Physical Model Studies on Breakwaters with Geotextile Sand Container Armour Units

Abstract

Breakwaters are essential constructions providing tranquillity to ports and harbour structures when natural protection is lacking. Traditionally, these massive structures are constructed using natural rocks weighing tonnes. In the present scenario, obtaining huge natural rocks is cumbersome and non-eco-friendly. Harnessing the advantages of geotextile sand containers (GSCs), numerous submerged breakwaters and shoreline protection structures have been constructed worldwide. But an emerged breakwater structure with geotextile armour units, capable of replacing the conventional structures, is rarely discussed. A 1:30 scaled, monochromatic wave flume physical experimentation is carried out as a preliminary investigation to test the feasibility of using GSCs as breakwater armour units. The structural design of the GSC breakwater evolved based on a comprehensive literature survey. Single-layer GSC breakwater structures armoured with sand-filled units differing in size and fill percentage (named Bag 1, Bag 2, Bag 3 and Bag 4 ) are investigated in the initial stage. Studies on hydraulic performance (wave runup, rundown and reflection) and stability of GSC breakwater are carried out to analyse the efficiency of the structure against the wave conditions of the Mangaluru coast. The study revealed that the reflection coefficient (Kr) for GSC structures could range from 0.26 to 0.69. Additionally, reducing GSC fill percentage from 100 to 80 is found to be more effective (up to 64%) in reducing reflection, runup and rundown rates than altering GSC size. As far as stability is concerned, the best-performing single-layer configuration comprising Bag 3 could withstand wave heights up to 2.7 m in the prototype. The effect of armour unit size and sand fill ratio on the stability of the structure is analysed, and it is concluded that changing the sand fill ratio from 80% to 100% shot up the structural stability to a maximum of 14%. Increasing bag size also resulted in increased stability by up to 8%. Stability curves for all tested configurations are projected as the significant research outcome and can serve as a practical guideline for coastal engineers in designing GSC breakwaters. It's efficacy to be used as the armour units of breakwaters motivated to advance further in GSC research by adding a second GSC layer to the breakwater model. These double- layered breakwater models with different placement modes are tested for its hydraulic iiiperformance and stability. Double layer placement showed 45 to 52% less Kr than single- layer placement and 38 to 42% lesser than the conventional breakwater, owing to its higher porosity. It is observed that the stability of the structure increased by up to 17% when supplemented with double layers. Structure tends to be stable with increasing armour unit size and fill percentage. Larger bags stacked in double layers is found to be the most stable configuration. 80% filled, slope parallel placement exhibited the least stability. Double layer placement with Bag 3 is found to be stable up to a wave height of 0.132 m on the model scale, which is 3.96 m in the prototype. Stability nomograms for all the tested double-layer GSC breakwater cases are obtained from physical experimentation. In the last stage, a pilot study is conducted by filling cement and sand to GSC units of the best-performing models from the above experiments. When GSC breakwaters are filled with sand and cement, up to 43% increased stability is observed with a considerable decrease in wave runup, rundown and reflection. As a result, cement-sand-filled GSC units can be suggested as a possible alternative to sand-alone-filled units where vandalism has to be countered.