Rapid rise in Effective Sea-Level in southwest Bangladesh: Its causes and contemporary rates

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