Rapid rise in effective sea-level in southwest Bangladesh: Its causes and contemporary rates

Abstract

Evidence is presented from three estuarine tide gauges located in the Sundarban area of southwest Bangladesh of relative sea level rise substantially in excess of the generally accepted rates from altimetry, as well as previous tide-gauge analyses. It is proposed that the difference arises from the use of Relative Mean Sea Level (RMSL) to characterise the present and future coastal flood hazard, since RMSL can be misleading in estuaries in which tidal range is changing. Three tide gauges, one located in the uninhabited mangrove forested area (Sundarban) of southwest Bangladesh, the others in the densely populated polder zone north of the present Sundarban, show rates of increase in RMSL ranging from 2.8mma−1 to 8.8mma−1. However, these trends in RMSL disguise the fact that high water levels in the polder zone have been increasing at an average rate of 15.9mma−1 and a maximum of 17.2mma−1. In an area experiencing tidal range amplification, RMSL will always underestimate the rise in high water levels; consequently, as an alternative to RMSL, the use of trends in high water maxima or ‘Effective Sea Level Rise’ (ESLR) is adopted as a more strategic parameter to characterise the flooding hazard potential. The rate of increase in ESLR is shown to be due to a combination of deltaic subsidence, including sediment compaction, and eustatic sea level rise, but principally as a result of increased tidal range in estuary channels recently constricted by embankments. These increases in ESLR have been partially offset by decreases in fresh water discharge in those estuaries connected to the Ganges. The recognition of increases of the effective sea level in the Bangladesh Sundarban, which are substantially greater than increases in mean sea level, is of the utmost importance to flood management in this low-lying and densely populated area.