Impact of Land Subsidence on Extreme Sea Levels and Floodings in the Ganges-Brahmaputra-Meghna Delta

The Ganges-Brahmaputra-Meghna Delta (GBMD) is a densely populated region regularly suffering from severe floodings. Several studies explored the impact on the future coastal floodings in the GBMD due to sea level rise, changes in the weather conditions and the tidal cycle. However, effect of land subsidence on future extreme sea levels in the GBMD, although widely recognized, has not yet been assessed quantitatively. Available estimates of the GBMD subsidence vary a lot and reveal a broad variety of subsidence drivers operating at very different space- and time-scales. Rates of the present-day GBMD subsidence induced by sediment loading alone were evaluated recently to range between 1 and 3 mm/yr which is comparable to, or larger than, the rate of the mean global sea level rise over the past century. This study is to analyze the consequences of land subsidence on changes of sea level extremes and floodings along the GBMD coast during the 21th century. We used (1) subsidence patterns derived from combination of satellite altimetry and tide gauge records and (2) those simulated by the Earth sediment loading models. By combining the subsidence patterns with climate-driven absolute sea level rise and changing weather conditions we demonstrate that not only the land subsidence increases significantly amplitude and frequency of sea level extremes but also it makes the eastern and western parts of the GBMD to react rather differently to the ongoing climate changes.