Synthesis of the distribution of subsidence of the lower Ganges-Brahmaputra Delta, Bangladesh

Deltas, the low-lying land at river mouths, are sensitive to the delicate balance between sea level rise, land subsidence and sedimentation. Bangladesh and the Ganges-Brahmaputra Delta (GBD) have been highlighted as a region at risk from sea-level rise, but reliable estimates of land subsidence have been limited. While early studies suggested high rates of relative sea-level rise, recent papers estimate more modest rates. Our objective is to better quantify the magnitude, spatial variability, and depth variation of sediment compaction and land subsidence in the lower GBD to better evaluate the processes controlling them and the pattern of relative sea level rise in this vulnerable region. We combine subsidence and compaction estimates from hand-drilled tube wells and historic sites (1-5 mm/y), GNSS and river gauges (4-8 mm/y) and RSET-MH and borehole vertical strainmeters (9-10 mm/y) in SW Bangladesh. The differences between the different types of measurements reflect the different timescales, spatial distribution and depth sensitivity of the different observations. Rates are lower for times > 300y providing data on the timescale of compaction. We also observe differences related to the degree to which different devices measure shallow and deep subsidence. Higher values reflect a greater component of subsidence from young shallow deposits from soil compaction and organic matter degradation. Thus, we observe different rates for different environments and physical settings. These differences indicate that in planning adaptation for rising sea level, hard construction with a solid foundation may experience different subsidence rates than open fields or reclaimed land with recent natural or anthropogenic sedimentation.& nbsp;Significance statement: Land subsidence increases the impact of sea level rise. We combine six different types of measurements that examine land subsidence in coastal Bangladesh. The results show that causes of subsidence, including compaction of the sediments varies both spatially and with depth, and that compaction and organic matter degradation from young shallow deposits is a significant contribution to subsidence. This suggests that hard construction with a solid foundation, such as buildings and embankments, may experience a lower subsi-dence rates than open fields or reclaimed land with recent natural or anthropogenic sedimentation.