The formation and spatio-temporal variations of a new sand spit in Vaan Island, India—An influence of the multipurpose artificial reefs deployed to restore the sinking island

Abstract

Sand spits constitute one of the depositional landforms that occur along the coast. This study (conducted from December 2015 to December 2020) investigated the spatio-temporal changes of the newly formed sand spit of Vaan Island in Gulf of Mannar, Southeast India. The investigation was based on morphological variations, bathymetry variations, wave climate, sediment nature and rate of deposition. Morphological variations were monitored using handheld Garmin GPS, and bathymetry variations were studied with low-cost bathymetry survey technique using Garmin echo-sounders. The freely available ECMWF wave data were used to obtain information on wave climate that approaches the study area. Satellite sun glitter imagery interpretation method was used to obtain information on wave direction around the Vaan Island for different time periods. Sand spit sediment characters and sedimentation rate were also estimated. The results indicate that the artificial reef modules, deployed on the windward side of the island for the purpose of restoring it from sinking, hinder the wave action, which has led to the formation of sand spit on the northwest by the long-shore sediment drift. Seasonal variations and morphological changes undergone by the sand spit are associated with the local wave climate. Bathymetry variation and shoreline configuration are also related. On shifting into deep contour the spit breaches into two or more pieces, whereas on shifting into shallow flat contour the area of exposure above sea level increases during the regular seasonal oscillation. The changes that this highly dynamic spit undergoes are cyclic, which indicates that the spit is undisturbed. Higher sediment supply from the nearby reef area will further increase the area of spit and island in future.