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Impervious surface encompasses land areas covered by
roofs, roads, parking lots and other materials which
keep rainfall and snow from penetrating the ground.
Watersheds with a high percentage of impervious
surface area are susceptible to increased stormwater
runoff and decreased water quality in nearby surface
waters. Impervious surfaces channel water into smaller
areas, which in turn helps to increase flow
velocities, sometimes resulting in localized flooding
and, frequently, in accelerated erosion of
streambanks. To estimate impervious surface on a
watershed basis, Maryland Office of Planning (OP) 1994
land use data were allocated to Maryland Department of
Environment-designated eight-digit watersheds.
Impervious surface was calculated from a combination
of "urban" land classifications (low, medium and high
density residential; commercial; industrial;
institutional, extractive and other urban; large lot
residential; and other) and lands classified as
barren. Each land cover class was assigned a
percent impervious factor based on the U.S. Soil
Conservation Service TR-55 Manual; the applicable
percentage was multiplied by the acres in each class
to derive total impervious area. The percent
impervious surface indicator was developed by
normalizing the impervious surface acres by the total
land acres in the watershed.
The Maryland Biological Stream Survey has related the
percent impervious surface in a watershed to the
health of aquatic resources. For areas with less than
4% impervious cover, streams generally rate "Fair" to
Good for both fish and instream invertebrates;
beyond about 12% impervious surface, streams generally
rate as poor to fair for both.
Indicator Use
The watersheds with the highest values for this
indicator offer the greatest potential for
implementation of best management practices whose
objective is to filter runoff and moderate runoff peak
velocities. Local planners, for example, might look
in these areas for opportunities to install or
retrofit stormwater management facilities or to take
steps to redesign roads and parking lots so they
become less effective in channeling rainfall.
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