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<br />This research has
<br />surprisingly similar c
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<br />lncpewious cover is a pvwe~ fnl i~rdicator of fulru•e stream quality
<br />^
<br />he emcreing field of urban watershed protcc-
<br />~~ tion often lacks a unifying theme to guide
<br />the efforts of its many participants-planners,
<br />engineers, landscape architects, scientists, and local
<br />officials. The lack of a common theme has often made
<br />it difficult to achieve a consistent result at either the
<br />individual development site or cumulatively, at the
<br />watershed scale.
<br />In this article a unifying theme is proposed based on
<br />a physically defined unit-imperviousness. Impervi-
<br />ousness here is defined as the sum of roads, parking
<br />lots, sidewalks, rooftops, and other impermeable sur-
<br />~~ faces of the urban landscape. This vari-
<br />yielded a able can be easily measured at all scales
<br />onclusion- of development, as the percentage of
<br />area that is not "green".
<br />s ream egra a ton occurs at
<br />relatively low le
<br />imperviousness
<br />~•
<br />vels of Imperviousness is a very useful
<br />10-20%). indicator with which to measure the
<br />~t impacts of land development on
<br />aquatic systems. Reviewed here is the
<br />scientific evidence that relates imperviousness to spe-
<br />cificchanges in the hydrology, habitat structure, water
<br />quality and biodiversity of aquatic systems. This re-
<br />search, conducted in many geographic areas, concen-
<br />trating on many different variables, and ..employing
<br />widely different methods, has yielded a surprisingly
<br />similarconclusion-stream degradation occurs at rela-
<br />tively low levels of imperviousness (10-20%). Most
<br />importantly, imperviousness is one of the few vari-
<br />ables that can be explicitly quantified, managed and
<br />controlled at each stage of land development. The
<br />remainder of this paper examines in detail the relation-
<br />ship between imperviousness and stream quality.
<br />The Components of Imperviousness
<br />Imperviousness represents the imprint of land de-
<br />velopment on the landscape. It is composed of two
<br />primary components-the rooftops under which we
<br />live, work and shop, and the trcuzsport system (roads,
<br />driveways, and parking lou) that we use to get from
<br />one roof to another. As it happens, the transport com-
<br />ponent now often exceeds the rooftop component in
<br />terms of total impervious area created. For example,
<br />transport-related imperviousness comprised 63% to
<br />70% of total impervious. cover at the site in l 1 residen-
<br />tial, multifamily and commercial areas where it had
<br />actually been measured" This phenomenon is ob-
<br />served most often in suburban areas and reflects the
<br />recent ascendancy of the automobile in both our cul-
<br />ture and iandscape. Tne sharp increases in per capita
<br />vehicle ownership, trips taken, and miles travelled
<br />have forced local planners to increase the relative size
<br />of the transport component over the last two decades.
<br />Traditional zoning has strongly emphasized and
<br />regulated the first component (rooftops) and lamely
<br />neglected the transport component. While tl~e rooftop
<br />component is largely fixed in density zoning, tl~e
<br />transport component is not. As an example, nearly all
<br />zoning codes set the maximum density for an area,
<br />based on dwelling units (rooftops). Thus, in a liven
<br />area, no more than one single family home can be
<br />located on each acre of land, and so forth.
<br />Thus a wide range in impervious cover is often seen
<br />for the same zoning category. For example, impervi-
<br />ous area associated with medium density single family
<br />homes can range from 25% to nearly 60%, depending
<br />on the layout of streets and parking. This suggests that
<br />significant opportunities exist to reduce the share of
<br />imperviousness from the transport component.
<br />Imperviousness and runoff
<br />The relationship between imperviousness and run-
<br />offmay be widely understood, but it is not always fully
<br />appreciated. Figure I illustrates the increase in the site
<br />runoff coefficient as a result of site imperviousness,
<br />developed From over~0 runoff monitoring sites across
<br />the nation. The runoff coefficient ranges from zero to
<br />one and expresses the fraction of rainfall volume that
<br />is actually converted into storm runoff volume. As can
<br />be seen, the runoff coefficient closely tracks percent
<br />impervious cover, except at low levels where soils and
<br />slope factors also become important. In practical terms
<br />this means that the total runoff volume for aone-acre
<br />parking lot (Rv = 0.95) is about 16 times that produced
<br />by an undeveloped meadow (Rv = 0.06).
<br />To put this in more understandable terns, consider
<br />the runoff from aone-inch rainstorm (see Table 1). The
<br />total runoff from aone-acre meadow would fill a
<br />standard size office to a depth of about two feet (218
<br />cubic feet). By way of comparison, if that same acre
<br />was completely paved, aone-inch rainstorm would
<br />completely fiil your office, as_ well as the ttivo next to it.
<br />The peak discharge, velocity and time of concen tration
<br />of stormwater runoff also exhibit a striking increase
<br />after a meadow is replaced by a parking lot (Tahle 1).
<br />Because infiltration is reduced in impervious areas,
<br />one would expect groundwater recharge to be propor-
<br />tionately reduced. This, in turn, should translate into
<br />lowerdry weatherstream flows. Actual data, however,
<br />that demonstrate this effect is rare. Indeed, Evett et ai."
<br />could not find any statistical difference in low stream
<br />100 ~rY l=f@.-~i~Xl°ra :a "a ~ i':® e~_ lre ~ •l j~c v i ~;.1'
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