Flood Risk Across US Much Higher Than Government Estimates Show.New data shows that approximately 14.6 million properties are at risk from what experts call a '100-year flood.' .It has a one percent chance of striking any given year.The federal government’s flood maps estimate a much lower number of 8.7 million properties being at risk.The calculation takes into account sea-level rise, rainfall and creeks not included in federal mapping.The data was compiled by the First Street Foundation and