Modelling of Resilient Rubble Mound Breakwater Against Tsunami

Abstract

The Rubble Mound (RM) breakwater is the most common type of breakwater constructed near the seacoasts of many countries across the globe. Most of these countries have a higher population density concentrated near their coastal lines. Tsunamis are one of the most devastating natural hazards a breakwater could encounter during its life time. Unfortunately, many breakwaters were damaged or collapsed in several countries during past tsunamis, such as the 2004 Indian Ocean tsunami and the 2011 Great East Japan tsunami. Due to the collapse of these breakwaters, the tsunamis were not blocked, giving them direct and unfettered access into the coastal areas, leading to tremendous loss of life and property. This underscores the urgent need for comprehensive research and development of effective countermeasures to make tsunami-resilient RM breakwaters. However, the existing literature is limited in addressing the behaviour of RM breakwaters during tsunamis and developing mitigation strategies. The present study aims to address these gaps by (i) investigating the behaviour of RM breakwaters under tsunami and (ii) proposing novel countermeasure techniques to enhance their resilience. To the end, rigorous methodology involving physical model tests, analytical studies, profile mapping and numerical simulations were performed on conventional and proposed reinforced breakwater models. The Northern breakwater at the Ennore Port, India, was chosen as the prototype for being the longest breakwater in the country to be impacted by the 2001 Indian Ocean tsunami. The small-scale model of the prototype breakwater was tested in the lab and revealed the failure mechanisms in conventional breakwaters during tsunami events, laying the groundwork for the development of innovative countermeasures. Novel techniques incorporating gabions, geobags, geogrids, sheet piles, and crown walls with shear key were devised and evaluated using tsunami overflow tests, analytical studies, profile mapping, and numerical simulations. As far as the author knows, it is the first time that gabions and geosynthetics have been employed in RM breakwaters to make a tsunami-resilient model. In the physical model tests, particular attention was given to critical scenarios observed in the past, such as the overflow of the Kamaishi breakwater during the 2011 Great East Japan Tsunami. This event is significant because the Kamaishi breakwater, which was considered tsunami-resistant, experienced severe damage due to the overflow of the tsunami that exceeded its design height. To address such crucial scenarios where tsunamis exceed the design height of breakwaters, the tsunami was allowed to overflow the breakwater in the physical model tests. This meticulous consideration of real-world scenarios in the testing phase ensured that the developed solutions were robust and adaptive to extreme events, enhancing the reliability ii and applicability of the research findings. In order to evaluate the effectiveness of the developed reinforced models, comparative analyses were conducted between reinforced and conventional (unreinforced) RM breakwaters in terms of settlement, horizontal displacement, pore water pressure, scouring, and seepage. The emerged portion of conventional RM above the mean sea level (MSL) was observed to be scoured entirely during the tsunami overflow. On the contrary, the reinforced models remained intact even after the tsunami overflow. It was observed during the tests that the reinforced models reduced settlement of the crest by more than 90.2% and lateral displacement by more than 94.6%. Additionally, the placement of sheet piles in the seabed soils at either end of the breakwater in the reinforced models reduced the incremental pore water pressure in the seabed during the tsunami by 54.2%. The quantification of damages on RM breakwaters by profile mapping revealed that 4.5 x 10-3 m3 of the conventional breakwater volume was scoured by tsunami overflow, which was prevented entirely in the reinforced models. Numerical analyses and analytical studies were also performed, which underscored the stability enhancement of the reinforced models and the effectiveness of sheet piles in arresting seepage through the seabed. The stability of the reinforced models was observed to be 2.2 times higher than the stability of conventional models. Further parametric studies analysing the influence of tsunami height, depth of shear key, and number of shear keys were also performed in the validated numerical model. The results of the numerical analysis revealed that the developed techniques are very successful in mitigating the damages on RM breakwaters caused by tsunamis.