OGLE COUNTY, ILLINOIS AND INCORPORATED AREAS

Transcription

1 OGLE COUNTY, ILLINOIS AND INCORPORATED AREAS COMMUNITY NAME COMMUNITY NUMBER Ogle County ADELINE, VILLAGE OF BYRON, CITY OF *CRESTON, VILLAGE OF *DAVIS JUNCTION, VILLAGE OF FORRESTON, VILLAGE OF HILLCREST, VILLAGE OF LEAF RIVER, VILLAGE OF *MONROE CENTER, VILLAGE OF *MOUNT MORRIS, VILLAGE OF OGLE COUNTY (UNINCORPORATED AREAS) OREGON, CITY OF *POLO, CITY OF ROCHELLE, CITY OF *ROCKFORD, CITY OF STILLMAN VALLEY, VILLAGE OF *NO SPECIAL FLOOD HAZARD AREAS IDENTIFIED IN OGLE COUNTY REVISED: AUGUST 17, 2016 FLOOD INSURANCE STUDY NUMBER 17141CV000B

2 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: December 17, 2010 Revised Countywide FIS Effective Date(s): August 17, 2016

3 TABLE OF CONTENTS 1.0 INTRODUCTION Purpose of Study Authority and Acknowledgements Coordination AREA STUDIED Scope of Study Community Description Principal Flood Problems Flood Protection Measures ENGINEERING METHODS Hydrologic Analyses Hydraulic Analyses Vertical Datum FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Floodways INSURANCE APPLICATIONS FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES 50 i

4 TABLE OF CONTENTS (continued) FIGURES Figure 1. FIRM Notes to Users Figure 2. Vertical Datum Conversions 27 Figure 3. Floodway Schematic 31 TABLES Table 1. Multi-County Communities 1 Table 2. CCO Meeting Dates for Pre-Countywide Studies 4 Table 3. Stream Name Changes 5 Table 4. Limits of Revised or New Zone AE Study 6-7 Table 5. Limits of Zone AE Study 8-10 Table 6. Incorporated Letters of Map Change (December 17, 2010) 11 Table 7. Historical Flood Data 15 Table 8. Summary of Discharges Table 9. Summary of Stillwater Elevations 23 Table 10. Roughness Coefficients (Manning's "n" Values) 25 Table 11. Vertical Datum Conversions 28 Table 12. Floodway Data Table 13. Community Map History ii

5 TABLE OF CONTENTS (continued) EXHIBITS Exhibit 1 - Flood Profiles Banning Lateral Beach Creek Creston Ditch Flagg Creek Flagg Creek Tributary Grubin Ditch Hilb Ditch Intermodal Channel Johns Creek Kyte River Kyte River Overflow Kyte River Tributary 1 Lake Mistake Drain Mill Creek North Creston Ditch Rock River Ryley Ditch South Branch Kishwaukee River Steward Creek Stillman Creek Tributary No. 1 Panel 01P-02P 03P 04P-05P 06P-08P 09P 10P 11P-12P 13P 14P-15P 16P-22P 23P 24P 25P 26P-27P 28P 29P-32P 33P-34P 35P 36P 37P-39P 40P Exhibit 2 - Flood Insurance Rate Map Index and Flood Insurance Rate Map iii

6 FLOOD INSURANCE STUDY OGLE 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 Ogle County, Illinois, including: the cities of Byron, Oregon, Polo, and Rochelle; the villages of Adeline, Creston, Davis Junction, Forreston, Hillcrest, Leaf River, Monroe Center, Mount Morris, and Stillman Valley; and the unincorporated areas of Ogle County (hereinafter referred to collectively as Ogle County) and aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 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 Ogle 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 The FIS and FIRMs show the portion of the Cities of Rochelle and Rockford that lie within Ogle County. The remaining portions of these communities lie within other counties as indicated in Table 1. Please see separately published FIS reports and FIRMs for the portions of these communities that do not lie in Ogle County. Table 1 - Multi-County Communities Community Adjacent Counties City of Rochelle Lee County City of Rockford Winnebago County Note that the City of Polo, the portion of the City of Rockford within Ogle County, and the villages of Creston, Davis Junction, Monroe Center, Mount Morris have no 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

7 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 The FIS includes the unincorporated areas of, and incorporated communities within, Ogle 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. Pre-Countywide FISs Ogle County (Unincorporated Areas): City of Oregon: City of Rochelle: The hydrologic and hydraulic analyses for the FIS report dated April 5, 1988 (Reference 1) were performed by the U.S. Army Corps of Engineers (USACE), Rock Island District (the Study Contractor) for the Federal Emergency Management Agency (FEMA), under Inter-Agency Agreement No. EMW-E , Project Order No. 1, Amendment No 23A. This study was completed in September The hydrologic and hydraulic analyses for the FIS report dated September 15, 1981 (Reference 2) were performed under the directives of the Federal Insurance Administration (FIA). Hydrologic and hydraulic analyses were obtained from the Soil Conservation Service (SCS) Flood Hazard Analysis (Reference 3) and were coordinated with the Illinois Department of Transportation, Division of Water Resources. This study was completed in January The hydrologic and hydraulic analyses for the FIS report dated October 1, 1981 (Reference 4) were performed by the U.S. Geological Survey (USGS), Water Resources Division, for the FIA, under Inter-Agency Agreement No. IAA-H-9-77, Project Order No. 23. This study was completed in September

8 The authority and acknowledgements for the cities of Byron, Polo, and Rochelle and the villages of Adeline, Creston, Davis Junction, Forreston, Hillcrest, Leaf River, Monroe Center, Mount Morris, and Stillman Valley are not included because there were no previously printed pre-countywide FISs for those communities or the studies included in those FISs have been superseded. December 17, 2010 Initial Countywide FIS For the initial countywide FIS, hydrologic and hydraulic analyses performed for the September 21, 2000 countywide DeKalb County FIS (Reference 5) were incorporated for a portion of the South Branch Kishwaukee River floodway extending through the northeastern corner of Ogle County, previously mapped as Zone A. The January 2, 2009 FIS for DeKalb County, Illinois and Incorporated Areas (Reference 6) should be referenced for additional information. Base map information was derived from the USGS digital orthophoto quarter quadrangles at a one-half meter ground resolution (Reference 7). The initial 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. IDNR August 17, 2016 Revised Countywide FIS This Physical Map Revision (PMR) incorporates new hydrologic and hydraulic analyses of the Rock River (Reference 8, 9). The hydrologic analysis was completed by the Illinois State Water Survey (ISWS) in October The hydraulic analysis of the Rock River was completed by the Rock Island District of the USACE in July of 2011 under FEMA Interagency Agreement No. HSFE05-09-X Also included are new hydrologic and hydraulic analyses of the Kyte River watershed prepared by Wills Burke Kelsey Associates, Ltd. (Reference 10). Base map information for the 29 panels affected by this PMR was provided in digital format by the USGS. This information was derived from digital orthoimagery at a spatial resolution of one foot from aerial photography dated 2011 (Reference 11). This PMR was performed under the CTP Partnership Agreement Nos. EMC CA-7007 and EMC-2011-CA-7012 between the ISWS and FEMA, per MAS Nos. ISWS 0904 and ISWS

9 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 Ogle County s incorporated communities and unincorporated areas are shown in Table 2, CCO Meeting Dates for Pre-Countywide Studies. Table 2 CCO Meeting Dates for Pre-Countywide Studies Community Initial CCO Date Final CCO Date Byron, City of * November 18, 1983 Ogle County (Unincorporated Areas) July 26, 1983 May 28, 1987 Oregon, City of * April 1, 1981 Rochelle, City of January 1977 April 14, 1981 *Initial meeting not held December 17, 2010 Initial Countywide FIS The initial CCO meeting was held on September 13, 2006 in Oregon, Illinois, and was attended by representatives of Ogle County, the cities of Byron and Rochelle, and IDNR. This meeting was intended to discuss various issues and concerns for the study area. An intermediate CCO meeting was held on October 12, 2006 in Oregon, Illinois, and was attended by representatives of Ogle County, the cities of Byron and Rochelle, and IDNR. The results of the study were reviewed at the final CCO meeting held on September 23, 2009 in Oregon, Illinois, and attended by representatives of Ogle County; the cities of Byron, Oregon, Polo, and Rochelle; the Village of Hillcrest; and IDNR. All problems raised at that meeting have been addressed in this study. 4

10 August 17, 2016 Revised Countywide FIS A Flood Risk Review meeting was held on April 5, 2011 in Oregon, Illinois to discuss various aspects of the Rock River watershed study. The meeting was attended by representatives of Ogle County, the cities of Byron and Oregon, ISWS, USACE, and the Illinois Department of Transportation (IDOT). The results of the PMR were reviewed at final CCO meetings held on March 26, 2015 and April 1, 2015 in Rochelle and Dixon, Illinois, respectively, and were attended by representatives of Ogle County, the cities of Byron, Oregon, and Rochelle, the Village of Hillcrest, FEMA, ISWS, and IDNR. All problems raised at those meetings have been addressed in this study. 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Ogle County including the incorporated areas listed in Section 1.1. Typically, areas studied by Zone AE methods are selected with priority given to all known flood hazards and areas of projected development or proposed construction. The following table lists streams that have names in this FIS other than those used in previously printed FISs. See Table 3, Stream Name Changes. Table 3 - Stream Name Changes Community Old Name New Name Village of Hillcrest Ogle County (Unincorporated Areas) Kyte River Tributary Flagg Creek City of Rochelle Ogle County (Unincorporated Areas) Unnamed Tributary No. 1 Banning Lateral 5

11 The streams, or portions of streams, listed in Table 4, Limits of Revised or New Zone AE Study, have new or revised hydrologic and hydraulic analyses for this countywide FIS. Table 4 - Limits of Revised or New Zone AE Study Study Limits Flooding Source Downstream Limits Upstream Limits Banning Lateral Confluence with Kyte River Approximately 13,050 feet upstream of the confluence with Kyte River (approximately 6,850 feet upstream of IL Route 38 / Lincoln Highway) Beach Creek Confluence with Kyte River Approximately 17,500 feet upstream of the confluence with Kyte River (just upstream of Brooklyn Road, in Lee County) Creston Ditch Confluence with Ryley Ditch Approximately 16,950 feet upstream of the confluence with Ryley Ditch (approximately 750 feet upstream of South Mulford Road) Flagg Creek Confluence with Kyte River Approximately 26,600 feet upstream of the confluence with Kyte River (approximately 300 feet upstream of Hemstock Road) Flagg Creek Tributary Confluence with Flagg Creek Approximately 4,250 feet upstream of the confluence with Flagg Creek (approximately 675 feet upstream of Hemstock Road) Approximately 10,100 feet Grubin Ditch upstream of the confluence with Confluence with Beach Beach Creek (approximately 550 Creek (in Lee County) feet upstream of Brooklyn Road, in Lee County) Hilb Ditch Confluence with Ryley Ditch Approximately 7,300 feet upstream of the confluence with Ryley Ditch (approximately 750 feet upstream of Interstate 39 / U.S. Route 51) Intermodal Channel Confluence with Flagg Creek Approximately 1,700 feet upstream of the confluence with Flagg Creek (approximately 1,000 feet upstream of Pond Access) 6

12 Table 4 - Limits of Revised or New Zone AE Study (continued) Study Limits Flooding Source Downstream Limits Upstream Limits Johns Creek Kyte River Upstream reach Kyte River Overflow Kyte River Tributary 1 North Creston Ditch Rock River Ryley Ditch Steward Creek Confluence with Steward Creek (in Lee County) Approximately 85,800 feet upstream of the confluence with Rock River (IL Route 38 / Lincoln Highway) Confluence with Kyte River Confluence with Kyte River Confluence with Creston Ditch Approximately miles above the confluence with Mississippi River (at Whiteside/Lee County Boundary, approximately 3.79 miles above 1st Avenue Bridge) Confluence with Kyte River Confluence with Kyte River Approximately 26,050 feet upstream of the confluence with Steward Creek (approximately 300 feet upstream of Paw Paw Road, in Lee County) Approximately 146,150 feet upstream of the confluence with Rock River (just downstream of Bethel Road) Approximately 3,950 feet upstream of the confluence with Kyte River (approximately 1,260 feet upstream of weir) Approximately 2,375 feet upstream of the confluence with Kyte River (just downstream of Queens Road) Approximately 10,650 feet upstream of the confluence with Creston Ditch (approximately 1,550 feet upstream of Interstate 39) Approximately miles above the confluence with Mississippi River (at Ogle/Winnebago County Boundary, approximately 7.58 miles above IL Route 72 Bridge / South Union Street) Approximately 18,400 feet upstream of the confluence with Kyte River (approximately 7,050 feet upstream of Interstate 39 / U.S. Route 51) Approximately 17,360 feet upstream of the confluence with Kyte River (just downstream of Elva Road, in Lee County) 7

13 The streams, or portions of streams, listed in Table 5, Limits of Zone AE Study, were studied in detail and are included in this report. The limits of Zone AE study are also indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2). Flooding Source Banning Lateral Beach Creek Creston Ditch Flagg Creek Flagg Creek Tributary Grubin Ditch Hilb Ditch Intermodal Channel Johns Creek Table 5 Limits of Zone AE Study Study Limits Downstream Limits Upstream Limits Confluence with Kyte River Confluence with Kyte River Confluence with Ryley Ditch Confluence with Kyte River Confluence with Flagg Creek Confluence with Beach Creek (in Lee County) Confluence with Ryley Ditch Confluence with Flagg Creek Confluence with Steward Creek (in Lee County) Approximately 13,050 feet upstream of the confluence with Kyte River (approximately 6,850 feet upstream of IL Route 38 / Lincoln Highway) Approximately 17,500 feet upstream of the confluence with Kyte River (just upstream of Brooklyn Road, in Lee County) Approximately 16,950 feet upstream of the confluence with Ryley Ditch (approximately 750 feet upstream of South Mulford Road) Approximately 26,600 feet upstream of the confluence with Kyte River (approximately 300 feet upstream of Hemstock Road) Approximately 4,250 feet upstream of the confluence with Flagg Creek (approximately 675 feet upstream of Hemstock Road) Approximately 10,100 feet upstream of the confluence with Beach Creek (approximately 550 feet upstream of Brooklyn Road, in Lee County) Approximately 7,300 feet upstream of the confluence with Ryley Ditch (approximately 750 feet upstream of Interstate 39 / U.S. Route 51) Approximately 1,700 feet upstream of the confluence with Flagg Creek (approximately 1,000 feet upstream of Pond Access) Approximately 26,050 feet upstream of the confluence with Steward Creek (approximately 300 feet upstream of Paw Paw Road, in Lee County) 8

14 Table 5 Limits of Zone AE Study (continued) Study Limits Flooding Source Downstream Limits Upstream Limits Approximately 73,390 feet Approximately 85,800 feet upstream Kyte River upstream of the confluence of the confluence with Rock River (Downstream reach) with Rock River (just (IL Route 38 / Lincoln Highway) upstream of Flagg Road) Kyte River (Upstream reach) Kyte River Overflow Kyte River Tributary 1 Lake Mistake Drain Mill Creek North Creston Ditch Rock River Ryley Ditch Approximately 85,800 feet upstream of the confluence with Rock River (IL Route 38 / Lincoln Highway) Confluence with Kyte River Confluence with Kyte River Approximately 100 feet above the confluence with Gale Creek Confluence with Rock River Confluence with Creston Ditch Approximately miles above the confluence with Mississippi River (at Whiteside/Lee County Boundary, approximately 3.79 miles above 1st Avenue Bridge) Confluence with Kyte River Approximately 146,150 feet upstream of the confluence with Rock River (just downstream of Bethel Road) Approximately 3,950 feet upstream of the confluence with Kyte River (approximately 1,260 feet upstream of weir) Approximately 2,450 feet upstream of the confluence with Kyte River (just downstream of Queens Road) Approximately 2,375 feet above confluence with Gale Creek (approximately 470 feet above confluence of Tributary No. 1) Approximately 13,200 feet above confluence with Rock River (approximately 360 feet above confluence of Middle Creek) Approximately 10,650 feet upstream of the confluence with Creston Ditch (approximately 1,550 feet upstream of Interstate 39) Approximately miles above the confluence with Mississippi River (at Ogle/Winnebago County boundary, approximately 7.58 miles above IL Route 72 Bridge / South Union Street) Approximately 18,400 feet upstream of the confluence with Kyte River (approximately 7,050 feet upstream of Interstate 39 / U.S. Route 51) 9

15 Flooding Source South Branch Kishwaukee River Steward Creek Stillman Creek Table 5 Limits of Zone AE Study (continued) Study Limits Downstream Limits Edson Road (Winnebago / Ogle County boundary) Confluence with Kyte River Confluence with Rock River Upstream Limits County Line Road (the Ogle / DeKalb County Boundary) Approximately 17,360 feet upstream of the confluence with Kyte River (just downstream of Elva Road, in Lee County) Approximately 24,900 feet upstream of the confluence with Rock River (County Highway 4 / Stillman Road) Tributary No. 1 Confluence with Lake Mistake Drain Approximately 1,150 feet above the confluence with Lake Mistake Drain December 17, 2010 Initial Countywide FIS For the initial countywide FIS, hydrologic and hydraulic analyses were for the first time incorporated for a portion of the South Branch Kishwaukee River floodway extending through the northeastern corner of Ogle County within the limits provided in Table 5. In addition, an approximate study of Gale Creek re-delineated the Zone A floodplain using data from the Reconnaissance Report For Section 205 Flood Control; Gale Creek and Lake Mistake Drain, Oregon, Illinois, dated January 1990 and performed by the USACE, Rock Island District (Reference 12). August 17, 2016 Revised Countywide FIS This PMR incorporates new studies of the Rock River (Reference 8, 9) and the Kyte River watershed (Reference 10). The Kyte River watershed study includes Zone AE study within the limits provided in Tables 4 and 5 for the following streams: Kyte River, Beach Creek, Creston Ditch, Flagg Creek Tributary, Grubin Ditch, Hilb Ditch, Intermodal Channel, Johns Creek, Kyte River Overflow, Kyte River Tributary 1, North Creston Ditch Tributary, and Steward Creek. In addition, backwater effects from the Rock River study have been applied to Mill Creek and Stillman Creek. 10

16 LOMC Type Letters of Map Revision A Letter of Map Revision (LOMR) is an official revision to the currently effective FEMA map. It is used to change flood zones, floodplain and floodway delineations, flood elevations and planimetric features. All requests for LOMRs should be made to FEMA through the chief executive officer of the community, since it is the community that must adopt any changes and revisions to the map. If the request for a LOMR is not submitted through the chief executive officer of the community, evidence must be submitted that the community has been notified of the request. To obtain an application for a LOMR, visit and download the form MT-2 Application Forms and Instructions for Conditional Letters of Map Revision and Letters of Map Revision. Visit the Flood Map-Related Fees section to determine the cost of applying for a LOMR. For more information about how to apply for a LOMR, call the FEMA Map Information exchange; toll free, at FEMA MAP ( ) to speak to a Map Specialist. At the time of this PMR, there were no existing LOMRs affecting the 29 revised FIRM panels. Table 6 provides a history of the incorporation of determination letters issued by FEMA that were included in the December 17, 2010 initial countywide FIS. It should be noted that all or portions of a given map change may be superseded by subsequent LOMRs or restudies. Table 6 - Incorporated Letters of Map Change (December 17, 2010) Case Number Effective Date LOMR B086P 2/15/2007 FIRM Notes to Users Community City of Rochelle Ogle County (Unincorporated Areas) Flooding Source Unnamed Pond Project Identifier Whispering Waters Subdivision 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 1- percent-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. 11

17 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 FEMA-MAP ( ) or visit the FEMA Map Service Center website at 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 13 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 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. 12

18 Figure 1 - FIRM Notes to Users (continued) PROJECTION INFORMATION: The projection used in the preparation of the map was State Plane Illinois West 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 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 or contact the National Geodetic Survey at the following address: NGS Information Services NOAA, N/NGS12 National Geodetic Survey SSMC-3, # East-West Highway Silver Spring, Maryland (301) 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 Ogle 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. 13

19 2.2 Community Description Ogle County is located in northwestern Illinois about 23 miles south of the Illinois-Wisconsin state line. It is bounded on the west by Carroll County, on the north by Stephenson and Winnebago Counties, on the east by DeKalb County, and on the south and southwest by Lee and Whiteside Counties, respectively. Ogle County is the seventeenth largest county in Illinois, comprised of 24 townships totaling 757 square miles, or 488,320 acres (Reference 13). In 2010, the county had a population of 53,497 (Reference 14), representing a 4.8 percent increase since the 2000 census. The community of Davis Junction demonstrated the greatest increase in population between 2000 and 2010, growing by 1,881 persons or percent, followed by Byron, which grew by 836 persons or 28.7 percent, and by Creston, growing by 119 persons or 21.9 percent (Reference 13). Transportation facilities are well developed throughout the county. Ogle County is traversed by Interstate Highways 39 and 88 and State Routes 2, 26, 38, 52 64, 72 and 251. The county is also served by three rail lines (Reference 13). Within the general vicinity of Byron, two railway bridges and one state highway bridge cross the Rock River. Much of Ogle County is devoted to agricultural production, and cropland covers 435,692 acres, or 89.2 percent of the county. Incorporated cities and villages account for 4.5 percent of land use in the county while industrial and commercial enterprises, occupying 3,469 acres and 1,598 acres respectively, account for one percent (Reference 13). Of the 102 Illinois counties, Ogle County ranks seventh in acreage covered in grassland (124,360 acres), nineteenth in acres covered by cropland (308,663 acres), and fifteenth in acres covered by coniferous forest, with 666 acres of pine or other coniferous forest (Reference 15). Along with agriculture, the county s major industries include printing, food distribution and processing, and implement manufacturing. A nuclear generating station is located in Ogle County south of Byron (Reference 16, 17). Ogle County s commercial/industrial mineral resources include sand and gravel, industrial sand, and crushed stone (Reference 13). In addition, the Rock River, which runs through the entirety of the county and the three State parks (Lowden, White Pines, and Castle Rock), makes the county a recreation and tourism destination. Castle Rock State Park/Lowden-Miller State Forest complex is home to more breeding pairs of forest birds (85) than any other location in the state, and one valley in Castle Rock State Park harbors 27 different species of fern alone. The Rock River between Oregon and Grand Detour has been designated a Biologically Significant Stream (Reference 13). Ogle County lies entirely within the Rock River drainage basin. The Rock River flows southwesterly through the center of the county. Most of Ogle County s tributaries flow into the Rock River within the boundaries of the county (Reference 13). The Kyte River, a tributary of the Rock River, drains 30 square 14

20 miles and flows south and southwest along the eastern edge and southern portion of the community of Rochelle. Ryley Ditch and Unnamed Tributary No. 1 flow west to join the Kyte River at mile 22.9 and 23.0, respectively. Ryley Ditch drains 12 square miles at an average gradient of 19 feet per mile, and Unnamed Tributary No. 1 drains 2.5 square miles at an average gradient of 11 feet per mile (Reference 4). Lake Mistake Drain originates approximately two miles northwest of Oregon and has a total drainage area of approximately three square miles. It flows in a southeast direction to the confluence with Gale Creek. Tributary No. 1 has a drainage area of less than 1 square mile and flows from north to south along the western corporate limit of Oregon where it meets Lake Mistake Drain (Reference 3). The climate of Ogle County is characterized by cold winters and warm, humid summers. The total annual precipitation is 34 inches. Average seasonal snowfall is 33 inches. Of this precipitation, 22 inches (65 percent) usually falls in April through September (Reference 13). 2.3 Principal Flood Problems The Rock River is the most developed stream corridor in Ogle County. Flood stages can rise rapidly and remain high for considerable lengths of time (Reference 13). Areas on the eastern and western edges of Byron that are lower in elevation and lying in the Rock River floodplain are subject to flood inundation (Reference 18). In Oregon, only minor flood problems occur along the Rock River (Reference 2). Table 7, Historical Flood Data summarizes the river stages of record on the Rock River at Byron, Illinois (USGS Gage Number ) between 2001 and Information for the table was collected from the USGS National Water Information System (Reference 19) and from the National Weather Service s Advanced Hydrologic Prediction Service (Reference 20). Table 7 - Historical Flood Data ( ) Rock River at Byron, Illinois USGS Gage Number Flood Stage: 13 feet Date Peak Streamflow (cfs) River Stage (feet) 04/20/ , /25/ , /25/ , /15/ , * 06/05/ , *Gage height not the maximum for the year 15

21 In Rochelle, the primary flood problems are associated with the Kyte River as the result of a combination of intense rainfall, excessive runoff, and inadequate channel and bridge capacities. The high rate of sedimentation from agricultural and other surface-disturbed lands compromises channel capacity. Extensive development along the Kyte River and its floodplain has aggravated flood problems by encroaching on natural flood drainage and storage areas, thereby increasing runoff and property damage susceptibility (Reference 21). In Oregon, the primary flood problems are associated with Lake Mistake Drain and Tributary No. 1 and are caused by intense rainfall on the agricultural watersheds located north and west of the community. The watersheds are steep and when combined with intense rainfall cause high peak discharges. Flood problems are further aggravated by the relatively flat gradient and small outlet of Lake Mistake Drain, which prevents large discharges from easily flowing out of the area (Reference 3). 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 (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. 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods in Ogle 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 16

22 period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potential based on conditions existing in Ogle 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 detailed methods affecting the county. Analyses that have not been superseded have been compiled and are summarized below. Pre-Countywide FISs A regional discharge-frequency analysis using equations developed by the USGS for the State of Illinois (Reference 22) was completed for the downstream reach of Kyte River (see Table 5 for limits of study), Mill Creek, and Stillman Creek. These discharge-frequency equations, developed by multiple regression analysis for unregulated rural streams, utilize the independent variables drainage area, slope, rainfall intensity, and an areal factor to compute the peak flood discharge for a selected frequency of recurrence. Drainage area and channel slope were determined using the USGS 7.5 minute quadrangle maps for the communities of Stillman Valley, Chana, Creston, Kings, Kishwaukee, and Rochelle, and a 15 minute quadrangle map for the City of Oregon (Reference 1, 4, 23, 24). On the Kyte River, divided flow occurs at the Treatment Plant Road, therefore, there is a decrease in downstream discharges (Reference 4). The SCS Computer Program for Project Formulation-Hydrology (TR-20) (Reference 25) was used to develop the hydrology for Lake Mistake Drain and Tributary No. 1. This is an advanced hydrologic model which simulates the historical flood stages. This model can develop hydrographs for local drainage areas, channel route these hydrographs and combine hydrographs. Parameters used in this model are rainfall, drainage areas, soils, land use, vegetation and channel and floodplain characteristics. A decrease in discharge for Lake Mistake Drain occurs because of the relatively flat gradient and small outlet of Lake Mistake Drain. The discharge cannot easily flow out of the area and therefore the floodwaters pond and are stored until Lake Mistake Drain can drain the floodwaters (Reference 2). 17

23 December 17, 2010 Initial Countywide FIS For the initial countywide FIS, hydrologic analyses were incorporated for a portion of the South Branch Kishwaukee River floodway extending through the northeastern corner of Ogle County. The January 2, 2009 FIS for DeKalb County, Illinois and Incorporated Areas (Reference 6) should be referenced for additional information. August 17, 2016 Revised Countywide FIS Peak discharge values for the Rock River were determined using PeakFQ version analysis and the 2004 regression equations (Reference 26) as determined by StreamStats version 2. The Rock River gage at Como analysis was based on the historical record from 1915 to Full details of the methodology are described in the October 2011 ISWS report Rock River Hydrologic Analysis, Ogle and Lee County, Illinois (Reference 8). Hydrologic analysis of the upstream reach of the Kyte River (see Table 5 for limits of study) and Kyte River tributaries (Reference 10) utilized HEC-HMS version 3.4 in conjunction with HEC-Geo HMS and a digital elevation model (DEM) of the watershed. The HEC-HMS model was calibrated to two storm events: a smaller event in 2010 and a larger event in A summary of the drainage area-peak discharge relationships for all the streams studied by detailed methods is shown in Table 8, Summary of Discharges. 18

24 Table 8 - Summary of Discharges Peak Discharges (cubic feet per second) Drainage Area 10-Percent- 4-Percent- 2-Percent- 1-Percent- 0.2-Percent- Flooding Source and Location (square miles) Annual-Chance Annual-Chance Annual-Chance Annual-Chance Annual-Chance Banning Lateral At confluence with Kyte River * ,034 At IL Route 38 / Lincoln Highway * Beach Creek At confluence with Kyte River * 1,482 1,738 1,834 Creston Ditch Just downstream of I-39 / U.S. Route * Flagg Creek At confluence with Kyte River * Approximately 2,100 feet downstream of IL Route 38 / Lincoln Highway (just upstream of railroad) * Approximately 5,050 feet downstream of Flagg Road (just upstream of railroad) * ,950 Approximately 3,200 feet downstream of Hemstock Road * Flagg Creek Tributary At confluence with Flagg Creek * Grubin Ditch Just upstream of Brooklyn Road (in Lee County) * ,330 1 Contributing drainage area varies at this discharge location due to inter-basin flow. The singular reported contributing drainage area is from the hydrologic model. * Not calculated for this FIS project 19

25 Table 8 - Summary of Discharges (continued) Peak Discharges (cubic feet per second) Drainage Area 10-Percent- 4-Percent- 2-Percent- 1-Percent- 0.2-Percent- (square miles) Annual-Chance Annual-Chance Annual-Chance Annual-Chance Annual-Chance Hilb Ditch At confluence with Ryley Ditch * Intermodal Channel At confluence with Flagg Creek * Johns Creek At confluence with Steward Creek (in Lee County) ,205 * 2,432 3,248 4,866 Just upstream of confluence of Johns Creek South (in Lee County) * 1,240 1,429 1,636 Just upstream of I-39 / U.S. Route * 1,109 1,311 1,683 Kyte River At IL Route 38 / Lincoln Highway ,547 * 3,898 4,520 6,539 Just downstream of confluence of Steward Creek ,852 * 6,152 6,996 8,514 Just downstream of confluence of Flagg Creek ,197 * 2,843 3,118 3,798 Just downstream of confluence of Ryley Ditch ,957 * 2,596 2,876 3,770 Just downstream of IL Route ,393 * 2,250 2,561 3,237 Just downstream of Hemstock Road 9.3 1,176 * 1,845 2,114 2,714 Just downstream of Bethel Road * 1,347 1,607 2,530 1 Contributing drainage area varies at this discharge location due to inter-basin flow. The singular reported contributing drainage area is from the hydrologic model. * Not calculated for this FIS project 20

26 Table 8 - Summary of Discharges (continued) Peak Discharges (cubic feet per second) Drainage Area 10-Percent- 4-Percent- 2-Percent- 1-Percent- 0.2-Percent- (square miles) Annual-Chance Annual-Chance Annual-Chance Annual-Chance Annual-Chance Kyte River Overflow At confluence with Kyte River * Kyte River Tributary 1 At confluence with Kyte River * Lake Mistake Drain At Polo Road * Upstream of confluence of Tributary No * ,054 Mill Creek At confluence with Rock River ,600 * 5,440 6,200 8,040 North Creston Ditch Just downstream of Dement Road * Rock River Just downstream of Pine Creek 8,575 42,600 51,000 56,800 62,400 74,500 Just downstream of Kyte River 8,405 41,700 50,200 56,100 61,700 74,100 At historical USGS Oregon gage 8,205 40,700 48,900 54,600 60,000 71,800 At Ogle/Winnebago County line 7,905 39,300 47,300 52,900 58,300 70,000 Ryley Ditch At confluence with Kyte River * 1,026 1,145 1,618 Just upstream of I-39 / U.S. Route * Contributing drainage area varies at this discharge location due to inter-basin flow. The singular reported contributing drainage area is from the hydrologic model. * Not calculated for this FIS project 21

27 Table 8 - Summary of Discharges (continued) Peak Discharges (cubic feet per second) Drainage Area 10-Percent- 4-Percent- 2-Percent- 1-Percent- 0.2-Percent- (square miles) Annual-Chance Annual-Chance Annual-Chance Annual-Chance Annual-Chance South Branch Kishwaukee River At the De Kalb/Ogle County Boundary ,554 * 12,825 14,670 19,004 Steward Creek At confluence with Kyte River ,273 * 3,754 4,445 5,506 Just upstream of confluence of Johns Creek (in Lee County) ,638 * 4,041 4,402 5,156 Stillman Creek At confluence with Rock River ,900 * 5,900 6,740 8,700 Just upstream of confluence of Black Walnut Creek ,775 * 4,200 4,800 6,200 Tributary No. 1 Upstream of confluence with Lake Mistake Drain * Contributing drainage area varies at this discharge location due to inter-basin flow. The singular reported contributing drainage area is from the hydrologic model. * Not calculated for this FIS project 22

28 Stillwater elevations determined for flooding sources studied by Zone AE methods are summarized in Table 9, Summary of Stillwater Elevations. Table 9 - Summary of Stillwater Elevations Elevation (feet NAVD 88) 10-Percent- 2-Percent- 1-Percent- 0.2-Percent- Flooding Source and Location Annual-Chance Annual-Chance Annual-Chance Annual-Chance Unnamed Pond Ponding area bounded by Kyte River to the north and east, and 7 th Street / Illinois Route 251 to the west * * * *Not calculated for this FIS report 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. Pre-Countywide FISs For the downstream reach of Kyte River (see Table 5 for limits of study), Mill Creek and Stillman Creek, cross-section data were obtained by channel survey soundings. Overbank elevations for the valley cross sections were obtained from 23

29 computer printouts used for photogrammetric mapping (Reference 27). Watersurface elevations of the selected recurrence intervals were computed using the USACE HEC-2 step-backwater computer program (Reference 28) Starting watersurface elevations were calculated using slope-area method. For Lake Mistake Drain and Tributary No. 1, stream cross section data were obtained by field survey. All bridges and culverts were surveyed in detail to obtain elevation data and structural geometry. Channel sections were taken at selected locations to represent the hydraulic characteristics of the stream system. Water-surface elevations for the selected recurrence intervals were computed through the use of the SCS Computer Program for Project Formulation, Hydrology, WSP-2. Starting water-surface elevations were computed using normal depth computations and channel slope (Reference 2, 29). December 17, 2010 Initial Countywide FIS For the initial countywide FIS, hydraulic analyses were incorporated for a portion of the South Branch Kishwaukee River floodway extending through the northeastern corner of Ogle County, previously mapped as Zone A. The January 2, 2009 FIS for De Kalb County, Illinois and Incorporated Areas (Reference 6) should be referenced for additional information. August 17, 2016 Revised Countywide FIS Hydraulic analysis of the Rock River (Reference 9) utilized a steady state HEC- RAS model to compute the 10-, 4-, 2-, 1-, and 0.2-percent event water surface profiles and floodway for the Rock River. Channel cross-section data were obtained from HEC-2 models previously utilized for Flood Information Reports/Flood Insurance Studies. Over-bank elevations were obtained from State of Illinois 2009 digital elevation LiDAR data made available through Illinois Height Modernization Program (ILHMP). The model was calibrated using the high-water mark data collected by the USACE for the July 26, 2010 flood event from the USGS gage at Como. Hydraulic analysis of the upstream reach of the Kyte River (see Table 5 for limits of study) and Kyte River tributaries was performed using an unsteady HEC-RAS model (Reference 10). Surveyed cross sections of the channel along with LiDAR data were used in the model. Results of the unsteady HEC-RAS modeling included water surface elevations for the 10-, 2-, 1-, and 0.2-percent-chance events. 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 24

30 coefficients for each stream is shown in Table 10, Roughness Coefficients (Manning s n Values). Table 10 Roughness Coefficients (Manning's "n" Values) Stream Channel "n" Overbank "n" Banning Lateral Beach Creek Creston Ditch Flagg Creek Flagg Creek Tributary Grubin Ditch Hilb Ditch Intermodal Channel Johns Creek Kyte River Kyte River Overflow Kyte River Tributary Lake Mistake Drain Mill Creek North Creston Ditch Rock River Ryley Ditch South Branch Kishwaukee River Steward Creek Stillman Creek Tributary No The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the Flood Profiles (Exhibit 1) are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. 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. 25

31 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 30) or visit the National Geodetic Survey website at The National Geodetic Survey may also be contacted at the following address: NGS Information Services NOAA, N/NGS12 National Geodetic Survey SSMC-3, # East-West Highway Silver Spring, Maryland (301) 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 Technical Support Data Notebook associated with the FIS report and FIRM for this county. 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) , or visit their website at The datum conversion locations and values that were calculated for Lee County are provided below: December 17, 2010 Initial Countywide FIS Effective information for this FIS was converted from NGVD 29 to NAVD 88 based on data presented in Figure 2 and Table 11. Computations show an average conversion factor of feet (NGVD = NAVD 88) 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. 26

32 Figure 2 Vertical Datum Conversionss USGSS Quadrangle Corner Intersections The change in elevation for each Point ID is listed in Table 11 27

33 Table 11 - Vertical Datum Conversions Single Conversion Factor (countywide) Method NAD83 Latitude (dec. deg.) NAD83 Longitude (dec. deg.) NGVD29 to NAVD88 Elevation Change (feet) Point ID# Quadrangle Name Corner 1 Lee NW Steward NW Ashton NW Franklin Grove NW Dixon East NW Dixon West NW Creston NW Rochelle NW Chana NW Daysville NW Grand Detour NW Polo NW Fairdale NW Kings NW Stillman Valley NW Oregon NW Mt. Morris NW Forreston South NW Range of conversion values through Average conversion values Maximum variance from the average conversion Maximum variance from a no-conversion value 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 1- percent-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. 28

34 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. Between cross sections, the boundaries were interpolated on the basis of available topography. Pre-Countywide FISs Zone AE floodplain boundaries were interpolated using topographic maps at a scale of 1:12,000 with a contour interval of 2 feet (Reference 27). December 17, 2010 Initial Countywide FIS For the initial countywide FIS, floodplain boundaries were delineated using Illinois State Geological Survey (ISGS) statewide ArcSDE raster mosaic of USGS digital raster graphics at a scale of 1:24,000 (Reference 31) and the USGS National Elevation Dataset (Reference 32). Topographic maps at a scale of 1:4800 with a contour interval of 1 foot (Reference 12) were additionally used to delineate the western portion of Lake Mistake Drain and Zone A floodplain boundaries for Gale Creek. August 17, 2016 Revised Countywide FIS A DEM (Reference 33) derived from the 2009 LiDAR dataset (Reference 34) provided by the ISGS was used for delineation of Zone AE floodplain. 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). 29

35 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 35). 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. 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. 30

36 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 sectionss (see Table 12, Floodway Data ). The computed floodways are shown on the FIRM (Exhibit 2). In cases where the floodwayy 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 agreee 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. 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 increasee in the base flood elevations that creates a potential for flood damages. 31