JACKSON COUNTY, GEORGIA AND INCORPORATED AREAS

Transcription

1 Jackson County JACKSON COUNTY, GEORGIA AND INCORPORATED AREAS COMMUNITY NAME COMMUNITY NUMBER ARCADE, CITY OF BRASELTON, TOWN OF COMMERCE, CITY OF HOSCHTON, CITY OF JACKSON COUNTY (UNINCORPORATED AREAS) JEFFERSON, CITY OF MAYSVILLE, CITY OF NICHOLSON, CITY OF PENDERGRASS, CITY OF TALMO, CITY OF EFFECTIVE: DECEMBER 17, 2010 Federal Emergency Management Agency FLOOD INSURANCE STUDY NUMBER 13157CV000A

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: December 17, 2010

3 TABLE OF CONTENTS Page 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 19 i

4 TABLE OF CONTENTS - continued Page FIGURES Figure 1 Floodway Schematic 16 TABLES Table 1 Summary of Discharges 6-8 Table 2 Floodway Data 15 Table 3 - Community Map History 18 EXHIBITS Exhibit 1 - Flood Profiles Big Bear Creek Cabin Creek Curry Creek East Tributary Indian Creek Little Bear Creek Middle Oconee River Mulberry River North Oconee River Tributary to Walnut Creek Walnut Creek Walton Creek West Tributary Panel 01P Panels 02P 03P Panel 04P Panel 05P Panel 06P Panels 07P 08P Panels 09P 11P Panel 12P 13P Panel 14P Panel 15P Panels 16P 19P Panel 20P Panel 21P Exhibit 2 - Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY JACKSON COUNTY, GEORGIA AND INCORPORATED AREAS 1.0 INTRODUCTION 1.1 Purpose of Study This FIS revises and updates information on the existence and severity of flood hazards in the geographic area of Jackson County, including the Cities of Arcade, Commerce, Hoschton, Jefferson, Maysville, Nicholson, Pendergrass, and Talmo; the Town of Braselton and; and the unincorporated areas of Jackson County (referred to collectively herein as Jackson 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 Town of Braselton is geographically located in Jackson, Barrow, Gwinnett, and Hall Counties. The portion of the Town of Braselton shown within Jackson County is included in this FIS report. See the separately published Barrow, Gwinnett, and Hall Counties FIS reports and Flood Insurance Rate Maps (FIRMs) for flood-hazard information for the portions of the Town of Braselton outside of Jackson County. Please note that the City of Maysville is geographically located in Jackson and Banks Counties. The portion of the City of Maysville shown within Jackson County is included in this FIS report. See the separately published Banks County FIS report and FIRM for flood-hazard information for the portion of the City of Maysville outside of Jackson County. 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

6 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: Jackson County (Unincorporated Areas): Jefferson, City of: the hydrologic and hydraulic analyses for the November 16, 1990, FIS report were performed by Law Environmental, Inc., for FEMA, under Contract No. EMW-87-C The work was completed in October the hydrologic and hydraulic analyses for the June 4, 1987, FIS report were obtained from Special Flood Hazard Information, Curry Creek, Jefferson, Georgia (USACE, 1976). The Cities of Arcade, Commerce, Hoschton, Maysville, Nicholson, Pendergrass, and Talmo and the Town of Braselton and have no previously printed FIS reports. Revised Analyses The hydrologic and hydraulic analyses for Walton Creek and the downstream reach of Middle Oconee River came from Athens-Clarke County, Georgia FIS report (FEMA, 2007). The hydrologic and hydraulic analyses for the downstream reach of Mulberry River came from the Barrow County, Georgia, Letter of Map Revision (LOMR), case number P, dated July 11, 2008 (FEMA, 2008). The hydrologic and hydraulic analyses for the approximate study streams were performed by Dewberry & Davis LLC, for FEMA, under Contract No. EMA CA This work was completed in June Base map information shown on the FIRM was derived from digital orthoimagery produced at a scale of 1:20,000, from National Agriculture Imagery Program dated The projection used in the preparation of this map is Georgia State Plane West FIPS Zone 1002 (feet), and the horizontal datum used is North American Datum 1983, GRS80 spheroid. 2

7 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. The initial and final meeting dates for previous FIS reports for Jackson County and its communities are listed in the following table: Community FIS Date Initial Meeting Final Meeting Jackson County (Unincorporated Areas) November 16, 1990 October 16, 1988 January 3, 1990 Jefferson, City of June 4, 1987 * July 23, 1986 *Data not available The initial meeting for this countywide revision was held on July 16, 2008, and attended by representatives of FEMA, the Georgia Department of Natural Resources, Jackson County, and the study contractor. The results of the study were reviewed at the final meeting held on September 18, 2009, and attended by representatives of Jackson County and Georgia Department of Natural Resources. 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 Jackson 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 July 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 29) to the North American Vertical Datum of 1988 (NAVD 88). 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 The following streams are studied by detailed methods in this FIS report: Big Bear Creek Mulberry River 3

8 Cabin Creek Curry Creek East Tributary Indian Creek Little Bear Creek Middle Oconee River North Oconee River Tributary to Walnut Creek Walnut Creek Walton Creek West Tributary The detailed studied streams, Big Bear Creek, Cabin Creek, Curry Creek, Little Bear Creek, Middle Oconee River, North Oconee River, and Walnut Creek, were in previous FIS reports prepared for the City of Jefferson and the unincorporated areas of Jackson County. Their hydrologic and hydraulic analyses remain valid; however, their floodplain delineations have been revised on the current FIRM. The limits of detailed study are indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2). 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 Jackson County. The following tabulation presents Letters of Map Change (LOMCs) incorporated into this countywide study: LOMC Case Number Date Issued Project Identifier LOMR P April 17, 2009 The Fields at Walnut Creek LOMR P April 16, 2009 Steeplechase LOMR BY83P June 27, 2007 Walnut Creek Flood Elevation Study LOMR BQ92P December 27, 2006 Scenic Falls LOMR P May 26, 2004 Middle Oconee River LOMR P April 1, 2004 Bramblett/Bryan Properties LOMR P April 10, 2003 Jeffco Properties 2.2 Community Description Jackson County, encompassing approximately 342 square miles, is located in northern Georgia, approximately 48 miles northeast of the City of Atlanta. The county is bordered on the north by Banks County; on the south by Barrow and Clarke Counties; on the east by Madison County; and on the west by Gwinnett and Hall Counties. Major transportation routes that serve Jackson County include Interstate 85; U.S. Highway 129 and 441; and State Highways 11, 15, 98 and 403; and the CSX Railroad and Norfolk Southern Railway. According to the U.S. Census Bureau, in 2008 the population estimate for Jackson County was 61,620 (U.S. Census Bureau, 2009). 4

9 Jackson County s moderate climate consists of mild winters and warm summers. The average annual rainfall is 50 inches. The wettest month is March while the driest months are September and October (National Weather Service, 2009). Land use within Jackson County is primarily agricultural open space with some light industry except for the urbanized areas in and near the City of Jefferson. 2.3 Principal Flood Problems The majority of land subject to flooding in Jackson County is presently forest, pasture, or farm lands. The watershed topography generally consists of rolling hills with steep slopes. Floods along Curry Creek in the City of Jefferson, caused by runoff from general rainfall, thunderstorms, and hurricane and tropical storm activities, have occurred during all seasons of the year. Past floods have caused erosion along the Curry Creek and damage to the City of Jefferson s water supply pumping facilities. Brush, debris, sand deposits, and other natural vegetation tend to block the waterways, thus restricting water passage and increasing upstream flood stages. Manmade structures on and over the Curry Creek also create more extensive flooding upstream than would otherwise occur. Past floods have caused damage because of development in the flood plain. Increased residential and commercial development has occurred in the City of Jefferson increasing demand for building sites, some of which are in floodprone areas. 2.4 Flood Protection Measures No major structural flood protection measures exist or are planned for Jackson County. 3.0 ENGINEERING METHODS For the flooding sources studied in the 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-, 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-percentannual-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 5

10 community at the time of completion of this study. Maps 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 The methods of hydrologic analyses developed in conjunction with the Expanded Flood Plain Information Report (USACE, 1977) were used in the hydrologic determinations of the Middle Oconee River, Big Bear Creek and Little Bear Creek in the unincorporated areas of Jackson County. Peak Discharges were determined from the U.S. Geological Survey (USGS) regional regression equations (U.S. Department of Interior, 1979). The drainage basins of the North Oconee River, Walnut Creek, and Cabin Creek are located in USGS Hydrologic Region 2. Drainage areas were determined at specific locations from topographic maps (USGS, 1964). A letter report dated July 1988 and entitled Hydrologic Analysis for Study Streams in Jackson County, Georgia presents the hydrologic portion of this study (Law Environmental, Inc., 1988). Discharges for Curry Creek in the City of Jefferson were determined by the USGS equations for runoff and by regression equations developed by the U.S. Army Corps of Engineers (USACE). Peak discharge-drainage area relationships for each flooding source studied in detail are shown in Table 1, Summary of Discharges. TABLE 1 - SUMMARY OF DISCHARGES FLOODING SOURCE AND LOCATION DRAINAGE AREA (sq. miles) PEAK DISCHARGES (cubic feet per second) 10-PERCENT ANNUAL CHANCE 2-PERCENT ANNUAL CHANCE 1-PERCENT ANNUAL CHANCE 0.2-PERCENT ANNUAL CHANCE BIG BEAR CREEK At mouth ,425 3,842 4,426 6,300 CABIN CREEK At mouth 6.6 1,400 2,200 2,500 3,500 6

11 TABLE 1 SUMMARY OF DISCHARGES continued FLOODING SOURCE AND LOCATION DRAINAGE AREA (sq. miles) 10-PERCENT ANNUAL CHANCE PEAK DISCHARGES (cubic feet per second) 2-PERCENT 1-PERCENT ANNUAL ANNUAL CHANCE CHANCE 0.2-PERCENT ANNUAL CHANCE CURRY CREEK Approximately 1.3 miles downstream of State Route 15 * 2,150 3,655 4,555 6,349 Approximately 1.2 miles upstream of State Route 15 * 1,650 2,800 3,550 5,587 EAST TRIBUTARY Just upstream of confluence with Middle Oconee River 1.42 * * 980 * Just upstream of confluence of West Tributary 0.65 * * 614 * INDIAN CREEK Just upstream of Tapp Wood Road (approximately 4,300 feet upstream of the confluence with Mulberry River) 7.4 * * 2,656 * LITTLE BEAR CREEK At mouth 6.1 1,370 2,200 2,600 3,700 MIDDLE OCONEE RIVER At U.S. Route ,500 20,000 22,700 30,000 At State Route * * 13,880 * Approximately 2.5 miles upstream of State Route * * 12,932 * MULBERRY RIVER At County Road * * 16,942 * Just downstream of the confluence of Indian Creek * * 14,108 * Approximately 1,300 feet upstream of the confluence of Indian Creek * * 13,436 * *Data not available 7

12 TABLE 1 SUMMARY OF DISCHARGES continued FLOODING SOURCE AND LOCATION DRAINAGE AREA (sq. miles) 10-PERCENT ANNUAL CHANCE PEAK DISCHARGES (cubic feet per second) 2-PERCENT 1-PERCENT ANNUAL ANNUAL CHANCE CHANCE 0.2-PERCENT ANNUAL CHANCE NORTH OCONEE RIVER At mouth ,900 16,500 19,054 26,000 At confluence of Cabin Creek * 8,160 12,670 14,300 19,100 Just upstream of confluence of Sandy Creek 210 7,150 10,600 12,250 18,200 TRIBUTARY TO WALNUT CREEK Approximately 40 feet downstream of Guy Cooper Road ,270 Approximately 600 feet upstream of Guy Cooper Road ,080 WALNUT CREEK At State Route ,600 7,200 8,200 11,100 At State Route * * 7,310 * Just upstream of New Cut Road 35.0 * * 6,541 9,574 Just downstream of the confluence of Tributary to Walnut Creek ,260 5,390 6,140 8,620 Just upstream of the confluence of Tributary to Walnut Creek ,160 5,210 5,950 8,350 At Cooper Bridge Road ,130 5,170 5,900 8,280 WALTON CREEK At confluence with North Oconee River 3.4 1,287 2,156 2,615 3,810 Approximately 2,800 feet upstream of Jefferson River Road WEST TRIBUTARY Just upstream of confluence of East Tributary to Middle Oconee River 0.35 * * 422 * Just upstream of Unnamed Road 0.27 * * 362 * *Data not available 8

13 Revised Analyses The hydrologic analyses for Walton Creek and the downstream reach of Middle Oconee River shown in Jackson County were studied as part of the Athens-Clarke County, Georgia FIS report (FEMA, 2007). The hydrologic analyses for the downstream reach of Mulberry River shown in Jackson County were studied as part of the Barrow County, Georgia LOMR (FEMA, 2008). Discharges for approximate studies were developed using regression equations for rural areas in Georgia contained in the USGS report and available USGS gage record data (where applicable) (Stamey and Hess, 1993). Drainage areas were developed from Jackson County 1.4-meter LiDAR bare earth Digital Elevation Models (DEMs) (Photo Science, 2009). 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-section data for the backwater analyses in the unincorporated areas of Jackson County were compiled from aerial photographs (Jack W. Berry & Associates, 1987) and field surveys. Below-water cross sections were obtained by field survey. Bridges and culverts within the study reach were surveyed to obtain elevation data and structural geometry. For the Middle Oconee River, Big Bear Creek, Little Bear Creek, Walnut Creek, Cabin Creek the North Oconee River, and Curry Creek water-surface elevations for floods at the selected recurrence intervals were computed using the HEC-2 step-backwater computer program (USACE, 1959, 1974, & 1984). Starting watersurface elevations were determined using the slope-area method. Backwater effects on Cabin Creek from the North Oconee River were estimated for each flood event by using HEC-2 to compute water-surface elevations for cross section on the North Oconee River near the confluence with Cabin Creek. Peak discharges for the North Oconee River were determined using USGS regional regression equations (U.S. Department of Interior, 1979). Channel geometry for the North Oconee River cross sections were approximated using data from 9

14 previous work on the North Oconee River upstream from Cabin Creek. Overbank geometry was determined from topographic maps (USGS, 1964). In the City of Jefferson, cross sections for Curry Creek overbank areas were obtained from aerial photographs. The channel sections were obtained by field measurement using normal survey methods; bridge and road profiles were field surveyed to obtain elevation and structural geometry. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1) and FIRM (Exhibit 2). Roughness coefficients (Manning s n ) used in the hydraulic computations were chosen by engineering judgment and were based on field observations and aerial photography of the streams and flood plain areas. The Manning s n values for selected detailed studied streams are listed in the following tabulation. Stream Channel n Overbank n Big Bear Creek Cabin Creek Curry Creek Little Bear Creek Middle Oconee River North Oconee River Walnut Creek Flood profiles were drawn showing the computed water-surface elevations for flood of the selected recurrence intervals. In cases where the 10-, 2-, 1-, or 0.2- percent-annual-chance flood elevations are close together, due to limitations of the profile scale, only the 1-percent-annual-chance flood profile has been drawn. Revised Analyses The hydraulic analyses of Walton Creek and the downstream reach of Middle Oconee River shown in Jackson County were studied as part of the Athens-Clarke County, Georgia FIS report (FEMA, 2007). The hydraulic analyses for the downstream reach of Mulberry River shown in Jackson County were studied as part of the Barrow County, Georgia LOMR (FEMA, 2008). 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. 10

15 For the streams studied by approximate methods, cross section data was obtained from the Jackson County 1.4-meter LiDAR bare earth DEMs (Photo Science, 2009). Hydraulically significant roads were modeled as bridges, with opening data approximated from available inventory data or approximated from the imagery. Top of road elevations were estimated from the best available topography. The studied streams were modeled using HEC-RAS version 4.0 (Hydrologic Engineering Center, 2008). Floodplains were delineated using the computer 1-percent annual chance watersurface elevations and the Jackson County 1.4-meter LiDAR bare earth DEMs (Photo Science, 2009). 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. Qualifying bench marks within a given jurisdiction that are cataloged by the National Geodetic Survey (NGS) and entered into the National Spatial Reference System (NSRS) as First or Second Order Vertical and have a vertical stability classification of A, B, or C are shown and labeled on the FIRM with their 6- character NSRS Permanent Identifier. Bench marks cataloged by the NGS and entered into the NSRS vary widely in vertical stability classification. NSRS vertical stability classifications are as follows: Stability A: Monuments of the most reliable nature, expected to hold position/elevation well (e.g., mounted in bedrock) Stability B: Monuments which generally hold their position/elevation well (e.g., concrete bridge abutment) Stability C: Monuments which may be affected by surface ground movements (e.g., concrete monument below frost line) Stability D: Mark of questionable or unknown vertical stability (e.g., concrete monument above frost line, or steel witness post) In addition to NSRS bench marks, the FIRM may also show vertical control monuments established by a local jurisdiction; these monuments will be shown on the FIRM with the appropriate designations. Local monuments will only be placed on the FIRM if the community has requested that they be included, and if the monuments meet the aforementioned NSRS inclusion criteria. To obtain current elevation, description, and/or location information for bench marks shown on the FIRM for this jurisdiction, please contact the Information Services Branch of the NGS at (301) , or visit their Web site at 11

16 It is important to note that 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 this FIS and FIRM. Interested individuals may contact FEMA to access this data. 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 29. With the finalization of NAVD 88, many FIS reports and FIRMs are being prepared using NAVD 88 as the referenced vertical datum. All flood elevations shown in this FIS report and on the FIRM are referenced to NAVD 88. Structure and ground elevations in the community must, therefore, be referenced to NAVD 88. It is important to note that adjacent communities may be referenced to NGVD 29. This may result in differences in Base Flood Elevations (BFEs) across the corporate limits between the communities. Prior version of the FIS report and FIRM were referenced to NGVD 29. When a datum conversion is effected for an FIS report and FIRM, the Flood Profiles, BFEs reflect the new datum values. To compare structure and ground elevations to BFEs shown in the FIS report and on the FIRM, the structure and ground elevations must be referenced to the new datum values. As noted above, the elevations shown in the FIS report and on the FIRM for Jackson County are referenced to NAVD 88. Ground, structure, and flood elevations may be compared and/or referenced to NGVD 29 by applying a standard conversion factor to the NAVD 88 values. The conversion factor to NGVD 29 is The BFEs shown on the FIRM represent whole-foot rounded values. For example, a BFE of will appear as 102 on the FIRM and will appear as 103. Therefore users that wish to convert the elevations in this FIS to NGVD 29 should apply the stated conversion factor to elevations shown on the Flood Profiles and supporting data tables in the FIS report, which are shown at a minimum to the nearest 0.1 foot. NAVD = NGVD 29 For additional information regarding conversion between NGVD 29 and NAVD 88, visit the National Geodetic Survey website at or contact the National Geodetic Survey at the following address: 12

17 NGS Information Services NOAA, N/NGS12 National Geodetic Survey SSMC-3, # East-West Highway Silver Spring, Maryland (301) FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local governments to adopt sound floodplain management programs. To assist in this endeavor, each FIS provides 1-percent-annual-chance floodplain data, which may include a combination of the following: 10-, 2-, 1-, and 0.2-percent-annualchance flood elevations; delineations of the 1- and 0.2-percent-annual-chance floodplains; and 1-percent-annual-chance floodway. This information is presented on the FIRM and in many components of the FIS, including Flood Profiles, Floodway Data tables, and Summary of Stillwater Elevation tables. Users should reference the data presented in the FIS as well as additional information that may be available at the local community map repository before making flood elevation and/or floodplain boundary determinations. 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the 1-percentannual-chance flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2-percent-annual-chance flood is employed to indicate additional areas of flood risk in the community. For the streams studied by detailed and approximate methods the boundaries were delineated using the Jackson County 1.4-meter LiDAR bare earth DEMs (Photo Science, 2009). For Walton Creek and the downstream reach of Middle Oconee River, the cross sections from Athens-Clarke County, Georgia were digitized and attributed with 1- percent-annual-chance flood elevation. The BFEs shown on the Athens-Clarke County, Georgia FIRM were extended from the Athens-Clarke County FIS and drawn perpendicular to flow of the stream in Jackson County. For the downstream reach of Mulberry River the cross sections of the Barrow County, Georgia LOMR were digitized and attributed with 1-percent-annualchance flood elevation. The BFEs shown on the Barrow County, Georgia LOMR were extended from the Barrow County FIS and drawn perpendicular to flow of the stream in Jackson County. Additional lines parallel to cross sections of Mulberry River, Walton Creek, and the downstream reach of Middle Oconee River were drawn to supplement the cross-section lines where they are far apart and attributed with 1-percent-annualchance flood elevation interpolated from the neighboring cross sections. These lines and cross sections were extended to fully cover the valley to ensure the correct backwater elevation was applied to the tributaries. These lines were used 13

18 to create a water-surface Triangular Irregular Network (TIN). The water-surface TIN was then subtracted from the ground surface TIN created from the USGS 10- meter DEM topographic data. The result was a depth of water TIN that was converted to a polygon with positive values (water above ground) to show the floodplain. This process was repeated for the 0.2-percent-annual-chance flood elevations of Walton Creek and the downstream reach of Middle Oconee River. 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-annual-chance 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 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 2). 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 water surface elevation of the 1-percent-annual-chance flood more than 1 foot at any 14

19 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY BASE FLOOD WATER-SURFACE ELEVATION (FEET NAVD) WITHOUT FLOODWAY WITH FLOODWAY INCREASE Big Bear Creek A 7, , B 8, Cabin Creek A 2, B 3, , C 4, , D 6, E 8, F 9, Little Bear Creek A 1, B 14, Walnut Creek A 10, , B 11, , C 13, , D 16, , E 18, , F 20, , G 23, H 26, , I 29, , Feet above mouth 4 Feet above mouth at Big Bear Creek 2 Feet above mouth at North Oconee River 3 Elevation computed without consideration of backwater effects from the North Oconee River TABLE 2 FEDERAL EMERGENCY MANAGEMENT AGENCY JACKSON COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA BIG BEAR CREEK CABIN CREEK - LITTLE BEAR CREEK WALNUT CREEK

20 point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in Figure 1. No floodways have been computed for Curry Creek, East Tributary, Indian Creek, Middle Oconee River, Mulberry River, North Oconee River, Tributary to Walnut Creek, Walton Creek, and West Tributary. Near the mouths of streams studied in detail, floodway computations are made without regard to flood elevations on the receiving water body. Therefore, Without Floodway elevations are presented in Table 2 for certain downstream cross sections of Cabin Creek are lower than the regulatory flood elevations in that area, which must take into account the 1-percent-annual-chance flooding due to backwater from other sources. FLOODWAY SCHEMATIC Figure 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: 16

21 Zone A Zone A is the flood insurance risk zone that corresponds to the 1-percent-annualchance 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 rate zone that corresponds to the 1-percent annual chance floodplains that are determined in the FIS by detailed methods. In most instances, whole-foot base flood elevations 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- percent-annual-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-percentannual-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 Jackson County. Previously, FIRMs were prepared for each incorporated community and the unincorporated areas of Jackson County identified as flood-prone. Historical data relating to the maps prepared for each community are presented in Table 3, Community Map History. 17

22 COMMUNITY NAME INITIAL IDENTIFICATION FLOOD HAZARD BOUNDARY MAP REVISIONS DATE FIRM EFFECTIVE DATE FIRM REVISIONS DATE Arcade, City of 1 April 23, 1976 None November 16, 1990 Braselton, Town of April 23, None June 15, November 2, Commerce, City of 1 April 23, 1976 None November 16, 1990 Hoschton, City of 1 April 23, 1976 None November 16, 1990 Jackson County April 23, 1976 None November 16, 1990 (Unincorporated Areas) Jefferson, City of June 28, 1974 June 4, 1976 June 4, 1987 Maysville, City of 1 April 23, 1976 None November 16, 1990 Nicholson, City of 1 April 23, 1976 None November 16, 1990 Pendergrass, City of 1 April 23, 1976 None November 16, 1990 Talmo, City of 1 April 23, 1976 None November 16, 1990 August 1, November 16, October 16, September 29, ,4 December 18, This community did not have its own FIRM prior to this countywide FIS. A portion of the land area for this community was previously shown on the FIRM for the unincorporated areas of Jackson County. Therefore, the map history dates associated with this community were taken from the FIRM for Jackson County. 2 Dates taken from Jackson County, GA 3 Dates taken from Gwinnett County, GA. 4 Dates taken from Hall County, GA 5 Dates taken from Barrow County, GA TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY JACKSON COUNTY, GA AND INCORPORATED AREAS COMMUNITY MAP HISTORY C

23 7.0 OTHER STUDIES Information pertaining to flood hazards for each jurisdiction within Jackson County has been compiled into this FIS. Therefore, this FIS supersedes all previously printed FIS reports, FHBMs, FBFMs, and FIRMs for all of the incorporated and unincorporated jurisdictions within Jackson County 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. (April 2, 2007). Flood Insurance Study, Athens-Clarke County, Georgia (All Jurisdictions). Federal Emergency Management Agency. (June 4, 1987). Flood Insurance Study, City of Jefferson, Jackson County, Georgia. Federal Emergency Management Agency. (November 16, 1990). Flood Insurance Study, Jackson County (Unincorporated Areas), Georgia. Federal Emergency Management Agency. (July 11, 2008). Letter of Map Revision, Barrow County, Georgia, Case No P. Hydrologic Engineering Center. (March 2008). HEC-RAS River Analysis System, Version U.S. Army Corps of Engineers. Davis, California. Jack W. Berry & Associates. (March 1987). Aerial Photographs. Peachtree City, Georgia. National Weather Service. (Accessed March 27, 2009) Georgia s Climatology Law Environmental Inc. (July 1988). Hydrologic Analysis for Study Streams in Jackson County, Georgia, Kennesaw, Georgia. Photo Science, Inc. (Collected 2009). 1.4-meter LiDAR bare earth DEMs. Stamey, T.C. and G. W. Hess. (1993). Techniques for Estimating Magnitude and Frequency of Floods in Rural Basins of Georgia, Water Resources Investigation Report U.S. Geological Survey. 19

24 U.S. Army Corps of Engineers. (May 1977). Expanded Flood Plain Information Report, Upper Oconee River Basin, Georgia. U.S. Army Corps of Engineers. (December 1959). Engineering Manual, Backwater Curves in River Channels. U.S. Army Corps of Engineers. (November 1976, updated May 1984). HEC-2 Water Surface Profiles, Generalized Computer Program, Hydrologic Engineering Center, Davis, California. U.S. Army Corps of Engineers (July 1976). Special Flood Hazard Information, Curry Creek, Jefferson, Georgia, Savannah District. U.S. Army Corps of Engineers. (June 1974). Training Document No. 6, Calculation of Water-Surface Profiles through Bridges. U.S. Census Bureau. (Accessed June 16, 2009) Population Estimate U.S. Department of the Interior. (1979). Geological Survey, Water Resources Investigation , Floods in Georgia, Magnitude and Frequency, U.S. Geological Survey, (Nicholson, Georgia, 1985; Commerce, Georgia, 1964; Pendergrass, Georgia, 1964). 7.5 Minute Series Topographic Maps, Scale 1:24000, Contour Interval of 20 feet. 20

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