Novel Techniques for Reinforcing Rubble-Mound Breakwater against Tsunamis

Abstract

The widespread use of rubble-mound (RM) breakwaters along coasts across the world highlights the importance of understanding their behavior during natural disasters such as tsunamis. The failure of these breakwaters during tsunamis can have far-reaching consequences, potentially causing damage to coastal infrastructure and loss of life. Many breakwaters failed during past tsunamis. Despite this, studies on the behavior of RM breakwaters during tsunamis are minimal. The present study thus attempts to elucidate the behavior of RM breakwater subjected to a tsunami. Furthermore, efforts were made to develop effective countermeasures that can safeguard the breakwater against tsunamis. To the end, a novel technique of using geogrids for reinforcing the RM is proposed. This study could be a pioneering application of geogrids as reinforcing elements in RM breakwaters to mitigate damages from tsunamis. Geogrid layers are provided on both the seaside and harborside to mitigate tsunami-induced damage to the breakwater. In addition, a crown wall (with shear keys) is also introduced to prevent the scouring of the crest and sheet piles from preventing excess seepage through the seabed. Physical model tests, analytical studies and numerical simulations were carried out to assess the performance of the proposed countermeasures by comparing it with the behavior of conventional RM breakwater during the tsunami. The tsunamis can overflow the breakwater, potentially exceeding its design limits. Hence, provision was made in the study for overflow, where the breakwater may overflow by the tsunami. It was observed that excess seepage through the body of the breakwater and the scouring of the crest were significant factors that led to the failure of RM breakwaters under tsunami overflow. A novel reinforced model was proposed to address these issues. This model effectively withstood tsunami-induced damages without significant deformations, demonstrating its potential as a reliable solution. In the past, tsunamis have caused significant damage to several rubble-mound breakwaters, leading to devastating impacts on coastal communities. The 2004 Indian Ocean tsunami and the 2011 Great East Japan tsunami are two well-known examples of the destructive power of tsunamis. A significant number of breakwaters across the globe are of the rubble-mound type, and finding a practical solution to make them resilient against tsunamis could protect many coasts against future tsunamis. This study proposes novel reinforcing techniques to make the rubble-mound breakwater tsunami-resilient. These techniques include adding geogrid layers on either side, providing a crown wall (with shear key) at the top, and inserting two sheet piles into the seabed at either end of the rubble-mound breakwater. Perhaps, this is the first-time geogrids were used on rubble-mound breakwaters to mitigate tsunami-induced damages. The reinforcing elements were chosen for their practicality in the real world. These elements can be practically applied to make existing and future breakwaters tsunami resilient. The study found that the proposed reinforced model, particularly Reinforced model III, remained intact without significant deformation even under a 15-m high tsunami.