Global River Flood Exposure Assessment Under Climate Change: How Many Asians Are Affected By Flood in the Future?

Abstract:

Physical exposure assessment in this study shows a methodological possibility to be used as a preliminary case study based on a global approach for flood risk assessment consisting of hazard, exposure, and vulnerability. The purpose of this preliminary study is to estimate potential flood inundation areas as a hazard (both present and future condition), and flood exposure change over the Asia region with consideration of climate change impacts.

A flood hazard was characterized by inundation area at the high-resolution of 500 m, location (lowland around rivers), and probability (floods with the 50-year return period). This study introduced a new approach to moderate the global flood hazard and the exposure calculation with significant limitations of current models for continental-scale flood risk assessment by using the flood inundation depth (FID) model based on Manning’s steady, uniform flow resistance formula in extreme case during 25-year simulations based on the global BTOP distributed hydrological model using precipitations from the MRI-AGCM 3.2S with SRES A1B emissions scenarios for present-day (daily data from 1980 to 2004), and end-of-the-21st century (daily data from 2075 to 2099). It effectively simplified the complexity between hydrological and topological variables in a flood risk-prone area with assumption of the effects of natural or artificial levees. Exposure was obtained by combining the hazards at the same resolution to identify affected population by calculating with urbanization ratio and population change ratio of Asian countries from a distributed data of global population (Landscan by the Oak Ridge National Laboratory).

As a result of the physical exposure assessment from present to the end-of-the-21st century, potential hazards area and affected population are projected to increase 4.2 % (approximately 75,900 km²) and 3.4 % (approximately 35.1 million people) respectively, because Asian population increases about 43% in the future. We found that Bangladesh was listed at the top as the most high risk country in the Asian region: the largest vulnerable population of the river floods by the impact of climate change. For the flood risk assessment, it is important to understand the characteristic of correlation between hazard and exposure among the Asian countries.