Managing runoff in urban areas offers many challenges for engineers, landscape architects, and planners. As cities grow, the amount of impermeable surfaces--those that do not allow water to infiltrate into the ground--increases. Examples of impervious surfaces are asphalt roads, concrete sidewalks, parking lots, building roofs, and areas of highly compacted soils such as in subdivisions. If not properly managed, the stormwater runoff produced by these impermeable surfaces can have negative effects on nearby surface waters.