Johan Reyns1,2, Mick van der Wegen1,2, Dano Roelvink1,2
1 IHE Delft. the Netherlands; 2Deltares, the Netherlands
* Corresponding author: j.reyns@un-ihe.org
Introduction
Among the world’s largest delta systems, the Ganges-Brahmaputra-Meghna (GBM) delta is an extremely complex channel system that experiences high rates of relative sea level rise (Becker et al, 2020), and ever increasing anthropogenic interventions (Paszkowski et al., 2021). Therefore, in the framework of sustainable delta management, assessment of the impacts of these disturbances on the hydrodynamics and sediment dynamics, at present and in the future under climate change and further human interventions is essential. Here, we present sediment budgets of the Bengal part of the GBM delta, including incoming sediment volumes, transport and distribution in the river system and the estuaries. Moreover, analysis of the multi-decadal morphodynamics, to get a comprehensive understanding on how the system functions as a whole, and to estimate the morphological impact of climate change and anthropogenic works at the delta scale.
Objective and Methods
With the aim of understanding these long-term and large-scale morphodynamics of the Ganges-Brahmaputra-Meghna (GBM) delta, we set up a 2DH large-scale coastal model using Delft3D FM (Kernkamp et al., 2011). This model has been developed to simulate hydrodynamics, sediment transport, and morphological change on the delta-scale, and the validation results for tidal propagation, discharges and bed level change compared favourably with observations over a 25-year period. Subsequently, we carried out scenario simulations over 80 years, until 2100, to investigate the effects of sea level rise, land subsidence, upstream discharge regime and the effects of a lower future upstream sediment concentration, mimicking the effect of sediment entrapment by dam building.
Results
Under current conditions, approximately 1/3th of the fluvial sediment input is exported to the deep sea, and the other 2/3th is deposited in the delta area, mainly through the eastern branches of the Lower Meghna estuary, making the system sediment-supply dominated. Changes of the sediment dynamics in the GBM delta are mainly influenced by the changes in the river flow regime and the sediment transport regime at the upstream river reaches. Under sea level rise (SLR) conditions, we see a general trend of sedimentation in the tidally influenced part of the delta, however an order of magnitude less than the sea level rise itself, with a tendency for net land loss after 2050. This effect is exacerbated by land subsidence, such that a SLR of 0.5 m by 2100 combined with subsidence approximately has the same effect as a SLR of 1m. Climate change is expected to lead to increased Ganges and Brahmaputra discharges. This increase will result in higher sedimentation in the Lower Meghna. Considering anthropogenic effects, we expect a strong reduction in sedimentation in the lower estuarine reaches and the delta front, with strong land loss after 2050.
References
Becker, M., Papa, F., Karpytchev, M., Delebecque, C., Krien, Y., Khan, J. U., ... & Shum, C. K. (2020). Water level changes, subsidence, and sea level rise in the Ganges–Brahmaputra–Meghna delta. Proceedings of the National Academy of Sciences, 117(4), 1867-1876.
Kernkamp, H. W., Van Dam, A., Stelling, G. S., & de Goede, E. D. (2011). Efficient scheme for the shallow water equations on unstructured grids with application to the Continental Shelf. Ocean Dynamics, 61, 1175-1188.
Paszkowski, A., Goodbred Jr, S., Borgomeo, E., Khan, M. S. A., & Hall, J. W. (2021). Geomorphic change in the Ganges–Brahmaputra–Meghna delta. Nature Reviews Earth & Environment, 2(11), 763-780.
