CATOOSA COUNTY, GEORGIA

Transcription

1 CATOOSA COUNTY, GEORGIA AND INCORPORATED AREAS Catoosa County Community Name Community Number Catoosa County (Unincorporated Areas) Fort Oglethorpe, City of Ringgold, City of Effective: September 11, 2009 FLOOD INSURANCE STUDY NUMBER 13047CV000A

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) report may not contain all data available within the Community Map Repository. Please 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 report by the Letter of Map Revision process, which does not involve republication or redistribution of the FIS report. Therefore, users should consult with community officials and check the Community Map Repository to obtain the most current FIS report components. Selected Flood Insurance Rate Map panels for this community contain information that was previously shown separately on the corresponding Flood Boundary and Floodway Map panels (e.g., floodways, cross sections). In addition, former flood hazard zone designations have been changed as follows: Old Zone(s) Al through A30 VI through V30 B C New Zone AE VE X X Initial Countywide FIS Effective Date: September 11, 2009

3 TABLE OF CONTENTS 1.0 INTRODUCTION Purpose of Study Authority and Acknowledgments 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...35 i

4 TABLE OF CONTENTS (Continued) FIGURES Figure 1 - Floodway Schematic TABLES Table 1 - Streams Studied by Approximate Methods... 4 Table 2 - Streams Studied by Limited Detailed Methods... 5 Table 3 - Incorporated Detailed Studies... 6 Table 4 - Gaging Stations Table 5 - Summary of Discharges Table 6 - Vertical Datum Conversion Table 7 - Floodway Data Table 8 - Community Map History Exhibit 1 - Flood Profiles EXHIBITS Black Branch Cherokee Creek East Chickamauga Creek Hurricane Creek Johnson Branch Little Chickamauga Creek Peavine Creek South Chickamauga Creek Spring Creek Tributary No. 1 to Black Branch Tributary No. 2 to Black Branch Tributary No. 3 to Black Branch West Chickamauga Creek Panels 01P-08P Panel 09P Panels 10P-12P Panel 13P Panel 14P Panels 15P-16P Panels 17P-20P Panels 21P-24P Panels 25P-27P Panel 28P Panel 29P Panels 30P-31P Panels 32P-35P Exhibit 2 - Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY CATOOSA COUNTY, GEORGIA AND INCORPORATED AREAS 1.0 INTRODUCTION 1.1 Purpose of Study This Flood Insurance Study (FIS) revises and updates information on the existence and severity of flood hazards in the geographic area of Catoosa County, including the Cities of Fort Oglethorpe and Ringgold and the unincorporated areas of Catoosa County (referred to collectively herein as Catoosa 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 community that will be used to establish actuarial flood insurance rates and to assist the community in its efforts to promote sound floodplain management. Minimum floodplain management requirements for participation in the National Flood Insurance Program (NFIP) are set forth in the Code of Federal Regulations at 44 CFR, Please note that the City of Fort Oglethorpe is geographically located in Catoosa and Walker Counties. The City of Fort Oglethorpe is included in its entirety in this FIS report. 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. The Digital Flood Insurance Rate Map (DFIRM) and FIS report for this countywide study have been produced in digital format. Flood hazard information was converted to meet the Federal Emergency Management Agency (FEMA) DFIRM database specifications and Geographic Information System (GIS) format requirements. The flood hazard information was created and is provided in a digital format so that it can be incorporated into a local GIS and be accessed more easily by the community. 1.2 Authority and Acknowledgments The sources of authority for this FIS are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of

6 Precountywide Analyses Information on the authority and acknowledgements for each jurisdiction included in this countywide FIS, as compiled from their previously printed FIS reports, is shown below: Catoosa County (Unincorporated Areas): The hydrologic and hydraulic analyses for the March 1979 FIS report (FIA, 1979) were performed by the Tennessee Valley Authority (TVA), for the Federal Insurance Administration (FIA), under Inter- Agency Agreement Nos. IAA-H-11-76, Project Order Nos. 6 and 6A2, and IAA-H-7-77, Project Order No. 1. The work was completed in August The hydrologic and hydraulic analyses for West Chickamauga Creek, for the July 5, 1984, FIS report (FEMA, 1984) were performed by the TVA, for FEMA. The work was completed in July City of Oglethorpe: City of Ringgold: The hydrologic and hydraulic analyses for the April 1, 1983 FIS report (FEMA, 1983) were performed by the TVA, for FEMA, under Interagency Agreement No. EMW-E-0771, Project Order No. 1. The work was completed in June The hydrologic and hydraulic analyses for the December 1977 FIS report FIA, 1977) were performed by the TVA, for the FIA, under Inter-Agency Agreement No. IAA-H-11-76, Project Order Nos. 6 and 6A2, and IAA-H-7-77, Project Order No. 1. The work was completed in April

7 This Countywide FIS Report The hydrologic and hydraulic analyses for approximately studied streams for this countywide FIS were performed by PBS&J, for the Georgia Department of Natural Resources (DNR), under Contract No. EMA-2006-CA-5615 with FEMA. All other approximately studied and detailed studied streams were redelineated. The work was completed in November The Georgia DNR conducted all hydrologic and hydraulic analyses for the limited detailed studied streams. The work was completed in November Base map information shown on this FIRM was provided for Catoosa County by MD-Atlantic Technologies, Inc., dated 2005 and captured at a resolution of one half foot. The projection used was Georgia State Plane West Zone (FIPS Zone 1002). The horizontal datum was NAD 83, GRS 80 spheroid. 1.3 Coordination An initial meeting is held with representatives from FEMA, the community, and the study contractor to explain the nature and purpose of a FIS, and to identify the streams to be studied or restudied. A final meeting is held with representatives from FEMA, the community, and the study contractor to review the results of the study. Information on the authority and acknowledgements for each jurisdiction included in this countywide FIS, as compiled from their previously printed FIS reports, is shown below: Community FIS Date Initial Meeting Final Meeting Catoosa County July 5, 1984 May 6, 1975 August 30, 1978 (Unincorporated Areas) City of Oglethorpe August 1, 1983 October 14, 1981 December 8, 1982 City of Ringgold December 15, 1977 May 1975 June 23, 1977 For this countywide FIS, a scoping meeting was held on January 20, 2006, and attended by representatives of Catoosa County, the Georgia DNR, Coosa Valley Regional Development Center, Georgia Environmental Protection Division and FEMA. The results of the study were reviewed at the final meeting held on May 29, 2008, and attended by representatives of PBS&J, Catoosa County, and the Georgia DNR. All problems raised at the meeting have been addressed. 3

8 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Catoosa County, Georgia, including the incorporated communities listed in Section 1.1. The areas studied by detailed methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction through the time of the study. The following streams are studied by detailed methods in this FIS report: Black Branch Cherokee Creek East Chickamauga Creek Hurricane Creek Johnson Branch Little Chickamauga Creek Peavine Creek South Chickamauga Creek Spring Creek Tributary No. 1 to Black Branch Tributary No. 2 to Black Branch Tributary No. 3 to Black Branch West Chickamauga Creek The limits of detailed study are indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2). All detailed and approximate streams were redelineated as part of this countywide FIS to match existing topography. For this countywide revision, reaches that have been studied by approximate methods using Watershed Information (WISE) are listed in Table 1. Table 1 Streams Studied by Approximate Methods Stream Cat Creek Cherokee Creek Reach Description From confluence with Little Creek to the county boundary From confluence with Tiger Creek to approximately 130 feet downstream of Cherokee Lake Drive / Dam and from approximately 2,070 feet upstream of Cherokee Lake Drive / Dam to approximately 8,650 feet upstream of Cherokee Lake Drive / Dam 4

9 Table 1 Streams Studied by Approximate Methods (Continued) Stream Cat Creek Dry Creek Little Chickamauga Creek Little Chickamauga Creek Tributary No. 35 Little Creek Little Tiger Creek Sugar Creek Tiger Creek Reach Description From confluence with Little Creek to the county boundary From confluence with East Chickamauga Creek to county boundary From State Highway 151 to county boundary From confluence with Little Chickamauga Creek to approximately 5,580 feet upstream of Colbert Hollow Road From confluence with Tiger Creek to approximately 6,500 feet upstream of Tiger Creek From confluence with Tiger Creek to county boundary From confluence with Tiger Creek to approximately 3,030 feet upstream of Keith Road From confluence with East and South Chickamauga Creeks to county boundary For this countywide revision, reaches that have been studied by limited detailed methods by the Georgia DNR are listed in Table 2. Table 2 Streams Studied by Limited Detailed Methods Streams Blue Spring Branch Dry Branch Detailed Description From confluence with West Chickamauga Creek to approximately 200 feet upstream of Burning Bush Road From confluence with Blue Spring Branch to approximately 2,800 feet upstream of Twin Cedars Road 5

10 Table 2 Streams Studied by Limited Detailed Methods (Continued) Streams Hurricane Creek Peavine Creek Peters Branch Unnamed Tributary to Little Chickamauga Creek Detailed Description From confluence with South Chickamauga Creek to approximately 7,160 feet upstream of Swanson Hollow Road From approximately 2,340 feet upstream of Boynton Drive to approximately 3,600 feet downstream of Poplar Springs Road From confluence with Hurricane Creek to approximately 350 feet upstream of Pleasant View Drive From confluence with Little Chickamauga Creek to approximately 2,370 feet upstream of Westbrook Road For this countywide revision, reaches studied by detailed methods were incorporated from the FIS text for Hamilton County, Tennessee and Incorporated Areas (FEMA, 2002). Table 3 Incorporated Detailed Studies Streams Hurricane Creek Johnson Branch Detailed Description From approximately 650 feet downstream of Cherokee Valley Road to confluence of Johnson Branch From confluence with Hurricane Creek to approximately 840 feet upstream of confluence with Hurricane Creek For this countywide FIS, the FIS report and FIRM were converted to countywide format, and the flooding information for the entire county, including both incorporated and unincorporated areas, is shown. Also, the vertical datum was converted from the National Geodetic Vertical Datum of 1929 (NGVD) to the North American Vertical Datum of 1988 (NAVD). In addition, the Transverse Mercator, State Plane coordinates, previously referenced to the North American Datum of 1927, are now referenced to the North American Datum of Approximate analyses were used to study those areas having low development potential or minimal flood hazards. The scope and methods of study were proposed to and agreed upon by FEMA and the communities. 6

11 The following tabulation presents Letters of Map Correction (LOMCs) incorporated into this countywide study: LOMC Case Number Date Issued Project Identifier LOMR P 10/22/1998 Black Branch-Greenleaf Road Crossing LOMR P 02/28/2000 Cherokee Creek - Just upstream of Cherokee Lake Drive to approximately 2,200 feet upstream of Cherokee Lake Drive LOMR P 01/05/2001 Spring Creek - Approximately 700 feet downstream of confluence of Black Branch to just downstream of Cloud Springs Road LOMR P 04/12/2001 West Chickamauga Creek Shipp s RV Center and Campground LOMR P 12/11/2002 Black Branch and Tributary No. 1 to Black Branch - New Fant Drive Bridge LOMR P 10/22/2003 West Chickamauga Creek - Brown and Bergethon Property 2.2 Community Description Catoosa County is located in the northwestern part of Georgia, approximately 130 miles north of Atlanta. The county is bordered by Walker County to the west and south, Whitfield County to the south and east, and Hamilton County, Tennessee to the north. The total area contained within the county is 163 square miles. According to the U.S. Census Bureau in 2000, Catoosa County s population was 53,282 (U.S. Census Bureau, 2007). The climate in the county can be classified as moderate. The average summer temperature is 88.3 degrees Fahrenheit ( F) and the average winter temperature is 34.3 F. The average monthly rainfall for the county is 4.5 inches with majority of the precipitation falling as rain during the winter and spring months (The Weather Channel, 2007). South Chickamauga Creek Valley is situated between Peavine Ridge to the west and White Oak Mountain and Taylor Ridge to the east. This valley is underlain by Chickamauga limestone, varying with depth from 1,400 to 2,300 feet thick. The rocks have been extensively folded and thrust faulted, and the formations range in age from Cambrian to Mississippian (WACADA Development Association and TVA, 1967). The U.S. Department of Agriculture (USDA) soil survey characterizes the region as being dominated by ridge and valley 7

12 topography, with continuous long, low ridges and intervening broad, shallow valleys. Taylor Ridge and White Oak Mountain are capped with thick beds of shale. This formation is greatly resistant to degradation (USDA, 1941). The main soil series located in the county include the Clarksville Series, derived from Ft. Payne Chert and Knox Dolomite; the Conasauga Series, consisting of shaley loam; and the Sequatchie Series, derived from limestone with lime siltstone (USDA, 1941). 2.3 Principal Flood Problems Data concerning historical floods have been collected for South, West, and East Chickamauga Creeks. On South Chickamauga Creek, the largest flood of record at the U.S. Geological Survey (USGS) stream gage at Chickamauga, Tennessee, occurred during March 1973 and was estimated as having a 2-percent-annualchance flood recurrence interval. Floodmarks have been recorded at approximately one mile above the Catoosa County line on West Chickamauga Creek and at stream mile 16 on East Chickamauga Creek for large floods which have occurred on these streams (TVA, 1968 and 1970). The City of Ringgold lies in a large bend of South Chickamauga Creek, which forms the city s limits on three of its sides and is the only significant flooding in the city. Floods within Ringgold have occurred most frequently in the winter and spring months. The flood of April 1977 had a discharge of 11,000 cubic feet per second (cfs) and a 2-percent-chance flood frequency. The most damaging flood known to have occurred was the flood of March 16, South Chickamauga Creek reached a stage of feet at the USGS partial record station at the U.S. Highway 41 bridge. Floodwaters entered six homes; the sewage treatment facility on the right side of the creek was under 6 to 8 feet of water, and State Highway 2 was under nearly 9 feet of water (TVA, 1974a). The estimated discharges and recurrence intervals for floods on East, South and West Chickamauga Creeks are as follows: Flooding Source Date Discharge (cfs) Recurrence interval (% Annual Chance) East Chickamauga Creek , , South Chickamauga Creek March ,400 * April , February ,500 * March , November , * Data not available 8

13 Flooding Source Date Discharge (cfs) Recurrence interval (% Annual Chance) West Chickamauga Creek , , , , , Flood Protection Measures There are no known structural flood protection measures which significantly affect flooding in Catoosa County. With the assistance of the TVA and the Coosa Valley Area Planning and Development Commission, the City of Ringgold adopted floodway zoning for South Chickamauga Creek in April 1971, based on the regional flood determined by the TVA. The floodway ordinance requires the first floor elevation of structures in the floodway fringe to equal or exceed the regional flood and the foundation of all structures to be designed to withstand flood conditions at the site (City of Ringgold, 1971). 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods in the community, 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-, 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-, 50-, 100-, and 500-year floods, have a 10-, 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 that equals or exceeds the 1-percent-annual-chance (100-year) flood in any 50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 9

14 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 community. Precountywide Analyses No formal stream gaging stations have been maintained on Black Branch, Tributary No. 1 through 3 to Black Branch, East Chickamauga Creek, Little Chickamauga Creek, Peavine Creek, the southern reaches of South Chickamauga Creek, Spring Creek and West Chickamauga Creek. To define the dischargefrequency relationships for these streams, regional relationships of peak discharge and drainage area were developed from analysis of stream flow records at gaging stations on streams in Catoosa County and on nearby streams with similar hydrologic characteristics (Table 4). Frequency curves for the gaged locations used to define the regional relationships were computed using the procedure outlined in Bulletins #17 and #17B (Water Resources Council, 1976 and 1982), including adjustment for historic floods. The adopted regional discharge estimates were about 25 percent greater than those published by the USGS (USGS, 1979). The difference in the results was attributed to the use of additional years of record available for this study and the specific attention that was given to the gaged watersheds near Fort Oglethorpe. Where stream discharge is affected by significant urban development, the discharge estimates from regional relationships were increased to account for urbanization effects using the following relationship: Q pu =I x Q pn Where: Q pu = Discharge for selected probability, p, for urban conditions I = Percent imperviousness of urban area x = A factor which varies with flood probability Q pn = Discharge for selected probability, p, for natural conditions The relationship and values of x, developed by Espey and Winslow (Espey and Winslow, 1974), relate discharge at selected frequencies to watershed and climatic factors. The relationships and x values were compared and found to be consistent with studies by others. Percent imperviousness (I) was estimated using ratios of imperviousness to urban area (USGS, 1975) and urban areas determined from 7.5-minute topographic maps and aerial photographs. 10

15 Floodmarks of unknown date(s) were also considered in the analysis (USGS, and , and TVA, ). Table 4 Gaging Stations Stream Gage No. Drinage Area (Sq. Miles) Period of Record Agency Bailey Creek Near Villanow USGS Brymer Creek Near McDonald, TN USGS Chattanooga Creek Near Flintstone USGS At Summerville USGS Consauga River Near Eaton USGS Dead Mans Branch Near Resaca USGS Duck Creek Above Lafayette USGS Little Chickamauga Creek Near Ringgold USGS Little Chickamauga Creek Tributary Near Ringgold USGS Middle Creek Below Highway 39 near Englewood, TN TVA Mill Creek Tributary Near Eton USGS Oostanaula Creek Near Sanford, TN USGS Sewee Creek Near Decatur, TN USGS South Chestuee Creek Near Benton, TN USGS South Chickamauga Creek At Ringgold USGS Near Chickamauga, TN USGS Below state boundary USGS 11

16 Table 4 Gaging Stations (Continued) Stream Gage No. Drinage Area (Sq. Miles) Period of Record Agency Storey Mill Creek Near Summerville USGS Sugar Creek Near Ringgold USGS West Armuchee Creek Near Subligna USGS West Chickamauga Creek Near Kensington USGS Wolftever Creek Near Ooltewah, TN USGS The USGS stream gaging station (No ) on South Chickamauga Creek at Ringgold (USGS, 1964 and ) was the principal source of data for defining discharge-frequency relationships for South Chickamauga Creek. The gage was operated as a partial record station from 1949 to Stream flow records for two stream gages on South Chickamauga Creek downstream from Ringgold were also considered. These were USGS gaging station No , South Chickamauga Creek near the Georgia-Tennessee State line (1953 to 1957), (USGS, ); and USGS gaging station No , South Chickamauga Creek near Chickamauga, Tennessee, which has been in continuous operation as a recording station since 1929 (USGS, 1964, , 1971, and 1973). Values of the 10-, 2-, 1-, and 0.2-percent-annual-chance flood peak discharges were estimated using the procedure outlined in Bulletin #17 (Water Resources Council, 1976), including adjustments for historic flood information. Upstream of Ringgold the regional relationship was used. To define discharge-frequency data for the main stream of West Chickamauga Creek, the partial-record station (USGS gage No ) on West Chickamauga Creek near Kensington, Georgia, was used in conjunction with the regionalized relationship. In 1977, the USGS updated the storm frequency flows based on the rating of this gage. The resulting flows were significantly increased. Consideration was given also to the record (USGS gage No ) on Chattanooga Creek near Flintstone, Georgia ( ). This relationship was compared to and agreed with that developed in 1976 by the USGS (USGS, 1962). The regional peak flow-drainage area relationships with drainage areas less then 10 square miles were compared with those developed by the USGS (USGS, 1976). The relationship developed by the USGS in 1976 were about 25 percent lower than those adapted for the study. The USGS relationship was developed from data over a large geographic area and from extending records by simulating runoff from rainfall. Rainfall used in the simulation was weighted for the two National Weather Service stations: Chattanooga (67 percent) and Atlanta (33 percent). The weighted average annual rainfall for these two stations is about 2.5 inches less than the mean average annual rainfall at nine stations in the immediate vicinity of the study area in the Tennessee Valley. Therefore, the regional peak 12

17 flow-drainage area relationship developed from station data in the northwest Georgia geologic area was adopted. This Countywide FIS Report For Hurricane Creek and Johnson Branch, discharge-frequency determinations were based on updated drainage area-peak discharge relationships developed from regional analyses using recent stream gage records. Flood-frequency curves for these stations were computed using procedures outlined in Bulletin #17B (Water Resources Council, 1982), including the skew map of Plate 1 and adjustments for historic flood information. For Cherokee Creek, the USACE s HEC-1 computer program Version 4.1 (HEC, 1998) was used to calculate flood discharges. For the streams studied by limited detailed methods, Blue Spring Branch, Dry Branch, Hurricane Creek, Peavine Creek, Peters Branch, and Unnamed Tributary to Little Chickamauga Creek, watershed physiographic features used in the discharge-frequency determinations were estimated using ESRI s ArcHydro (ESRI, 2007). The 1-percent-annual-chance flood discharges were determined using the USGS flood frequency relations (USGS, 1999). For the approximately studied streams, peak flows were determined using the rural regression equations for Georgia (Stamey and Hess, 1993). Peak discharge-drainage area relationships for are presented in Table 5. Table 5 - Summary of Discharges Peak Discharges (cubic feet per second) Flooding Source and Location Drainage Area (square miles) 10-Percent- Annual-Chance 2-Percent- Annual-Chance 1-Percent- Annual-Chance 0.2-Percent- Annual-Chance BLACK BRANCH At confluence with Spring ,650 5,000 5,500 6,900 Creek At mile ,150 3,000 3,400 4,350 At mile 2.6 (above Tributary ,600 2,300 2,750 3,470 No. 1 to Black Branch) At mile 3.2 (above Tributary ,350 1,950 2,220 2,780 No. 2 to Black Branch) At mile ,370 1,600 2,150 CHEROKEE CREEK At Cherokee Lake Drive/Dam 3.43 * * 2,626 * *Data not available 13

18 Table 5 - Summary of Discharges (Continued) Peak Discharges (cubic feet per second) Flooding Source and Location Drainage Area (square miles) 10-Percent- Annual-Chance 2-Percent- Annual-Chance 1-Percent- Annual-Chance 0.2-Percent- Annual-Chance EAST CHICKAMAUGA CREEK At confluence with South ,500 12,100 14,000 18,000 Chickamauga Creek At confluence of Dry Creek ,800 9,700 11,000 15,000 At confluence of Tanyard ,100 7,400 8,400 11,300 Creek Approximately 7,700 feet ,900 7,100 8,100 11,000 upstream of Dogwood- Tunnel Hill Road HURRICANE CREEK At confluence of Johnson Branch JOHNSON BRANCH At confluence with Hurricane Creek ,500 2,300 2,700 3, ,250 1,450 1,950 LITTLE CHICKAMAUGA CREEK At confluence with South ,600 9,700 11,000 14,800 Chickamauga Creek At State Highway ,500 7,800 9,000 12,000 PEAVINE CREEK At confluence with South ,300 7,700 8,800 11,800 Chickamauga Creek At Old Mill Road ,400 6,400 7,300 9,800 Approximately 1,239 feet ,700 4,000 4,600 6,200 upstream of Ringgold-Rock Springs Road SOUTH CHICKAMAUGA CREEK At state boundary ,000 25,000 28,500 36,500 Approximately 8,160 feet ,000 23,000 26,000 34,000 upstream of CSX railroad At mile ,000 23,000 26,000 34,000 At mile ,500 18,000 20,000 27,000 SPRING CREEK At state boundary ,300 7,000 7,650 9,200 TRIBUTARY NO. 1 TO BLACK BRANCH At confluence with Black ,050 1,170 1,450 Branch At mile TRIBUTARY NO. 2 TO BLACK BRANCH At confluence with Black ,280 Branch At mile

19 Table 5 Summary of Discharges (Continued) Peak Discharges (cubic feet per second) Flooding Source and Location Drainage Area (square miles) 10-Percent- Annual-Chance 2-Percent- Annual-Chance 1-Percent- Annual-Chance 0.2-Percent- Annual-Chance TRIBUTARY NO. 3 TO BLACK BRANCH At mile ,470 2,100 2,300 2,930 At mile ,120 1,640 1,830 2,330 WEST CHICKAMAUGA CREEK At state boundary ,217 21,333 24,417 33,833 At mile 6.5 (State Highway ,800 20,500 24,000 33, /Cloud Springs Road) At mile 11.1 (Old Ringgold ,500 20,000 23,500 32,500 Road) 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. Users should be aware that flood elevations shown on the FIRM represent rounded whole-foot elevations and may not exactly reflect the elevations shown on the Flood Profiles or in the Floodway Data Table in the FIS report. 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. Precountywide Analyses Cross sections for Black Branch, Peavine Creek, South Chickamauga Creek, Spring Creek, Tributary No. 1 to Black Branch, Tributary No. 2 to Black Branch, and Tributary No. 3 to Black Branch were field surveyed at bridges and other strategic locations at sufficiently close intervals to accurately compute water surface elevations (WSELs). The field surveys were supplemented with valley cross sections developed by photogrammetric methods. All cross sections for East, Little and West Chickamauga Creeks were field surveyed. WSELs of floods for the selected recurrence intervals for Black Branch, Tributary Nos. 1 through 3 to Black Branch, Peavine Creek, and Spring Creek were computed through use of the US. Army Corps of Engineers (USACE s) HEC-2N computer program (HEC, 1977). WSELs for floods for East Chickamauga Creek, Little Chickamauga Creek, South Chickamauga Creek, and West Chickamauga Creek were computed through use of a standard-step method backwater computer program developed by the TVA (TVA, 1974b) which is similar to, but with some alteration to, the USACE s HEC-2 step-backwater program (HEC, 1973). 15

20 Tributary No. 1 to Black Branch was started by slope-area calculations using the slope of the streambed. Tributary No. 2 and Tributary No. 3 to Black Branch were started by slope-area calculations using the slope of surveyed floodmarks. Starting WSELs for Black Branch were based on backwater elevations from Spring Creek. Flood profiles for East Chickamauga and Spring Creeks were started using backwater elevations from South Chickamauga Creek. Starting WSELs for Peavine Creek and Little Chickamauga Creek were obtained by slope-area calculations using the slope of the 1970 flood for Peavine Creek and the 1969 flood for Little Chickamauga Creek. Starting WSELs for South Chickamauga Creek were taken from rating curves upstream of bridges at stream station mile 2.91, based on flood profile computations for Hamilton County, Tennessee. Starting WSEL for West Chickamauga Creek were based on a detailed study which traversed Catoosa County. This Countywide FIS Report For Cherokee Creek, cross sections were obtained through field survey. Cross sections for Hurricane Creek and Johnson Branch were obtained from statistical analyses of elevation data drawn from TVA flood records, field observations, and high water marks or previous floods (TVA, 1959). Cross sections for the streams studied by limited detailed methods, Blue Spring Branch, Dry Branch, Hurricane Creek, Peavine Creek, Peters Branch, and Unnamed Tributary to Little Chickamauga Creek, were obtained using digital topography and field survey of the hydraulic structures. For Cherokee Creek, the WSELs were computed using the HEC-2 computer program (HEC, 1991). WSELs for Hurricane Creek and Johnson Branch were computed using a modified version of the HEC-2 step-backwater computer program (HEC, 1980, 1991 and TVA, 1974a). WSELs for the limited detailed studied streams Blue Spring Branch, Dry Branch, Hurricane Creek, Peavine Creek, Peters Branch, and Unnamed Tributary 16

21 to Little Chickamauga Creek were computed using HEC-RAS Version (HEC, 2005). Starting WSELs for Cherokee Creek and Hurricane Creek were calculated by the slope-area method. Johnson Branch starting WSELs were assumed to be coincident with Hurricane Creek. Starting WSELs for the limited detailed studied streams Blue Spring Branch and Dry Branch were based on the effective WSEL obtained at West Chickamauga Creek. Starting WSELs for the limited detailed studied streams Hurricane Creek and Peters Branch were based on the effective WSEL obtained at South Chickamauga Creek. The starting WSEL for the limited detailed studied stream Peavine Creek was based on the effective WSEL obtained at the downstream end of the limited detailed study. The starting WSEL for the limited detailed studied stream Unnamed Tributary to Little Chickamauga Creek was based on the effective WSEL obtained at Little Chickamauga Creek. For the approximately studied streams, cross section data was obtained from the topography. Roads were modeled as weirs, using elevations from the topography. The studied streams were modeled using HEC-RAS version (HEC, 2005). 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). Channel roughness factors (Mannings n ) used in the hydraulic computations were chosen by engineering judgment and based on field observations of the streams and floodplain areas. The Manning s n values for all detailed studied streams are listed in the following table: Manning's "n" Values Stream Channel n Overbank n Black Branch Cherokee Creek * * East Chickamauga Creek *Data Not Available 17

22 Manning's "n" Values (Continued) Stream Channel n Overbank n Hurricane Creek Johnson Branch Little Chickamauga Creek Peavine Creek South Chickamauga Creek Spring Creek Tributary No. 1 to Black Branch Tributary No. 2 to Black Branch Tributary No. 3 to Black Branch West Chickamauga Creek The profile baselines depicted on the FIRM represent the hydraulic modeling baselines that match the flood profiles on this FIS report. As a result of improved topographic data, the profile baseline, in some cases, may deviate significantly from the channel centerline or appear outside the Special Flood Hazard Area. 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 in use for newly created or revised FIS reports and FIRMs was NGVD. With the finalization of NAVD, many FIS reports and FIRMs are being prepared using NAVD as the referenced vertical datum. All flood elevations shown in this FIS report and on the FIRM are referenced to NAVD. Structure and ground elevations in the community must, therefore, be referenced to NAVD. It is important to note that adjacent communities may be referenced to NGVD. This may result in differences in Base Flood Elevations (BFEs) across the corporate limits between the communities. In this revision, an average vertical datum conversion of foot was calculated and used to convert all elevations in Catoosa County from NGVD to NAVD using the National Geodetic Survey s VERTCON online utility (NGS, 2007). The data points used to determine the conversion are listed in Table 6. 18

23 Table 6 Vertical Datum Conversion Conversion from Quad Name Corner Latitude Longitude NGVD to NAVD Fort Oglethorpe NE Kensington NE East Ridge NE Nickajack Gap NE Nickajack Gap SE Ringgold NE Tunnel Hill NE Average: For additional information regarding conversion between NGVD and NAVD, visit the National Geodetic Survey website at or contact the National Geodetic Survey at the following address: Vertical Network Branch, N/CG13 National Geodetic Survey, NOAA Silver Spring Metro Center 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 community. Interested individuals may contact FEMA to access these data. To obtain current elevation, description, and/or location information for benchmarks shown on this map, please contact the Information Services Branch of the NGS at (301) , or visit their website at FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each FIS provides 1-percent-annual-chance (100- year) flood elevations and delineations of the 1- and 0.2-percent-annual-chance (500- year) 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 Table, and Summary of Stillwater Elevations Table. Users should reference the data presented in the FIS report as well as additional information that may be available at the local map repository before making flood elevation and/or floodplain boundary determinations. 19

24 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 each stream studied by detailed methods except Spring Creek from the confluence of Black Branch to approximately 170 feet downstream of Cloud Springs Road, 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 using LIDAR topographic maps at a scale of 1:100 with a contour interval of 2 feet (MD-Atlantic Technologies, 2005). For Spring Creek from the confluence of Black Branch to approximately 170 feet downstream of Cloud Springs Road, a topographic map with a scale of 1:300 and a contour interval of 2 feet (Price, 2000a) and a topographic map with a scale of 1:500, and contour interval of 5 feet (Price, 2000b) were used to interpolate flood boundaries between cross sections. For Cherokee Creek, a topographic map with a scale of 1:100 with a contour interval of 2 feet (Mesa Associates, Inc., 1999) were used to interpolate flood boundaries between cross sections. For Hurricane Creek and Johnson Branch, a topographic map with a scale of 1:4,800 with a contour interval of 4 feet (TVA, 1985) were used to interpolate flood boundaries between cross sections. The floodplains for all streams studied by approximate or limited detailed methods were delineated on topographic maps developed from LIDAR with a scale of 1:100 with a contour interval of 2 feet (MD-Atlantic Technologies, 2005). The 1- and 0.2-percent-annual-chance 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 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-annual-chance floodplain boundaries are close together, only the 1-percent-annual-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 approximate methods, only the 1-percent-annualchance floodplain boundary is shown on the FIRM (Exhibit 2). 20

25 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of 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, 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 foot, provided that hazardous velocities are not produced. The floodways in this study are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The floodways presented in this FIS report and on the FIRM were 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 (Table 7). In cases where the floodway and 1-percent-annual-chance floodplain boundaries are either close together or collinear, only the floodway boundary has been shown. The area between the floodway and 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 WSEL of the 1-percent-annual-chance flood more than 1 foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in Figure 1. 21

26 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) BLACK BRANCH A , B 2, , C 2, , D 4, , E 7, , F 9, , G 10, , H 11, , I 13, , J 14, , K 15, , L 17, M 18, N 18, Feet above confluence with Spring Creek 2 Elevation computed without consideration of backwater effects from Spring Creek TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CATOOSA COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA BLACK BRANCH

27 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) EAST CHICKAMAUGA CREEK A 4, , B 6, , C 7, , D 11,299 1,070 12, E 14, , F 16,949 1,370 7, G 22, , H 24, , I 27,720 1,190 11, J 30,730 1,500 10, K 33,792 1,170 7, L 35, , M 37, , N 39, , O 43, , P 45, , Q 47, / , R 50, , S 52, , T 55,018 1,000 9, Feet above confluence with South Chickamauga Creek 2 Total width/width within Catoosa County TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CATOOSA COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA EAST CHICKAMAUGA CREEK

28 FLOODING SOURCE CROSS SECTION DISTANCE WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) HURRICANE CREEK A 30, / JOHNSON BRANCH A / LITTLE CHICKAMAUGA CREEK A 3, , B 5, , C 7, , D 8, , E 12, , F 14, , G 15, , H 17, , I 17, , J 19, , K 22, , L 23, ,280 9, M 26, , N 28, , O 29, , Feet above confluence with South Chickamauga Creek 2 Total Width/width within Catoosa County 3 Feet above confluence with Hurricane Creek TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CATOOSA COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA HURRICANE CREEK JOHNSON BRANCH LITTLE CHICKAMAUGA CREEK

29 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) LITTLE CHICKAMAUGA CREEK (CONTINUED) P 32, , Q 34, , R 36, , S 39, , T 41, , U 44, , V 48, , PEAVINE CREEK A 6, , B 12, , C 14, , D 17, , E 22, , F 28, , G 31, , H 32, , I 35, , J 38, , K 40, , L 55, , Feet above confluence with South Chickamauga Creek 2 Elevation computed without consideration of backwater effects from South Chickamauga Creek TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CATOOSA COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA LITTLE CHICKAMAUGA CREEK PEAVINE CREEK