Modeling of Shoreline Configuration along Rameshwaram Island, East Coast of India

Abstract

The shoreline, a line of contact between the land and ocean, undergoes continuous changes mainly due to the action of waves and longshore current. The geographical formation of Tamil Nadu coast plays a vital role in the movement of littoral drift around the Indian peninsular tip across the east and west coasts of India. Rameshwaram Island is one of the most ecologically sensitive island ecosystems of India and is surrounded by Palk Bay (PB) in the north and the Gulf of Mannar (GoM) in the south. Wave characteristics around Dhanushkodi are studied based on the measured data in the GoM and in the PB. Wave spectra are mainly double-peaked in the GoM and single peaked in the PB. High waves are observed in the GoM compared to PB. Wave heights are more in the GoM during the southwest monsoon period and the waves are from south-southwest. In the PB, high waves are found during the northeast monsoon period. Swells dominate the GoM, except during the southwest monsoon period, whereas wind seas exist in the PB as distant swells cannot reach the bay due to the protection by Indian peninsula and northern extremity of Sri Lanka. Wave-induced Longshore Sediment Transport (LST) plays an important role in the dynamics of the Dhanushkodi sand spit located southeast of Rameshwaram. The LST along the Dhanushkodi coast is studied based on data collected simultaneously in GoM and PB using directional wave rider buoys. The numerical model REF/DIF1 was used to calculate the nearshore waves and the LST rate was estimated using three different formulae. The model validation was done based on the measured nearshore waves using InterOcean S4DW. Examination of shoreline changes indicates that accretion was more compared to erosion. A comparison between the 1953 toposheet and the 2010 satellite image shows accretion at the eastern tip of land, leading the sand spit along Dhanushkodi to grow by 1.03 km during this period. Numerical model LITPACK was also used for simulating noncohesive sediment transport and the LITLINE module was used to study the shoreline evolution over 5 years. Low net annual LST along PB (~ 0.01 x 106 m3), compared to the GoM region (0.3 x106 m3), was due to the weak waves. Accretion in the region led to the growth of the Dhanushkodi sand spit by 65 m during the period 2010-2015.