Simulation of Freshwater Transport and controls on salt intrusion in the Ganges-Brahmaputra-Meghna (GBM) delta

Yujuan Sun1, Lucy Bricheno1, Kevin Horsburgh2 and Marta Payo Payo1, (1)National Oceanography Centre, Liverpool, United Kingdom, (2)National Oceanography Center, Liverpool, United Kingdom
The Ganges-Brahmaputra-Meghna (GBM) delta is one of the world’s largest and most complex riverine systems, covering most of Bangladesh and part of West Bengal, India. The delta supports a high population density through fisheries, shrimp farming, and subsistence rice production. Two-thirds of Bangladesh is less than 5 m above sea level, exposing these coastal areas to risk from sea-level rise. The GBM delta is in a delicate hydrodynamic balance, experiencing tidal flooding in the dry season and riverine flooding in the wet season. Freshwater flow into this delta brings a large supply of sediments from upstream, causing blockages in some narrow river channels and further modifying the bathymetry and river flow paths. The modification of the delta morphology, combines with reduced river flows from upstream to alter the delivery of freshwater to the GBM delta. The consequences of these physical changes combine with the growing saltwater shrimp farming industry to exacerbate severe saltwater intrusion and degrade the ecosystem.

We will simulate the complex circulation in the GBM delta, to better understand the freshwater transports, and controls on salt-intrusion over multiple time scales. By applying an unstructured-grid modelling approach with the finest resolution of 100 m, we can investigate volume and salt flux transport at a range of scales from non-tidal rivers to the open ocean. The volume and salt flux transports across several river channels are examined in different schemes of bathymetry to analyse channel connectivity. The model is then applied to wet and dry seasons, leading to a better understanding of the competition between controlling processes of riverine flow and saltwater intrusion in Bangladesh.