North Carolina National Estuarine Research Reserve
Technical Paper Series:
*_ - — _ \
Stormwater Runoff from
Impervious Surfaces
Why is Urban Stormwater a Problem?
When rainwater washes
over farmlands or urban streets, it
collects a wide variety of pollutants
from the surface of the land and
carries them into streams, lakes,
and estuaries. Runoff from
developed areas, construction
sites, rooftops, roads, and
highways is categorized as urban
stormwater runoff. As areas
become more densely
developed, a larger percentage of
land is covered by these paved or
hardened surfaces, and the
severity of water pollution grows
worse.
A typical family house
covers about 5,000 square feet
with impervious surfaces —
including a roof, driveway, and
maybe a deck or patio1. In
addition, an enormous system of
roads, highways, and parking lots
expands to serve new facilities.
Roads and parking lots contribute
a far greater amount of paved
Figure 1. This stenciled storm drain
educates the public about where
stormwater flows. Photo courtesy of
the City of Kinston.
surfaces; 60 to 70 percent of all
impervious surfaces are related to
the automobile2.
Paved areas and rooftops
are impervious surfaces —
impenetrable materials that
prevent infiltration of
water into soil.
Increasing
impervious
coverage is
И
a significant
threat to North
Carolina water quality. Rain that
would have been absorbed by
plants or filtered into groundwater
aquifers instead flows into
stormdrains. Though many people
believe that storm drains carry
water from streets to wastewater
treatment plants, these drains
usually carry runoff directly to
nearby streams, rivers, lakes, or
coastal waters. Our extensive
network of parking lots, rooftops,
and especially roads, creates a
'stormwater superhighway’ that
carries pollutants quickly into the
aquatic environment.
Because there is no
opportunity for plants to absorb the
moisture that falls on pavement, a
much larger volume of stormwater
drains into streams that flow from
urban areas. This large quantity of
water reaches streams too quickly,
flowing across roads and through
pipes that do not offer the
resistance to surface flow that the
natural vegetation of meadows
and forests would.
During dry periods,
streambase flow is substantially
reduced. Because none of the past
rains were able to saturate the
ground, shallow groundwater
storage is diminished. As a result,
less water is available to gradually
percolate into
streams. Drastic
fluctuations
in stream
and river flow
levels increase
the potential for
destructive flooding.
To understand just how
much stormwater reaches streams
from impervious surfaces,
compare an acre of undeveloped
land to a parking lot. The
stormwater runoff from one inch of
rain falling on one acre of meadow
would fill a room with two feet of
standing water. The same amount
of rainfall on an acre of parking lot
would generate enough runoff to
fill three rooms with water from
floor to ceiling2.
The Center for Watershed
Protection has found that stream
degradation occurs at relatively
low levels of imperviousness — only
10 or 20 percent impervious
coverage2. Cumulative effects
from increasing stormwater runoff
include public health threats,
economic losses in commercial
fisheries and tourism, and
damaged aquatic environments.
Rain that
would have been absorbed
by plants or filtered into ground-
water aquifers instead flows
into local waterbodies
