What are impervious surfaces?
Impervious surfaces - mainly artificial structures–such as pavements (roads, sidewalks, driveways and parking lots) that are covered by impenetrable materials such as asphalt, concrete, brick, and stone–and rooftops. Soils compacted by urban development are also highly impervious. (via Wikipedia)
Did you know that 65% of DC’s natural ground cover has been covered over by hard (impervious) surfaces such as roads, parking lots, and buildings? Impervious surfaces are harmful because they prevent storm water from seeping back into the ground slowly. In a highly impervious area like DC, water flows off of buildings and down streets, eventually making its way to the Anacostia and Potomac rivers, both directly and via the city’s drainage system. Toxins and pollutants such as oil, grease, tailpipe emissions, phosphorus, nitrogren, herbicides, pesticides, and bacteria get picked up along the way and are also deposited in our rivers.
This problem is made even worse because of DC’s antiquated sewage system. Like other older cities, DC does not have separate systems for storm water runoff and for sewage. Instead, both storm water and sewage travel the same pipes, eventually making their way to sewage treatment facilities. Because of this, even a small storm can overwhelm the system, forcing raw sewage and pollutants from runoff to be purged directly into the Anacostia and Potomac Rivers. This issue is known as Combined Sewage Overflow (CSO). Solving the CSO problem is a long term goal of the District government, but many other things can be done in the meantime to help mitigate the problem:
- Green roofs absorb water that would otherwise make its way into storm drains. Every additional green roof in the District helps reduce pollution in our rivers.
- Redirecting downspouts at residential homes away from storm drains and towards rain gardens and rain barrels.
- Using pervious pavers such as brick or cobblestone instead of asphalt and concrete also helps reduce impervious surfaces. Many of DC’s sidewalks have been redone in brick, but the vast majority are still concrete block. Virtually all of DC’s roads are still asphalt.
- Increasing green space gives stormwater more opportunity to seep back into the ground. Any land devoted to green space is by definition, pervious.
While Park(ing) Day shows the recreational and livability benefits of adding green space to our streets, it also serves to demonstrate another way we can help clean up our rivers and improve the health of our local environment.
