Recent trends in inland water level change in coastal Bangladesh–implications of sea level rise in low-lying deltas

Accurately constraining local variation in tidal water levels is the vital first step in managing the risk of coastal hazards associated with sea level rise (SLR). Here we show sub-regional patterns in delta tidal dynamics and coastal SLR along the northern Bay of Bengal based on long-term (1990 to 2017) in-situ measurement data. Seasonal trend decomposition combined with robust linear modelling are applied to determine rates of water level change, with an average rise in coastal extreme sea levels of 6.9 ± 2.1 mm yr⁻¹ translating to increases of 5.1 ± 4.2 mm yr⁻¹ in tidal water levels within the delta. Mean increases of 18.3 ± 4.1 mm yr⁻¹ and 18.2 ± 6.8 mm yr⁻¹ at high and low-tide respectively in tidally dominated rivers of southwest Bangladesh contrast with the fluvially active central and eastern sections of the delta where falling trends of −6.5 ± 2.3 mm yr⁻¹ (high-tide) and −10.2 ± 3.1 mm yr⁻¹ (low-tide) are recorded. Rapid changes in river level at individual sites can occur over periods of ca. 3 to 10 years, most likely resulting from short-term hydro-morphological changes. An acceleration of change in high-tides is indicated by a rise from the long term (1950 to 2018) rate of 4.4 ± 1.5 mm yr−1 to 5.9 ± 3.2 mm yr⁻¹ between 1990 and 2017, leading to increased risk for the local population. At local and regional scales, the inland impact of SLR within the delta is observed to be strongly modulated, necessitating more detailed analysis of driving mechanisms at the site specific level. Our results can be used to aid development of adaptation strategies in Bangladesh and as a call for increased monitoring of hydro-morphological parameters within the Bengal delta and other large deltas in Asia and worldwide.