Mega-delta regions in Southeast Asia are sensitive to multiple flood risks such as
flood inundation caused by overbank flow from rivers, and storm surges generated by
cyclones (IPCC AR5 WG2, 2014). Moreover, global warming would pose additional
risks in form of frequent fluvial floods deriving from runoff increase (Hirabayashi et
al. 2013) and sea level rise due to ocean thermal expansion and glacier melting
(Ericson et al. 2006). Thus, it is significant to analyze both river and coastal flooding
together with future sea level rise at a large-scale such as mega-deltas. However, most
studies that handle river and coastal flooding have been limited to a relatively small
scale application, and global- or large-scale studies have not dealt with both of them.
This study aims to clarify the current and future risks of fluvial flooding and storm
surges in the Ganges-Brahmaputra-Meghna (GBM) Delta, one of the largest megadelta
regions located in Southeast Asia. We employed a state-of-the-art global river
routing model. Forced by dynamic variation of water level boundary condition at
river mouths, the model could represent storm surge effects on river flooding.
We took one specific cyclone event, Sidr, which made its landfall in November 2007,
Bangladesh along with severe damages on the country. Our results showed that the
surge height of 6 m at river mouth could propagate about 200 km upstream along
with 1 m increase in water levels and its increase > 0.5 m continued for about 10
hours. Also we found that the increase in water levels was greater when upstream
discharge was large. Furthermore, we conducted sensitivity experiments under
different scenarios. While the actual Sidr hit Bangladesh in the dry season, we
investigated Sidr’s impacts in case of the flood season. The increase in water levels
due to cyclonic surges was about 1 m larger in flood seasons than dry seasons.
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