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Universities Council on Water Resources
Journal of Contemporary Water Research & Education
Issue 146, Pages 103-109, December 2010
Stormwater Management for a
Record Rainstorm at Chicago
Stanley A. Changnon
University of Illinois, Champaign, IL
Abstract: A record setting 24-hour rainstorm hit Chicago in September 2008. It created an opportunity to
assess its impacts and to compare the storm water management with that used in other recent rainstorms.
Flooding impacted all forms of transportation and thousands of homes and businesses, 10,000 homes
had to be evacuated. One lesson learned is that this storm’s magnitude was enhanced by urban and
lake influences on the atmosphere that extended over the large suburban areas west of the city. This
helped intensify the rainfall over a large area. A second lesson learned is when 6-hour and 24-hour rainfall
amounts exceed once-in-25-years recurrence values, the water management facilities cannot handle the
large volume of water. Hence, major diversions of flood waters were needed to be made into Lake Michigan
and the Illinois River.
Keywords: Urban hydrology, flood impacts, urban-induced rainfall.
C
hicago and its suburbs experienced a record
heavy 24-hour rainstorm on September
13, 2008. Rainfall reaching 16.9 cm,
exceeding recurrence interval values of once in 50
years, occurred over 485 square kilometers. This
flash flood quickly created massive flooding that
stopped surface transportation, caused evacuation
of 10,000 homes, and led to property damages
totaling $155 million. The city was aware of the
approaching storm and acted to remove waters
stored in the city’s deep tunnel storm water
reservoir system. The emptied tunnel waters plus
early stormwater totaling 49 Mm3 were released
into the Illinois River. However, the management
system could not handle the extreme volume of
storm water, and as urban flooding developed,
managers chose to release 341 Mm3 into Lake
Michigan, creating water supply problems from
the polluted storm water. The impacts from the
urban flooding included all forms of transportation,
thousands of flooded homes and businesses, and
10,000 homes had to be evacuated. This storm
and the management actions employed illustrate
the sizable difficulties large cities face in handling
waters from major rainstorms.
Journal of Contemporary Water Research & Education
Chicago’s official raingage at O’Hare Airport
recorded 16.9 cm (6.64 inches), a new Chicago
record for a 24-hour period. The atmospheric
remnants of Hurricane Ike moved northeast and
crossed central Illinois on September 12; the
passage of its low pressure center with abundant
moist air along a cold front boundary resulted in
heavy rainfall across northern Illinois (Figure 1).
Amounts ranged from 5 to 7 cm eastward from
the Mississippi River. The rains intensified on
September 13 in the Chicago area, with some
amounts exceeding 15 cm.
This record-setting rainstorm at Chicago
presented an opportunity to examine two
key questions. First, what were the types and
magnitudes of impacts of the event? Second, what
water management actions were used to handle a
record amount of water in the urban area? The
record-setting rainstorm in Chicago in September
2008 also provided an opportunity to examine the
effects of improvements since 2001 in Chicago’s
water management and treatment system. Flood
waters from two severe rainstorms in 2001
(Changnon and Westcott 2002a) overwhelmed the
city’s developing deep tunnel-reservoir storage
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Changnon
104
Figure 1. Total storm rainfall (cm) in Illinois for September 12-13, 2008.
system, but the system has been enlarged since
2001. The city had also developed a Rain Blocker
system, at a cost of $75 million, to slow the inflow
of flood waters into the sewage treatment system,
prolonging time for water treatment to occur.
This system and the deep tunnel system had not
experienced extremely large rainfall amounts since
the two 2001 storms. Chicago has long had serious
water problems, some related to the amount of
water diverted from Lake Michigan, and some
related to the management of storm runoff.
Major previous rainstorm impacts have included
extensive flooding of residences and businesses,
and major problems for transportation systems
in the city. The vast quantities of storm water in
September 2008 led to the flooding of several
major highways, many streets, and rail lines. The
storm waters also flooded thousands of buildings,
leading to evacuations of more than 10,000 homes.
Property insurance losses totaled $155 million,
identified as a catastrophe by the insurance
industry (Property Claim Services 2008). These
losses included damages to homes, businesses, and
vehicles. Stormwater storage was inadequate to
handle the vast quantity of water, and diversions of
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stormwaters away from the city were necessary to
lessen the flooding.
Rainfall Assessment
Rainfall totals in Cook County, home of
Chicago, for the storm period that began between
1900-2100 CST on September 12 and ended by
2200 CST on September 13, are shown on Figure
2. Rain began first in the southern urban area
(1900 CST) and ended latest in the north sector
of Cook County at 2200. This rainfall pattern
is based mainly on a network of 25 recording
raingages (Westcott 2008). The 24-hour rain
pattern is also defined by data from the National
Weather Service (NWS) raingages in Cook
County (at O’Hare Airport, Midway Airport, and
Chicago Heights) and those in the surrounding
area, mainly suburbs. The 25-gage network
consists of recording raingages spaced in a grid
with 10 km between gages, and these allowed
assessment of peak rainfall amounts for periods
during the storm. The average storm rainfall over
Chicago was 11.3 cm, and the average over Cook
County was 12.6 cm.
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Stormwater Management for a Record Rainstorm
105
Figure 2. Total storm rainfall (cm) for September 13, 2008, in and around the Chicago
metropolitan area.
The record setting rainstorm on the 13th was
preceded by light rains that occurred in midday
on the 12th. Amounts across the Cook County
area on the 12th were 0.25 to 0.63 cm. Rain
began again during the morning of September
14 as a cold front passed through the area. This
system produced 2.5 cm of rain in northern Cook
County, and up to 6.4 cm in the south end of
Cook County. This rain added to the flooding
problems created on the 13th. The rainstorm on
the 13th was the third rainstorm during 2008,
five less than the record of eight rainstorms set
in 2001 (Changnon and Westcott, 2002b). A
moderately heavy rainstorm (5 to 7 cm) occurred
in the Chicago area in August 2007 (Angel and
Changnon 2008).
The heaviest rainfall on the 13th occurred in the
northern part of the Chicago area where amounts
Journal of Contemporary Water Research & Education
ranged from 15 cm up to 18.2 cm at gage #3
(Figure 2). Amounts in the central section of Cook
County were less, 7.5 to 10 cm, and amounts were
heavier in the southern sections of the urban area
where totals exceeded 12 cm.
This high-low-high sequence was also found in
the patterns for the maximum 6-hour and 3-hour
rainfall amounts. The 6-hour maximum amounts
exceeded 10 cm in the northern sector and were
7.5 cm or more in the south and west, with 5 cm in
the central portions of the city. The peak 6-hour
rainfall occurred from midnight on September 12
to 0600 local standard time on September 13. The
peak 3-hour amounts had values greater than 5
cm in the northern and southern parts of the urban
area with 2.5 cm in the central section.
The statistical frequencies of the maximum
rainfall amounts were determined based on recently
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Changnon
developed values for the area (Huff and Angel
1992). The 24-hour rainfall amounts revealed
that the 18.2 cm at gage #3 rated as a once in
100-year event, and amounts at six surrounding
gages were rated as 50- to 75-year events.
In the city’s center area, the amounts rated as
once in 5-year occurrences, with once in 25year amounts at five gages in the south. Peak
amounts for 12-hours had similar recurrence
interval values. The maximum 6-hour amounts
achieved recurrence intervals of 25-years in the
north, whereas those in the center of Chicago
had 5-year recurrence interval levels.
The temporal distribution of rainfall during
the storm revealed that the heaviest amounts at
all gages occurred during the 2400-0700 LST
period. Such nocturnal maximums of rainfall
occurred in 87 percent of all flash floods in
Illinois (Changnon 1978). This timing meant
that when many residents awakened, they were
surprised to find serious flooding in and around
their homes. Many commuters unexpectedly ran
into flooded streets and underpasses, leading to
traffic jams, accidents, and great delays. The
remaining rainfall occurred between 0600 and
2100 on September 13.
Urban and lake effects on the atmosphere
at Chicago often act to increase precipitation
(Huff 1995). A 3-year meteorological field study
of weather conditions in and around Chicago
identified how urban effects, sometimes aided by
lake effects on the atmosphere, acted to increase
warm season rainfall and often led to heavier rains
in heavy rainstorms (Changnon 1980).
An issue relevant to the 2008 storm relates
to climate shifts that lead to more rainstorms
over time. Studies of historic floods and floodproducing rain events in the Midwest revealed a
temporal increase since 1930 (Kunkel et al. 1993).
Assessment of heavy rainstorms in the Midwest
since 1930 found increasing frequencies. The
three 2008 storms, plus the eight in 2001 and one
in 2007 at Chicago, reflect this climate-related
increase in storm frequency, an important issue
for water management planning and operations.
Studies of global warming and its effects on
climate concluded that heavier rainfalls would
continue to increase in the 21st century (National
Assessment Synthesis Team 2000).
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Storm Impacts
Some past studies addressed the variety of
problems that heavy rainstorms caused in Chicago
(Changnon 1980). A common problem relates to
flooding of viaducts and streets. Chicago is the
national hub for commercial air traffic, the nation’s
railroads and trucking industry. Hence, heavy rains
that disrupt and slow or stop traffic flow create
large costs. Flood waters also disrupt movement of
the city’s commuter rail and bus systems, another
major problem. Floods also have always produced
extensive damage to property and businesses,
and are a major challenge for the water treatment
facilities.
The heavy September 2008 rains, which largely
fell in a 6-hour period early on the 13th, created
flash flood conditions and major impacts resulted.
Record high river flows and allied flooding quickly
resulted from the heavy rains, particularly along
the rivers in the urban-suburban area (Figure 2).
The flooding was particularly bad along the North
Branch of the Chicago River, and also along the
Des Plaines, Du Page, and Fox Rivers (Figure 2).
The Illinois River at Morris (Figure 2) reached
7.55 m on the 16th, exceeding the flood stage of
4.9 m, and breaking the old record of 7.1 m set in
1957. This created flooding many kilometers from
the Chicago metropolitan area. The Illinois River
at La Salle (Figure 1) peaked at 7.1 m on the 17th (1
day later) which was higher than its flood stage of
6.1 m. The Des Plaines River (Figure 2) crested at
3.1 m at Riverside, a suburb, much above the flood
stage of 0.9 m, creating flooding along the river in
the western suburbs. The Fox and Du Page Rivers
also flooded along their courses, creating damages
along the Fox from Elgin (Figure 2) southward
to Yorkville and Ottawa. Flooding was bad along
the Du Page at Addison and other suburbs (Figure
2). The mean flow of the lower Fox River was
seven times greater than the average flow. Similar
flooding in the western suburbs also occurred in
August 2007 after several days of moderate to
heavy rains west and north of Chicago (Angel and
Changnon 2008).
Flooded homes, with extensive damages in
basements, caused more than 10,000 homes to be
evacuated and thousands of persons were placed
in shelters or homes of friends. Many evacuees
Journal of Contemporary Water Research & Education
Stormwater Management for a Record Rainstorm
were housed in the four Red Cross shelters in
Chicago. Insured property losses to 22,500
homes in the flooded zones totaled $125 million.
Communities with sizable evacuations along the
Des Plaines River (Figure 2) included Glenview
(3,000 persons), Des Plaines (1,000), Rosemont
(250), and Riverside (400). Along the North
Branch of the Chicago River, 2,800 persons had to
be evacuated. Evacuations also occurred at Sugar
Grove (75 persons), Morris (144), and Addison
(750). Cleanup of flooded homes and damaged
areas was done largely by local communities, and
these efforts took three months (Chicago Tribune
October 21). Cleanups cost $3.5 million in Cook
County, $1.4 million in Will County, and $547,690
in Kendall County.
Several major transportation arteries were
blocked. Two major commuter rail lines were
closed for 3 days, and several railroads had
freight trains delayed for 2-3 days (Trains 2008).
Numerous streets, highways, and Interstates were
closed for several days. Interstate 80-94, one of
the region’s busiest, was blocked for 4 days, and I94 was closed in north Chicago (Chicago Tribune
September 17). Closed streets led to major traffic
congestion on non-flooded streets and highways,
and thousands were late to work. Flood damages
to vehicles created 5,500 insurance claims and
amounted to losses of $18 million. Several city and
suburban schools were closed from September 13th
to the 16th. Commercial airline flights at O’Hare
and Midway Airports were delayed 6 to 8 hours on
September 13.
The Governor of Illinois acted after the flooding
began and identified seven counties as Disaster
Areas. These included Cook, DeKalb, DuPage,
Grundy, Kane, La Salle, and Will (Chicago
Tribune October 21).
President Bush later
identified the seven counties as Federal Disaster
Areas, allowing residents with flood damages to
seek federal aid. These seven counties were where
the heaviest rain fell (Figure 2) and in downstream
areas (Will, Grundy, and LaSalle counties) where
massive flood waters had moved from the Chicago
area. In mid-November the federal government
began providing disaster aid to local government
agencies in nine counties to cover 75 percent of
the costs for the removal of flood debris, to repair
streets and roads, and to repair public properties.
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107
Five weeks after the flood, thousands of persons
were still relying on food stamps to cover losses
from the flood. More than 5,000 appeals had been
approved for federal disaster aid by October 20
(Chicago Tribune October 21).
Stormwater Management
Water management endeavors reflected
reactions to the evolving flooding on the 13th.
Chicago has constructed a deep tunnel and
reservoir storage system, which consists of 109
miles of large tunnels under the city’s rivers plus
several surface reservoirs. Their construction
cost $3 billion. Normally, storm waters stored in
the tunnel system are extracted, treated at sewage
plants, and then released into the Sanitary and Ship
Canal which connects the Chicago River to the
Illinois River.
Chicago has a peculiar storm water management
situation related to the diversion of waters from
Lake Michigan (Changnon and Changnon 1996),
and the reversal of its in-lake drainage system. The
city’s three river exits into the lake are blocked by
movable gates, and water taken in from the lake,
and from rain falling over the city, is moved down
the Illinois River. The annual amount of water
diverted by Chicago for various purposes (local
water supplies and flow down the Illinois River to
dilute pollution and maintain barge traffic) is under
federal control and cannot exceed a fixed level.
Thus, to handle storm water, the city has three
choices: 1) lower the waters in local rivers and the
storage system in advance of a storm and divert it
down the Illinois River; 2) store storm waters in
its special deep tunnel system; or 3) divert flood
waters into Lake Michigan. The in-lake diversion
is undesirable because it moves polluted storm
water into the lake where waters are withdrawn
for city and suburban domestic supplies. The
pollution of lake waters led water treatment plants
in and around Chicago to add chlorine to minimize
any problems with the potentially polluted water
(Chicago Tribune September 16).
Research directed at storm forecasting and
nowcasting of severe rainstorms at Chicago has
shown that it is possible to forecast and thus
anticipate rainstorms (their time, location, and
amount) with enough time before the storm to
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Changnon
lower the waters in the Chicago River system (Huff
et al. 1980). This action was used to direct treated
water down the Illinois River prior to the start of
the September 2008 storm.
In response to the September 2008 rainstorm,
the city filled the deep tunnel system to its capacity
with 9.5 M m3 gallons of water, but that did not
reduce the rapid increase in flooding. Cook and
DuPage counties received rainfall totaling 469.4
M m3 of storm water. Chicago began releasing
untreated flood waters into Lake Michigan on the
morning of the 13th, and by the morning of the 15th
had released 340.7 M m3 of flood water into the
lake. An additional 49.2 M m3 of treated water
were released down the Sanitary and Ship Canal
to the Illinois River, done prior to the storm to help
enhance in-city storage of the storm waters.
These 2008 diversions revealed that the recent
improvements in the tunnel-reservoir system since
the 2001 storms were not sufficient to handle the
flood waters from a storm of the magnitude of that
in September 2008. The 2008 storm had a flooding
outcome similar to those with two extremely heavy
rainstorms in 2001 (Changnon and Westcott
2002b). All three storms produced excessive
flooding in the city and its suburbs. All three
storms had 6- and 24-hour rainfall amounts that
equaled or exceeded the 50-year return values and
that covered more than 450 square kilometers. In
contrast, six other rainstorms in 2001 with rainfall
amounts at the 5- and 10-year return levels did not
create any serious flooding. Thus, even with an
extensive flood water storage system, Chicago’s
system can not handle the water generated by rains
achieving the 50-year level over a large area, and
the only option is to divert untreated water into
Lake Michigan.
Summary
The record high 2008 rainstorm in Chicago
produced the highest rainfall amounts in the
northern metropolitan area, least in the city’s center,
and heavier rains in the city’s southern sections.
The resulting huge mass of water amounted to 469
M m3. The temporal distribution of rainfall during
this 24-hour rainstorm began with light rain,
followed by the heaviest rains for 6 hours at night,
and then moderate amounts until the storm ended
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12 hours later. Overall, the urban area averaged
12.6 cm of rain, and the stormwater management
actions could not halt some flooding, particularly
along the region’s rivers. Storm impacts were
similar to those in past heavy rainstorms with
extensive home flooding and creation of major
transportation problems. However, the number of
evacuations was larger than ever before and the
problems were in the city and the suburbs west of
the city. Insured property losses amounted to $155
million.
The magnitude of the September 2008 storm
and the use of diversions of flood waters into Lake
Michigan and the Illinois River illustrate what will
be needed when major rainstorms occur at other
large cities. They should be prepared under heavy
rainstorm situations to divert stormwaters into
an adjacent lake, an oceanic body, and/or a large
adjacent river system. The September storm reveals the large and
costly deep tunnel-reservoir system in Chicago
is not adequate to handle the magnitude of water
produced by rainstorms that create 6 to 24hour amounts having once in 50-year or greater
frequencies and extending over several hundred
square kilometers. Lowering the river system by
added inflow to the Illinois River before a rainstorm
strikes the area will help but is not sufficient to
handle waters generated by more major storms.
Thus, future major rainstorms in the Chicago area
will require sizable releases of water into Lake
Michigan to minimize flooding in the city.
Acknowledgments
The property loss data were provided by Gary Kerney
of the Property Claims Service, and Dan Injerd of the
Illinois Natural Resources Department provided valuable
information about the water management actions. Maria
Peters and Nancy Westcott of the Midwest Regional
Climate Center provided extensive rainfall data, and
Eileen Deremiah assisted in the analysis.
Author Bio and Contact Information
Stanley A. Changnon has pursued and directed
atmospheric and hydrospheric research for 57 years.
He directed the atmospheric research and services
program of the Illinois State Water Survey for 15
years and served as the Survey’s Chief for six years.
As Chief Emeritus, he also serves as a Professor of
Journal of Contemporary Water Research & Education
Stormwater Management for a Record Rainstorm
Geography at the University of Illinois. His interests
include: hydrology; weather and climate extremes;
climate variability and change; atmospheric effects on
agriculture, water resources, and society; and accidental
and planned weather modification. He can be contacted
at the Illinois State Water Survey, 2204 Griffith Drive,
Champaign, IL 61820. Telephone (217) 586-5691.
Email: [email protected].
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