xvEPA
United States
Environmental Protection
Agency
Office of Water
(4503F)
Washington, DC 20460
841-R-97-009
December 1997
Urbanization and Streams:
Studies of Hydrologic Impacts
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URBANIZATION AND STREAMS: STUDIES OF HYDROLQGIC IMPACTS
INTRODUCTION
Hydrologic impacts due to urbanization are reported to cause water quality problems such as,'
sedimentation, increased temperatures, habitat changes, and the loss of fish populations.
Although there is widespread recognition that these problems are caused-by increased runoff
volumes and velocities from urbanization and associated increases in watershed
imperviousness, much of the reported information has been anecdotal. The summaries and
analyses of reports and case studies in this report are intended to go beyond the anecdotal and
provide documentation of problems and sources, as well as a foundation for further
investigation. , .
Planners, engineers, water quality specialists, and government officials should find this study a
useful introduction to understanding the potential hydrologic impacts of urbanization on
streams. ., • .. '
t • • '•'>''• . '
This report was derived from a literature search to find and document physical impacts and
indications of water quality problems. United States Geological Survey reports; American
, Water Resources Association publications; federal, state, and local agency reports; journal
articles; conference proceedings; and consultations with experts provided the documentation
and case study examples cited in this report.
L •' '
FINDINGS AND ANALYSIS x
Examination of published
literature revealed a large
amount of anecdotal
information that identifies
hydrologic impacts on streams
caused by increased impervious
area (e.g., roads, driveways,
parking lots, and rooftops) in
urban developments. Figure 1
graphically depicts the impacts
of urbanization on stream flow
documented in the literature,
and Table 1 summarizes the
relationship between these
changes in flow and other
impacts hi receiving streams.
These impacts include increased
frequency of flooding and peak
flow volumes, increased
sediment loadings, loss
,4-Short,
peak discharge
Increased total runoff volume
^Baseline peak discharge
Gradual recession
Figure 1. Impacts of urbanization on stream flow (Schueler, 1987).
Urbanization and Streams: Studies of Hydrologic Impacts
Page. 1
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Table 1. Impacts from Increases in Impervious Surfaces.
Increased
Imperviousness leads
to:
Increased volume
Increased peak flow
Increased peak flow
duration
Increased stream
temperature
Decreased base flow
Changes in sediment
loadings
'....:: ; /: :: Resulting Jmpacfe; ; '.'.'•,.
Flooding
*
#
*
*
Habitat loss
(e.g., inadequate
substrate, loss of '
riparian areas, etc.)
#
*
#
*
*
*
Erosion
'#
*
*
#
Channel
widening
*
*
# '
*
Streambed
alteration
*
#
*
*
of aquatic/riparian habitat, changes in stream physical characteristics (channel width and
depth), decreased base flow, and increased stream temperature.1
Nine case studies that contained quantitative documentation linking urbanization to. hydrologic
impacts on streams were identified. They are summarized in Table 2 and are described in the
appendix in more detail. It should be noted that some of the impacts identified hi Table 2 are
inferred from the presence of other indicators. For example, the Valley Stream, Pines Brook,
and Bellmore and Massapequa creeks case studies from Long Island, New York, revealed a
significant decrease hi stream base flow resulting from increased urbanization within the
contributing watersheds. Although habitat loss, average stream temperatures, and low
dissolved oxygen concentrations were not reported hi the study, these impacts typically occur
as a result of decreased base flow and can be assumed (Horner et al., 1994; Klein, no date).
1 For more information on impacts on streams due to urbanization, refer to the following:
Fundamentals of Urban Runoff Management (Horner et al., 1994), Site Planning for Urban Stream
Protection (Schueler, 1995), Effects of Urbanization on Aquatic Resources (Klein, no date),
Environmental Indicators to Assess Control Programs and Practices (Claytor and Brown, 1996),
Clearing and Grading Strategies for Urban Watersheds (Corish, 1995), and several articles in Watershed
Protection Techniques (Center for Watershed Protection).
Page 2
EPA 841-R-97-009
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Table 2. Results of Case Study Reviews
Case Study
Location
Documented impacts
Inferred Impacts
Pheasant Branch Basin
Middleton, Wl '• Stream incision
• Increase in bankfull events
• Sedimentation
• Flooding
« Habitat loss
• Erosion
• Channel-widening
•.Streambed alteration
Holmes Run Watershed
Fairfax, VA • Frequent flooding
• Severe stream bank erosion
• Sedimentation
• Flooding
• Habitat loss
• Erosion
« Channel widening
Streambed alteration
Peachtree Creek
Pipers Creek
Valley Stream, Pines
Brook, Bellmore Creek, and
Massapequa Creek
East Meadow Brook
Kelsey Creek
Several Creeks
Patuxent River System
Atlanta, GA
Seattle, WA
Nassau
County, NY
Nassau
County, NY
Bejlvue, WA
Dekalb County,
.GA
Maryland ,
• Increased bankfull events
• Decreased base flow
• Increased peak flows
• Loss of fish populations '
» Aesthetic degradation '
• Decreased base flow
) • •
» Increased peak flows
» .Degradation of designated uses
• Decreased base flow
• Loss of fish populations
• Stream enlargement
» Stream incision
• Increased sediment transport
/ ' "
• Increased instream sediment load
»' Changes in morphology, of urban
channels
• Flooding ' ,
• Habitat loss
» Erosion '
• Channel widening .
• Streambed alteration
• Flooding
• Habitat loss
• Erosion
• Channel widening
• Streambed alteration
» Habitat loss
• Flooding
• Habitat loss
• Erosion •
• Channel widening
» Streambed alteration
• Habitat loss
• Channel widening . . . . .
• • Habitat loss
• Erosion
• Channel widening
» Streambed alteration
• Habitat loss
• Erosion
Urbanization and Streams: Studies ofHydrologic Impacts
Page 3
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CONCLUSIONS
There are documented case studies that conclusively link urbanization and increased watershed
imperviousness to hydrologic impacts on streams. Existing reports and case studies provide
strong evidence that urbanization negatively affects streams and results in water quality
problems such as loss of habitat, increased temperatures, sedimentation, and loss of fish
populations.
However, relatively few case studies have assembled detailed quantitative information to
document these phenomena. This is due, in part, to (1) the heavy reliance on engineered
approaches to runoff management that can transfer hydrologic impacts (e.g., habitat loss,
flooding, channel widening, and erosion) to downstream areas through the construction of
paved channels, stormwater pipes, and bank stabilization (e.g., riprap, cutbacks, plantings,
bulkheads) and (2) the difficulty and high costs associated with long-term watershed
monitoring. Furthermore, the installation of drainage structures, such as pipes and concrete
channels, is the final step in removing urban streams from the landscape. Classically, many of
these activities have resulted in urban streams being "written off' as virtually nonexistent;
therefore, the resulting impacts on water quality and habitats are being ignored.
It is anticipated that in the future the literature will be supplemented with additional studies that
document the relationship between urbanization, impervious surfaces, and problems in streams.
Future investigations might include Federal Emergency Management Agency (FEMA)
floodplain management activities. FEMA trend analysis of widespread changes in 100-year
floodplain delineations or increased claims for financial assistance in specific watersheds might
add increased evidence of hydrologic impacts due to urbanization. In the meantime, it is hoped
that existing information proves sufficient to allow planners, engineers, and local officials to
recognize potential hydrologic impacts due to urbanization and to take steps to prevent water
quality problems while allowing for sensible development.
Page 4 EPA 841-R-97-009
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LITERATURE CITED , .
Claytor, Richard A., and Whitney E. Brown. 1996. Environmental Indicators to Assess Stormwater ,
Control Programs and Practices. Prepared by the Center for Watershed Protection, Silver, :
Spring, Maryland, in cooperation with the U.S. Environmental Protection Agency.
Corish, Kathy. 1995. Environmental Land Planning (ELP) Series: Clearing and Grading Strategies for
Urban Watersheds. Metropolitan Washington Council of Governments, Washington, DC;
Homer, Richard R., Joseph J. Skupien, Eric H. Livingston, and H. Earl Shaver. 1994. Fundamentals
of Urban Runoff Management: Technical and Institutional Issues. Prepared by the Terrene
•Institute, Washington, DC, in cooperation with the U.S. Environmental Protection Agency.
Klein, Richard D.. (No date). Effects of Urbanization Upon Aquatic Resources. Report by the -
Tidewater Administration, Maryland Department of Natural Resources.
Schueler, Thomas. 1995. Environmental Land Planning Series: Site Planning for Urban Stream
Protection. Prepared by the Metropolitan Washington Council of Governments and the
Center for Watershed Protection, Silver Spring, Maryland.
Schueler, Thomas. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing
Urban BMPs. Metropolitan Washington Council of Governments, Washington, DC.
U.S. Environmental Protection Agency. 1995. Economic Benefits of Runoff Controls. U.S
Environmental Protection Agency, Office of Wetlands, Oceans and Watersheds, Washington,
DC. i
RELATED LITERATURE
Barbour, Michael T., Jerome Diamond, and Christopher Yoder. 1996. Effects of Watershed
Development and Management on Aquatic Ecosystems. SETAC Press, Pensacola, Florida.
Driver, Nancy E., and Gary D. Tasker. 1990. Techniques for Estimation of Storm-Runoff Loads,
Volumes, and Selected Constituent Concentrations in Urban Watersheds in the United States.
U. S. Geological Survey Water-Supply Paper 2363. U.S. Geological Survey, Washington,
' ' - DC- • .• - . ' •:••-''•• .-'.•.-
James, Williams. 1995. Modern Methods forModeling the Management oj'Stormwater Impacts.
"Computational Hydraulics International, Guelph, Ontario.
Jones, R. Christian, and Donald P. Kelso. 1994. Bioassessment ofNqnpdint Source Impacts in Three
Northern Virginia Watersheds. George Mason University, Fairfax, Virginia.
Leopold, Luna B. 1994. A Field Example: Watts Branch. In A View of the River, pp. 148-167.
Harvard University Press, Cambridge, Massachusetts.
Urbanization and Streams: Studies of Hydrologic Impacts Page-5
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Mead, Bstyn R. (Date unknown). Addressing Hydrologic Modification and Habitat Loss: Tools to
Assess the Impacts of Hydrologic Modification on Aquatic Communities. U.S. Fish and
Wildlife Service, Division of Habitat Conservation, Arlington, Virginia.
Nevvbury, Robert. 1995. Rivers and the Art of Stream Restoration. In Natural and Anthropogenic
Influences in Fluvial Geomorphology, pp. 137-149. Newbury Hydraulics Ltd., Gibsons,
British Columbia, Canada.
Sauer, V.B., W.O. Thomas, Jr., V.A. Strieker, and K.V. Wilson. 1983. Flood Characteristics of
Urban Watersheds in the United States. U.S. Geological Survey Water-Supply Paper 2207.
Prepared by the U.S. Geological Survey in cooperation with U.S. Department of
Transportation, Federal Highway Administration.
Schueler, Thomas R. 1994. The Stream Protection Approach: Guidance for Developing Effective
Local Nonpoint Source Control Programs in the Great Lakes Region. Prepared by the Center
for Watershed Protection, Silver Spring, Maryland, in cooperation with the U.S.
Environmental Protection Agency.
Spinello, Anthony G., and Dale L. Simmons. 1992. Base Flow of 10 South-Shore Streams, Long
Island, Ne\v York, 1976-85, and the Effects of Urbanization on Base Flow and Flow Duration.
USGS Water Resources Investigations, Report 85-4068. Prepared by U.S. Geological Survey
in cooperation with Nassau County Department of Public Works and Suffolk County
Department of Health Services.
Yoder, Christopher, and Edward Rankin. 1995. Biological criteria program development and
implementation in Ohio. In Biological Assessment and Criteria: Tools for Risked-based
Planning and Decision Making, ed. W.S. Davis and T. Simon. CRC Press/Lewis Publishers,
Ann Arbor, Michigan.
PERSONAL CONTACTS
Finley, Stuart. Lake Barcroft Watershed Improvement District, Fairfax County, Virginia.
Henry, William. Northern Virginia Soil and Water Conservation District, Fairfax, Virginia.
Muncy, Joy. U.S. Army Corps of Engineer, Fort Belvoir, Virginia.
Powell, Rocky. Baltimore County Department of Environmental Protection, Baltimore, Maryland.
Shreeve, Robert. Maryland Department of Highway Administration, Annapolis, Maryland.
Turlinger, Margaret. Waterways Experiment Station, U.S. Army Corps of Engineers, Vicksburg,
Mississippi.
City of Fairfax, Department of Environmental Resources, Fairfax, Virginia.
Page 6 EPA 841-R-97-009
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APPENDIX: CASE STUDIES
The following case studies demonstrate the impacts that increased flow due to urbanization
can have on urban streams. Like urban streams, each case study is unique. The case
studies look at different attributes such as habitat, stream stability, .and sedimentation. In .
some cases; where field data did not quantify the impacts, models were applied to estimate
impacts. When available, cost information related to,the impacts and restoration is
included. These summaries reflect the level of detail available in the published reports.
PHEASANT BRANCH BASIN
MIDDLETON, WISCONSIN
Background . .
The USGS completed a 5-year data collection and modeling study on Pheasant Branch, a
stream that drains 24.5 square miles (mi2) of rolling hills, agricultural land, and rapidly
urbanizing areas around Middleton, Wisconsin. The stream is a tributary to Lake Mendota,
which requires maintenance dredging because of sedimentation. The area changed in
\ population by 44 percent (8,246 to-11,851) from 1970 to 1980 and is projected to have a
population of 18,000 by 2000. Problems of stream channel erosion and suspended
sediment have developed in Pheasant Branch because of land use changes in the drainage
basin. Urbanization in this area has consisted of residential development as well as
industrial and commercial development. The purposes of the study were to demonstrate
that urbanization does cause adverse impacts on streams within the watershed and to
provide information to city planners and engineers for use when they are evaluating the
consequences of development within the drainage basin.
Impacts on Development
During the 1970s, Pheasant Branch exhibited observed impacts from increased urbanization
(change in morphology, increased erosion and sediment loadings, lowering of mean
streambed elevation by almost 2 feet, t and widening of mean channel width by 35 percent).
A rainfall runoff model was calibrated and applied to the stream to simulate 68 years of
. summer flood hydrographs for three conditions—current land use, projected urban
development, and complete urban development. Analysis of simulated flood flows indicates
that projected urban development would double the mean annual flood peaks in portions of
the streams. Complete development of the basin would increase the mean annual flood
peaks by a factor of 2.4 without mitigation. , ••
As the watershed .became urbanized, significant sedimentation occurred, as well as
widening and incision of the stream channel. Table A-l shows the percent increase of the
2-year flood, barikfull width, and barikfull depth from present conditions to urbanized
conditions.
Urbanization and Streams: Studies of Hydrologic Impacts Appendix-1
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Table A-1. Percent Increase of 2-year Flood, Bankfuil Width, and Bankfull
Depth from Present Conditions to Urbanized Conditions (based
on modeling results).
Site
Sitel
Site 2
SiteS
Projected Urbanization
2-yr Flood Width Depth
(Percent Increase from
Preurbanization)
99 40 ' 30
324 110 80
32 10 10
Complete Urbanization
2-yr Flood Width Depth
(Percent Increase from
Preurbanization)
140 60 40
361 110 80
224 80 ' 60
*Most heavily urbanizing subwatershed.
Source: William R. Krug and Gerald L. Goddard. Effects of Urbanization on Stream flow,
Sediment Loads, and Channel Morphology in Pheasant Branch Basin near Middleton,
W7scons//7. USGS Water Resources Investigations, Report 85-4068. July 1986. U.S.
Geological Survey in cooperation with the University of Wisconsin Extension—Geological and
Natural History Survey and the City of Middleton.
HOLMES RUN WATERSHED
FAIRFAX COUNTY/FALLS CHURCH, VIRGINIA
Background
The Holmes Run drainage basin is a 14.5-mi2 watershed with a population of approximately
60,000 (1990). The city of Falls Church composes 14 percent of the watershed; the
remaining 86 percent is in Fairfax County. Overall, the watershed is an older suburban
region, with the highest densities occurring in Falls Church. In 1995, the'Lake Barcroft
Watershed Improvement District received Clean Water Act section 319 funds to develop
and implement a retrofit program for mitigating the impacts of 30 years of development in
the watershed.
!
Impacts of Development
The flow-related impacts of unmitigated development within the Holmes Run watershed
include the following:
• Frequent flooding from snowmelt and storm runoff. Floods occur several times a year
and can be intense enough to endanger the lives of people trapped in cars. Because of
urbanization, this flooding has affected private property.
• Severe stream bank erosion within subwatersheds, which has resulted in severe
undercutting of stream banks and deposition of sediment downstream (Figure A-1).
Appendix-2
EPA 841-R-97-009
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Figure A-1. Severe Stream Bank Erosion in Holmes Run Watershed.
• Significant sediment problems in Lake Barcroft. Dredging of two internal silt basins
. must be done four times a year at an average cost of $150,000 for each dredging. Since
1961, approximately 376,000,cubic yards (yd3) of sediment has been dredged in the
watershed at a total cost of more than $2 million.
• Debris from intense storm scour is washed into Holmes Run and its tributaries, blocking
flow and impairing water quality. ..
Source: Lake Barcroft, Watershed Improvement District. Holmes Run Watershed Best
Management Practice Implementation Project. Final report. Lake Barcroft Watershed
Improvement District, Fairfax County, Virginia. 1997.
PEACHTREE CREEK
ATLANTA, GEORGIA
Background
The Peachtree Creek watershed near Atlanta, Georgia, is an ideal location to monitor the
response of stream flow to urbanization. A major portion of the watershed, covering 86.8
mi2, lies upstream of a U.S. Geological Survey (USGS) gauging station where stream
Urbanization and Streams: Studies ofHydrologic Impacts
Appendix-3
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runoff data have been collected continuously since 1958. This corresponds roughly to the .
period of rapidly increasing urbanization in the watershed.
Prior to urbanization in the watershed, which began slowly in the early part of the century,
the area was covered primarily by wooded land. Early increases in imperviousness were
primarily due to conversions of this woodland to buildings or pavement. By the middle of
the century the watershed had a substantial amount of impervious cover—about 28 percent
in 1958. More rapid urbanization began at about this time, and the rate of conversion to
impervious cover increased. By 1968 imperviousness had increased to 35 percent.
Population of the area increased rapidly as well—from 215,450 in 1960 to 473,600 in 1985.
Researchers decided to use the stream flow data that had been collected over 30 years in the
watershed to determine .if correlations between increases in imperviousness and stream flow
volume could be found. Stream flow data, annual runoff data, and information on the state
of imperviousness in the watershed were collected and analyzed together. The results
demonstrated just how closely a change from southern woodland to southern city is related
to impacts on streams and rivers.
Results of the Analysis
Annual runoff and rainfall data for the watershed from 1958 to 1988 indicate the
urbanization and impacts on streams are closely correlated. During the latter half of those
30 years (1973 to 1988), the analysis indicated that urbanization had resulted in stream
runoff volumes even greater than those which had been expected based on the relationship
derived from the data. During dry years in the same period, in contrast, the data pointed to
a decrease in stream flow during low flow periods as a result of urbanization, to levels
below normal. This result was not surprising and is an expected result of urbanization,
which typically decreases the quantity of water that seeps into the ground to replenish
ground water supplies. It is the level of ground water, not rainwater runoff, that is
primarily responsible for keeping streams running during periods of low rainfall. The
ground water reserves in the Peachtree Creek watershed had probably dwindled over the
years due to progressive urbanization. Increased evaporation during these dry years could
also have contributed to the low flows.
Perhaps the most important finding from the data analysis in terms of the effects of
urbanization on stream flow was that peak runoff flows for a given intensity of storm
increased in the Peachtree Creek watershed as the watershed became more urbanized. That
means that the Peachtree Creek .today has to carry far more water in—or beyond—its banks
during a storm event than it did before urbanization of the surrounding watershed.
Source: Bruce Ferguson and Philip Suckling. Changing Rainfall-Runoff Relationships in the
Urbanizing Peachtree Creek Watershed, Atlanta, Georgia. Water Resources Bulletin
(AWRA). ApriM 990.
Appendix-4 , EPA841-R-97-009
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PIPERS CREEK .
SEATTLE, WASHINGTON ,
Background
The Pipers Greek Watershed is located in the Seattle, Washington, area and covers
approximately 3 mi2. The upper reaches of the watershed are 100 percent
developed—primarily with shopping centers, residences, and commercial
development—with a high percent of impervious surfaces.
The lower reaches of the watershed are surrounded by steep slopes in a park. The creek
discharges to Puget Sound with an average 1-year peak flow of 330 cubic feet per second
(ft3Is) and a 100-year event flow of 1,000 ft3/s. Although no predevelopment rates have
been quantified, it is estimated that they did not exceed 20 ftVs for the 1-year event. Under
natural conditions, it is believed that Pipers Creek was dominated by pools and drops and
provided excellent habitat for several aquatic species, including trout and salmon.
Impacts of Development
-f - ' ' ' - ,
In the early 1970s the city of Seattle built a storm drain pipe system to serve the heavily
developed .portion of the watershed. The Pipers Creek watershed averages 10 housing units
per acre. This led to peak storm flows in excess of 300 ft3/s. Because of the development
of the watershed and increased flow, boulders originally installed to control runoff impacts
'downstream became traps for sediment and debris. During low flows, the stream lacked
concentrated flows to move sediment through the system. Because of large stormwater
volumes over many years, the stream channel was straightened. Due to these conditions,
fish populations were restricted by limited quality habitat, limited food, and difficult
passage up and down the stream. The stream was also aesthetically unappealing.
Actions Taken
The city has taken actions to restore the stream. This program is based on a relatively low
cost maintenance approach ($35,000 for 1 mile of stream) that stabilizes the channel and
rebuilds fish habitat. Some of the actions taken include protecting the eroding portions of
the stream channel, installing "step-downs" to create pools and riffles for habitat,-clearing
fish passages, through the boulders, and deepening the channel to allow a fairly steady
, consolidated stream flow to remove fine sediments.
Sources: Richard Gustav, Douglas Sovem, and Percy Washington. Maintaining Fish Habitat
in Urban Streams. Water Environment and Technology. June 1994.
Douglas Severn, Richard Gustav, and Percy Washington. Effects of Urban Growth on Stream
Habitat. In Conference Proceedings - Effects of Watershed Development and Management
on Aquatic Ecosystems. 1996.
Urbanization and Streams: Studies of Hydrologic Impacts" , Appendix-5
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VALLEY STREAM, PINES BROOK, AND BELLMORE AND. MASSAPEQUA CREEKS .
LONG ISLAND, NEW YORK
Background
The USGS conducted a study of the impacts of urbanization on base flow in four urban
streams on the southwest shore of Long Island, New York. The purpose of the study was
to quantify the changes in base flow hi the streams resulting from urbanization. Because of
the permeable glacial soils (sand and gravel) in the area, ground water seepage made up
approximately 95 percent of the area's stream flow. The balance was from runoff'from
storm events.
Impacts of Urbanization
The urbanization that began hi the 1940s and continued through the 1970s led to
construction of stormwater conveyance systems and sanitary sewers. This resulted in more
water being discharged to tide and not seeping into the ground to recharge the aquifer, thus
reducing base flow to the streams. Table A-2 shows the impact of urbanization on base
flow by comparing two streams hi each of three areas—an urbanized sewered area, an
urbanized unsewered area, and a rural unsewered area. As shown hi the table, urbanization
since the 1940s has resulted in significant loss of ground water flow to streams hi the area.
Table A-2. Average Percent Base Flow of Selected Streams on Long Island
by Area.
Years
1948-1953
1953-1964
1964-1970
Urbanized Sewered Area
(% Flow from Base Flow)
Stream 1 Stream 2
(no data) 86
63 69
17 22
Urbanized Unsewered Area
(% Flow from Base Flow)
Stream 1 Stream 2
84 94
89 89
83 84
Rural Unsewered Area
(% Flow from Base Flow)
Stream 1 Stream
2
96 95
95 97
96 97
Source: Dale. Simmons and Richard Reynolds. Effects of Urbanization on Base Flow of
Selected South-Shore Streams, Long Island, New York. U.S. Geological Survey. AWRA
Water Resources Bulletin. October 1982.
EAST MEADOW BROOK
NASSAU COUNTY, LONG ISLAND, NEW YORK
Background
A study was conducted on the southward-flowing East Meadow Brook in Nassau County,
Long Island, New York, to determine the impact of increased urbanization on the direct
Appendix-6
EPA 841-R-97-009
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runoff to the stream. The purposes of the study were to relate urban development to the ; .
increases in the volume of annual runoff to the stream, to compare hydrograph features of
preurbanization and posturbanization, and to compare, rainfall-runoff relationships for
periods before and after urban development. The East Meadow Brook drainage area covers
approximately 31 mi2. The area experienced intense urbanization from 1944 to 1962. This
development included construction of storm sewers that discharge to the stream. The area
was developed when.the main focus of stormwater management was to move the water. out
of an area and prevent flooding: • . .'
Impacts of Urbanization
The .study showed that an increase in the volume of direct runoff closely corresponded to an
increase in the area having storm sewers that drained directly, to East Meadow Brook. The
development area increased by 530 percent from 1943 to 1962. During this same period,
annual direct runoff to East Meadow Brook increased by 270 percent. . One-hour
hydrographs of storms in the watershed showed that the average peak discharge increased
from 3 13 ftVsm 1939 to approximately 776
Source: G.E. Seaburn. Effects of Urbanization on Direct Runoff to East Meadow Brook,
' Nassau County, Long island, New York. U.S. Geological Survey Professional Paper 627-B.
U.S. Government Printing Office, Washington, DC. 1969.
KELSEY CREEK x
BELLEVUE, WASHINGTON
/* " '' '
Background
Kelsey Creek is a heavily urbanized watershed in Bellevue, Washington. Over the years,
degradation of its designated uses has occurred.
Impacts of Urbanization
Although degraded water quality has been a factor in the declining quality of Kelsey Greek,
aquatic organism impacts are mostly associated with increased peak flow and the resultant
sediment carrying capacity and channel instability in the stream. Kelsey Creek has extreme
hydrologic responses to storms. .Flooding has substantially increased due to urbanization;
the peak annual discharge has almost doubled in 30 years, and the flooding frequency also
has increased. This has resulted in the greater sediment transport and channel instability.
The stream has also exhibited lower base flows (when compared to urbanized streams)
between storms. This factor might have affected the stream's ability to flush toxic spills or
other dry-weather pollutants from the creek systems. All of. these factors might have
resulted in a change hi the dominant fish species from coho salmon to the less pollutant-
Urbanization and Streams: Studies of Hydrologic Impacts Appendix-7
-------
sensitive cutthroat trout. This lower "flushing" during dry periods might also have reduced
the movement of smaller fish and other aquatic organisms through the system.
Source: Robert Pitt. Biological Effects of Urban Runoff Discharges. Presented at the
Engineering Foundation conference Urban Runoff and Receiving Systems: An
Interdisciplinary Analysis of Impact, Monitoring, and Management, Mt. Crested Butte,
Colorado. August 1991.
ATLANTA METROPOLITAN AREA
DEKALB COUNTY, GEORGIA
Background
Observations and studies of several creeks in and around the Atlanta, Georgia, area have
demonstrated the impact of increased stormwater flow on urban stream morphology,
primarily incision and enlargement of stream channels. Despite city and county stormwater
regulations requiring that peak discharges following development be controlled to
predevelop'ment rates for the 2-, 5-, 10-, 25-, 50-, and 100-year storms, degradation is
occurring.
Impacts of Development
The following are two documented examples of changes in stream morphology in the
Atlanta area:
• A first-order stream that was stable before the construction of a 12-acre apartment
complex now exhibits channel enlargement where it receives outfall from a detention
pond constructed to control impacts from the development. The detention facility, sized
to accommodate peak runoff rates calculated by the rational method, was designed with
a maximum storage capacity of 40,000 ft3. However, abundant vegetative growth in the
pond has reduced its capacity, resulting in more water being discharged to the stream.
Efforts to reduce the channel degradation have been ineffective.
• When the area was used for agricultural production at the turn of the century, several
small gullies formed on hillsides. After the abandonment of agriculture more than 40
years ago, the gullies stabilized. They often contain 30- to 40-year-old trees, which
were able to grow because the gullies received only intermittent flows during times of
severe rain events. As urbanization increased, these areas became conveyance systems
for stormwater from impervious surfaces. Active downcutting is taking place in these
areas, resulting in undercut trees, headcuts, and the export of large amounts of
sediment.
Source: Nelson R. Nunnaly. Channel Incision in the Atlanta Metropolitan Area. In
Management of Landscapes Disturbed by Channel Incision, edited by S. Wang, E.
Langendoen, and F. Shields, Jr. The University of Mississippi. 1997.
Appendix-8 - EPA841-R-97-009
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PATUXENT RIVER SYSTEM • ... .
MARYLAND
Background
The Patuxent River system was studied by the Maryland Department of Natural Resources
in the 1970s because it had both rural and urbanizing areas.
Impacts of Urbanization . ,
.The study concluded .that subwatersheds within suburbanizing areas are markedly different
in physical characteristics and behavior from,rural watersheds. Urbanizing basins yield
approximately 986.6 tons of sediment/mi2/yr, compared to 63.7 tons produced by the same
area in a rural watershed. Such extensive sediment loads can choke streams, and "sand
bars" can occur as far downstream as 3.5 miles. The size and shape of urban channels
changed at rates at least three times greater than those found hi comparable rural areas.
Source: Helen L Fox. The Urbanizing River: A Case Study in the Maryland Piedmont. In
Geomorphology and Engineering, edited by D.R. Coates. Dowden, Hutchinson, and Ross,
Inc., Stroudsburg, Pennsylvania. 1976.
VARIOUS STREAMS
NORTH CAROLINA PIEDMONT
Background
Historical stream flow data were analyzed for a number of streams in North Carolina. The
intent was to see if a correlation could be drawn between low stream flows and
urbanization. The data were compared for both urbanizing watersheds and watersheds hi
areas that are still rural. -
Results
While there was some support for the premise that urbanization could lead to low. stream
flow, the statistical analysis of the data proved inconclusive. It appeared that both urban
and rural small streams were experiencing decreasing stream flows over time.
Source: Evett, J.B. Effects of Urbanization and Land Use Changes on Low Stream Flows.
University of North Carolina, Charlotte, College of Engineering, Department of Civil
Engineering. June 1994. .
Urbanization and Streams: Studies of Hydrologic Impacts Appendix-9
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