In general, urban areas have an abundance of infrastructure. This heavy amount of infrastructure takes the place of natural surfaces, such as grass and soil. Paved roadways are sealed at the surface, leaving rain water with no choice but to run along the pavement's dirty surface into the nearest drainage system. This method pictured above of laying porous concrete and asphalt is a relatively new technology (past decade or two). While this is method has proven to yield positive results, it is also very costly, which is why it has not yet been vastly implemented into major cities.