1. science
2. weather
3. meteorological disaster
4. flood

STEPHENSON COUNTY,
ILLINOIS
AND INCORPORATED AREAS
COMMUNITY
NAME
CEDARVILLE, VILLAGE OF
*DAKOTA, VILLAGE OF
*DAVIS, VILLAGE OF
FREEPORT, CITY OF
*GERMAN VALLEY, VILLAGE OF
LENA, VILLAGE OF
ORANGEVILLE, VILLAGE OF
PEARL CITY, VILLAGE OF
RIDOTT, VILLAGE OF
*ROCK CITY, VILLAGE OF
STEPHENSON COUNTY
(UNINCORPORATED AREAS)
WINSLOW, VILLAGE OF
COMMUNITY
NUMBER
Stephenson
County
170842
170843
171338
170640
171339
171340
170641
170642
170643
171341
170639
170644
*NO SPECIAL FLOOD HAZARD AREAS IDENTIFIED
REVISED:
MARCH 16, 2015
FLOOD INSURANCE STUDY NUMBER
17177CV000B
NOTICE TO
FLOOD INSURANCE STUDY USERS
Communities participating in the National Flood Insurance Program have established
repositories of flood hazard data for floodplain management and flood insurance purposes. This
Flood Insurance Study (FIS) may not contain all data available within the Community Map
Repository. It is advisable to contact the Community Map Repository for any additional data.
The Federal Emergency Management Agency (FEMA) may revise and republish part or all of
this FIS report at any time. In addition, FEMA may revise part of this FIS by the Letter of Map
Revision process, which does not involve republication or redistribution of the FIS. It is,
therefore, the responsibility of the user to consult with community officials and to check the
Community Map Repository to obtain the most current FIS components.
Initial Countywide FIS Effective Date:
March 3, 2011
Revised Countywide FIS Effective Date(s): March 16, 2015
TABLE OF CONTENTS
1.0 1 INTRODUCTION
1.1 Purpose of Study
1 1.2 Authority and Acknowledgements
1 1.3 Coordination
4 2.0 AREA STUDIED
5 2.1 Scope of Study
5 2.2 Community Description
10 2.3 Principal Flood Problems
11 2.4 Flood Protection Measures
12 3.0 13 ENGINEERING METHODS
3.1 Hydrologic Analyses
13 3.2 Hydraulic Analyses
16 3.3 Vertical Datum
19 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS
23 4.1 Floodplain Boundaries
23 4.2 Floodways
24 5.0 INSURANCE APPLICATIONS
32 6.0 FLOOD INSURANCE RATE MAP
33 7.0 OTHER STUDIES
33 8.0 LOCATION OF DATA
33 9.0 BIBLIOGRAPHY AND REFERENCES
35 i
TABLE OF CONTENTS (continued)
FIGURES
8-9 Figure 1. FIRM Notes to Users
Figure 2. Vertical Datum Conversions
21 Figure 3. Floodway Schematic
25 TABLES
Table 1.
CCO Meeting Dates for Pre-Countywide Studies
4 Table 2.
Limits of Revised or New Detailed Study
5 Table 3.
Limits of Detailed Study
6 Table 4.
Incorporated Letters of Map Change (March 3, 2011)
7 Table 5.
Historical Flood Data
12 Table 6.
Summary of Discharges
15 Table 7.
Summary of Stillwater Elevations
16 Table 8.
Roughness Coefficients (Manning's "n" Values)
18 Table 9a. Vertical Datum Conversions (Single Conversion Factor Method)
22 Table 9b. Vertical Datum Conversions (Multiple Conversion Factors Method)
22 27-31 Table 10. Floodway Data
34 Table 11. Community Map History
ii
TABLE OF CONTENTS (continued)
EXHIBITS
Exhibit 1 - Flood Profiles
Panel
Goldmine Road Tributary
Indian Creek
Pecatonica River
Yellow Creek
01P
02P
03P-09P
10P-14P
Exhibit 2 - Flood Insurance Rate Map Index and Flood Insurance Rate Map
iii
FLOOD INSURANCE STUDY
STEPHENSON COUNTY, ILLINOIS AND INCORPORATED AREAS
1.0
INTRODUCTION
1.1
Purpose of Study
This Flood Insurance Study (FIS) revises and supersedes the FIS reports and/or
Flood Insurance Rate Maps (FIRMs) and/or Flood Hazard Boundary Maps
(FHBMs) in the geographic area of Stephenson County, Illinois, including: the
City of Freeport; the villages of Cedarville, Dakota, Davis, German Valley, Lena,
Orangeville, Pearl City, Ridott, Rock City and Winslow; and the unincorporated
areas of Stephenson County (hereinafter referred to collectively as Stephenson
County) and aids in the administration of the National Flood Insurance Act of
1968 and the Flood Disaster Protection Act of 1973. This study has developed
flood risk data for various areas of the county that will be used to establish
actuarial flood insurance rates. This information will also be used by Stephenson
County to update existing floodplain regulations as part of the Regular Phase of
the National Flood Insurance Program (NFIP), and by local and regional planners
to further promote sound land use and floodplain development. Minimum
floodplain management requirements for participation in the NFIP are set forth in
the Code of Federal Regulations at 44 C.F.R § 60.3.
Note that the villages of Dakota, Davis, German Valley, and Rock City have no
special flood hazard areas (SFHAs) identified.
In some states or communities floodplain management criteria or regulations may
exist that are more restrictive or comprehensive than the minimum Federal
requirements. In such cases, the more restrictive criteria take precedence and the
State (or other jurisdictional agency) will be able to explain them.
1.2
Authority and Acknowledgements
The sources of authority for this FIS are the National Flood Insurance Act of 1968
and the Flood Disaster Protection Act of 1973.
The FIS includes the unincorporated areas of, and incorporated communities
within, Stephenson County. Information on the authority and acknowledgments
for each jurisdiction included in this FIS, as compiled from their previously
printed FIS reports, is shown below.
1
Pre-Countywide FISs
City of Freeport:
The hydrologic and hydraulic analyses for
the FIS report dated November 1976
(Reference 1) were conducted by Carl C.
Crane, Inc. at the request of the Federal
Insurance Administration, U.S. Department
of Housing and Urban Development. The
source of authority for the study is the
national Flood Insurance Act of 1968, as
amended. Authority and financing are
contained in Contract No. H-3681 between
the contractor and the Federal Insurance
Administration.
Village of Pearl City:
The hydrologic and hydraulic analyses for
the FIS report dated September 29, 1989
(Reference 2) were performed by the U.S.
Geological Survey (USGS) (the Study
Contractor) for the Federal Emergency
Management Agency (FEMA), under InterAgency Agreement No. EMW-86-E-2224,
Project Order No. 2. This study was
completed in October 1987.
Stephenson County
(Unincorporated Areas):
The hydrologic and hydraulic analyses for
the FIS report dated July 1982 (Reference 3)
were performed by the U.S. Department of
Agriculture, Soil Conservation Service, for
the Federal Insurance Administration, under
Inter-Agency Agreement No. IAA-H-4-73,
Project Order No. 6. This work was
completed in July 1973.
Village of Winslow:
The hydrologic and hydraulic analyses for
the FIS report dated May 17, 1982
(Reference 4) were performed by the U.S.
Department of Agriculture, Soil Conservation
Service for FEMA, under Inter-Agency
Agreement No. IAA-H-11-79, Project Order
No. 10. This study was completed in August
1980.
2
The authority and acknowledgements for the villages of Cedarville, Dakota,
Davis, German Valley, Lena, Orangeville, Ridott and Rock City are not included
because there were no previously printed pre-countywide FISs for those
communities.
March 3, 2011
Initial Countywide FIS
For the initial countywide FIS, base map information was provided in digital
format by the USGS. Digital orthoimagery with a spatial resolution of 0.5 meter
ground sample distance were photogrammetrically compiled from aerial
photography acquired during the leaf-off period of spring 2005 (Reference 5).
This countywide FIS was performed under the Cooperating Technical Partners
(CTP) Partnership Agreement No. EMC-2006-CA-7023 between the Illinois
Department of Natural Resources (hereinafter referred to as IDNR) and FEMA,
per the Mapping Activity Statement (MAS) No. IDNR06-21.
March 16, 2015
Revised Countywide FIS
This Physical Map Revision (PMR) incorporates new hydrologic and hydraulic
analyses of the Pecatonica River and Yellow Creek performed by the Illinois State
Water Survey (ISWS). The Pecatonica River hydrologic study was completed in
2010 (Reference 6). The Pecatonica River hydraulic study and Yellow Creek
hydrologic and hydraulic studies were completed in 2012 (Reference 7, 8).
Base map information for the 19 panels affected by this PMR was provided by the
USGS. Digital orthoimages with one-foot spatial resolution were compiled from
aerial photography acquired in 2011 (Reference 9).
The coordinate system used for the production of the digital FIRMs is State Plane
Coordinate System North American Datum of 1983 (NAD 83) Geodetic
Reference System 1980 (GRS80) spheroid.
This Physical Map Revision was performed under the Cooperating Technical
Partners (CTP) Partnership Agreement Nos. EMC-2009-CA-7007 and EMC2012-CA-7014 between the ISWS and FEMA, per the Mapping Activity
Statement (MAS) Nos. ISWS 09-03 and ISWS 12-06.
3
1.3
Coordination
Coordination and outreach activities were performed to create a climate of
understanding and ownership of the mapping process at the state and local levels.
These activities were ongoing throughout the entirety of the project.
The purpose of an initial consultation coordination officer (CCO) meeting, or
project team meeting, is to discuss the scope of the project. An intermediate CCO
meeting, or scoping meeting, is meant to continue outreach and create a climate of
understanding throughout the process. A final CCO meeting, or open house, is
held with public officials and the general public to review the results of the study.
Pre-Countywide FISs
The dates of the initial and final CCO meetings held for the pre-countywide
studies for Stephenson County’s incorporated communities and unincorporated
areas are shown in Table 1, “CCO Meeting Dates for Pre-Countywide Studies.”
Table 1 - CCO Meeting Dates for Pre-Countywide Studies
Community
Freeport, City of
Pearl City, Village of
Stephenson County
(Unincorporated Areas)
Winslow, Village of
Initial CCO Date
*
*
Final CCO Date
December 17, 1975
August 24, 1988
*
April 29, 1980
June 22, 1976
October 22, 1981
*Initial meeting not held
March 3, 2011
Initial Countywide FIS
The initial CCO meeting was held on September 12, 2006 in Freeport, Illinois,
and was attended by representatives of Stephenson County, the City of Freeport,
and IDNR. This meeting was intended to discuss various issues and concerns for
the study area. An intermediate CCO meeting was held on October 19, 2006 in
Freeport, Illinois, and was attended by representatives of Stephenson County, the
City of Freeport, the Illinois State Water Survey, and IDNR.
A preliminary FIRM and FIS were prepared by merging effective FIS text, tables,
and profiles with new study data. A preliminary Summary of Map Actions
(PSOMA) was also prepared for all affected communities. The PSOMA lists
pertinent information regarding Letters of Map Change (LOMCs) that will be
affected by the issuance of the FIRM (i.e., superseded, incorporated, and
revalidated). Preliminary copies of the FIRM, FIS, and SOMA were distributed to
community officials for public review and comment.
4
The results of the study were reviewed at the final CCO meeting held on
September 24, 2009 in Freeport, Illinois, and attended by representatives of
Stephenson County, the City of Freeport, the Illinois State Water Survey, and
IDNR. All problems raised at that meeting have been addressed in this study.
March 16, 2015
Revised Countywide FIS
The results of the Physical Map Revision were reviewed at the final CCO meeting
held on February 25, 2014 in Freeport, Illinois and was attended by
representatives of Stephenson County, the City of Freeport, the villages of
Orangeville and Ridott, ISWS, IDNR, and FEMA. All problems raised at that
meeting have been addressed in this study.
2.0
AREA STUDIED
2.1
Scope of Study
This FIS covers the geographic area of Stephenson County including the
incorporated areas listed in Section 1.1.
Typically, areas studied by detailed methods are selected with priority given to all
known flood hazards and areas of projected development or proposed
construction.
The streams or portions of streams listed in Table 2 have new or revised
hydrologic and hydraulic analyses for this revised countywide FIS.
Table 2 - Limits of Revised or New Detailed Study
Flooding Source
Pecatonica River
Limits of Revised or New Detailed Study
From 36.73 miles above the confluence with Rock River (at
Winnebago County Boundary) to 93.26 miles above the
confluence with Rock River (at Wisconsin/Illinois State
boundary, approximately 0.93 miles above Bridge Street)
Yellow Creek
From confluence with Pecatonica River to 43,613 feet above
confluence with Pecatonica River (approximately 619 feet above
Forest Road/Fairgrounds Road)
The streams, or portions of streams, listed in Table 3, “Limits of Detailed Study,”
were studied in detail and are included in this report. The limits of detailed study
are also indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2).
5
Table 3 - Limits of Detailed Study
Flooding Source
Goldmine Road Tributary
Limits of Detailed Study
From 0.04 miles above mouth at Yellow Creek (approximately
0.25 miles below Pearl City Road) to approximately 0.69 miles
above mouth at Yellow Creek (approximately 0.27 miles above
Main Street)
Indian Creek
From confluence with Pecatonica River to 4,300 feet above the
confluence at Pecatonica River (approximately 3,700 feet above
State Route 73)
Pecatonica River
From 36.73 miles above the confluence with Rock River (at
Winnebago County Boundary) to 93.26 miles above the
confluence with Rock River (at Wisconsin/Illinois State
boundary, approximately 0.93 miles above Bridge Street)
Yellow Creek
From confluence with Pecatonica River to 43,613 feet above
confluence with Pecatonica River (approximately 619 feet above
Forest Road/Fairgrounds Road)
AND
From 133,479 feet above the confluence with Pecatonica River to
138,335 feet above the confluence with Pecatonica River
(approximately 4,752 feet above Pearl City Road)
The countywide FIRM may show detailed studies where previously Zone A
studies were shown, and/or may show flood hazard areas where previously no
flood hazard areas were shown.
Also, special flood hazard areas were delineated for limited distances between
study reaches using additional information from the model or interpolated for
consistency.
March 16, 2015
Revised Countywide FIS
This Physical Map Revision incorporates new studies of the Pecatonica River and
Yellow Creek (Reference 7, 8). The Pecatonica River study supersedes two sets of
hydrologic discharges that had resulted in conflicting hydraulic data along the
Pecatonica River between Stephenson County, Winnebago County, Illinois and
Green County, Wisconsin.
In addition, the regulatory Base Flood Elevations (BFEs) for Indian Creek have
been revised based upon the backwater effects of the Pecatonica River study as
shown in the Floodway Data table (Table 10) and on the Indian Creek flood
profile and FIRM panel.
6
Letters of Map Revision
At the time of this Physical Map Revision, there were no existing Letters of Map
Revision (LOMRs) affecting the 19 revised FIRM panels.
Table 4 provides a history of the incorporation of determination letters issued by
FEMA for the March 3, 2011 initial countywide FIS. It should be noted that all or
portions of a given map change may be superseded by subsequent Letters of Map
Revision or restudies.
Table 4 - Letters of Map Change (March 3, 2011)
LOMC
Type
LOMR
Case
Number
97-05-165P
Effective
Date
06/06/1997
Flooding
Community
Source
Stephenson County
Lake
(Unincorporated Areas) Summerset
Project
Identifier
Mead & Hunt
Study
LOMR
04-05-0757P
01/14/2004
Stephenson County
Pecatonica
(Unincorporated Areas) River
FEMA Initiated
correction to FIS
FIRM Notes to Users
Each FIS Report provides floodplain data, which may include a combination of the
following: 10-, 4-, 2-, 1-, and 0.2-percent-annual-chance flood elevations (the 1percent-annual-chance flood elevation is also referred to as the Base Flood Elevation
[BFE]); delineations of the 1-percent-annual-chance and 0.2-percent-annual-chance
floodplains; and 1-percent-annual-chance floodway. This information is presented on
the FIRM and/or in many components of the FIS Report, including Flood Profiles and
Floodway Data tables.
Figure 1 presents important considerations for using the information contained in this
FIS report and the FIRM and is provided in response to changes in format and content.
7
Figure 1 - FIRM Notes to Users
NOTES TO USERS
For information and questions about this map, available products associated with this FIRM
including historic versions of this FIRM, how to order products, or the National Flood Insurance
Program in general, please call the FEMA Map Information eXchange at 1-877-FEMA-MAP (1877-336-2627) or visit the FEMA Map Service Center website at http://msc.fema.gov. Available
products may include previously issued Letters of Map Change, a Flood Insurance Study Report,
and/or digital versions of this map. Many of these products can be ordered or obtained directly from
the website. Users may determine the current map date for each FIRM panel by visiting the FEMA
Map Service Center website or by calling the FEMA Map Information eXchange.
Communities annexing land on adjacent FIRM panels must obtain a current copy of the adjacent
panel as well as the current FIRM Index. These may be ordered directly from the Map Service
Center at the number listed above.
For community dates, refer to Table 11 in this FIS Report.
To determine if flood insurance is available in the community, contact your insurance agent or call
the National Flood Insurance Program at 1-800-638-6620.
The map is for use in administering the NFIP. It may not identify all areas subject to flooding,
particularly from local drainage sources of small size. Consult the community map repository to find
updated or additional flood hazard information.
BASE FLOOD ELEVATIONS: For more detailed information in areas where Base Flood
Elevations (BFEs) and/or floodways have been determined, consult the Flood Profiles and
Floodway Data and/or Summary of Stillwater Elevations tables within this FIS Report. Use the
flood elevation data within the FIS Report in conjunction with the FIRM for construction and/or
floodplain management.
FLOODWAY INFORMATION: Boundaries of the floodways were computed at cross sections and
interpolated between cross sections. The floodways were based on hydraulic considerations with
regard to requirements of the National Flood Insurance Program. Floodway widths and other
pertinent floodway data are provided in the FIS Report for this jurisdiction.
Floodways restricted by anthropogenic features such as bridges and culverts are drawn to reflect
natural conditions and may not agree with the model computed widths listed in the Floodway Data
table in the Flood Insurance Study.
In the State of Illinois, any portion of a stream or watercourse that lies within the floodway fringe of a
studied (AE) stream may have a state regulated floodway. The FIRM may not depict these state
regulated floodways.
FLOOD CONTROL STRUCTURE INFORMATION: Certain areas not in Special Flood Hazard
Areas may be protected by flood control structures.
8
Figure 1 - FIRM Notes to Users (continued)
PROJECTION INFORMATION: The projection used in the preparation of the map was State
Plane Illinois West 1202. The horizontal datum was NAD83, GRS1980 spheroid. Differences in
datum, spheroid, projection or State Plane zones used in the production of FIRMs for adjacent
jurisdictions may result in slight positional differences in map features across jurisdiction boundaries.
These differences do not affect the accuracy of the FIRM.
ELEVATION DATUM: Flood elevations on the FIRM are referenced to the North American Vertical
Datum of 1988. These flood elevations must be compared to structure and ground elevations
referenced to the same vertical datum. For information regarding conversion between the National
Geodetic Vertical Datum of 1929 and the North American Vertical Datum of 1988, visit the National
Geodetic Survey website at http://www.ngs.noaa.gov/ or contact the National Geodetic Survey at
the following address:
NGS Information Services
NOAA, N/NGS12
National Geodetic Survey
SSMC-3, #9202
1315 East-West Highway
Silver Spring, Maryland 20910-3282
(301) 713-3242
Local vertical monuments may have been used to create the map. To obtain current monument
information, please contact the appropriate local community.
BASE MAP INFORMATION: Base map information is panel-specific. The map panels should be
referenced for this information.
Corporate limits shown on the map are based on the best data available at the time of publication.
Because changes due to annexations or de-annexations may have occurred after the map was
published, map users should contact appropriate community officials to verify current corporate limit
locations.
NOTES FOR FIRM INDEX
REVISIONS TO INDEX: As new studies are performed and FIRM panels are updated
within Stephenson County, IL, corresponding revisions to the FIRM Index will be
incorporated to reflect the effective dates of those panels.
FLOOD RISK REPORT: A Flood Risk Report (FRR) may be available for many of the
flooding sources and communities referenced in this FIS Report. The FRR is provided to
increase public awareness of flood risk by helping communities identify the areas within their
jurisdictions that have the greatest risks. Although non-regulatory, the information provided
within the FRR can assist communities in assessing and evaluating mitigation opportunities
to reduce these risks. It can also be used by communities developing or updating flood risk
mitigation plans. These plans allow communities to identify and evaluate opportunities to
reduce potential loss of life and property. However, the FRR is not intended to be the final
authoritative source of all flood risk data for a project area; rather, it should be used with other
data sources to paint a comprehensive picture of flood risk.
9
2.2
Community Description
Stephenson County is located within the Pecatonica River HUC-8 subbasin in
northwestern Illinois. The county extends approximately 20 miles from north to
south and 25 miles from east to west. It is bounded on the west by Jo Daviess
County, on the north by the State of Wisconsin, on the east by Winnebago
County, and on the south by Carroll and Ogle Counties. Stephenson County
includes 18 townships that make up 567 square miles, or 361,390 acres
(Reference 10). In 2010, the county’s population was 47,711 and that of the City
of Freeport, the county seat and largest community, was 25,638 (Reference 11).
Much of Stephenson County is devoted to agricultural production, and cropland
covers 186,627 acres, or 51.7 percent of the county. The remainder of the county
is covered by grassland (39 percent), forest/woodland (4.8 percent), wetland (1.4
percent), urban/built-up land (1.7 percent), open water (1.4 percent), and
barren/exposed land (0.1 percent). Of the 102 Illinois counties, Stephenson
County ranks third highest in grassland acreage (140,708 acres) and seventeenth
in perennial streams (Reference 12).
Along with agriculture, other established industries in the county include food
manufacturing and processing (including milk and milk products), lumber and
wood products, furniture and fixtures, chemicals, rubber and plastic products,
primary metal industries, fabricated metal industries, machinery, electrical
equipment and supplies, printing and publishing, stone and glass products, and
agricultural services. While industry is most highly developed in the City of
Freeport, it can also be found in many of the smaller towns (Reference 10).
The topography of Stephenson County is characterized by many knobs and
depressions, which are the result of glacial deposition. Glaciers covered this area
twelve to fifteen thousand years ago and left behind a residue of rock debris after
receding, and numerous small streams were created by melting of ice (Reference
3). The highest point in Stephenson County is located at the crest of a mound in
the northwestern part of the county at an elevation of approximately 1,160 feet
above sea level. The lowest elevation is approximately 690 feet above sea level
and occurs where the East Plum River leaves the county, about three-fourths of a
mile east of the southwest corner of the county. Level to sloping land surfaces,
which dominate most of the upland areas of Stephenson County, are conducive to
the development of poorly drained and somewhat poorly drained soils that have a
high water table (Reference 10).
The Pecatonica River is the principal stream in the county, and its average slope,
within Stephenson County, is less than 1 foot per mile (Reference 3). Major
drainage in Stephenson County is generally to the south and east by the
Pecatonica River and its numerous tributaries. The Pecatonica River flows
eastward into Winnebago County where it joins the Rock River, which in turn
discharges into the Mississippi River (Reference 10).
10
Stephenson County has a typical mid-western continental climate characterized
by cold winters and hot summers. According to records from the weather station
located in the City of Freeport (station 113262), the average annual temperature
for the county is 52.1 degrees Fahrenheit. The coldest average temperatures occur
in January and the warmest average temperatures in July. The average annual
total precipitation for the county is 34.79 inches (period of record: 1971-2000).
The monthly high precipitation record (period of record: 1948-2006) was set at
11.48 inches in June 1993, and the largest daily precipitation total of 9.12 inches
was recorded on August 22, 2002 (Reference 13).
2.3
Principal Flood Problems
The entirety of Stephenson County lies within the Mississippi River watershed,
and the Pecatonica River is the major tributary (Reference 14). Pecatonica River
has a large drainage area with limited capacity channel and a flat channel
gradient. The watershed topography is rolling, which causes discharges to rise
quickly (Reference 4). Yearly flooding occurs along the course of the Pecatonica
River (Reference 15), and floods in Stephenson County can be caused by both
summer and winter storms. Rainfall and freezing weather along with ice jams at
bridges also contribute to flood stages (Reference 3). The most severe floods
normally occur during the spring runoff period as a result of rainfall during the
snowmelt season (Reference 15).
Flooding of the Pecatonica River has occurred in numerous years. During the
flood of 1929, the Pecatonica River attained a river stage of 19.76, which remains
the highest stage ever recorded for this river (Reference 1). More recently, the
Pecatonica River reached major flood stage in 1990, 1993, and 1997, reaching
16.24, 16.61, and 16.40 feet, respectively, at the Freeport gage (Reference 16, 17).
The City of Freeport, which is contained exclusively within the Pecatonica River
HUC-8 subbasin (Reference 1), experienced flooding in the 1990s of a severity
requiring evacuation of large portions of the area, and virtually all neighborhood
streets have flooded since 1990 (Reference 18). In Winslow, development within
floodplain areas of the Pecatonica River and two of its tributaries has aggravated
flood problems (Reference 19). The areas flooded along Indian Creek in Winslow
are due to backwater from the Pecatonica River (Reference 4).
Table 5, “Historical Flood Data” summarizes the river stages of record on the
Pecatonica River at Freeport, Illinois. Information for the table was collected from
the National Weather Service’s Advanced Hydrologic Prediction Service
(Reference 20) and the USGS National Water Information System’s, Peak
Streamflow for Illinois (Reference 17).
11
Table 5 - Historical Flood Data
Pecatonica River at Freeport, Illinois - USGS Gage Number 05435500
Flood Stage: 13 feet
Date
March 16, 1929
March 28, 1916
February 25, 1922
September 16, 1914
April 6, 1923
March 16, 1919
April 3, 1933
February 27, 1915
July 3, 1969
March 25, 1975
Peak Streamflow (cfs)
18,400
17,000
14,500
13,000
13,100
10,000
8,600
9,070
11,000
13,900
River Stage (feet)
19.76
19.40
18.82
18.45
18.36
17.60
17.41
17.27
17.16
17.13
Yellow Creek and Goldmine Road Tributary also have the potential to flood
during any season, but flooding most commonly occurs during the winter and
spring when it is possible for floating ice to jam and partially dam the river,
causing water levels to rise (Reference 2). The overflow of Yellow Creek and
Goldmine Road Tributary are the primary cause of flood problems in the
community of Pearl City.
2.4
Flood Protection Measures
For purposes of the NFIP, FEMA only recognizes levee systems that meet, and
continue to meet, minimum design, operation, and maintenance standards that are
consistent with comprehensive floodplain management criteria. The Code of
Federal Regulations, Title 44, Section 65.10 (44 CFR 65.10) describes the
information needed for FEMA to determine if a levee system reduces the risk
from the 1-percent-annual-chance flood. This information must be supplied to
FEMA by the community or other party when a flood risk study or restudy is
conducted, when FIRMs are revised, or upon FEMA request. FEMA reviews the
information for the purpose of establishing the appropriate FIRM flood zone.
Levee systems that are determined to reduce the risk from the 1-percent-annualchance flood are accredited by FEMA.
Levees may exist in the study area that provide some degree of protection against
flooding. However, there are no levees in Stephenson County that protect against
the 1-percent-annual-chance flood.
12
3.0
ENGINEERING METHODS
For the flooding sources studied by Zone AE methods in Stephenson County, standard
hydrologic and hydraulic study methods were used to determine the flood hazard data
required for this study. Flood events of a magnitude that are expected to be equaled or
exceeded once on the average during any 10-, 25-, 50-, 100-, or 500-year period
(recurrence interval) have been selected as having special significance for floodplain
management and for flood insurance rates. These events, commonly termed the 10-, 25-,
50-, 100-, and 500-year floods, have a 10-, 4-, 2-, 1-, and 0.2-percent chance,
respectively, of being equaled or exceeded during any year. Although the recurrence
interval represents the long term, average period between floods of a specific magnitude,
rare floods could occur at short intervals or even within the same year. The risk of
experiencing a rare flood increases when periods greater than 1 year are considered. For
example, the risk of having a flood which equals or exceeds the 1-percent-annual-chance
flood in any 50-year period is approximately 40 percent (4 in 10), and, for any 90-year
period, the risk increases to approximately 60 percent (6 in 10). The analyses reported
herein reflect flooding potential based on conditions existing in Stephenson County at the
time of completion of this study. Maps and flood elevations will be amended periodically
to reflect future changes.
3.1
Hydrologic Analyses
Hydrologic analyses were carried out to establish peak discharge-frequency
relationships for each flooding source studied by Zone AE methods affecting the
county. Hydrologic analyses that have not been superseded have been compiled
and are summarized below.
Pre-Countywide FISs
The peak discharges for Indian Creek were obtained using regional equations for
flood analysis found in Technique for Estimating Magnitude and Frequency of
Floods in Illinois (Reference 21). The peak discharges were calculated for
different frequencies at State Route 73 and at the western corporate limit of
Winslow at the time of the study (approximately 3,500 feet upstream of State
Route 73).
Discharges for Goldmine Road Tributary and the Pearl City reach of Yellow
Creek were obtained using regional equations for flood analysis found in
Technique for Estimating Magnitude and Frequency of Floods in Illinois
(Reference 21) and from historic data.
Hydrologic information for the Freeport reach of Yellow Creek can be found
below in the subsection, “March 16, 2015, Revised Countywide FIS”.
13
March 3, 2011
Initial Countywide FIS
No new detailed hydrologic studies were conducted for the initial countywide
FIS.
March 16, 2015
Revised Countywide FIS
Hydrologic modeling of the Pecatonica River and of the Freeport reach of Yellow
Creek establishes peak discharges for the 10-, 4-, 2-, 1-, and 0.2-percent-annualchance flood events. An updated flow frequency analysis at stream gage locations
reflects additional data and is based on Bulletin 17B, using the USGS PeakFQ
software (Reference 22). The generalized skew coefficients were obtained from
the 2004 USGS report, Estimating Flood-Peak Discharge Magnitudes and
Frequencies for Rural Streams in Illinois (Reference 23). Regression discharge
values were determined using USGS StreamStats version 2 software (Reference
24). StreamStats version 2 was also used to calculate regression discharge values
at ungaged locations using Illinois regression equations (Reference 23) in
conjunction with the National Elevation Dataset (Reference 25). The results of the
Pecatonica River study are detailed in Pecatonica River Hydrologic Analysis,
Stephenson County, Illinois (Reference 6). The results of the Yellow River study
are detailed in Hydrology and Hydraulic Modeling and Floodplain Mapping for
Yellow Creek in Stephenson County, Illinois (Reference 7).
Hydrologic information for the Pearl City reach of Yellow Creek can be found
above in the subsection, “Pre-Countywide FISs”.
A summary of the drainage area-peak discharge relationships for all the streams
studied by detailed methods is shown in Table 6, “Summary of Discharges.”
14
Table 6 - Summary of Discharges
Peak Discharges (cubic feet per second)
4-Percent2-Percent1-PercentAnnual-Chance
Annual-Chance
Annual-Chance
Drainage Area
(square miles)
10-PercentAnnual-Chance
Goldmine Road Tributary
At mouth
2.2
*
*
*
957
*
Indian Creek
At State Route 73
14.1
2,198
*
3,399
3,910
5,126
Just downstream of
Stephenson-Winnebago
County line
1,664
11,200
14,400
17,000
19,700
26,600
Downstream of
confluence with Yellow Creek
1,534
10,500
13,600
16,100
18,800
25,500
At USGS Gage No.
05435500 at Freeport, IL
1,327
9,270
12,200
14,580
17,160
24,000
Downstream of
confluence with Richland
Creek
1,297
9,440
12,350
14,740
17,320
24,150
Downstream of
confluence with Cedar and
Indian Creek
1,055
9,790
13,193
16,040
19,130
27,490
At USGS Gage No.
05434500 at Martintown, WI
1,034
9,800
13,200
16,100
19,300
27,600
196
5,120
6,610
7,750
8,880
11,600
Flooding Source and Location
0.2-PercentAnnual-Chance
Pecatonica River
Yellow Creek
At the confluence with
Pecatonica River
* Data not available
15
Stillwater elevations determined for flooding sources studied by Zone AE
methods are summarized in Table 7, “Summary of Stillwater Elevations.”
Table 7 - Summary of Stillwater Elevations
Flooding Source and Location
Lake Summerset
Located between the Winnebago county
boundary and a point approximately 350
feet upstream of Davis Road
10-PercentAnnual-Chance
*
Elevation (feet NAVD 88)
2-Percent1-PercentAnnual-Chance
Annual-Chance
*
828.0
0.2-PercentAnnual-Chance
*
* Data Not Available
3.2
Hydraulic Analyses
Analyses of the hydraulic characteristics of flooding from the sources studied
were carried out to provide estimates of the elevations of floods of the selected
recurrence intervals. Base flood elevations on the FIRM represent the elevations
shown on the Flood Profiles and in the Floodway Data tables in the FIS Report.
Rounded whole-foot elevations may be shown on the FIRM in coastal areas, areas
of ponding, and other areas with static base flood elevations. These whole-foot
elevations may not exactly reflect the elevations derived from the hydraulic
analyses.
Flood elevations shown on the FIRM are primarily intended for flood insurance
rating purposes. For construction and/or floodplain management purposes, users
are cautioned to use the flood elevation data presented in this FIS Report in
conjunction with the data shown on the FIRM. The hydraulic analyses for this FIS
were based on unobstructed flow. The flood elevations shown on the profiles are
thus considered valid only if hydraulic structures remain unobstructed, operate
properly, and do not fail.
Locations of selected cross sections used in the hydraulic analyses are shown on
the Flood Profiles (Exhibit 1). For stream segments for which a floodway was
computed (Section 4.2), selected cross section locations are also shown on the
FIRM (Exhibit 2).
Hydraulic analyses that have not been superseded have been compiled and are
summarized below.
16
Pre-Countywide FISs
Water-surface elevations for Goldmine Road Tributary were determined using the
flood-depths method described in Depth and Frequency of Floods in Illinois
(Reference 26). Ground elevations were obtained from topographic maps
(Reference 27).
For Indian Creek, cross section data for streams and bridges in the area were
obtained from field surveys made by the SCS. Additional sections were obtained
based on topographic maps at a scale of 1:24000, with a contour interval of 10
feet (Reference 27). The starting water-surface elevations for Indian Creek were
determined using the 10-percent-annual-chance flood elevation for Pecatonica
River. Water-surface elevations for floods of the selected recurrence intervals
were determined using the SCS “Water-Surface Profile Computer Program”
(WSP-2) (Reference 28).
Water-surface elevations for the Pearl City reach of Yellow Creek were
determined using the flood-depths method described in Depth and Frequency of
Floods in Illinois (Reference 26). Ground elevations were obtained from
topographic maps (Reference 27).
Hydraulic information for Freeport reach of Yellow Creek can be found below in
the subsection, “March 16, 2015, Revised Countywide FIS”.
March 3, 2011
Initial Countywide FIS
No new detailed hydraulic studies were conducted for the March 3, 2011 initial
countywide FIS.
March 16, 2015
Revised Countywide FIS
For the Pecatonica River and the Freeport reach of Yellow Creek, separate HECRAS (Version 4.1.0) computer models (Reference 29) using a steady flow routing
methodology were prepared. The model results were used to produce water
surface profiles for the 10-, 4-, 2-, 1-, and 0.2- percent-annual-chance events, and
floodway analysis was performed for the 1-percent-annual-chance event.
A digital elevation model (DEM) with a 3-foot cell size was produced by the
ISWS. The DEM was derived from a 2009 countywide light detection and
ranging (LiDAR) dataset distributed by the Illinois State Geological Survey
(ISGS) through the Illinois Height Modernization Program (ILHMP). The data
support a contour interval of 2 feet (Reference 30).
17
Cross-sectional, bridge and hydraulic structures were surveyed by the U.S. Army
Corps of Engineers for these studies on the Pecatonica River and the Freeport
reach of Yellow Creek. Detailed survey information is available in the ISWS
report, Pecatonica River Hydraulics, Stephenson County, Illinois (Reference 8).
Additional survey data was collected by Fehr-Graham Engineering for the
Freeport reach of Yellow Creek, consisting of both overbank and channel
geometry. Detailed survey information is available in the ISWS report, Hydrology
and Hydraulic Modeling and Floodplain Mapping for Yellow Creek in
Stephenson County, Illinois (Reference 7).
Field-surveyed data, channel and overbank if provided, were merged into cross
sections derived from the DEM using HEC-GeoRAS 10 for ArcGIS 10.0. For
non-surveyed cross sections, channel geometry was interpolated in HEC-RAS
between sections with field-surveyed channel data.
The water surface elevations at the upstream cross section from the Winnebago
County, Pecatonica River HEC-RAS model detailed in the report titled, Hydraulic
Modeling and Floodplain Mapping for Pecatonica River Winnebago County,
Illinois (Reference 31) were used as the downstream boundary conditions in the
Stephenson County Pecatonica River HEC-RAS model. For the Freeport reach of
Yellow Creek, normal depth was used as the downstream boundary condition
(Reference 7).
Hydraulic information for the Pearl City reach of Yellow Creek can be found in
the preceding subsection, “Pre-Countywide FISs”.
Channel and overbank roughness factors (Manning’s “n”) used in the hydraulic
computations were chosen by engineering judgment and were based on field
observations of the stream and floodplain areas. The range of the Manning’s “n”
coefficients for each stream is shown in Table 8, “Roughness Coefficients
(Manning’s ‘n’ Values).”
Table 8 - Roughness Coefficients (Manning's "n" Values)
Stream
Channel "n"
Overbank "n"
Goldmine Road Tributary
*
*
Indian Creek
0.04 - 0.054
0.055 - 0.075
0.035
0.040 - 0.100
Pecatonica River
0.035
0.040 - 0.100
Yellow Creek
* Data Not Available
18
3.3
Vertical Datum
All FIS Reports and FIRMs are referenced to a specific vertical datum. The
vertical datum provides a starting point against which flood, ground, and structure
elevations can be referenced and compared. Until recently, the standard vertical
datum used for newly created or revised FIS Reports and FIRMs was the National
Geodetic Vertical Datum of 1929 (NGVD29). With the completion of the North
American Vertical Datum of 1988 (NAVD88), many FIS Reports and FIRMs are
now prepared using NAVD88 as the referenced vertical datum.
Flood elevations shown in this FIS Report and on the FIRMs are referenced to
NAVD88. These flood elevations must be compared to structure and ground
elevations referenced to the same vertical datum. For information regarding
conversion between NGVD29 and NAVD88 or other datum conversion, see
FEMA’S Guidelines and Specifications for Flood Hazard Mapping Partners
Appendix B: Guidance for Converting to the North American Vertical Datum of
1988 (Reference 32) or visit the National Geodetic Survey website at
www.ngs.noaa.gov. the National Geodetic Survey may also be contacted at the
following address:
NGS Information Services
NOAA, N/NGS12
National Geodetic Survey
SSMC-3, #9202
1315 East-West Highway
Silver Spring, Maryland 20910-3282
(301) 713-3242
Temporary vertical monuments are often established during the preparation of a
flood hazard analysis for the purpose of establishing local vertical control.
Although these monuments are not shown on the FIRM, they may be found in the
archived project documentation associated with the FIS Report and the FIRMs for
this community. Interested individuals may contact FEMA to access these data.
To obtain current elevation, description, and/or location information for
benchmarks in the area, please contact information services branch of the NGS at
(301) 713-3242, or visit their website at www.ngs.noaa.gov.
The datum conversion locations and values that were calculated for Stephenson
County are provided below:
19
March 3, 2011
Initial Countywide FIS
Effective information for the March 3, 2011 FIS was converted from NGVD29 to
NAVD88 based on data presented in Figure 2 and Table 9a. Computations show
an average conversion factor of -0.283 feet (NGVD29 – 0.283 = NAVD88) for
the county. The conversion factor was applied uniformly across the county and
used to prepare the Summary of Stillwater Elevations Table, Floodway Data
Tables, Flood Profiles, and FIRMs.
However, there was one instance (Pecatonica River) where this countywide
conversion factor could not be used and instead, the Multiple Conversion Factors
(stream-by-stream) method was required. The stream-by-stream conversion factor
was applied when a stream was located in two or more counties (multi-county
stream), whereby the average conversion factor determined for each county
differed from each other by more than one-tenth of a foot. For the stream-bystream method, each studied stream was assigned an average conversion factor
based on the conversion factors at three points along the stream. These results are
shown in Table 9b.
For this Revision
Study information for this PMR is referenced to NAVD88 and required no
conversion.
20
Figure 2 - Vertical Datum Conversions
USGS Quadrangle Corner Intersections
The change in elevation for each Point ID is listed in Table 9a
21
Table 9a - Vertical Datum Conversions
Single Conversion Factor (countywide) Method
Stephenson County
Point
ID#
Quadrangle Name
Corner
NAD83
Latitude
(dec. deg.)
1
Seward
NW
42.250
2
German Valley
NW
42.250
3
Forreston North
NW
42.250
4
Shannon
NW
42.250
5
Boone Branch
NW
42.250
6
Pecatonica
NW
42.375
7
Ridott
NW
42.375
8
Freeport East
NW
42.375
9
Freeport West
NW
42.375
10
Pearl City
NW
42.375
11
Durand
NW
42.500
12
Davis
NW
42.500
13
Dakota
NW
42.500
14
Orangeville
NW
42.500
15
Lena
NW
42.500
Range of conversion values
Average conversion values
Maximum variance from the average conversion
Maximum variance from a no-conversion value
NAD83
Longitude
(dec. deg.)
NGVD29 to NAVD88
Elevation Change
(feet)
89.375
89.500
89.625
89.750
89.875
89.375
89.500
89.625
89.750
89.875
89.375
89.500
89.625
89.750
89.875
-0.180
-0.295
-0.322
-0.249
-0.190
-0.240
-0.335
-0.367
-0.335
-0.262
-0.246
-0.266
-0.308
-0.338
-0.315
-0.367 through -0.18
-0.283
0.103
0.367
Table 9b - Vertical Datum Conversions
Multiple Conversion Factors (stream-by-stream) Method
Stephenson County
Point Location
Pecatonica River
Downstream
Intermediate
Upstream
County
NAD83
Latitude
(dec. deg.)
NAD83
Longitude
(dec. deg.)
NGVD29 to NAVD88
Elevation Change
(Feet)
Stephenson
Stephenson
Stephenson
42.322013
42.331019
42.505585
89.397754
89.677804
89.795039
-0.262
-0.364
-0.341
22
Maximum
Offset
Average
Conversion
0.060
-0.322
4.0
FLOODPLAIN MANAGEMENT APPLICATIONS
The NFIP encourages state and local governments to adopt sound floodplain management
programs. Therefore, each FIS provides 1-percent-annual-chance flood elevations and
delineations of the 1- and 0.2-percent-annual-chance floodplain boundaries and 1percent-annual-chance floodway to assist communities in developing floodplain
management measures. This information is presented on the FIRM and in many
components of the FIS report, including Flood Profiles, Floodway Data tables, and
Summary of Stillwater Elevation tables. Users should reference the data presented in the
FIS as well as additional information that may be available at the local community map
repository before making flood elevation and/or floodplain boundary determinations.
4.1
Floodplain Boundaries
To provide a national standard without regional discrimination, the 1-percentannual-chance flood has been adopted by FEMA as the base flood for floodplain
management purposes. The 0.2-percent-annual-chance flood is employed to
indicate additional areas of flood risk in the community. For the flooding sources
studied by detailed methods, the 1- and 0.2-percent-annual-chance floodplain
boundaries have been delineated using the flood elevations determined at each
cross section.
Pre-Countywide FISs
Boundaries were interpolated between cross sections using topographic maps at a
scale of 1:24,000 and contour interval of 10 feet (Reference 27).
March 3, 2011
Initial Countywide FIS
For the initial countywide FIS, Illinois State Geological Survey statewide
ArcSDE raster mosaic of USGS digital raster graphics at a scale of 1:24,000 and
the USGS National Elevation Dataset were used (Reference 25, 33).
A portion of an existing Zone A along Richland Creek in the Village of
Orangeville was re-delineated using information from hydrologic and hydraulic
analyses performed for the waste water treatment plant (Reference 34).
For streams studied by Zone A methods, the boundary of the 1-percent-annualchance flood was delineated with the use of USGS Flood-Prone Area Maps
(Reference 35) and a Surface Drainage and Flooding Map for Stephenson County
prepared by W.S. Lawrence and Associates (Reference 36). Additional references
considered include the USACE’s Survey Report for Flood Control on Pecatonica
River (Reference 37) and the official zoning maps for Stephenson County
(Reference 3, 38).
23
March 16, 2015
Revised Countywide FIS
For this Physical Map Revision, a Digital Elevation Model (DEM) with a 3-foot
cell size produced by the ISWS was used for re-delineation. The DEM was
derived from a 2009 countywide LiDAR dataset (Reference 30) distributed by the
Illinois State Geological Survey (ISGS) through the Illinois Height Modernization
Program (ILHMP). The data support a contour interval of 2 feet.
The 1- and 0.2-percent floodplain boundaries are shown on the FIRM (Exhibit 2).
On this map, the 1-percent-annual-chance floodplain boundary corresponds to the
boundary of the areas of special flood hazards (Zones A, AH, AO, and AE); and
the 0.2-percent-annual-chance floodplain boundary corresponds to the boundary
of areas of moderate flood hazards. In cases where the 1- and 0.2-percent-annualchance floodplain boundaries are close together or collinear, only the 1-percentannual-chance floodplain boundary has been shown. Small areas within the
floodplain boundaries may lie above the flood elevations but cannot be shown due
to limitations of the map scale and/or lack of detailed topographic data.
For the streams studied by Zone A methods, only the 1-percent-annual-chance
floodplain boundary is shown on the FIRM (Exhibit 2).
4.2
Floodways
Encroachment on floodplains, such as structures and fill, has the potential to
reduce flood-carrying capacity, increase flood heights and velocities, and increase
flood hazards in areas beyond the encroachment itself. For purposes of the NFIP,
a floodway is used as a tool to assist local communities in floodplain
management. Under this concept, the area of the 1-percent-annual-chance
floodplain is divided into a floodway and a floodway fringe.
The floodway is the channel of a stream, plus any adjacent floodplain areas (see
Figure 3, “Floodway Schematic”) that must be kept free of encroachment so that
the 1-percent-annual-chance flood can be carried without substantial increases in
flood heights. Minimum federal standards limit such increases to 1.0 foot,
provided that hazardous velocities are not produced. In Illinois, however, under
the Rivers, Lakes and Streams Act (615 ILCS 5/23, 29 & 30 and 615 ILCS 5/18),
encroachment in the floodplain is limited to that which will cause only an
insignificant increase in flood heights (Reference 39). The State of Illinois has
adopted this more stringent criterion which limits the increase in flood heights to
0.1 foot, no more than a 10 percent reduction in floodplain volume, and no more
than a 10 percent increase in average velocity. This has generally been interpreted
as the least surcharge measurable, consistent with the encroachment option of the
computer program utilized for the floodway determination. The floodways in this
FIS are presented to local agencies as a minimum standard that can be adopted
directly or that can be used as a basis for additional floodway studies.
24
The area between the floodway and the 1-percent-annual-chance floodplain
boundaries is termed the floodway fringe. The floodway fringe encompasses the
portion of the floodplain that could be completely obstructed without increasing
the water-surface elevation of the 1-percent-annual-chance flood by more than 0.1
foot at any point. Typical relationships between the floodway and the floodway
fringe and their significance to floodplain development are shown in Figure 3,
“Floodway Schematic.”
Figure 3 - Floodway Schematic
The floodway presented in this FIS report and on the FIRM was computed for
certain stream segments on the basis of equal conveyance reduction from each
side of the floodplain. Floodway widths were computed at cross sections.
Between cross sections, the floodway boundaries were interpolated. The results of
the floodway computations have been tabulated for selected cross sections (see
Table 10, “Floodway Data”). The computed floodways are shown on the FIRM
(Exhibit 2). In cases where the floodway and 1-percent-annual-chance floodplain
boundaries are either close together or collinear, only the floodway boundary is
shown
In the State of Illinois, any portion of a stream or watercourse that lies within the
floodway fringe of a studied (AE) stream may have a state regulated floodway.
The FIRM may not depict these state regulated floodways.
Floodways restricted by anthropogenic features such as bridges and culverts are
drawn to reflect natural conditions and may not agree with the widths listed in the
floodway data table in the Flood Insurance Study. The floodway as shown on the
FIRM should be used for regulatory purposes.
25
In Illinois, along streams where floodways have not been computed, the
community must obtain state permit approval (when applicable) for development.
This ensures that the cumulative effect of development in the floodplain will not
cause an increase in the base flood elevations that creates a potential for flood
damages.
March 16, 2015
Revised Countywide FIS
As a part of this Physical Map Revision, the floodway for the entire studied reach
of the Pecatonica River within Stephenson County was revised.
In addition, a new floodway was computed for the Freeport reach of Yellow
Creek.
26
FLOODING SOURCE
1-PERCENT-ANNUAL-CHANCE FLOOD
WATER SURFACE ELEVATION (FEET NAVD88)
FLOODWAY
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
WITHOUT
FLOODWAY
WITH
FLOODWAY
INCREASE
*
*
777.9
778.4
778.7
0.1
0.1
0.1
CROSS SECTION
DISTANCE
WIDTH
(FEET)
Goldmine Road Tributary
*
*
*
*
*
*
Indian Creek
A
B
C
730
1
1,730
1
2,580
374
400
355
1,637
2,400
2,316
2.4
1.6
1.7
778.3
778.3
778.6
777.8
778.3
778.6
1
2
1
Feet above confluence with Pecatonica River
Elevation computed without consideration of backwater effects from Pecatonica River
* Data Not Available
FEDERAL EMERGENCY MANAGEMENT AGENCY
2
TABLE 10
STEPHENSON COUNTY, IL
AND INCORPORATED AREAS
FLOODWAY DATA
GOLDMINE ROAD TRIBUTARY
INDIAN CREEK
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE FLOOD
WATER SURFACE ELEVATION (FEET NAVD88)
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
WITHOUT
FLOODWAY
WITH
FLOODWAY
INCREASE
7,090
2
7,292
4,855
4,193
5,154
3,333
2,113
2
6,451
4,908
3,110
3,219
3,406
3,996
6,279
5,763
3,869
3,975
3,708
3,469
2,226
3,051
1,397
20,563
30,278
22,272
22,723
24,029
11,410
9,530
24,367
24,108
13,031
13,989
15,735
25,368
37,520
30,378
23,578
23,160
17,072
14,396
22,300
24,421
11,152
3.1
1.1
0.9
0.9
1.0
1.8
2.0
1.3
0.7
1.3
1.2
1.1
0.7
0.6
0.7
1.4
0.9
1.1
1.3
0.9
0.8
1.7
746.0
749.3
749.9
751.9
752.1
753.2
754.5
755.2
757.0
757.8
758.2
761.4
762.8
763.9
764.0
765.1
765.7
766.3
766.9
772.3
772.7
772.7
746.0
749.3
749.9
751.9
752.1
753.2
754.5
755.2
757.0
757.8
758.2
761.4
762.8
763.9
764.0
765.1
765.7
766.3
766.9
772.3
772.7
772.7
746.0
749.3
749.9
751.9
752.1
753.2
754.6
755.3
757.0
757.9
758.2
761.5
762.8
763.9
764.0
765.1
765.7
766.3
766.9
772.3
772.7
772.8
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.1
0.0
0.1
0.0
0.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
Pecatonica River
39.33
43.38
44.96
48.05
48.87
50.15
51.83
54.22
59.27
60.86
61.46
63.04
65.07
67.69
68.23
71.20
72.43
73.83
75.61
78.25
80.94
81.17
1
2
39.33
43.38
44.96
48.05
48.87
50.15
51.83
54.22
59.27
60.86
61.46
63.04
65.07
67.69
68.23
71.20
72.43
73.83
75.61
78.25
80.94
81.17
TABLE 10
Miles above confluence with Rock River
Floodway width includes areas of high ground
FEDERAL EMERGENCY MANAGEMENT AGENCY
STEPHENSON COUNTY, IL
AND INCORPORATED AREAS
FLOODWAY DATA
PECATONICA RIVER
FLOODING SOURCE
CROSS SECTION
Pecatonica River
(continued)
82.32
83.92
86.34
88.62
90.48
92.37
93.21
1
2
DISTANCE
82.32
83.92
86.34
88.62
90.48
92.37
93.21
1-PERCENT-ANNUAL-CHANCE FLOOD
WATER SURFACE ELEVATION (FEET NAVD88)
FLOODWAY
1
WIDTH
(FEET)
5,432
1,964
885
2,232
1,646
883
736
2
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
WITHOUT
FLOODWAY
WITH
FLOODWAY
INCREASE
47,101
14,997
7,250
24,683
13,818
8,708
6,554
0.9
1.3
2.6
0.8
1.4
2.2
2.9
774.6
774.7
776.0
777.6
777.8
778.7
779.1
774.6
774.7
776.0
777.6
777.8
778.7
779.1
774.6
774.7
776.0
777.6
777.8
778.7
779.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
TABLE 10
Miles above confluence with Rock River
Floodway width includes areas of high ground
FEDERAL EMERGENCY MANAGEMENT AGENCY
STEPHENSON COUNTY, IL
AND INCORPORATED AREAS
FLOODWAY DATA
PECATONICA RIVER
FLOODING SOURCE
CROSS SECTION
Yellow Creek
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
1
DISTANCE
6,212
7,608
8,601
11,436
14,733
17,240
19,025
21,704
22,983
23,707
25,831
27,202
28,430
29,245
30,131
31,836
34,122
34,540
35,581
35,855
36,146
36,610
1-PERCENT-ANNUAL-CHANCE FLOOD
WATER SURFACE ELEVATION (FEET NAVD88)
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
WITHOUT
FLOODWAY
WITH
FLOODWAY
INCREASE
4,860
4,700
3,215
2,740
2,130
1,920
2,300
3,400
1,480
785
1,099
1,540
970
465
750
930
590
500
280
315
300
315
16,246
36,985
31,506
24,875
16,005
13,436
14,286
19,686
9,648
4,694
6,852
8,422
4,161
3,745
4,671
4,899
3,883
3,549
2,337
2,749
2,496
2,308
3.5
2.3
0.9
0.4
0.6
1.8
0.8
1.8
1.7
1.9
1.7
1.3
3.2
2.6
1.9
1.8
2.4
2.5
4.0
3.2
4.4
3.9
760.2
761.9
762.8
762.8
762.9
762.9
763.8
763.9
765.4
765.5
767.0
767.1
767.4
769.2
769.5
770.1
771.6
772.5
773.2
774.5
774.6
775.7
760.2
761.9
762.8
762.8
762.9
762.9
763.8
763.9
765.4
765.5
767.0
767.1
767.4
769.2
769.5
770.1
771.6
772.5
773.2
774.5
774.6
775.7
760.2
761.9
762.8
762.8
763.0
763.0
763.8
763.9
765.5
765.5
767.0
767.2
767.5
769.2
769.5
770.1
771.6
772.6
773.2
774.6
774.6
775.8
0.0
0.0
0.0
0.0
0.1
0.1
0.0
0.0
0.1
0.0
0.0
0.1
0.1
0.0
0.0
0.0
0.0
0.1
0.0
0.1
0.0
0.1
TABLE 10
Feet above confluence with Pecatonica River
FEDERAL EMERGENCY MANAGEMENT AGENCY
STEPHENSON COUNTY, IL
AND INCORPORATED AREAS
FLOODWAY DATA
YELLOW CREEK
FLOODING SOURCE
CROSS SECTION
Yellow Creek
(continued)
W
X
Y
Z
AA
AB
1
DISTANCE
37,451
38,357
39,151
40,825
42,702
43,230
1-PERCENT-ANNUAL-CHANCE FLOOD
WATER SURFACE ELEVATION (FEET NAVD88)
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
WITHOUT
FLOODWAY
WITH
FLOODWAY
INCREASE
275
400
410
435
550
275
2,538
3,046
3,518
3,349
4,178
2,791
3.5
3.1
2.6
2.7
3.4
3.2
776.5
776.8
777.7
778.4
779.4
781.0
776.5
776.8
777.7
778.4
779.4
781.0
776.6
776.9
777.8
778.5
779.5
781.0
0.1
0.1
0.1
0.1
0.1
0.0
TABLE 10
Feet above confluence with Pecatonica River
FEDERAL EMERGENCY MANAGEMENT AGENCY
STEPHENSON COUNTY, IL
AND INCORPORATED AREAS
FLOODWAY DATA
YELLOW CREEK
5.0
INSURANCE APPLICATIONS
For flood insurance rating purposes, flood insurance zone designations are assigned to a
community based on the results of the engineering analyses. The zones are as follows:
Zone A
Zone A is the flood insurance rate zone that corresponds to the 1-percent-annual-chance
floodplains that are determined in the FIS by Zone A methods. Because detailed
hydraulic analyses are not performed for such areas, no base flood elevations or depths
are shown within this zone.
Zone AE
Zone AE is the flood insurance rate zone that corresponds to the 1-percent-annual-chance
floodplains that are determined in the FIS by detailed methods. In most instances, wholefoot base flood elevations derived from the detailed hydraulic analyses are shown at the
selected intervals within this zone.
Zone AH
Zone AH is the flood insurance rate zone that corresponds to the areas of 1-percentannual-chance shallow flooding (usually areas of ponding) where average depths are
between 1 and 3 feet. In most instances, whole-foot base flood elevations derived from
the detailed hydraulic analyses are shown at selected intervals within this zone.
Zone AO
Zone AO is the flood insurance rate zone that corresponds to the areas of 1-percentannual-chance shallow flooding (usually sheet flow on sloping terrain) where average
depths are between 1 and 3 feet. Average whole-foot depths derived from the detailed
hydraulic analyses are shown within this zone.
Zone X (Shaded)
Zone X is the flood insurance rate zone that corresponds to areas outside the 0.2-percentannual-chance floodplain, areas within the 0.2-percent-annual-chance floodplain, and to
areas of 1-percent-annual-chance flooding where average depths are less than 1 foot,
areas of 1-percent-annual-chance flooding where the contributing drainage area is less
than 1 square mile, and areas protected from the 1-percent-annual-chance flood by levees.
No base flood elevations or depths are shown within this zone.
32
6.0
FLOOD INSURANCE RATE MAP
The FIRM is designed for flood insurance and floodplain management applications.
For flood insurance applications, the map designates flood insurance rate zones as
described in Section 5.0 and, in the 1-percent-annual-chance floodplains that were
studied by detailed methods, shows selected whole-foot base flood elevations or average
depths. Insurance agents use the zones and base flood elevations in conjunction with
information on structures and their contents to assign premium rates for flood insurance
policies.
For floodplain management applications, the map shows by tints, screens, and symbols,
the 1- and 0.2-percent-annual-chance floodplains. Floodways and the locations of
selected cross sections used in the hydraulic analyses and floodway computations are
shown where applicable
The current FIRM presents flooding information for the entire geographic area of
Stephenson County. The countywide FIRM also includes flood hazard information that
was presented separately on Flood Boundary and Floodway Maps (FBFM), where
applicable. Historical data relating to the community maps prepared is presented in Table
11, “Community Map History.”
7.0
OTHER STUDIES
Flood Insurance Studies have been prepared for Carroll, Jo Daviess, Ogle, and
Winnebago Counties, Illinois and Green and Lafayette Counties, Wisconsin (Reference
40).
Information pertaining to revised and unrevised flood hazards for each jurisdiction, or the
portions of each jurisdiction, within Stephenson County has been compiled into this FIS.
Therefore, this FIS supersedes all previously printed FISs, FHBMs, FBFMs, and FIRMs
for all of the incorporated and unincorporated jurisdictions in Stephenson County.
8.0
LOCATION OF DATA
Information concerning the pertinent data used in the preparation of this FIS Report can
be obtained by submitting an order with any required payment to the FEMA Engineering
Library. For more information on this process, see http://www.fema.gov.
33
COMMUNITY NAME
Cedarville, Village of
Dakota, Village of
1, 2
1, 2
Davis, Village of
Freeport, City of
German Valley, Village of
Lena, Village of
2
Orangeville, Village of
Pearl City, Village of
Ridott, Village of
2
Rock City, Village of
1, 2
Stephenson County
(Unincorporated Areas)
1, 2
INITIAL IDENTIFICATION
FLOOD HAZARD
BOUNDARY MAP
REVISION DATE(S)
FLOOD INSURANCE
RATE MAP
EFFECTIVE DATE
April 11, 1975
None
N/A
N/A
N/A
N/A
N/A
N/A
N/A
November 2, 1973
None
May 16, 1977
N/A
N/A
N/A
N/A
N/A
N/A
August 16, 1974
March 26, 1976
N/A
May 3, 1974
December 5, 1975
September 29, 1989
N/A
N/A
N/A
N/A
N/A
N/A
December 27, 1974
None
February 15, 1978
TABLE 11
Winslow, Village of
March 15, 1974
July 2, 1976
1
No special flood hazard areas identified
2
This community does not have map history prior to the first countywide mapping
N/A - Not Applicable
FEDERAL EMERGENCY MANAGEMENT AGENCY
STEPHENSON COUNTY, IL
AND INCORPORATED AREAS
FLOOD INSURANCE
RATE MAP
REVISION DATES(S)
July 30, 1982
November 17, 1982
COMMUNITY MAP HISTORY
9.0
BIBLIOGRAPHY AND REFERENCES
1.
U.S. Department of Housing & Urban Development, Federal Insurance Administration.
Flood Insurance Study: City of Freeport, Illinois, Stephenson County. Washington D.C.:
November 1976.
2.
Federal Emergency Management Agency. Flood Insurance Study: Village of Pearl City,
Illinois, Stephenson County. Washington, D.C.: September 29, 1989.
3.
Federal Emergency Management Agency. Flood Insurance Study: Stephenson County,
Illinois, Unincorporated Areas. Washington, D.C.: July 1982.
4.
Federal Emergency Management Agency. Flood Insurance Study: Village of Winslow,
Illinois, Stephenson County. Washington, D.C.: May 17, 1982.
5.
Surdex Corporation and Science Applications International. USGS Digital Orthophoto
Quarter-Quadrangles. U.S. Geological Survey, Sioux Falls, South Dakota: 2005.
6.
Markus, M., Flegel, A. J., and McConkey, S. A. Pecatonica River Hydrologic Analysis,
Stephenson County, Illinois. Prepared by the Illinois State Water Survey for the Federal
Emergency Management Agency, July 2010, Revised July 2012.
7.
Thomas, Aaron, P.E. and Abdou, Sherif, F. E. Hydrology and Hydraulic Modeling and
Floodplain Mapping for Yellow Creek in Stephenson County, Illinois. Prepared by the
Illinois State Water Survey for the Illinois Department of Natural Resources, November
27, 2012, Rev. March 5, 2013.
8.
Thomas, Aaron, P.E. and Gambill, Dan, P.E. Pecatonica River Hydraulics, Stephenson
County, Illinois. Prepared by the Illinois State Water Survey for the Illinois Department
of Natural Resources, August 3, 2012, Revised March 07, 2013, Revised August 25,
2013, Revised January 14, 2014.
9.
Aero-Metric, Inc. Digital Orthoimagery, Stephenson County, Illinois. Provided by U.S.
Geological Survey. Spatial resolution: 1 foot; Aerial photography: Spring 2011.
10.
Gerhard, Erik A. and Frank E. Heisner. Soil Survey of Stephenson County, Illinois.
United State Department of Agriculture, Natural Resources Conservation Service, in
cooperation with the Illinois Agricultural Experiment Station, 2008.
11.
U.S. Census Bureau. State and County Quick Facts - Stephenson County, Illinois.
[Accessed: August 2013]. Available from: http://quickfacts.census.gov/qfd/states/17/
17177.html.
35
12.
Illinois Department of Natural Resources. Illinois Land Cover Summary Data by
Counties. 2007. http://dnr.state.il.us/orep/ctap/map/counties.htm
13.
Illinois State Climatologist Office. Illinois Climate Summaries (normals and extremes).
Illinois State Water Survey, Institute of Natural Resource Sustainability, University of
Illinois at Urbana-Champaign, 2007. [Accessed: August 2013]. Available from:
http://www.isws.illinois.edu/atmos/statecli/Summary/Illinois.htm.
14.
Partners in Planning Division, Freeport Area Economic Development Foundation. Future
Land Use Plan for Stephenson County, Illinois. July 12, 2000.
15.
U.S. Department of Agriculture, Soil Conservation Service in cooperation with State of
Illinois, Department of Transportation, Division of Water Resources. Final Flood Hazard
Analysis Reconnaissance Study, Village of Ridott, Stephenson County, Illinois.
Champaign, Illinois: November 1980.
16.
Illinois Department of Natural Resources, Office of Water Resources. River Stages in
Illinois. Springfield, Illinois: August 2004.
17.
U.S. Geological Survey. Peak Streamflow for Illinois. [Accessed: March 2009].
Available from: http://nwis.waterdata.usgs.gov/il/nwis/peak.
18.
Zar, Carol B., John L. Lewis, Gerald H. Veach, and John Nevenhoven. Freeport Eastside
Community Improvement Project. August 1996.
19.
State of Illinois, Division of Water Resources. Reconnaissance Study for Flood Control:
Village of Winslow, Stephenson County, Illinois. 1979.
20.
National Weather Service. Advanced Hydrologic Prediction Service - Pecatonica River
at Freeport. [Accessed: March 2009]. Available from: http://www.crh.noaa.gov/ahps2/
hydrograph.php?wfo=dvn&gage=feei2&view=1,1,1,1,1,1,1,1.
21.
U.S. Geological Survey. Technique for Estimating Magnitude and Frequency of Floods
in Illinois. Water Resources Investigations 77-117, Curtis, G. W., July 1977.
22.
Flynn, K. M., Kirby, W., and Hummel, P. R. User's manual for program PeakFQ, annual
flood frequency analysis using Bulletin 17B guidelines. U.S. Geological Survey
Technical Methods, 4-B4, 2006.
23.
Soong, D. T., Ishii, A. L., Sharpe, J. B., and Avery, C. F. Estimating Flood-Peak
Discharge Magnitudes and Frequencies for Rural Streams in Illinois. U.S. Geological
Survey Science Investigations Report 2004-5103, 2004.
36
24.
U.S. Geological Survey. StreamStats. 2010. [Accessed: 5/22/2010]. Available from:
http://water.usgs.gov/osw/streamstats/.
25.
U.S. Geological Survey (USGS) EROS Data Center. National Elevation Dataset. Sioux
Falls, South Dakota: 1999.
26.
Illinois Department of Transportation, Division of Water Resources. Depth and
Frequency of Floods in Illinois. B. J. Prugh, Jr., 1976.
27.
U.S. Geological Survey. 7.5 Minute Series Topographic Maps. Scale 1:24000, Contour
Interval 10 Feet: Pearl City, Illinois, 1971.
28.
U.S. Department of Agriculture, Soil Conservation Service. WSP-2 Water-Surface
Profile Computer Program. Technical Release No. 61, Champaign, Illinois: May 1976.
29.
U.S. Army Corps of Engineers, Hydrologic Engineering Center. Hydrologic Engineering
Center’s River Analysis System (HEC-RAS) Computer Program, Version 4.1.0. 2010.
30.
Aero-Metric, Inc. for the Illinois Department of Transportation. Stephenson County,
Illinois LiDAR. Contour Support: 2 feet; Vertical Datum: NAVD88; Horizontal Datum:
NAD83. 2009.
31.
Thomas, Aaron, P.E. and Gambill, Dan, P.E. Hydraulic Modeling and Floodplain
Mapping for Pecatonica River, Winnebago County, Illinois. Prepared by the Illinois State
Water Survey for the Illinois Department of Natural Resources, July 30, 2012, Revised
March 07, 2013.
32.
Federal Emergency Management Agency. Guidelines and Specifications for Flood
Hazard Mapping Partners. Appendix B: Guidance for Converting to the North American
Vertical Datum of 1988. April 2003. http://www.fema.gov/media-library-data/201307261543-20490-5385/frm__gsab.pdf
33.
Illinois State Geological Survey. Statewide ArcSDE Raster Mosaic of USGS DRGs.
Champaign, Illinois: 2003.
34.
Willett, Hofmann & Associates, Inc. Richland Creek Flood Plain Study, Village of
Orangeville, Stephenson County, Illinois. 1996.
35.
U. S. Geological Survey. Flood-Prone Area Maps. Browntown, Wisconsin-Illinois
Quadrangle (1970); Freeport, Illinois Quadrangle (1971); Pecatonica, Wisconsin-Illinois
Quadrangle (1972),
37
36.
Lawrence, W.S. and Associates. A Comprehensive Plan for Stephenson County, Illinois.
Surface Drainage and Flooding Map, July 1970.
37.
U.S. Army Corps of Engineers, Rock Island District. Survey Report for Flood Control on
Pecatonica River, Illinois and Wisconsin. Revised February 14, 1962.
38.
Stephenson County, Illinois. Stephenson County Zoning Ordinance. May 1, 1971.
39.
Illinois Compiled Statutes. Waterways Rivers, Lakes and Streams Act, 615 ILCS 5/23,
5/29 & 5/30; 615 ILCS 5/18. 2005.
40.
Federal Emergency Management Agency. Map Service Center: Product Catalog - Flood
Insurance Studies (FIS). [Accessed: 2009]. Available from: http://msc.fema.gov/
webapp/wcs/stores/servlet/StoreCatalogDisplay?storeId=10001&catalogId=10001&langI
d=-1&userType=G.
38