher hydraulic stability of Cubipods in comparison with cube flection and a high hydraulic stability. elements, which was proved in earlier executed tests [3]. Keywords—Cubipod - armour unit - mound breakwater - hydraulic sta- bility - breaking conditions I. INTRODUCTION Mound breakwaters play an important role in the protection of harbours. They have many failure modes, but the most im- portant one is the loss of hydraulic stability of the armour layer under wave attack. This can be caused by direct extraction of armour units, or by excessive settlement causing Heterogeneous Packing of the armour layer as described by Gomez-Marton & Medina [2]. Generally, mound breakwaters are placed in shallow water Fig. 1. The new armour unit: the Cubipod and thus subjected to breaking conditions. An important fac- tor influencing the hydraulic stability is the maximum incident III. EXPERIMENTS wave height. Hydraulic stability of armour layers has been in- tensively studied in literature and several formulae have been Regular experiments on five different physical model break- proposed for predicting armour damage. The first models were waters were carried out in the 2D wave flume of the laboratory only valid for stationary conditions. In 1988, Van der Meer [8] of Ports and Coasts in the Polytechnic University of Valencia. A proposed a first formula for irregular waves. Medina [7] pro- section with a double layer of Cubipods, one with a single layer posed a method applicable to nonstationary conditions, based of Cubipods, each with and without toe berm were considered. on an exponential model for individual waves of the storm. The Finally, experiments were carried out on a section consisting of most frequently cited armour stability formula was published by a cube layer covered by a Cubipod layer. Hudson in 1959[4] for regular waves, and later popularized for The unit weight of the Cubipods is 128g, and they have a 3 irregular waves by SPM using the equivalences H1/3 and H1/10 density of 2300kg/m . The water depth changes from 30cm to as representative of the wave height. 42 cm near the model. For every water depth different periods were considered, lancing waves with increasing wave height for II. ARMOUR UNITS every period. The wave height was increased until breaking oc- Originally, harbours were built with wooden or stone con- cured. Registered wave heights were separated in incident and structions. The continuous growing of the harbours meant a reflected waves with the LASA V-method (Figueres & Medina need for higher stones and design of artificial concrete armour [1]), and the reflection coefficient was obtained as CR [%] = units was forced. Many different breakwater armour units ex- Hr/Hi. Damage progression was analysed visually, establish- ist, each with their own advantages and disadvantages. Their ing the damage levels Initiation of Damage, Iribarren Damage characteristics have an important influence on the hydraulic sta- and Destruction, as well as quantitatively, using the Virtual Net bility of the mound breakwater and explains why improvement Method proposed by Gmez-Martn & Medina [2], which allows and development of armour units is still an important subject of to measure also the failure mode of Heterogeneous Packing, and research. not only extraction of armour units. IV. RESULTS for a double layer of Cubipods with toe berm and KD=23 for a single layer were found. K =18 was found for a combined A. Breaking wave height D armour layer with cubes and Cubipods. Comparison between The incident wave height is an important factor influencing the damage progression in deepwater conditions and in shallow the design of coastal structures. An overly conservative estima- water shows us that KD in shallow water is less than in deepwa- tion of this value can greatly increase costs and make projects ter conditions. Waves with higher energy reach the breakwater, uneconomical, whereas underestimation could result in struc- which means that the damage will initiate earlier than in deep- tural failure or significant maintenance costs. A short study con- water conditions. In Fig. 3, the damage progression for the dif- cerning the maximum wave height in breaking conditions was ferent breakwater sections are shown, with D0,2 the linearized executed. dimensionless damage proposed by Medina [6] and indication Different theories exist to estimate this maximum value. of the Initiation of damage and Initiation of Iribarren damage. Many theories however, overestimate this value. Further, they suppose mostly that the energy from the broken waves is con- centrated in the breaking wave height, which means that all the broken waves have the breaking wave height in the surfing zone. This statement however didn’t correspond with the reality. The energy from the broken waves was distributed back over the smaller wave heights in the distribution. In Fig. 2 is the theory of Le Roux (2007) [6] shown to estimate the real wave heights. The estimation is similar to the measured values, however, he underestimates the breaking wave height and supposes a con- stant wave height after breaking, independent of the wave pe- riod. Fig. 3. Linearised dimensionless equivalent damage as a function of dimension- less wave height for the different studied breakwater sections V. CONCLUSION Calculating a mound breakwater in breaking conditions, spe- cial attention has to paid to the maximum wave height. Many existing theories overestimate this wave height, which can re- sult in uneconomical results.
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