Modeling urban development and its exposure to river flood risk in Southeast Asia

Countries in Southeast Asia have been developing quickly from a predominantly rural to predominantly urban society, leading to a rapid increase in urban land. This increase in urban land has mainly occurred in river deltas and floodplains, exposing humans and human assets to flood hazard. Here we present an assessment of current and future flood risk in five countries of mainland Southeast Asia, using a new modeling approach that accounts for differences in urban land systems. To that effect we mapped urban land on a rural-urban gradient and projected urban development until the year 2040 in two contrasting scenarios. The urban expansion scenario mainly projects the development of new urban areas, while the intensification emphasizes an increase in the number of inhabitants in already existing urban areas. Subsequently, we assessed the expected annual damage due to flood risk, using country specific exposure values for different land-system classes along the rural-urban gradient, based on typical construction materials. Results indicate that expected annual flood damage will in-crease in all countries and in both scenarios, ranging from +8% in Thailand to +211% in Laos. We showed that preferable development pathways are context dependent. In Cambodia and Laos, the increase in flood risk was largest for the intensification scenario, while for Myanmar, Thailand and Vietnam, the increase in flood risk was largest in the urban expansion scenario.