Geomorphological Evolution and Vulnerability of Low- Lying Coasts in Bangladesh: The Case Study of Sandwip Island

Bangladesh is ranked as the “5th most at-risk country” in the world in terms of disasters with a world risk index of 20.22% in the World Risk Report 2012. Coast and island flooding induced by extreme storms and cyclones born in the Northern Indian Ocean passing through the funnel shaped shallow northern Bay of Bengal are the most dangerous natural hazards in the area. Additionally, remarkable geomorphological change of the lower GBM active delta and relative Sea Level Rise (SLR) have drawn special attention to the environmental scientists and respective managers for the alarming more intense and more frequent upcoming climate change vulnerabilities.The Lower Meghna River Estuary (LMRE) is an extremely dynamic estuarine system with dramatic geomorphological changes of the offshore islands. The erosion and accretion rates are remarkably high and the islands are reducing their original sizes. As populations flock to the coast and offshore islands at threat from erosions, storm inundation and sea level rise, intensive studies must be carried out on the vulnerability of these coastal regions. Sandwip Island, located at the confluence of the Lower Meghna River Estuary, shaped and characterized by both tidal actions of the Bay of Bengal and river streams of the Meghna, was chosen as the only international case study site outside Europe in the framework of the Risc-KIT FP7 European project (http://www.risckit.eu/). The project aims to develop tools and methods to reduce risk and increase resilience of coastal areas. The toolkit will be implemented for each case study. The present works aims to describe the state of the art of knowledge on the island covering geomorphology, sediment characteristics, hydrodynamics and socioeconomics. Community perceptions and indigenous coping strategies aspects of the island are included in a coastal vulnerability perspective to be useful in further research, development of effective preparedness, management and actions plan. To reach this goal, an attempt was taken to find out the answers to the following questions: 1) what is the status of geomorphological changes? 2) What are the sediment characteristics of the erosion-accretion prone shorelines? 3) What is the status of the sea level rise relative to the island 4) What is the total vulnerability/hazard ranking by local perceptions? 5) How much has the land use changed due to erosion and associated hazards ? and finally 6) What strategic changes should be taken? The research questions were answered using topographic surveys, lab based sediment analysis, tide gauge data analysis, erosion-accretion mapping and development of high resolution DEM and TIN using Remote Sensing and GIS-based tools and techniques and questionnaire survey results interpreted with national disaster management plans. The study found that the island is more physically prone to coastal erosion, coupling with storm surges and sea level rise making it more vulnerable in future. The erosion rate considering both shoreline and area changes is much more intense exceeding accretion in the silty clay sedimentary parts and the sea level rise trend is alarming in comparison with other nearby coastal areas in Bangladesh.Tidal data analysis for Sandwip shows semi-diurnal components higher in amplitude than Chittagong. Diurnal amplitudes are comparable and phases are generally higher for Sandwip with a 47 minute delay than Chittagong. The DEM demands more intensive bathymetric information that should be used for more accurate hydrometeorological models for the low elevated island, where as the island is almost flat with the average topography is only 5 meter above MSL and the north-western part is slightly higher than the south-western part as observed in the TIN justified by Topographic survey. The physical assets have been declining stressing on the socioeconomy of the community as found by the social survey as well as land use satellite mapping. So, a relatively high earthen embankment encircling the island, a crossdam connecting north side of the island to mainland is immediately needed to protect the community from erosion as well as relative sea level rise and storm surge inundation hazards. A site specific hydrometeorological warning should be developed and announcement dissemination should be maintained in the local language. Development of roads and transportation system and the health training should be enhanced to make people more aware and adaptive with the associate disasters.