The Potomac Highlands Watershed School 

Stream Cleaner Environmental Forum

Why Does Urban Matter?

Frank Rodgers
Urban Forester
Certified Arborist, International Society of Arborists 
Cacapon Institute, Director of Education and Outreach

 

First, what does "urban" mean?

 

The U.S. Census Bureau defines "urban" as any location where 500 or more people live within one square mile.  Urban, for U.S. federal agencies, is identified on the scale of census blocks.  Every decade a census is taken to collect data on the U.S. population and the country is divided into census blocks.  Since we know the population and size of each census block, we can determine population density.  Urban pressure has an "edge effect," with areas that were urban becoming more urban and surrounding areas converting to urban.   The Chesapeake Bay Program's website has a graph that shows us how rapidly population is growing in the Bay's watershed since 1950.

 

 

 How does urban expansion affect the Bay?

 

 It is important to realize that "urban" does not necessarily mean "city".  Suburban development often allows for more than 500 people per square mile so a "suburb" can be "urban" too.  The illustration above of urban areas is from a U.S. Forest Service presentation.  It shows how the Washington - Philadelphia corridor’s population grew between 1990 and 2000.  The blue-gray represents the census blocks that were urban in 1990, the red represents the census blocks that turned urban by the 2000 census - just ten short years.   Notice how urbanization has an “edge effect” with land on the border of existing urban land becoming urban over time.  When areas become urban, other land uses diminish (USFS footnote).   For example, between 1982 and 1997 the Chesapeake Bay watershed lost over 750,000 acres of forestland to development (CBP footnote (1)).  

 

The harmful "point sources" of pollution that come with urbanization such as sewage treatment plants and industry are regulated.  What are the non-point sources pollution of urbanization that impact the Chesapeake Bay?  Two of the major factors associated with non-point source sediment and nutrient pollution from urban areas are impervious surfaces and deforestation.

 

Impervious surfaces

 

Surfaces that water cannot penetrate are called impervious.  Impervious surfaces include natural surfaces like solid rock, and man-made surfaces such as parking lots, buildings, and roads.  The percentage of the total land area that is impervious increases as areas become urbanized.  That increase in turn increases and accelerates storm water run off following precipitation, causing erosion of stream banks and increasing sedimentation.  The rain, naturally, is not the problem; if the land were in its natural condition (grass and forest), much of the water would be absorbed into the soil. 

 

Impervious surfaces associated with urbanization have tremendous impacts on hydrology and water quality.   They increase storm water runoff, cause more frequent flooding, and cause lower flows in dry weather.  Why lower flows?  Impervious surfaces decrease the percentage of rainfall that soaks into the ground, which reduces "recharge" of the ground water that supports stream flow in dry weather.  In other words, because of impervious surfaces, streams in urban areas are "flashy" (they rise and fall rapidly) and go dry quickly. 

 

As water runs off impervious surfaces, it quickly gathers both volume and speed before reaching the stream.  This un-natural "storm surge" destroys the physical structure of natural stream beds by widening and eroding them.  It doesn't take very much impervious area to severely impact stream habitat.  For example, native brook trout begin to disappear with as little as 4% impervious land cover – that’s as little as a 1,700 square foot building (30’x58’) on a one acre lot.  When the percentage of impervious land reaches 25%, streams is considered "non-supporting" and has no value as habitat for fish (SMRC footnote (1)).  Urbanization removes the forest that would otherwise naturally reduce erosion.

 

Deforestation

 

A second major impact of urbanization is reduced tree cover.  Trees are important to the Chesapeake Bay because they reduce erosion by holding soil together with their roots, and because they take up nutrients.  In fact, as Native Guide Al Todd noted that "retaining and expanding forests across the watershed is one of the most cost effective strategies for ensuring long term reductions in nutrient loads to the Bay."  So, the tree loss associated with urbanization is a big problem for the Bay and local streams as well.

 

In addition to the urban areas, where the population density is 500 or more people per square mile, the land between the urban centers is also impacted by urban growth.  The roads, power lines, and infrastructure required to support city and suburban residents cut across the forest between urban areas.  We know that a healthy forest is the best assurance for a healthy watershed but the process of urbanization fragments the forest, making it harder to maintain a healthy forest in and around urbanized areas

 

You can learn what area of your state was classified as urban in 2000 and see data on the forest within your state and municipality as defined by the Urban Natural Resources Institute of the USDA Forest Services.  Click on the link below for information by state.

 Worse than dense urban development is low-density development, often referred to as "sprawl", that fragments and segments the forest.  This makes it harder to maintain the healthy forest that is needed for a healthy watershed.    Fragmentation (i.e.: disconnected land use) harms the forest by essentially "turning the forest inside out".  What used to be the forest's interior becomes an edge as roads and utilities cut through the center, exposing it to harmful influences like invasive species, pests, and damaging winds.  In addition to fragmentation, segmentation, or "parcelization", breaks up the forest by dividing it from large land ownership into smaller ones.  In the Potomac Valley of West Virginia, more than 80% of farmers have a nutrient management plan in place on their farm.  Do you think 80% of suburban land owners have a management plan in place for their woodlands?  Divided ownership of the forest has, so far, led to a decline in the percentage of forestland that is actively managed.  Unmanaged and fragmented, the forest is being lost and that is significant to the Bay because it represents a loss of erosion control and nutrient uptake capacity, a permanent loss in some cases (CBP footnote (2)).

 

What can we do reduce the impact of urban expansion?

 

There are numerous best management practices  (BMPs) that reduce the negative impact of urbanization on the Chesapeake Bay.  All of the Tributary Strategies contain an Urban Strategy section that details the practices that each state considers most important in helping them achieve their goals.  What BMPs address urban non-point source pollution?  BMPs that address the problems caused by impervious surfaces and loss of forest canopy.   Some best management practices that address these issues are:

·         Reduce the amount of impervious surface - Pervious pavement, and higher density development designs (i.e. shared driveways) reduce the need for impervious surfaces and allow more infiltration of water.  (SMRC footnote (2))

·         Manage runoff from impervious surfaces - Water retention ponds, constructed wetlands, rain gardens, and rain barrels help reduce the effect of impervious surfaces by slowing the flow of storm water run off and allowing for more infiltration.  (PG County footnote)

·         Increase forest cover - Municipalities are more and more recognizing the value of the urban forest for increased shade (lower temperatures), improved air quality, and now storm water management.  A healthy tree's canopy can absorb one-quarter inch of rain.  That means the first 1/4 inch of every storm never even reaches the ground under a tree.  Reforesting parks, school grounds, and open areas reduces storm water runoff and allows for water and nutrient uptake by the trees.  (CWP footnote)

·         Improved tree health - Many trees in urban areas are harmed by impervious surfaces restricting their root growth.  Impervious surfaces also reduce the available ground water.  Expanded tree wells, and improved parking lot and tree box designs will foster healthier tree growth which leads to a wider tree canopy and increased nutrient uptake.  (CWP footnote)

 

Footnotes

 

The footnote links below provide opportunities for additional, in depth, investigations of urban-specific issues. 

After completely reading the SCE Forum’s main web page urban stakeholders are encouraged to followed these leads as a team or individually.

 

PG County footnote:  SCE Forum participants are lucky because one of the nations foremost manuals on bioretention facilities, also known as rain gardens, was developed in Prince George’s County, Maryland.  The Rain Garden Guidelines is a good resource http://www.princegeorgescountymd.gov/sites/StormwaterManagement/Resources/BMP/Pages/default.aspx

SMRC footnote (1):  To learn more about the impacts of urbanization on streams and forests visit the Stormwater Manager's Resource Center, a web site created by The Center for Watershed Protection at www.stormwatercenter.net,  See the slide show on "The Impacts of Urbanization".  The show is not a large download but there are 75 slides so plan to spend at least 15 minutes reviewing it.  It is a good introduction to the impacts of impervious surfaces.  (back)

 

SMRC footnote (2):  To learn more about reducing storm water runoff from urbanization visit the Stormwater Manager's Resource Center, a web site created by The Center for Watershed Protection at www.stormwatercenter.net.  See the slide show on "Introduction to Better Site Design".  The show is not a large download but there are 110 slides so plan to spend at least 25 minutes reviewing it.  It provides insight on how to structure a watershed restoration panel (i.e. stakeholder group) and describes various best management practices.  (back)

 

CWP footnote:  Planting trees, even in the city, is one of the best ways to reduce both sediment and nutrient pollution.  You can learn more about urban forestry by visiting this website of the Center for Watershed Protection and and the U.S. Forest Service at http://www.forestsforwatersheds.org .  While the information is for a national audience, some of the source material was drawn from Baltimore City and the surrounding area so it is very relevant to the Chesapeake Bay Program.  (back) 

 

CBP footnote (1):  The State of the Chesapeake Forest, Page 7.   A wealth of information is available in the Chesapeake Bay Programs newly released The State of the Chesapeake Forests at

http://www.na.fs.fed.us/watershed/socf.shtm.   This unique publication of the U.S. Forest Service and the Conservation Fund includes overviews on why the forest is important, the historical and projected impact of human influence, and information on composition of the Chesapeake Bay watershed's forests.  It can be downloaded as a complete report (34MB) or sections can be downloaded individually.  The Introduction, Chapter 2–The Human-Influenced Forest; and Chapter 4–Forests: The Key to Watershed Function are most closely related to the SCE Forum.  (back)  

CBP footnote (2):  The State of the Chesapeake Forest, Page 20.  A wealth of information is available in the Chesapeake Bay Programs newly released The State of the Chesapeake Forests at http://www.na.fs.fed.us/watershed/socf.shtm.  This unique publication of the U.S. Forest Service and the Conservation Fund includes overviews on why the forest is important, the historical and projected impact of human influence, and information on composition of the Chesapeake Bay watershed's forests.  It can be downloaded as a complete report (34MB) or the chapters can be downloaded individually – Chapter 2 – The Human-Influenced Forest and Chapter 4 – Forests: The Key to Watershed Function are most closely related to the SCE Forum.  (back)  

 

U.S. Forest Service footnote:  To see the complete U.S. Forest Service presentation from the Urban Natural Resource Institute visit:  http://www.unri.org/webcasts/archive/may-2006/ and watch the May 2006
Urban Growth, Opportunities and Air Quality by Dr. David Nowak, USDA Forest Service, Syracuse, NY.  This presentation is 7.5MB, requires RealPlayer software, and requires 20 minutes.  It is a recording of an on-line presentation by Dr. Nowak, one of the country's foremost urban forest scientists.  In the first 20 minutes he shares research on urban growth patterns and how that is expected to impact forested lands across the country.  The presentation concludes with an additional 40 minutes of discussion on the value of the forest in terms of air quality.  Dr. Nowak is currently working on models projecting the hydrological value of the forest.  (back)