Rapidly expanding urban areas both increase the local flood regimes and the exposed riverine vulnerability. The 2007 United Nations report on the world population trend indicates that by 2050 it is expected that 80% of the population will live in urban areas. The present ratio is of 50%. Mainly mid size cities will be concerned by the rapid population increase when mega-cities, over millions of inhabitants, will stagnate. This presentation presents a study that aims to evaluate the impact of a rapidly expanding urban area on the flood risk. This research was conducted on the catchment of the Yzeron River in western Lyon, a city of 1.6 million of inhabitants expected to reach 2 millions by 2030.