905R90117
United States Watershed Management Unit
Environmental Protection Water Division, Region V
Agency Chicago, IL
December 1990
xvEPA
Impacts of Changes in
Hydrology Due to
Urbanization
The Problem
hen agricultural or natural areas
become urbanized, bare soil or
vegetation such as grasses,
sedges, shrubs, and trees are
replaced by impervious surfaces
such as asphalt, concrete, and roof
tops. Urbanization results in
increased flooding of local streams and channels,
washoff and transport of pollutants to the stream,
scour of the stream bottom, and streambank
caving or sloughing (Figure 1). Urbanization can
change watershed hydrology and subsequently
stream water quality and aquatic life.
Paving & Buildings
Cause
Increased Runoff,
Sediment, fc
Chemical Transport
Higher
Water Elevations,
Faster Flow
Sediment
Vegetation Scour
Figure 1.—Effects of urbanization on streams.
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The Cause
s an area becomes urbanized, more
roads are built and paved, houses,
buildings, parking lots, and storm
sewers are constructed, and areas
become impervious to rainfall.
Urbanization can result in two
important changes in watershed
hydrology:
1. The amount of runoff is increased because
rainfall no longer percolates into the soil or
is stored in small depressions in the soil, and
2. Runoff reaches the stream much faster and
peak flows are higher because the
resistance encountered as it flows overland
is reduced.
In general, the higher the peak rate of runoff,
the faster the flow velocity and the higher the
water levels in the stream. A stream that floods
only once a year with natural conditions is more
likely to have larger floods or flood more frequently
with urbanized surroundings. The energy
associated with the increased runoff is also likely
to be greater.
The increased energy can cause sediment,
fertilizers, pesticides, metals, and oil and grease to
be transported into and down the stream. With the
increased energy of the flow in the channel,
sediments and vegetation can be scoured from the
bottom. The increased frequency of flooding, the
deepening of the channel and loss of rooted
vegetation can cause decreased bank stability and
eventually bank erosion.
Storm sewers can aggravate the problem since
they collect stormwater from a large area and
deliver it to a single discharge point, often at a rate
faster than that of a natural stream.
Impacts
ncreased flow and pollutant loadings
into and down the stream can have
an adverse effect on aquatic life in
the stream. Flow itself can flush
established aquatic life downstream,
especially if vegetation that provides
shelter from the current is uprooted.
Pollutants such as oil and grease can be washed
off the surface of parking lots and roads and can
smother many aquatic organisms. Surface oil also
inhibits oxygen transfer between the air and the
water. In addition, some of the degradation
products of oil and grease are toxic to aquatic
organisms. Nitrates from fertilizers are easily
leached from the soil by runoff and can promote
aquatic plant growth; if the growth is sufficient,
oxygen depletion by plant respiration at night can
kill aquatic organisms unable to move out of the
affected reach.
Sediment erosion and deposition can have
several impacts on aquatic life. The sediment that
is eroded will eventually deposit in an area of
slower flow and can smother bottom-dwelling
organisms and fish eggs. Deposition can also
cover critical aquatic habitats.
If trees along the banks are lost due to bank
erosion (or clearing during urbanization), shading
is lost and stream temperatures rise. Stream
temperatures can get too warm for some stream
organisms. In addition, water holds less oxygen as
it gets warmer so decreased dissolved oxygen
also may make the stream uninhabitable for
stream organisms.
Sediment may also carry adsorbed phosphorus
from fertilizers and/or pesticides from residential
areas. Phosphorus can promote algae and aquatic
plant growth just as nitrate does and can have a
similar impact. Both phosphorous and nitrate
create problems in lakes because of nutrient
enrichment (i.e., eutrophication). Pesticides are
toxic to some organisms and also can
bioaccumulate in them.
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Stabilization and Remediation
ften bank and channel erosion are
dealt with by using stream
stabilization measures that further
change the ecology of the stream.
Dredging to remove sediment
accumulation physically removes
bottom dwellers and can resuspend
fine soils that eventually settle again
and smother downstream organisms. Associated
with the resuspended material may be
contaminants that were previously not impacting
aquatic life. Resuspension could release these
contaminants to the environment. Replacing a
natural earthen section with a wide concrete lined
channel eliminates the natural habitat and makes it
difficult for re-establishment of organisms. A better
approach is to use rip rap (i.e., rocks) which not
only stabilize the channel but also provides
physical habitat for aquatic organisms. Holes also
can be dredged to create endearing or permanent
pools in the stream to act as a refuge for aquatic
organisms during dry periods.
The installation of sedimentation, retention, or
detention basins in the channel can disrupt
resident aquatic life and replace desirable
organisms with nuisance organisms. However, the
basins can be designed to continuously contain
water and once established, become a natural
resource such as a lake or pool.
Example Case Study
he Northeastern Illinois Planning
Commission studied the
effectiveness of existing and
alternative detention practices in
limiting downstream peak flood
events and in controlling minor flood
events which cause channel erosion.
They found that runoff volumes increased by
25 percent and that peak flows more than doubled
after urbanization for the 2 year frequency flow.
They also found that properly designed detention
facilities reduced peak flow rates to
predevelopment levels and prevented downstream
scour and bank destabilization.
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TERRENE
INSTITUTE
This project was funded by the U.S. Environmental Protection Agency Office of Water Enforcement
and Permits-Water Permits Division and managed by Region V Watershed Management Unit-
Water Division. Prepared by Dynamac Corporation, FTN Associates, and JT&A, Inc. For copies of
this publication, contact The Terrene Institute, 1000 Connecticut Avenue, NW, Suite 300,
Washington, DC 20036, (202) 833-3380.
Printed on Recycled Paper
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