Experimental Investigation of an Offshore Detached Submerged Breakwater Made of Geotextile Tubes for a Barrier Island Along East Coast of India

Abstract Barrier islands are shore parallel sand deposits formed by the action of waves and winds and are separated from the mainland by tidal creeks, bays, and lagoons. These narrow strip of sand bars are most vulnerable to sea level rise and storm surges which are the consequences of changing Earth’s climate. As barrier islands are formed because of wind and waves, which shift according to weather patterns and local geographical features, they are most dynamic and complex in nature. Non-shore connected structures such as submerged reefs and offshore breakwaters helps in development of the coastline by interfering in the near shore sediment transport processes. Usage of shore connected structures such as groins, seawall, revetment etc., shifts the problem to the adjacent areas due to blocking of longshore sediment transport. Installed at a significant distance, offshore breakwaters can produce calm region landward of the structure. Traditional coastal structures like breakwaters, groins, revetments, and seawalls have historically been built using materials such as wood, rock, and concrete. However, the growing emphasis on environmental considerations and the scarcity of natural rock resources in specific areas have contributed to a rising use of geosynthetics in coastal protection. Geotextiles are permeable synthetic fabrics made from polypropylene and polyester which has wide range of engineering applications. The present study is an experimental investigation on a 1m cross section of an offshore detached submerged breakwater made of Geotextile tubes with a scaled model of 1:10 for water depths of 0.45 m, and 0.5 m. The wave heights varied in the range of 0.05 m to 0.2 m with corresponding wave periods of 1.6 to 3.0 seconds with an interval of 0.2 seconds. The relative water depth (d/L) is varied from 0.07385 – 0.1624 with relative wave steepness (H/L) ranging from 0.008206 – 0.06496. The performance of the submerged breakwater has been assessed for regular waves by analysing the wave reflection, transmission and energy dissipation. Three probe method suggested by Mansard and Funke (1980) has been adopted to analyze the wave reflection. The experimental study has been carried out in a wave flume of 2 m wide, 1.8 m deep and 30 m long with hydraulic piston type wave maker installed at one end to generate predefined wave characteristics. The results of the experimental study showed the efficiency of Geo synthetics as coastal protection structures for a barrier island.