Hard Truth About Impervious Surfaces

Here’s a little science observation for you.

Look out the window the next time it rains. Spend a few minutes looking at the streets and the parking lots. Watch all the water flowing into the storm drain.

Now look at the grass. You’ll have a tough time spotting any water flowing out of there. Now you can understand the challenges facing scientists as they study the health of tidal creeks in North Carolina. The amount of stormwater runoff has a tremendous effect on water quality. 

As you saw, stormwater runoff consists of all the rain that flows down streets, sidewalks and parking lots, through yards and playgrounds, and into the storm sewer system. That water, which carries with it petroleum products, fertilizers, animal waste along with who knows what else, never gets treated in a wastewater treatment plant. It flows right into a river, creek, or some other body of water. 

You should also consider all of the rain that doesn’t soak into farm fields and forests and gradually flows downhill into a body of water as stormwater runoff as well. That water also carries fertilizer and animal waste into rivers and streams as well.

The tough thing about calculating stormwater runoff is that the more impervious surfaces there are around a body of water, the more runoff there will be.

So what’s an impervious surface?

In general, an impervious surface is something that’s man-made, such as pavement (think roads, sidewalks, driveways and parking lots) that is covered by hard, impenetrable materials such as asphalt, concrete or brick. You can also add rooftops to the list. And here’s something you might not think of — soil that has been packed down by development is also pretty impervious.

So far, studies sponsored by the National Oceanic and Atmospheric Administration (NOAA) show that if more than 10-30% of the watershed (the area of land that drains into a body of water) is covered by an impervious surface, the health of the tidal creek is impacted. That explains why natural buffers around tidal creeks and rain gardens are so important. For that matter, this also explains why it is so important to preserve forests and green space.

- Frank Graff

Frank Graff is a producer/reporter with UNC-TV, focusing on North Carolina Science Now, a weekly science series that airs Wednesdays, beginning in August 2013, as part of North Carolina Now on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog!

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