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Impervious
Surface � an Environmental
Impervious
surfaces are one result of community growth that can be directly measured.
It is an important indicator -- an understandable measure of our
surroundings. It is used to show changes in environmental conditions and
to gauge the health of our natural resources. This article will show how
impervious surfaces can alter natural features. Urban
uses change the local water balance. As is illustrated in Figure 1,
removal of natural land cover disrupts the water balance. Imperviousness
changes the routing and timing for water to reach a lake or stream. Trees,
shrubs and grasses are natural land covers. They shelter the soil surface
from rain, wind and surface erosion, intercept precipitation, and filter
rainwater. When rain reaches the ground, leaf litter and shallow roots are
there to absorb it, as if they were a sponge, and recycle rainwater.
Some rainwater strikes a surface, wets it, and some of the water
eventually evaporates into the atmosphere.
This absorption and recycling is called evapo-transpiration.
Natural land covers encourage the lateral movement of shallow
infiltrated precipitation into wetlands, lakes and streams.
This movement of water is called �interflow.�
Natural covers also enhance the deeper water movement, or �base
flow.� On the average Wisconsin receives
Land use consists of many
different land covers. Some are impervious to water while others are not.
For example, impervious roof, driveway and sidewalk land covers along with
pervious lawns, flower gardens, trees, and shrubs, define residential land
use. Compacted lawn and
agricultural field soils are a middle ground between hard, impervious
surface runoffs and spongy natural land covers. Where a parking lot may be
95% impervious, a residential lawn may be 40% imperious and natural land
covers are nearly zero (Anacostia, 1991). Several
studies have estimated imperviousness for different urban land use
categories. The table below summarizes two such studies. The percentages
estimated in the studies are reflective of the general urban use category,
but only define a percentage range. Each
community should determine values that truly reflect their specific
situation. Table
1: Range of Reasonable Impervious Cover Percentages in Urban Environments
1
Bannerman, 2001. The
unintended results of urban development attributed to imperviousness are:
Urban development requires the
removal of some natural land cover to create a reliable hard surface and
to facilitate access to and from our homes, work, schools, commercial, and
recreational opportunities. It is essential that a system be developed
which quickly and efficiently drains water away from these human activity
areas. Flooding or ponding,
whether temporary or longer termed, are on acceptable. In urbanizing communities,
impervious surfaces have replaced roots, leaf litter, and forest canopies
that were once available to absorb and recycle precipitation. Where
precipitation was able to percolate into the ground and infiltrate to the
watertable or contribute to stream and lake base flows, now most
precipitation runs off directly into our wetlands, lakes, and streams.
Natural processes are no longer available to absorb and recycle rainwater
and snow melt. Under natural conditions, overland runoff is a relatively
minor component of the water balance. Urbanization suddenly makes runoff a
significant and probably the most visible component of the hydrologic
cycle. The absolute change resulting from a single parking lot may not
seem significant, but the cumulative impact of several parking lots, roof
tops, roads, divided highways and the like are significant. More water is
able to reach a stream or lake more quickly. Existing stream channels will
likely not be able to effectively handle the added stormwaters. Waters
flow over channel banks. What
had been small rain showers, which often never reached the streams as
overland flow, now result in bank full floods or worse. Our neighbors to the west in the
Minneapolis area have been witness to these problems. Since 1993 and the
Mississippi River flooding, communities in the metropolitan region have
experienced three 100-year floods. This is either a very unfortunate
statistical aberration, or the imperviousness of the watersheds has indeed
altered the frequency and magnitude of flood events. Costs amounted to
several million dollars to cities which involved removing residences and
installing storm water retention facilities. Community Actions
Sources. Anacostia
Restoration Team, 1991. Watershed
restoration handbook, Department of Environmental Programs,
Metropolitan Washington Council of Governments. Arnold,
Chester L. and C. James Gibbons, 1996.
Impervious surface coverages, Journal of the American Planning
Association, Vol. 62(2), pages 243 to 258. Bannerman,
2001, Ultra-urban connected impervious cover percentage.
Unpublished data from an email communication. Cappiella
and Brown, 2001. Land use and impervious cover in the Chesapeake Bay
region, Urban Lake Management, pages 835-840. NEMO,
2001. Addressing
imperviousness in plans, site design, and land use regulations, Nonpoint
Education for Municipal Officials, University of Connecticut, College
of Agriculture and Natural Resources.
http://www.caur.uconn.edu/ces/nemo. SEMCOG,
2000. Putting Southeast
Michigan�s water quality plan into action, tools for local governments.
Southeast Michigan Council of Governments, Detroit,
Michigan. Steuer,
J.J. and R.J. Hunt, 2001. Use
of a watershed modeling approach to assess hydrologic effects of
urbanization, North Fork Pheasant Branch Basin near Middleton, Wisconsin, U.S.G.S.
Water Resources Investigation Report 01-4113.
Middleton, Wisconsin. John
Haack, John Panuska, Alicia Acken, Kevin Struck, Sue O�Halloran and
Brook Wallen reviewed this article for form and content.
Any errors, mistakes and omissions remain the responsibility of the
author.
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