Philadelphia has launched an ambitious 20-year, $1.6-billion stormwater control plan that its creators hope will transform the city’s combined-sewer system and its urban landscape with new shades of green, including bio-mimetic systems. Photo: Courtesy of Philadelphia Water Dept. Natural rooftop plantings at Philadelphia’s Central Library have reduced stormwater runoff entering the city’s combined-sewer system. Photo: Courtesy of Philadelphia Water Dept. America’s sixth-largest city aims to convert a third of its impervious asphalt surface—about 4,000 acres—into absorptive green spaces through the use of constructed wetlands, floodplain restoration and the use of rain barrels, porous concrete and green roofs. The goal is
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.