A runoff footprint is the
total surface runoff that a site produces over the course
of a year. According to the United States
Environmental Protection Agency (EPA), stormwater is "rainwater and
melted snow that runs off streets, lawns, and other sites." Urbanized areas with high concentrations of impervious surfaces like buildings, roads, and
driveways produce large volumes of runoff which can lead to flooding, sewer
overflows, and poor water quality. Since soil in urban areas can be
compacted and have a low infiltration rate, the surface runoff
estimated in a runoff footprint is not just from impervious surfaces, but also
pervious areas including yards. The total runoff is a measure of the site’s
contribution to stormwater issues in an area, especially in urban areas with
sewer overflows. Completing a runoff footprint for a site allows a property
owner to understand what areas on his or her site are producing the most runoff
and what scenarios of stormwater green solutions like rain
barrels and rain gardens are most effective in mitigating this
runoff and its costs to the community.
Significance
The runoff footprint is the
stormwater equivalent to the carbon/energy footprint. When homeowners or
business owners complete an energy audit or carbon footprint, they understand
how they are consuming energy and learn how this consumption can be reduced
through energy efficiency measures. Correspondingly,
the runoff footprint allows someone to calculate their baseline annual runoff
and assess what the impact of ideal stormwater green solutions would be for
their site. Since the passage of the Clean
Water Act in 1972, the EPA has monitored and regulated stormwater issues in
urban areas. Municipalities across the United States are now required to
upgrade sanitary and stormwater systems to meet EPA mandates. The total cost
for these upgrades across the United States exceeds $3000 billion. The stormwater
runoff from every property in an area can contribute to the overall stormwater
issues including overflows and water pollution. Stormwater runoff carries
nonpoint source pollution which is a leading cause of water quality issues.
By completing a runoff footprint,
homeowners and business owners can consider how stormwater green solutions can
reduce runoff on-site. Stormwater green solutions (also called green infrastructure)
use "vegetation, soils, and natural processes to manage water and create
healthier urban environments. At the scale of a city or county, green
infrastructure refers to the patchwork of natural areas that provides habitat,
flood protection, cleaner air, and cleaner water. At the scale of a
neighborhood or site, green infrastructure refers to stormwater management
systems that mimic nature by soaking up and storing water". Stormwater
green solutions include bioswales (directional rain gardens), cisterns, green
roofs, permeable pavement, rain barrels, and rain gardens. According to the
EPA, onsite stormwater green solutions or low-impact developments (LIDs) can
significantly reduce runoff and costly stormwater/sewer infrastructure
upgrades.
Stormwater green solutions can also
reduce energy consumption. Treating and pumping water is an energy-intensive
activity. According to the River Network, the U.S. consumes at least 521
million MWh a year for water-related purposes which is the equivalent to 13% of
the nation’s electricity consumption Potable water must be treated and then pumped
to the consumer. Wastewater is treated before being discharged. In areas with
combined sewer systems or old separate sewer systems with high inflow and infiltration, stormwater is also
treated at the wastewater treatment facilities. By capturing stormwater runoff
onsite in rain barrels and cisterns, the consumption of potable water for
irrigation and its corresponding energy impact can be reduced. The reduction of
runoff from all types of stormwater green solutions reduces the stormwater that
may end up at the wastewater treatment facility in areas with combined sewer
systems or old separate sewers.
Completing a Runoff Footprint
There are number of methods
available to complete a Runoff Footprint. The simplest methods involve using a
runoff coefficient, which according to the State Water Resources Control Board
of California is "a dimensionless coefficient relating the amount of
runoff to the amount of precipitation received. It is a larger value for areas
with low infiltration and high runoff (pavement, steep gradient), and lower for
permeable, well vegetated areas (forest, flat land)."The runoff
coefficients for different surface types on a site can be multiplied with the
area for each surface along with the annual precipitation to generate a rough
runoff footprint. If the runoff coefficient and areas of proposed stormwater
green solutions like rain gardens and bioswales for the site are known, the
reduction in overall runoff from these improvements can be estimated.
More accurate Runoff Footprint
tools exist. By using computer modeling and detailed weather data, complex
runoff footprints can be made easy. The amounts of pollution in the stormwater
runoff can be estimated, and the effects of combinations of stormwater green
solutions can be assessed. The James River Association of central Virginia
provides an online
tool where property owners in the James River watershed can generate a
site-specific runoff pollution report. MyRunoff.org provides an online runoff footprint calculator for property
owners across the United States to estimate their baseline runoff and the
reduction from different scenarios of rain barrels and rain gardens. The EPA
launched the National Stormwater Calculator in July, 2013, which is a desktop
application for Windows allowing users to model the annual impact of a range of
stormwater green solutions.
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