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Ganges-Brahmaputra-Meghna (GBM) delta is characterized by abundance of natural resources and consists of large areas of low flat lands formed by the deposition of sediment from the three continental river systems, the Ganges, the Brahmaputra-Jamuna and the Meghna and their numerous tributaries and distributaries. At present the delta is vulnerable to climatic factors such as eustatic and regional sea level rise. In addition to these, subsidence is also a common phenomenon in deltas caused by various natural processes and accelerated by anthropogenic interventions. So, the combined effect of sea level rise and subsidence referred as Relative Sea Level Rise (RSLR) will accelerate the submergence of the low-lying deltaic region and pose a big threat to the population living in these regions. The only natural resource for offsetting RSLR in this region is the continuous supply, dispersal, and deposition of incoming sediment load from the GBM river systems and the net effect of sediment thickness and RSLR is termed as Effective Sea Level Rise (ESLR) on the delta surface. Due to upstream intervention, supply of sediment is decreasing and will continue to reduce further if all the planned interventions are fully implemented. This reduction will have a significant impact on deltas sustainability. Though GBM is one of the largest sediment laden deltas, very limited studies have been done describing and clarifying the sediment circulation, dynamics and deposition of sediment and considers the role of sediment deposition against the submergence of delta caused by RSLR. This study seeks to summarize data on local level RSLR, sediment flux and its future projections and test the potential of variable incoming sediment influx and its dispersion process to offset RSLR in the GBM delta of Bangladesh.
From this study it has been found that RSLR in the lower deltaic region varies in the range of 3.5 mm/yr to 17 mm/yr with an average estimate of 9 mm/yr. By applying a calibrated validated morphology model the inundation and sedimentation pattern, sediment circulation process and percentage of sediment retention has also been analyzed during different flood conditions at intervened and natural state of the delta. From model simulations it has been found that during average flood conditions about 22% of incoming sediment is retained on the southwestern and south-central part of the delta surface. Sediment retention as well as areal extent of inundation is increased in natural state of the delta. Recent research and available documents summarized that the range of incoming sediment to the GBM system varies between 500-1000 MT/yr with decreasing trend of around 5 MT/year. Within such context, it is important to assess up to which extent, such incoming sediment can offset the RSLR within the delta system. For example, a very rough estimate shows that if the incoming sediment is considered as 1000 MT/yr with decreasing trend of 5 MT/yr, it can counterbalance 5 mm/yr RSLR until the end of the 21st century. But, if the sediment is reduced to 500 MT/yr, then the GBM delta will face the threat of submergence from now. The above fact implicitly explains the importance of the incoming sediment to the delta system which should be one of the important considerations in the large-scale planning (say, Bangladesh Delta Plan 2100) and designing of the development activities in the GBM system. The outputs of this study will be helpful for delta planners, policy makers, coastal scientists and play a significant role in sustainable delta management plan. |
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