Submerged Geosynthetic Reef as Shore Protection Measure for Islands

Abstract

The Sagar Island, located north of the Bay of Bengal, intercepts the flow in the Hoogly estuary that comprises a network of several estuarine distributaries and creeks, which is considered to be one of the largest estuarine systems in the world. The Hooghly River experiences a tidal range in the order of about 4 m, due to which the tide-generated currents drive the sediments which are continuously set in motion. The temple, Kapil Muni (21°38′15.35″N, 88°4′30.56″E) is located on the south-western side of Sagar Island, where an annual religious festival and rituals with about a million pilgrims is conducted. The pertinent erosion problem at a rate of about 5 m/year is prevalent at the site has considerably reduced the beach width, thereby, resulting in reduced space for religious as well as recreational activities along the coast. A novel cross-section for the proposed submerged reef using geosynthetic materials is designed considering the different site-specific, environmental, and socio-economic conditions. The submerged reef can effectively be devised to redistribute the current circulation pattern and trap the sediment for beach restoration. The performance of such a structure depends on its geometrical and structural characteristics, the location of the reef (i. e.) the water depth at the toe, distance from the coastline, wave-structure interaction, sediment transport and local morpho dynamics. The aforesaid criteria were optimized using a numerical model which predicted the average residual velocity in the site to be in the order of about 1 m/s. Owing to logistical constraints geosynthetic materials had to be employed. The detailed design of such a system arrived through numerical modelling and field measurements are presented and discussed in this paper.

Keywords

Island coastal erosion / Submerged reef / Tide dominant currents / Sediment movement / Geosynthetic materials