The urgent evacuation of 10,000 people from communities below two failing dams in central Michigan last month prevented the loss of life, but the collapsed dams expelled billions of gallons of water from two large lakes, sending them hurtling downstream in a powerful rush of destruction. Water ripped buildings off their foundations, smashed and twisted roads and bridges, damaged or destroyed an estimated 2,500 properties and triggered fears of contamination as it swept by a chemical plant and hazardous waste sites and submerged downtown Midland — a city of 40,000 people — under 9 feet of water.