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1. What is impervious cover?

Impervious cover refers to any man made surfaces, like asphalt, concrete, and rooftops, that water cannot penetrate. With these surfaces rain and snow that would otherwise soak into the ground turns into stormwater runoff when it comes into contact with impervious surfaces.

The Falls Church stormwater utility fee further defines impervious cover as:

• Asphalt driveways / walkways
• Compacted soil with no vegetation
• Compacted stone driveways
• Concrete driveways / walkways / stairs / patios
• Decks over concrete slabs (decks over uncompacted stone are considered pervious)
• Garages
• Pools and concrete pool decks
• Porous pavers (paver systems are open to a credit discount)
• Rooftops of homes
• Sheds / storage containers

2. How does impervious cover impact volume of stormwater runoff?

Impervious cover increases the quantity of runoff by keeping stormwater from soaking into the ground naturally. Increased volumes and velocities of runoff then erode stream channels and reduce the health of existing streams. Because water runs more rapidly off of an impervious area, flooding becomes both more common and more intense downstream. Meanwhile, because less water is soaking into the ground, water tables can drop and streams and wells fed by groundwater can begin to dry up.

3. Why is stormwater runoff polluted?

When rain hits the ground it does so with enough energy to dislodge tiny particles from barren soil and impervious surfaces. The stormwater runoff then carries pollutants found on the ground, such as sediments, nutrients, pathogens, pesticides, hydrocarbons, and metals into our catch basins, streams, and ultimately the Chesapeake Bay. Polluted stormwater runoff is widely recognized by scientists as the greatest threat to water quality.

4. What can I do to reduce my impact on stormwater runoff?

The simplest way to reduce stormwater runoff is to eliminate unneeded impervious cover, like concrete slabs from an old shed or unused brick walkways. Another easy way to change the amount of runoff from a home is to redirect downspouts away from driveways and to pervious areas. This gives runoff an opportunity to naturally infiltrate into the ground instead of draining into the storm sewer system.

5. Where can I find more information about stormwater?

Visit the City of Falls Church stormwater fund section to learn more about stormwater runoff, how it impacts water quality in the Chesapeake Bay and water quantity (flooding) in Falls Church, and how to mitigate stormwater runoff on your site and get credit toward the stormwater utility fee.