The nature and impact of relative sea level rise on the coastal areas of Bangladesh: trend analysis and vulnerability assessment

Sea level rise is one of the most apparent and widespread consequences of climate change. Estimates of global past and future sea level changes have been the focus of many scientific studies throughout the last decades as the number of global coastal population is quite high and among them the low lying deltaic populations are especially exposed to the hazard of sea level rise. Bangladesh is a small deltaic country of South Asia which is about 12 times less than the size of Queensland. Extending from the Himalayan foothills in the north to the Bay of Bengal in the south, most of the country has low-gradient flat topography, suffers from deltaic subsidence with reduced sedimentation due to upstream dams, therefore often cited as one of the most vulnerable countries to sea level rise. However, to date few studies have quantitatively examined the decadal changes of sea level and its response along the coast of Bangladesh. This study aims to fill this knowledge gap by answering three questions: what is the nature and rate of sea level rise around the Bay of Bengal coast of Bangladesh, how the relative sea level rise (RSLR) has physically affected the coastal areas of Bangladesh over the past two decades (1989-2015) and how the relative sea level rise influences the physical vulnerability of the area and how the vulnerability can be measured.

The long-term sea level trend was calculated from the satellite altimetry and tide gauge data. The global Absolute Sea level (ASL) trend from the altimetry data indicates that over the period of 1993-2015, the ASL rose on average about 3.2 mm per year. During the same period, the trend of Bay of Bengal is also steadily rising with a rate about 3.69 mm/year, about 0.5 mm more than the global rate. The Relative Sea Level (RSL) changes are linked with global climatic events as well as local land conditions. Based on the tide gauge data, it is evident that the RSL rate increases toward the northern part of the Bay of Bengal. The rate ranges from almost zero at the southernmost part of the Bay of Bengal and the highest is found along the Bangladesh coast. The spatial variations are caused by local land movements. The highest subsidence rate of about - 4 mm/year is found in the deltaic coast and in contrast, the eastern hilly coast is uplifting at a rate of about +0.5 mm/year.

The seasonal variations of sea level are remarkable along the Bangladesh coast where the difference between the highest and lowest seasonal sea levels is about 1.01 m, and it is the highest in the world. The highest sea level is found in the month of July-August and lowest in February. When the monthly sea level is compared with the sea level pressure, a distinct inverse relationship is clearly visible. The pressure is lowest in the months of June and July and highest in December and January and the corresponding sea levels show the opposite pattern. The small contribution of pressure on sea level changes indicates the influence of other hydrological factors like the huge monsoon freshwater discharge by the Himalayan Ganges, Brahmaputra and Meghna Rivers. The transient sea levels appear as spikes in the residual time series and mainly indicate cyclone induced storm surges.

The coastal change detection is focused on the deltaic western and central coast, and hilly lower eastern coast. Satellite image analysis indicates that from 1989 to 2015, the erosion amount is about 103 square km in the western coast and 283 square km in the central coast. During the same period, the accretion amount is about 44 square km in the western coast and 403 square km in the central coast i.e. erosion predominates in the western coast and accretion in the central coast. When the sediment ages are overlaid on the erosion-accretion map it is evident that the accretion predominates in the younger central part of the delta and its magnitude gradually decreases towards the older western part. The older western part with age of about 4,000 to 1,800 years BP, is predominated by erosion. In the short-length coast of Cox's Bazar area, the spatial distribution of erosion is not continuous; rather it occurs in specific points while the adjacent areas are not affected much. The discontinuous erosion of this area seems to be related with monsoon induced longshore current. The erosion rate in the deltaic coast ranges from 2.72 to 16.73 m/year and in the eastern hilly Cox's Bazar coast it is about 0.11 to 24.23 m/year. Generally, the erosion rate is 2000 times more than the rate of relative sea level rise.

Accretion is mainly observed along the deltaic coast especially in the central part due to Ganges-Brahmaputra-Meghna River sedimentation. However, several land reclaiming processes have influence on the accretion process especially around the central and upper eastern coast. Accretion is also observed far inland in the western part of the delta especially around the tidal rivers. This part of the coastal zone is the oldest, has lost connections with the main rivers, and receives little fluvial discharge and sedimentation. Tidal asymmetry induced sedimentation is mainly responsible for this type of hinterland accretion.

The coastal vulnerability is quantitatively measured by the Coastal Vulnerability Index (CVI) method using four variables viz. rate of relative sea level changes, wave height, mean tidal range and lithology. The calculated CVI values along the coastline of Bangladesh range from 4 to 11. About 3 to 4% of the Bangladesh coast show very low to low vulnerability. About half of the coast (52%) is dominated by moderate vulnerability and about 17% of the coast show high vulnerability. The very high vulnerability areas cover about 24% of the total coastal area. The vulnerability analysis provides useful insights about the spatial variability of risk due to relative sea level rise along the coast of Bangladesh and can be used for prioritizing areas for mitigation and land use zonation.