GORDON COUNTY, GEORGIA

Transcription

1 GORDON COUNTY, GEORGIA AND INCORPORATED AREAS Gordon County Community Name Community Number Calhoun, City of Fairmount, City of Gordon County (Unincorporated Areas) Plainville, City of Resaca, Town of Effective: September 26, 2008 FLOOD INSURANCE STUDY NUMBER 13129CV000A

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. Initial Countywide FIS Effective Date: September 26, 2008

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...21 i

4 TABLE OF CONTENTS (Continued) FIGURES Figure 1 - Floodway Schematic TABLES Table 1 Streams Restudied by Approximate Methods... 4 Table 2 Summary of Discharges Table 3 Vertical Datum Conversion Table 4 Floodway Data Table 5 Community Map History EXHIBITS Exhibit 1 - Flood Profiles Oostanaula River Oothcalooga Creek Tributary No. 1 Tributary No. 2 Panels 01P-02P Panel 03P Panel 04P Panel 05P Exhibit 2 - Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY GORDON 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 Gordon County, including the Cities of Calhoun, Fairmount, and Plainville; the Town of Resaca; and the unincorporated areas of Gordon County (referred to collectively herein as Gordon 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, 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 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: 1

6 City of Calhoun: The hydrologic and hydraulic analyses for the Oostanaula River and Oothcalooga Creek in the July 2, 1991, FIS report (FEMA, 1991a) were performed by Post, Buckley, Schuh, & Jernigan, Inc., for FEMA, under Contract No. EMW-87-C The work was completed in September The hydrologic and hydraulic analyses for Tributary Nos. 1 and 2 were performed by City of Calhoun, for FEMA. The work was completed in Gordon County (Unincorporated Areas): The hydrologic and hydraulic analyses for the Oostanaula River and Oothcalooga Creek in the July 2, 1991, FIS report (FEMA, 1991b) were performed by Post, Buckley, Schuh, & Jernigan, Inc., for FEMA, under Contract No. EMW-87-C The work was completed in September The hydrologic and hydraulic analyses for Tributary Nos. 1 and 2 were performed by City of Calhoun, for FEMA. The work was completed in There were no previously printed FIS reports for the Cities of Fairmount and Plainville and the Town of Resaca. This Countywide FIS Report For this countywide FIS report, streams restudied by approximate methods were performed by PBS&J, for the Georgia Department of Natural Resources (DNR) under Contract No. EMA-2006-CA-5615, with FEMA. All other streams previously studied by approximate methods were redelineated to better match existing topography. The work was completed in June Base map information shown on the Flood Insurance Rate Map (FIRM) was derived from U.S. Department of Agriculture photography, produced at 1 foot per pixel, dated The projection used in the preparation of this map is NAD 2

7 1983 State Plane Georgia West FIPS 1002 Feet, and the horizontal datum used is NAD 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. The initial and final meeting dates for previous FIS reports for Gordon County and its communities are listed in the following table: Community FIS Date Initial Meeting Final Meeting Calhoun, City of July 2, 1991 * August 9, 1990 Gordon County (Unincorporated Areas) July 2, 1991 October 10, 1986 August 9, 1990 * Data not available For this countywide FIS, the initial meeting was held on January 20, 2006, and attended by representatives of FEMA, Coosa Valley Regional Development Center, City of Calhoun, Gordon County, Georgia Environmental Protection Division, Georgia DNR and PBS&J,. The purpose of an initial meeting is to discuss the scope of the FIS. The results of the study were reviewed at the final meeting held on January 30, 2008, and attended by representatives of Gordon County, the Georgia DNR, and PBS&J. All problems raised at that meeting have been addressed. 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Gordon 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 streams that were studied by detailed methods were the Oostanaula River, Oothcalooga Creek and Tributary No. 1 and Tributary No. 2 to Oothcalooga Creek. 3

8 The limits of detailed study are indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2). Existing areas studied by approximate methods were redelineated as part of this countywide study. USGS 10-foot topographic maps were obtained from the USGS (USGS, various dates). The areas restudied by approximate methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction through June The streams restudied by approximate methods are presented in Table 1. Table 1 Streams Restudied by Approximate Methods Stream Becky Branch Becky Branch Tributary No. 1 Becky Branch Tributary No. 1.1 Becky Branch Tributary No. 2 Reach Description From confluence with Pinhook Creek to approximately 10,980 feet upstream of Pittman Road Northeast From confluence with Becky Branch to approximately 11,830 feet upstream of Pittman Road Northeast From confluence with Becky Branch Tributary No. 1 to county boundary From confluence with Becky Branch to approximately 7,070 feet upstream of Becky Branch Blackwood Creek From U.S. Highway 41/State Highway 3 to Marine Drive Southeast Cedar Creek Coosawattee River Coosawattee River Tributary No. 26 Coosawattee River Tributary No. 30 From confluence with Pine Log Creek to county boundary From confluence with Oostanaula River to county boundary From confluence with Coosawattee River to approximately 3,520 feet upstream of Thompson Road Northeast From confluence with Coosawattee River to approximately 4,840 feet upstream of Reese Road Northeast 4

9 Table 1 Streams Restudied by Approximate Methods (Continued) Stream Dry Creek Dry Creek Tributary No. 3 Dry Creek Tributary No. 6 Dry Creek Tributary No. 9 Reach Description From confluence with Coosawattee River to approximately 10,910 feet upstream of U.S. Highway 411/State Highway 61/Tennessee Highway From confluence with Dry Creek to approximately 1,230 feet upstream of Hightower Road From confluence with Dry Creek to approximately 8,790 feet upstream of confluence with Dry Creek From confluence with Dry Creek to approximately 12,340 feet upstream of U.S. Highway 411/State Highway 61/Tennessee Highway Dry Creek Tributary No. 9.1 From confluence with Dry Creek Tributary No. 9 to approximately 3,860 feet upstream of confluence with Dry Creek Tributary No. 9 Dry Creek Tributary No. 12 Dry Creek Tributary No. 13 From confluence with Dry Creek to approximately 10,440 feet upstream of confluence with Dry Creek From confluence with Dry Creek to approximately 6,720 feet upstream of Apple Road Dry Creek Tributary No From confluence with Dry Creek Tributary No. 13 to approximately 4,270 feet upstream of Apple Road Lick Creek Lick Creek Tributary No. 7 Lick Creek Tributary No. 17 From confluence with Salacoa Creek to approximately 14,470 feet upstream of U.S. Highway 411/State Highway 61/Tennessee Highway From confluence with Lick Creek to approximately 7,100 feet upstream of Park Drive From confluence with Lick Creek to approximately 2,240 feet upstream of Red Bone Ridge Road 5

10 Table 1 Streams Restudied by Approximate Methods (Continued) Stream Lick Creek Tributary No Lick Creek Tributary No. 23 Little Dry Creek Little Dry Creek Tributary No. 3 Little Pine Log Creek Marlowe Branch Ninetynine Branch Noblet Creek Reach Description From confluence with Lick Creek Tributary No. 17 to approximately 2,750 feet upstream of Red Bone Ridge Road From confluence with Lick Creek to approximately 4,900 feet upstream of Craig Road Northeast From confluence with Dry Creek to approximately 1,100 feet upstream of Oakman Road Northeast From confluence with Little Dry Creek to approximately 1,000 feet upstream of Oakman Road Northeast From confluence with Pine Log Creek to county boundary From confluence with Salacoa Creek to approximately 3,670 feet upstream of Covington Bridge Road From confluence with Salacoa Creek to county boundary From confluence with Coosawattee River to county boundary Oostanaula River From Interstate Highway 75/State Highway 401 to divergence from Coosawattee River Oostanaula River Tributary No. 50 Oothcalooga Creek From Maudlin Road Northwest to approximately 130 feet upstream of U.S. Highway 41/State Highway 3 From Salem Road Southwest to county boundary Oothcalooga Creek Tributary No. 3 From Beamer Road Southeast to approximately 3,020 feet upstream of Beamer Road Southeast Oothcalooga Creek Tributary No. 8 From McDaniel Station Road Southwest to approximately 5,570 feet upstream of McDaniel Station Road Southwest 6

11 Table 1 Streams Restudied by Approximate Methods (Continued) Stream Oothcalooga Creek Tributary No. 8.1 Oothcalooga Creek Tributary No. 22 Pinhook Creek Pinhook Creek Tributary No. 5 Pinhook Creek Tributary No. 5.3 Redbud Creek Reach Description From McDaniel Station Road Southwest to approximately 1,930 feet upstream of Flora Avenue Southwest From confluence with Oothcalooga Creek to county boundary From confluence with Salacoa Creek to county boundary From confluence with Pinhook Creek to approximately 10,270 feet upstream of Pittman Road Northeast From confluence with Pinhook Creek Tributary No. 5 to approximately 4,660 feet upstream of the confluence with Pinhook Creek Tributary No. 5 From confluence with Lick Creek to approximately 3,240 feet upstream of U.S. Highway 411/State Highway 61/Tennessee Highway Tributary No. 1 From Peters Street to approximately 840 feet upstream of Saddlebrook Drive Southeast 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, Gordon County, and the Georgia DNR. The following tabulation presents Letters of Map Change (LOMCs) incorporated into this countywide study: LOMC Case Number Date Issued Project Identifier LOMR-F A 12/11/2002 Brookstone Subdivision 7

12 2.2 Community Description Gordon County is located in northwest Georgia, approximately 51 miles northeast of Atlanta. The county is boarded by Murray and Whitfield Counties to the north, Gilmer County to the northeast, Pickens County to the east, Cherokee County to the southeast, Bartow County to the south, Floyd County to the west, and Walker County to the northeast. The total area contained within the county is 358 square miles. According to the U.S. Census Bureau, in 2006, the population for Gordon County was 51,419 (U.S. Census Bureau, 2007). The climate in northwestern Georgia is classified as moderate. The average winter temperature is 30.7 degrees Fahrenheit ( F) and the average summer temperature is 88.7 F. The average monthly precipitation is approximately 4.47 inches with the wettest month being March and the driest October. It is also common for tropical storms and hurricanes to affect this region, dramatically altering precipitation totals. The entire county is contained in the Oostanaula River basin. The Oostanaula River flows southwest through Gordon County and joins with the Etowah River in Rome to form the Coosa River. The land in Gordon County is typically wooded uplands giving way to rolling pasture and farmlands at the lower elevations and wide floodplain areas in the vicinity of the Oostanaula River. At the northwestern boundary of the county are the Horn Mountains. The county possesses rich rock and mineral resources with significant quantities of marble, bauxite, limestone, and iron. Today, textiles are the largest industry located in the county. 2.3 Principal Flood Problems The USGS has recorded historic peak floods at its gaging station on the Oostanaula River at State Highway 136 near Calhoun, Georgia (No ), and at the gaging station on Oothcalooga Creek near Adairsville, Georgia. Peak flood-flows were recorded on the Oostanaula River on February 28, 1951, showing a record peak of 54,800 cubic feet per second (cfs) for the county. The USGS also recorded the Oostanaula River significant flooding on the following dates: February 4, 1957, 32,800 cfs; March 20, 1973, 29,000 cfs; April 7, 1977, 29,000 cfs; and March 24, 1980, 27,900 cfs. At Gage No , located in the City of Calhoun, peak flood-flows were recorded on the following dates: January 6, 1982, 25,300 cfs; and February 19, 1990, 45,500 cfs. Peak flood-flows were recorded on Oothcalooga Creek on March 23, 1964, March 4, 1966, April 4, 1977, and on March 4, Most major flooding occurs in the winter and spring and typically results from slow moving or multiple low pressure systems, tropical storms, or hurricanes. It 8

13 is estimated that approximately half of the major river rises have occurred in March and April (NOAA, 1980). Flooding has also occurred during the warmer months from tropical storms or heavy, prolonged thunderstorms. Often tornadoes will be associated with this frontal-thunderstorm activity. An average of 18 tornadoes were reported in Georgia each year, with over 50 percent occurring during the months of March and April (NOAA, 1980). Overloading of storm sewers and obstructions at culverts appear to be the main factors affecting the severity of recent flood events. 2.4 Flood Protection Measures Flood protection measures are not known to exist within the study area. 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. 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 Discharge-frequency relationships along Oothcalooga Creek were determined using USGS regional regression equations (USGS, 1979) and adjusted for urbanization using established techniques (USGS, 1983). 9

14 Discharge-frequency relationships along the Oostanaula River were determined using gage data for a 94-year period of record, at the gage near Resaca, and applying the log-pearson Type III distribution in accordance with Bulletin #17B (Water Resources Council, 1982). Peak discharges determined using the log-pearson Type III methodology were then weighted with peak discharges computed using the regression equation and transposed downstream to the ungaged nodes in the study, in accordance with USGS procedures (USGS, 1979). Since flood records were not available for the small drainage areas of Tributary Nos. 1 and 2 to Oothcalooga Creek, peak discharges were developed using Technical Release No. 55, an empirical guide for estimating the effects of urbanization on peak rates of discharge, taking into account synthetic rainfallrunoff relationships based on soils, watershed slope, land use, and other hydrologic factors (SCS, 1975). This Countywide FIS Report For the streams studied by approximate methods listed in Table 1, peak flows were determined using the rural regression equations for Georgia (Stamey and Hess, 1993). Peak discharge-drainage area relationships for the 10-, 2-, 1-, and 0.2-percentannual-chance floods of each flooding source studied in detail in the county are shown in Table 2. Table 2 - 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 OOSTANAULA RIVER At State Highway 156/ 1, * * 68,402 88,362 Roland Hayes Parkway Southeast Just downstream of 1, * * 65,604 84,445 confluence of Oothcalooga Creek Just upstream of 1, * * 61,139 78,205 confluence of Oothcalooga Creek At approximately 2,700 feet 1, * * 59,431 75,821 upstream of Interstate Highway 75/State Highway 401 *Data not available 10

15 Table 2 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 OOTHCALOOGA CREEK At confluence with ,027 11,949 13,725 18,530 Oostanaula River Just downstream of ,434 11,061 12,707 17,137 confluence of Tributary No. 1 Just downstream of ,221 9,285 10,669 14,387 confluence of Blackwood Creek TRIBUTARY NO. 1 At confluence with 2.90 * * 1,670 2,250 Oothcalooga Creek TRIBUTARY NO. 2 At confluence with 3.25 * * 1,300 1,800 Oothcalooga Creek *Data not available 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to provide estimates of the elevations of floods of the selected recurrence intervals. 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 Overbank cross sections for the backwater analyses for the Oostanaula River, Oothcalooga Creek, and Tributary Nos. 1 and 2 to Oothcalooga Creek were obtained from aerial photography (Woolpert Consultants, 1987). The belowwater sections were obtained by field measurement at bridge structures and interpolated or estimated between structures. All bridges, dams, and culverts were field surveyed to obtain elevation and structural geometry. Water surface elevations (WSELs) for Oostanaula River, Oothcalooga Creek, and Tributary Nos. 1 and 2 to Oothcalooga Creek were computed using the HEC-2 step-backwater computer program (HEC, 1984). 11

16 Starting WSELs for the Oostanaula River and Oothcalooga Creek were determined using the slope-area method. The starting WSELs for Tributary Nos. 1 and 2 were taken at the confluences. 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 observation of the streams and floodplain area. 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 Oostanaula River Oothcalooga Creek Tributary No Tributary No This Countywide FIS Report For the approximate study streams, cross section data was obtained from the topography. Low flow channels were added to the cross section data, based on the estimated depth of the 50-percent-annual-chance flow. Roads appearing on the topographic maps were modeled as weirs; top of road elevations were estimated from the topography. The studied streams were modeled using HEC- RAS version (HEC, 2004). 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. 12

17 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. The average conversion factor that was used to convert the data in this FIS report to NAVD was calculated using the National Geodetic Survey s VERTCON online utility (NGS, 2007). The data points used to determine the conversion are listed in Table 3. Table 3 Vertical Datum Conversion Conversion from Quad Name Corner Latitude Longitude NGVD to NAVD Sugar Valley NE Sugar Valley SE Plainville NW Plainville SW Plainville SE Calhoun North NE Calhoun North SE Calhoun South SE Red Bud NE Red Bud SE Sonoraville SE Fairmount NW 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: 13

18 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. 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, 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 topographic maps at a scale of 1:4,800, with a contour interval of 4 feet (USGS, 1987). For streams studied or restudied by approximate methods presented in Table 1, and for redelineated areas studied by approximate methods, the 1-percent-annual- 14

19 chance floodplain boundaries were delineated using topographic maps at a scale of 1:24,000 with a 10-foot contour interval (USGS, various dates). 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). 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 FIRMs 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 4). 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. 15

20 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) OOTHCALOOGA CREEK A 1, , B 4, , C 5, , D 7, , E 11, , F 12, , G 13, , H 16,955 1,040 10, I 21, , J 22, , K 27, , L 30, , M 31,900 1,275 6, N 33,925 1,117 10, O 36,420 1,312 10, P 41,210 1,257 6, Feet above confluence with Oostanaula River 2 Elevation computed without consideration of flooding controlled by Oostanaula River 3 Elevation computed without consideration of backwater effects from Oostanaula River TABLE 4 FEDERAL EMERGENCY MANAGEMENT AGENCY GORDON COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA OOTHCALOOGA CREEK

21 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) TRIBUTARY NO. 1 A 1, , B 3, , C 4, D 5, * * TRIBUTARY NO. 2 A 1, , B 1, , C 2, D 3, E 6, Feet above confluence with Oothcalooga Creek 2 Elevation computed without consideration of backwater effects from Oothcalooga Creek *Data not available TABLE 4 FEDERAL EMERGENCY MANAGEMENT AGENCY GORDON COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA TRIBUTARY NO. 1 TRIBUTARY NO. 2

22 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. Figure 1 - Floodway Schematic No floodways were computed for Oostanaula River. 5.0 INSURANCE APPLICATIONS For flood insurance rating purposes, flood insurance zone designations are assigned to a community based on the results of the engineering analyses. These zones are as follows: 18

23 Zone A Zone A is the flood insurance risk zone that corresponds to the 1-percent-annual-chance floodplains that are determined in the FIS by approximate methods. Because detailed hydraulic analyses are not performed for such areas, no BFEs or base flood depths are shown within this zone. Zone AE Zone AE is the flood insurance risk zone that corresponds to the 1-percent-annual-chance floodplains that are determined in the FIS by detailed methods. In most instances, wholefoot BFEs derived from the detailed hydraulic analyses are shown at selected intervals within this zone. Zone X Zone X is the flood insurance risk zone that corresponds to areas outside the 0.2-percentannual-chance floodplain, areas within the 0.2-percent-annual-chance floodplain, areas of 1-percent-annual-chance flooding where average depths are less than 1 foot, areas of 1- percent-annual-chance flooding where the contributing drainage area is less than 1 square mile, and areas protected from the 1-percent-annual-chance flood by levees. No BFEs or base flood depths are shown within this zone. 6.0 FLOOD INSURANCE RATE MAP The FIRM is designed for flood insurance and floodplain management applications. For flood insurance applications, the map designates flood insurance risk zones as described in Section 5.0 and, in the 1-percent-annual-chance floodplains that were studied by detailed methods, shows selected whole-foot BFEs or average depths. Insurance agents use the zones and BFEs in conjunction with information on structures and their contents to assign premium rates for flood insurance policies. For floodplain management applications, the map shows by tints, screens, and symbols, the 1- and 0.2-percent-annual-chance floodplains, floodways, and the locations of selected cross sections used in the hydraulic analyses and floodway computations. The countywide FIRM presents flooding information for the entire geographic area of Gordon County. Previously, FIRMs were prepared for each incorporated community and the unincorporated areas of the County identified as flood-prone. Historical data relating to the maps prepared for each community are presented in Table 5. 19

24 COMMUNITY NAME INITIAL IDENTIFICATION FLOOD HAZARD BOUNDARY MAP REVISION DATE FIRM EFFECTIVE DATE FIRM REVISION DATE Calhoun, City of March 29, 1974 None April 1, 1977 July 2, 1991 Fairmount, City of September 26, 2008 September 26, 2008 Gordon County (Unincorporated Areas) October 29, 1976 July 16, 1987 July 2, 1991 Plainville, City of April 18, 1975 None June 18, 1987 Resaca, Town of September 26, 2008 September 26, 2008 TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY GORDON COUNTY, GA AND INCORPORATED AREAS COMMUNITY MAP HISTORY Table 2 - Community Map Histor

25 7.0 OTHER STUDIES This report either supersedes or is compatible with all previous studies on streams studied in this report and should be considered authoritative for purposes of the NFIP. 8.0 LOCATION OF DATA Information concerning the pertinent data used in the preparation of this study can be obtained by contacting FEMA, Federal Insurance and Mitigation Division, Koger Center Rutgers Building, 3003 Chamblee Tucker Road, Atlanta, Georgia BIBLIOGRAPHY AND REFERENCES Federal Emergency Management Agency, Flood Insurance Study, City of Calhoun, Gordon County, Georgia, July 2, 1991a. Federal Emergency Management Agency, Flood Insurance Study, Gordon County, Georgia (Unincorporated Areas), July 2, 1991b. Hydrologic Engineering Center, HEC-2 Water Surface Profiles, U.S. Army Corps of Engineers, Davis, California, Hydrologic Engineering Center, HEC-RAS River Analysis System, Version 3.1.3, U.S. Army Corps of Engineers, Davis, California, April National Geodetic Survey, VERTCON-North American Vertical Datum Conversion Utility, Retrieved February 15, 2007, from National Oceanic and Atmospheric Administration, Climates of the States, Volume 2, Soil Conservation Service, Urban Hydrology for Small Watersheds, Technical Release No. 55, U.S. Department of Agriculture, Stamey, T.C., and C.W. Hess, Techniques for Estimating Magnitude and Frequency of Floods in Rural Basins of Georgia, Water Resources Investigations Report , U.S. Geological Survey, U.S. Census Bureau, American Fact Finder, Gordon County, Georgia, Retrieved February 16, 2007, from 21

26 U.S. Department of Agriculture, Aerial Photography, Scale 1 foot per pixel, U.S. Geological Survey, Floods in Georgia, Magnitude and Frequency: Techniques for Estimating the Magnitude and Frequency of Floods in Georgia with Compilation of Flood Data through 1974, Water-Resources Investigation Report , U.S. Department of the Interior, U.S. Geological Survey, Flood Characteristics of Urban Watersheds in the United States, Techniques for Estimating Magnitude and Frequency of Urban Floods, Water Supply Paper 2207, U.S. Department of the Interior, U.S. Geological Survey, 7.5 Minute Series Topographic Map, Scale 1:4,800, Contour Interval 4 feet, Gordon, Georgia, U.S. Department of the Interior, U.S. Geological Survey, 7.5 Minute Series Topographic Maps, Scale 1:24,000, Contour Interval 10: Berner, 1973; Dames Ferry, 1964; Dry Branch, 1973; East Juliette, 1973; Gordon, 1973; Gray, 1964; Haddock, 1978; Hillsboro, 1964; James, 1973; Macon East, 1956, Macon NE, 1973; Macon NW, 1973; Resseaus Crossroad, 1978; Stanfordville, 1964, U.S. Department of the Interior, various dates. Water Resources Council, Hydrology Committee, Guidelines for Determining Flood Flow Frequencies, Bulletin #17B, Revised September 1981, Editorial Corrections March Woolpert Consultants, Aerial Photography, Gordon County, Georgia, Scale1:48,000, Contour Interval 4 feet, City of Mobile, Alabama, March

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