Imperviousness refers to the inability of a surface to allow water to percolate through. A sponge is pervious, a countertop is impervious, cardboard is somewhere in between. Sandy soils are pervious; asphalt is not. On an impervious surface, water is forced to travel downhill until it finds a place it can sink into soil or enter a wetland.
Applies to: Parking Lots, Streets & Roads