Assessment of the impact of sea level rise on salinity intrusion in coastal areas of Bangladesh

Abstract

Salinity intrusion has become a major problem in the south west coastal region of Bangladesh over the past few decades. Together with sea level rise, the reduction of upstream flow of major rivers of Bangladesh has potential negative effect on salinity intrusion. Salinity intrusion will have devastating effects on coastal life, particularly on agriculture, water supply and ecosystem. The main objective of this research was to assess the effect of sea level rise on salinity intrusion in the coastal areas of Bangladesh. A widely used flow simulation model Delft3D has been set up and validated for the coastal part of Ganges, Brahmaputra and Meghna (GBM) delta; the hydrodynamic module Delft3D-FLOW has been used in this study. Using the model, salinity pattern of Bangladesh coastal zone has been assessed for 0.5m, 1m and 1.5m sea level rise (SLR) along with one additional scenario in which 20 percent decrease in Brahmaputra discharge has been considered with 1m SLR.

This model has been calibrated with different versions of bathymetry starting from the General Bathymetric Chart of the Oceans (GEBCO) data. Model performance has been observed by comparing output to observed data in the form of tidal constituents. The model has been run to assess its performance on salinity by comparing model output with observed salinity at different BWDB monitoring stations. The Delft3D model has been able to capture the tidal phenomenon of the complex coastal Bengal Delta with a moderate level of success. Bathymetric data appears to have a significant influence on model performance. Use of improved bathymetric data improved model output appreciably. Comparision of predicted salinity and water level with available data at different stations suggests that the model performance for salinity is comparatively poor than it is for water level.

Model predictions suggest that stations (river reaches) located in the Meghna and the Tentuli river systems would experience increase in salinity in response to increase in sea level. But all other stations located in the small distributories of Ganges, Meghna and Tentulia would undergo a decrease in salinity in response to sea level rise. It appears that under extra pressure of tide from sea (due to sea level rise), the fresh water would face difficulty to flow downstream through Meghna. So, it is likely that a part of the upstream flow would find their way into the distributoties of Ganges, Padma,Meghna and Tentulia. So, fresh water availability in Meghna and Tentulia should decrease, while all other small distributories would get higher fresh water compared to existing condition. Therefore, salinity in Meghna and Tentulia would increase with sea level rise, while salinity in smaller rivers would decrease. This result was explaied by model simulation by comparing seasonal variation of mean water level for different scenarios at different stations, with the understanding that the seasonal variation of water level is representative of fresh water availability at different times of a year. The model simulated mean water levels at different scenarios show that fresh water would decrease in estuary of Meghna and Tentulia and increase in all other small distributories with increase in sea level rise; this explains why salinity would increases in estuary of Meghna and Tentulia and decreases in other places. Finally, spatial pattern of salinity has been prepared based on observed data and model output for different sea-level rise scenarios. These results could be utilized in devising management strategies for mitigation of adverse impacts of sea level rise in coastal areas of Bangladesh.