From the Homeland Security Digital Library, this article on How Urbanization is Affecting Natural Disaster Risk Assessment. It provides some background and the citation for a new report. Here is the lead paragraph:
The United Nations Population Fund predicts that by the year 2050 86% of the population in developed nations (64% in developing nations) will live in urban areas. This ongoing migration from rural areas to more densely inhabited cities catalyzes urban sprawl, which subsequently makes a greater portion of the population susceptible to the risks of natural disasters.