Contribution of campaign GNSS toward parsing subsidence rates by time and depth in coastal Bangladesh - Archive ouverte HAL
Article Dans Une Revue Frontiers in Earth Science Année : 2024

Contribution of campaign GNSS toward parsing subsidence rates by time and depth in coastal Bangladesh

Résumé

Coastal regions are vulnerable to rising seas, increasing storm magnitude, and decimation of ecologically fragile areas. Deltas are particularly sensitive to the balance between sea level rise, land subsidence and sedimentation that determine relative elevation. Bangladesh has been highlighted as being at risk from sea level rise, but reliable estimates of land subsidence are limited. Subsidence rates vary spatially, and with depth and time. However, integrating measurements from different methods can approach a more complete understanding of factors controlling spatially and temporally varying rates. To augment our compilation of rates from stratigraphic wells, historic buildings and structures, vertical strainmeters, RSET-MH, and continuous GNSS sites, we resurveyed geodetic monuments in coastal Bangladesh. We resurveyed 48 sites ~18 years after the monuments were installed. Approximately ½ the sites had high rates that we strongly suspected to result from unstable monuments, confirmed by a later resurvey of 4 sites. Remaining sites show subsidence rates of 13-15mm/y, tapering to 0-2mm/y in the northwestern part of the study area. Sites with rates <2 mm/y overlie thin (≤35m), sandy Holocene deposits. Remaining sites overlie thick (70 to >90m) muddy deposits within the incised valleys of the Ganges, Brahmaputra and Meghna Rivers. Together with earlier measurements, we parse the different rates and mechanisms of subsidence. We estimate 2-3mm/y correspond to deep processes, such as isostasy. 1-5mm/y is estimated to result from compaction of Holocene strata, with thicker and muddier strata undergoing greater compaction. Finally, 5-7 mm/y result from shallow, edaphic effects such as tree roots, burrows, organic matter decomposition, and shallow (≤10m) sediment consolidation on short timescales (10$^0$ -10$^2$ y) in the upper few meters. Subsidence rates in areas of active sedimentation, such as rice fields and mangrove forests, are greater than at buildings and structures without active sedimentation. Subsidence on timescales >300y, which does not include edaphic effects, are up to ~5mm/y. We note shallow subsidence can be offset by active deltaic sedimentation, and do not necessarily indicate elevation loss. Collectively, the integration of these approaches allows us to better quantify the varied contributions to land subsidence from edaphic effects, Holocene sediment compaction, lithology, and time.
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Dates et versions

hal-04684573 , version 1 (02-09-2024)

Identifiants

Citer

Michael Steckler, Md. Hasnat Jaman, Céline Jo Grall, Steven Goodbred, Carol Wilson, et al.. Contribution of campaign GNSS toward parsing subsidence rates by time and depth in coastal Bangladesh. Frontiers in Earth Science, 2024, 12, pp.1354686. ⟨10.3389/feart.2024.1354686⟩. ⟨hal-04684573⟩
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