In a move that business praised and environmentalists decried, President Obama told the Environmental Protection Agency on Sept. 2 to withdraw an ozone rule issued in July. He said the rule would harm the economy. The rule required utilities in 27 states to trim emissions or buy pollution allowances starting in 2012. Then, in 2014, sulfur dioxide and nitrogen oxide emissions would have needed to be cut by 73% and 54%, respectively, from 2005 levels.
Arch dams, classically curved monoliths situated in narrow valleys, are not as common as other types of dams, such as embankment dams and gravity dams. ENR takes a look at some of the biggest arch dams in the world, both completed and under construction.