Conference Paper Representation of the impact of Cyclone Sidr on fluvial flooding in the GangesBrahmaputra-Meghna Delta

池内, 寛明  ,  Ikeuchi, Hiroaki  ,  山崎, 大  ,  Yamazaki, Dai  ,  平林, 由希子  ,  Hirabayashi, Yukiko  ,  鼎, 信次郎  ,  Kanae, Shinjiro

Mega-delta regions in Southeast Asia are sensitive to multiple flood risks such asflood inundation caused by overbank flow from rivers, and storm surges generated bycyclones (IPCC AR5 WG2, 2014). Moreover, global warming would pose additionalrisks in form of frequent fluvial floods deriving from runoff increase (Hirabayashi etal. 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 floodingtogether with future sea level rise at a large-scale such as mega-deltas. However, moststudies that handle river and coastal flooding have been limited to a relatively smallscale 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 stormsurges in the Ganges-Brahmaputra-Meghna (GBM) Delta, one of the largest megadeltaregions located in Southeast Asia. We employed a state-of-the-art global riverrouting model. Forced by dynamic variation of water level boundary condition atriver 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 thesurge height of 6 m at river mouth could propagate about 200 km upstream alongwith 1 m increase in water levels and its increase > 0.5 m continued for about 10hours. Also we found that the increase in water levels was greater when upstreamdischarge was large. Furthermore, we conducted sensitivity experiments underdifferent scenarios. While the actual Sidr hit Bangladesh in the dry season, weinvestigated Sidr’s impacts in case of the flood season. The increase in water levelsdue to cyclonic surges was about 1 m larger in flood seasons than dry seasons.

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