Imperviousness: What Is It? Why Is It a Problem?
What Are We Doing About It?

-Shannon Moore, NPDES Program Manager
Frederick County DPW, Watershed Management Section

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It’s hard to talk about environmental restoration in an urban area without talking about impervious surface.  What is imperviousness?  Why is it a problem?  What are we doing about it?

What is imperviousness?
Pervious land allows water to absorb.  Impervious land does not.  Impervious surfaces are very common in developed areas and include roads, parking lots, building footprints, sidewalks, driveways, and compacted soils.  Forests and meadows, on the other hand, act as sponges and tend to be very pervious.  The problem with impervious surface is that rain water (in an urban area, it’s known as stormwater), instead of absorbing into the ground, will run off the ground surface; collect in volume, temperature and force; and then pose a problem to the surfaces it contacts.  Impervious surface not only decreases groundwater recharge, it can also damage streams that are not used to the volume, temperature and force of water.  The water can also pose flooding risks.  The percent of effective, or untreated, impervious cover is a strong indicator of the health of water bodies including their stability and ability to support aquatic life.  Impervious cover is an indicator for a large suite of urban impacts including increased water force and volume, increased pollution, stream degradation, loss of habitat, etc. 

Why is imperviousness a problem?
At 10% imperviousness in the watershed, the area of land that drains to a stream, stream health generally begins to decline.  At 25%, streams are heavily degraded, and often can no longer support the organisms like fish and bugs that used to live there.  Some streams are even more sensitive; for example, brook trout require very pristine, cold water for survival and generally cannot survive in a watershed with greater than 1% imperviousness.  For more information on effects of impervious area, refer to the Center for Watershed Protection, www.cwp.org.

Figure 1: Impervious Cover Model, Cappiella 2001 (www.cwp.org)

Some older development in Frederick County has no stormwater treatment.  All new development has stormwater treatment.  Our physical and biological monitoring studies show that new development, even with protections to address problems from stormwater, is causing major negative impacts to streams in Bush Creek.  New development is regulated by the Maryland 2000 Stormwater Management Act and will soon also be regulated by a stormwater law passed in 2007.  How this new law will work is still unclear.

What are we doing to address impervious area?
In 2002, the County Government calculated impervious area in Frederick County by associating a standard percent impervious to each urban land use type.  This standard method was developed by the Center for Watershed Protection.

The results were published in the County’s 2002 Annual Report dated March 11, 2003 pages 7-15 through 7-18, for its NPDES MS4 permit.   The number of effective untreated impervious acres in the County is calculated to be 6,725.  The County strives to treat ten percent of this area every five years.  Ten percent of this number is 672 acres; therefore 672 acres must be treated every five years.  This number does not include prevention of imperviousness impacts from new development, which is regulated by Stormwater Management Acts of 2000 and 2007 described above.

The County and its partners in the Monocacy and Catoctin Watershed Alliance are working to address environmental impairments including imperviousness through restoration.  To correct imperviousness, we first try to correct the source of the stormwater by “retrofitting” areas.  We go into areas that were developed under old stormwater regulations that did not protect against the impacts of impervious cover, and we provide better stormwater treatment through environmental restoration techniques known as Best Management Practices.  We also try to correct problems downstream by restoring stream corridors to filter pollutants, slow the water, and correct stream stability issues caused by stormwater. 

Some good news: At the end of 2007, Frederick County and its partners had reduced untreated impervious area by 1,302 acres.  Click here to view a table that lists all of the projects on record to reduce impervious surface in urban areas in Frederick County.  Know of more projects? Let us know!