GC045-06
Recent measurements of subsidence in the Ganges-Brahmaputra Delta, Bangladesh

Wednesday, 9 December 2020: 07:15
Virtual
Michael S Steckler1,2, Bar Oryan1,2, Dhiman Ranjan Mondal3, Céline Grall4, Syed Humayun Akhter5, Carol Wilson6, Md. Hasnat Jaman7, Valerie Ballu8 and Eric Lindsey9, (1)Columbia University, Palisades, NY, United States, (2)Columbia University of New York, Lamont-Doherty Earth Observatory, Palisades, NY, United States, (3)MIT Haystack Observatory, Westford, MA, United States, (4)LIENSs, La Rochelle University, La Rochelle, France, (5)University of Dhaka, Department of Geology, Dhaka, Bangladesh, (6)Louisiana State University, Department of Geology and Geophysics, Baton Rouge, LA, United States, (7)Barisal University, Barisal, Bangladesh, (8)Laboratoire Littoral Environnement et Sociétés (LIENSs), La Rochelle, UMR 7266, La Rochelle, France, (9)Earth Observatory of Singapore, Singapore, Singapore
Abstract:
Deltas, the low-lying land at rivers 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 in the GBD suggested high rates of relative sea level rise, recent papers estimate more modest rates.

Our objective is to better quantify the magnitude and spatial variability of subsidence in the GBD, to better evaluate the processes controlling it and the pattern of relative sea level rise in this vulnerable region. With the support of the Bangladesh Water Development Board, we have rehabilitated previously installed GPS and installed new GPS co-located with Rod Surface Elevation Tables (RSET) to better understand the balance of subsidence and sedimentation in the coastal zone in SW Bangladesh, which is less affected by the active tectonic boundaries to the north and the east. The continuous GPSs installed in 2003 and 2012 were mounted on reinforced concrete building roofs. GPS stations in the area yield subsidence rate estimates of 3-7 mm/y. To densify the subsidence data, in early 2020 we resurveyed 48 concrete Survey of Bangladesh geodetic monuments in SW Bangladesh that were installed in 2002. Although only measured at the start and end of the period, the time between the two measurements is ~18 years enabling us to estimate average subsidence over this span.

Preliminary results show that about ½ the sites yielded very high subsidence rates; we suspect that the monuments at these sites are unstable and have undergone localized subsidence from settling or anthropogenic activity. The remaining sites show an increase in subsidence from the NW to the SE, consistent with estimates of average Holocene subsidence (Grall et al., 2018). However, rates from the campaign stations are still much higher than those from continuous GPS sites. It is possible that the continuous building GPS omits very shallow compaction-related subsidence, or that there was initial subsidence of the concrete monuments. While initial results require further investigation, we highlight the importance of methodology for interpreting subsidence rates--deep, shallow, natural, anthropogenic--in vulnerable delta regions.

<< Previous Abstract | Next Abstract >>