Salinity in surface water and groundwater is a pervasive issue along coastal Bangladesh,
a low-lying megadelta where around 35 million people live. A large amount of this land
has been reclaimed using a network of low-lying polders. The area is particularly susceptible
to flooding from tropical cyclones. Cyclone induced storm surges coupled with
the low-lying reclaimed land can breach polder embankments and cause extensive flooding,
resulting in excess salinity in soil and surface water. Salinity in drinking water is
known to cause adverse effects on human health. It is, therefore, important to identify
the controls surface water and groundwater salinity in these coastal areas.
A fully coupled surface-subsurface model of a coastal polder by using HydroGeo-
Sphere is developed to investigate the impact of storm surge events on groundwater
salinity. The hydrological parameters were calibrated from the fieldwork at a field site
in the Dacope Upazila, in the southwest coastal region of Bangladesh. The results suggest
that sudden salt fluxes in the pond are likely to build up salinity in the underlying
sediment.
A set of scenarios were considered: a cyclone induced storm surge during both the
monsoon and dry seasons, and both with and without remediation. The results show that
surge events caused a rise in salinity in drinking water and near-surface groundwater.
However, rapid remediation after a surge event could help mitigate the severity of the
impact on drinking water. This provides suggestions for water resources management
planning.
The 2D cross-section model was extended to the 3D model to improve the understanding
of the salinity process. Climate change scenarios were then used to evaluate the effects of episodic cyclone surges on shallow groundwater salinity. This study suggests that more
frequent cyclones would worsen not only salinity in near-surface groundwater but lateral
saltwater intrusion at the shallow or deep aquifers.