OCONEE COUNTY, GEORGIA

Transcription

1 OCONEE COUNTY, GEORGIA AND INCORPORATED AREAS Oconee County Community Name Community Number *BISHOP, TOWN OF *BOGART, CITY OF NORTH HIGH SHOALS, TOWN OF OCONEE COUNTY (UNINCORPORATED AREAS) WATKINSVILLE, CITY OF *No Special Flood Hazard Areas Identified Effective: September 2, 2009 FLOOD INSURANCE STUDY NUMBER 13219CV000A

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 2, 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 i

4 TABLE OF CONTENTS (Continued) FIGURES Figure 1 - Floodway Schematic TABLES Table 1 - Streams Studied by Detailed Methods...4 Table 2 - Redelineated Streams Studied by Detailed Methods...4 Table 3 - Streams Studied by Limited Detailed Methods...5 Table 4 - Streams Studied by Approximate Methods...6 Table 5 - Summary of Discharges...9 Table 6 - Vertical Datum Conversion...14 Table 7 - Floodway Data...17 Table 8 - Community Map History...22 EXHIBITS Exhibit 1 - Flood Profiles Apalachee River Barber Creek Calls Creek Lampkin Branch McNutt Creek Oconee River-Middle Oconee River Parker Branch Porters Creek Shoal Creek Unnamed Tributary to Calls Creek West Fork Parker Branch Panels 01P-06P Panels 07P-09P Panels 10P-12P Panels 13P-14P Panels 15P-17P Panels 18P-19P Panels 20P-22P Panels 23P-26P Panels 27P-28P Panel 29P Panel 30P Exhibit 2 - Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY OCONEE 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 Oconee County, including the Cities of Bogart and Watkinsville; the Towns of Bishop and North High Shoals; and the unincorporated areas of Oconee County (referred to collectively herein as Oconee 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 Bogart is geographically located in Oconee and Clarke Counties. Only the portion of the City of Bogart located in Oconee County is included in its entirety in this FIS report. Please note that the Town of Bishop and the City of Bogart have no mapped flood hazard areas. 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 report, as compiled from their previously printed FIS reports, are shown below: North High Shoals, Town of: Oconee County (Unincorporated Areas): For the May 16, 1995, FIS report (FEMA, 1995a), the hydrologic and hydraulic analyses for the Apalachee River were performed by Neel Schaffer, Inc., for FEMA, under Contract No. EMW-90-C The work was completed in May For the July 17, 1989, FIS report (FEMA, 1989), the hydrologic and hydraulic analyses for Barber Creek were performed by W.L. Jorden & Company, Inc., for FEMA, under Contract No. EMA-86-C The work was completed in September Additional hydraulic information was obtained from the Expanded Flood Plain Information Report, Upper Oconee River Basin, Georgia (USACE, 1977). For the April 17, 1995, FIS report (FEMA, 1995b), the hydrologic and hydraulic analyses for the Apalachee River were performed by Neel Schaffer, Inc., for FEMA, under Contract No. EMW-90-C The work was completed in May The Cities of Bogart and Watkinsville and the Town of Bishop have no previously printed FIS reports. This Countywide FIS Report The hydrologic and hydraulic analyses and redelineation for this study were performed by PBS&J under Contract No. EMA-2006-CA-5615 and the Georgia Department of Natural Resources (DNR). This work was completed in December The new hydraulics, hydrology, and redelineation were completed by PBS&J, while the Georgia DNR completed the limited detailed studies. Base map information shown on the Flood Insurance Rate Map (FIRM) was derived from aerial photography provided by Oconee County, dated March

7 The projection used in the preparation of this map is State Plane Georgia East, and the horizontal datum used is the North American Datum of 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. Precountywide Analyses The initial and final meeting dates for previous FIS reports for Oconee County and its communities are listed in the following table: Community FIS Date Initial Meeting Final Meeting North High Shoals, Town of Oconee County (Unincorporated Areas) *Notified by a letter May 16, 1995 May 4, 1993* April 26, 1994 July 17, 1989 April 17, 1995 **No data available January 22, 1986 April 14, 1993* August 25, 1988 ** 2.0 AREA STUDIED This Countywide FIS Report For this countywide FIS, the initial scoping meeting was held on January 18, 2005, at the Barrow County Recreational facility, and attended by representatives of FEMA, Georgia DNR, PBS&J, and the communities. The results of the study were reviewed at the final meeting held on December 16, 2008, and attended by representatives of FEMA, Georgia DNR, PBS&J, and the communities. All problems raised at that meeting have been addressed. 2.1 Scope of Study This FIS covers the geographic area of Oconee 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 Streams studied by detailed methods in this FIS report are indicated in Table 1. Limits of detailed study are shown on the Flood Profiles (Exhibit 1), and on the FIRM (Exhibit 2). 3

8 Table 1 - Streams Studied by Detailed Methods Apalachee River Barber Creek Calls Creek Lampkin Branch McNutt Creek Oconee River-Middle Oconee River Parker Branch Porters Creek Shoal Creek Unnamed Tributary to Calls Creek West Fork Parker Branch For this initial countywide FIS, existing areas studied by detailed methods were redelineated as part of this countywide revision. Oconee County provided PBS&J with countywide digital topographic data dated 2005 (Oconee County, 2005). The topographic data was provided as a Digital Elevation Model (DEM) in raster grid format and the elevation data in the North American Vertical Datum of 1988 (NAVD). The DEM was created from 2-foot contour data, with a vertical accuracy of 6 inches, generated from a countywide airborne LIDAR survey completed in The streams redelineated as part of this countywide revision are listed in Table 2. Table 2 - Redelineated Streams Studied by Detailed Methods Stream Reach Limits Apalachee River Barber Creek From approximately 10,070 feet upstream of Price Mill Road to approximately 165 feet upstream of U.S. Highway 78/State Highway 10/Monroe Highway From the confluence with McNutt Creek to approximately 6,450 feet upstream of Old Hodges Mill Road Calls Creek From the confluence with Middle Oconee River to approximately 3,665 feet upstream of U.S. Highway 441 / 129 / State Highway 24 / 186 Lampkin Branch From confluence with Calls Creek to Experiment Station Road / State Highway 53 McNutt Creek From the confluence with Middle Oconee River to State Highway 316 / U.S. Highway 29 / State Highway 8 / University Parkway Oconee River - Middle Oconee River Parker Branch Porters Creek From approximately 30,000 feet downstream of confluence of Wildcat Creek to the confluence of McNutt Creek From the confluence with Barber Creek to Autry Road From the confluence with Oconee River to approximately 4,200 feet upstream of McCrees Gin Road 4

9 Table 2 Redelineated Streams Studied by Detailed Methods (Continued) Stream Reach Limits Shoal Creek West Fork Parker Branch From the confluence with Oconee River to the county boundary From the confluence with Parker Branch to Autry Road A floodway has been added to Oconee River Middle Oconee River from Barnett Shoal s Road to approximately 9,360 feet upstream of Simonton Bridge Road. This floodway was added using data from the Athens-Clarke County FIS (FEMA, 2007). The areas studied by limited detailed methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction through June The streams studied by limited detailed methods are listed in Table 3. Table 3 - Streams Studied by Limited Detailed Methods Stream Freeman Creek Greenbrier Creek Lane Creek Reach Description From approximately 320 feet upstream of confluence with Apalachee River to approximately 2,350 feet upstream of Freemans Creek Road From approximately 720 feet upstream of county boundary to approximately 2,330 feet upstream of U.S. Highway 441 / 129 / State Highway 24 / 186 / Macon Highway From confluence with Apalachee River to approximately 7,090 feet upstream of Snows Mill Road Rose Creek From county boundary to approximately 9,950 feet upstream of Astondale Road Wolf Creek From approximately 250 feet upstream of confluence with Apalachee River to approximately 230 feet upstream of Tappan Spur Road Approximate analyses were used to study those areas having low development potential or minimal flood hazards. Existing areas studied by approximate methods were redelineated using the new two foot contours as part of this countywide FIS. The scope and methods of study were proposed to and agreed upon by FEMA and Oconee County. Twenty-five miles of new approximate analyses was completed for Oconee County. The streams restudied by approximate methods are presented in Table 4. 5

10 Table 4 - Streams Studied by Approximate Methods Stream Barber Creek Dove Creek Frazer Creek Middle Robinson Creek Porters Creek Robinson Creek Reach Description From approximately 6,460 feet upstream of Old Hodges Mill Road to the county boundary From confluence with Apalachee River to approximately 3,450 feet upstream of Osborne Road From the confluence with Robinson Creek to approximately 11,515 feet upstream of Rays Church Road From the confluence with Robinson Creek to approximately 1,030 feet upstream of High Shoals Road / State Highway 186 From approximately 4,200 feet upstream of McCrees Mill Road to approximately 290 feet upstream of Norfolk Southern Railway From the confluence with Apalachee River to approximately 650 feet upstream of Old Watkinsville Road All other streams studied by approximate methods were redelineated based on the new topography. 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 NAVD. In addition, the Universal Transverse Mercator coordinates, previously referenced to the North American Datum of 1927, are now referenced to the North American Datum of The following tabulation presents Letters of Map Change (LOMCs) incorporated into this countywide study: LOMC Case Number Date Issued Project Identifier LOMR P May 20, 2004 *Unnamed Tributary of Butler Creek - Christian Lake *Name has been changed to Unnamed Tributary to Calls Creek 6

11 2.2 Community Description Oconee County is located in northeastern Georgia, approximately 7 miles south of the City of Athens-Clarke County and 62 miles east of the City of Atlanta. It is bordered on northeast by Athens-Clarke County, on the east by Oglethorpe County, on the south by Greene and Morgan Counties, on the west by Walton County, on the northwest by Barrow County, and on the north by Jackson County. The City of Watkinsville is the county seat. According to the U.S. Census Bureau, the estimated 2005 population of Oconee County and incorporated areas was 29,748 (U.S. Census Bureau, 2007). This represents an increase of 13.4% from the population of 26,225 in The climate of the area is generally warm, with humid summers and mild, short winters. The average summer high temperature in July is 80 degrees Fahrenheit ( F) and the average winter low temperature in January is 42 F. The average annual precipitation is 47.8 inches with March being the wettest month (The Weather Channel, 2007). The county encompasses 186 square miles within the Piedmont physiographic province, in the Oconee River Basin. The terrain of the county is, for the most part, sharply rolling hills with narrow floodplains. The streams are relatively narrow, shallow, and fast-flowing, with steep gradients. 2.3 Principal Flood Problems Most flooding problems in Oconee County result from the overflow of Barber Creek, Calls Creek, the Oconee River, and Porter Creek. Most flood damage reported has been to pastures and croplands (Northeast Georgia Area Planning and Development Commission, 1986). Flooding problems that occur in the Town of North High Shoals are a result of the overflow of the Apalachee River. Damaging floods in the area occur during winter and spring, when flooding is most likely. Major flooding in the region is caused primarily by runoff from rain and thunderstorms, and the occasional large flood is a result of hurricanes and tropical storms. 2.4 Flood Protection Measures Barnett Shoals Dam is located upstream of the confluence of Shoal Creek on the Oconee River but does not provide any flood protection for downstream areas. 7

12 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 The discharge-frequency relationships for the 10-,2-, and 1-percent-annual-chance floods for Apalachee River, Barber Creek and McNutt Creek were estimated using the regional frequency equations and methodologies developed by the U.S. Geological Survey (USGS) for rural basins (USGS, 1979). The USGS method of analysis estimates the peak discharges developed for rural streams with adjustments made with gage data. The 0.2-percent-annual-chance peak discharges were estimated by extrapolation of the flow-frequency relationship. Streamflow hydrographs for the 1-percent-annual-chance flood for Calls Creek, Lampkin Branch, Oconee River-Middle Oconee River, Parker Branch, Porters Creek, Shoal Creek, and West Fork Parker Branch were computed by applying synthetic storm precipitation to a flood hydrograph model calibrated for the appropriate conditions. The rainfall runoff parameters were developed by semiautomated analysis using data from the data banks (land use, soil type, land surface slope, and sub-basin boundaries), and data from observed runoff events, when available. 8

13 This Countywide FIS Report For Unnamed Tributary to Calls Creek peak discharges were estimated using the approach described in the Natural Resource Conservation Service s Technical Realse-55 model (SCS, 1986). For the streams studied by limited detailed and approximate methods listed in Table 3 and Table 4, the 1-percent-annual-chance flood discharges were determined using the USGS regression equation for rural watersheds in 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 Oconee County are shown 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 APPALACHEE RIVER Approximately 500 feet ,530 14,600 16,400 21,300 downstream of the confluence of Big Robinson Creek At the confluence of ,220 13,900 15,800 19,500 Robinson Creek At State Highway 186 / High ,150 13,800 15,600 18,700 Shoals Road Approximately 10,730 feet ,110 13,700 15,500 18,700 downstream of Lane Creek At the confluence of Lane ,000 13,500 15,300 17,200 Creek At Snows Mill Road ,880 13,300 15,100 16,600 Approximately 0.9 mile ,830 13,200 15,000 16,400 upstream of Snows Mills Road At the confluence of ,710 13,000 14,700 16,300 Turkey Creek Just downstream of U.S ,530 12,700 14,400 16,200 Highway 78 / State Highway 10 / Monroe Highway Just upstream of U.S. Highway 78 / State Highway 10 / Monroe Highway ,470 12,600 14,300 16,100 BARBER CREEK Approximately 3.8 miles downstream of Daniels Bridge Road ,074 6,397 7,306 9,878 9

14 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 BARBER CREEK (CONTINUED) Approximately 2.4 miles downstream of Daniels Bridge Road Approximately 0.7 miles downstream of Daniels Bridge Road Just downstream of Daniels Bridge Road Approximately 1.6 miles feet downstream of Mars Hill Road Approximately 400 feet downstream of Old Hodges Mill Road Approximately 0.5 mile upstream of Old Hodges Mill Road Approximately 1 mile upstream of Old Hodges Mill Road ,989 6,265 7,158 9, ,917 6,155 7,034 9, ,858 6,063 6,930 9, ,749 5,894 6,741 9, ,607 5,675 6,494 8, ,434 5,408 6,194 8, ,329 5,245 6,010 8,154 CALLS CREEK * * * * * LAMPKIN BRANCH * * * * * MCNUTT CREEK At the confluence with Middle Oconee River Approximately 2,350 feet upstream of U.S. Highway 441 / 129 / State Highway 15 Just upstream of Unnamed Crossing Approximately 0.5 mile upstream of Jimmie Daniel Road OCONEE RIVER MIDDLE OCONEE RIVER ,525 8,629 9,804 13, ,210 3,506 4,046 5, ,834 2,918 3,378 4, ,405 2,246 2,611 3,700 * * * * * PARKER BRANCH * * * * * PORTERS CREEK * * * * * SHOAL CREEK * * * * * UNNAMED TRIBUTARY TO CALLS CREEK Just upstream of Christian Lake Dam 0.26 * * 513 * *Data not available 10

15 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 UNNAMED TRIBUTARY TO CALLS CREEK (CONTINUED) Approximately 1,037 feet upstream of Christian Lake Dam WEST FORK PARKER BRANCH 0.15 * * 326 * * * * * * *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 Cross-section data were obtained from field surveys. All bridges, culverts, and dams were surveyed to obtain elevation and structural geometry data. Water surface elevations (WSELs) of the floods of the selected recurrence intervals for Barber Creek, Calls Creek, Lampkin Branch, McNutt Creek, Oconee River-Middle Oconee River, Parker Branch, Porters Creek, Shoal Creek, and West Fork Parker Branch were computed using the USACE HEC-2 stepbackwater computer program (HEC, 1984). The WSELs for the Apalachee River were computed using a more recent USACE HEC-2 step-backwater computer program (HEC, 1991). The Apalachee River has a natural falls in the vicinity of the Town of North High Shoals. The bottom slope is very steep in this reach of the river; the elevation drops from to NAVD in 525 feet. The HEC-2 model calculates critical depth at cross sections in the reach. Two low-head dams in the study reach were modeled using the HEC-2 Special Bridge Method, as outlined in the HEC-2 User s Manual (HEC, 1984). A small opening area was specified in the model 11

16 to satisfy the special bridge modeling routine, correctly forcing the majority of the stream discharge to pass over the weir. Starting WSELs for Barber Creek are taken from the FIS Report for Clarke County, Georgia (FIA, 1978). Starting WSELs for the Apalachee River were estimated using the slope/area method. Information concerning the starting WSELs for all other detailed studied streams is not available. This Countywide FIS Report The cross sections for Unnamed Tributary to Calls Creek were obtained by field survey and the WSEL was computed using the USACE s HEC-RAS River Analysis System, Version (HEC, 2001). Starting WSELs were computed using normal depth. For streams studied by limited detailed methods listed in Table 3, cross sections were obtained using digital topography and field surveys. WSELs of floods for the limited detailed studies were computed using the USACE HEC-RAS computer program (HEC, 2005). The hydraulic model was prepared using digital elevation data without surveying bathymetric data. Field measurements were conducted to approximate the geometry of the hydraulic structures in the hydraulic models. A limited detailed study is a buildable product that can be upgraded to a full detailed study at a later date by verifying stream channel characteristics, bridge, and culvert opening geometry, and by analyzing multiple recurrence intervals. For the streams studied by approximate methods listed in Table 4, 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). Structure geometry data included in the hydraulic model was inferred using aerial photography and topographic data. The hydraulic models do not include either field surveys of the channel and floodplain nor bridge and culvert geometry. 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 field inspection and based on engineering judgment. The Manning s n values for all detailed studied streams are listed in the following table: 12

17 Manning's "n" Values Stream Channel n Overbank n Apalachee River Barber Creek Calls Creek * * Lampkin Branch * * McNutt Creek Oconee River * * Oconee River-Middle Oconee River * * Parker Branch * * Porters Creek * * Shoal Creek * * Unnamed Tributary to Calls Creek West Fork Parker Branch * * *Data not available Flood profiles were drawn showing computed WSELs for floods of the selected recurrence intervals. 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. Some of the data used in this study were taken from the prior effective FIS reports and adjusted to NAVD. The average conversion factor that was used to convert the data in this 13

18 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 6. Table 6 - Vertical Datum Conversion Quad Name Corner Latitude Longitude Conversion from NGVD to NAVD Winder South SE feet Statham SE feet Athens West SE feet Athens East SE feet Watkinsville SE feet Barnett Shoals SE feet Average: feet 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 14

19 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each FIS provides 1-percent-annual-chance (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 except Unnamed Tributary to Calls Creek, 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 digital topography, with a contour interval of 2 feet (Oconee County, 2005). For Unnamed Tributary to Calls Creek, the floodplain boundaries were interpolated using topographic data, with a contour interval of 2 feet (Brewer and Dudley, L.L.C., 2003). For all streams studied by approximate methods and limited detailed methods, the 1-percent-annual-chance floodplain boundaries were delineated using digital topography, with a contour interval of 2 feet (Oconee County, 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. 15

20 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 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. 16

21 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH2 (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) APALACHEE RIVER A 10, / 60 2, B 14, / 93 2, C 16, / 58 2, D 19, / 75 3, E 23, / 215 4, F 25, / 109 2, G 26, / 110 1, H 28, / 48 2, I 31, / 121 3, J 34, / 112 3, K 36, / 303 5, L 39, / 143 5, M 41, / 95 3, N 44, / 140 3, O 46, / 95 3, P 49, / 120 3, Q 49, / 111 2, R 60, / 64 3, S 53, / 81 1, T 56, / 115 3, U 57, / 128 3, V 59, / 79 3, W 62, / 110 3, X 65, / 750 7, Y 67, / 184 6, Z 70,335 1,306 / , Distance in feet above Price Mill Road 2 Total width / Width within Oconee County TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY OCONEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA APALACHEE RIVER

22 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) APALACHEE RIVER (CONTINUED) AA 72, / , AB 77, / , BARBER CREEK A 3, , B 5, , C 7, , D 8, E 10, , F 11, , G 11, H 13, , I 16, , J 20, , K 22, , L 25, M 26, , N 28, , O 29, P 31, Q 37, Distance in feet above Price Mill Road 2 Total width / Width within Oconee County 3 Distance in feet above confluence with McNutt Creek TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY OCONEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA APALACHEE RIVER BARBER CREEK

23 FLOODING SOURCE CROSS SECTION DISTANCE 2 WIDTH3 (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) OCONEE RIVER A-C 1 D 1,189, /191 5, E 1,195, /393 9, F 1,198, /98 9, G 1,201, /112 6, H 1,204, /188 8, MIDDLE OCONEE RIVER I 1,208, /21 3, J 1,211, /363 8, K 1,218, /99 3, L 1,225, /160 5, Floodway not computed 2 Distance in feet above confluence with Altamaha River 3 Total width/ Width within Oconee County TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY OCONEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA OCONEE RIVER MIDDLE OCONEE RIVER

24 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 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 Calls Creek, Lampkin Branch, McNutt Creek, Parker Branch, Porters Creek, Shoal Creek, Unnamed Tributary to Calls Creek, and West Fork Parker Branch. 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: 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 20

25 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 Oconee 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 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. 21

26 COMMUNITY NAME INITIAL IDENTIFICATION FLOOD HAZARD BOUNDAY MAP REVISION DATE FIRM EFFECTIVE DATE FIRM REVISION DATE *Bishop, Town of N/A None N/A None *Bogart, City of N/A None N/A None North High Shoals, Town of August 19, 1977 None September 1, 1986 May 16, 1995 Oconee County (Unincorporated Areas) January 13, 1978 None July 17, 1989 April 17, 1995 Watkinsville, City of April 11, 1975 None August 19, 1986 None *No special flood hazard areas identified TABLE 8 FEDERAL EMERGENCY MANAGEMENT AGENCY OCONEE COUNTY, GA AND INCORPORATED AREAS COMMUNITY MAP HISTORY mmunity Map History

27 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 Brewer and Dudley, L.L.C., Christian Dam, Scale 1:200, Contour Interval 2 feet, Watkinsville, Georgia, July Federal Emergency Management Agency, Flood Insurance Study Report, Town of High Shoals, Oconee County, Georgia, May 16, 1995a. Federal Emergency Management Agency, Flood Insurance Study Report, Oconee County, Georgia (Unincorporated Areas), July 17, Federal Emergency Management Agency, Flood Insurance Study Report, Oconee County, Georgia (Unincorporated Areas), April 17, 1995b. Federal Insurance Administration, Flood Insurance Study Report, Clarke County, Georgia (Unincorporated Areas), June 15, Federal Insurance Administration, Flood Insurance Study Report, Athens-Clarke County, Georgia (All Jurisdictions), April 2, Hydrologic Engineering Center, HEC-2 Water Surface Profiles, U.S. Army Corps of Engineers, Davis, California, April Hydrologic Engineering Center, HEC-2 Water Surface Profiles, U.S. Army Corps of Engineers, Davis, California, May Hydrologic Engineering Center, HEC-RAS River Analysis System, Version 3.0.1, U.S. Army Corps of Engineers, Davis, California, April Hydrologic Engineering Center, HEC-RAS River Analysis System, Version 3.1.3, U.S. Army Corps of Engineers, Davis, California, May National Geodetic Survey, VERTCON-North American Vertical Datum Conversion Utility. Retrieved January 10, 2007, from Northeast Georgia Area Planning and Development Commission, Economic Profile of Oconee County, Georgia, April

28 Oconee County, Georgia, Digital Topographic Data, Oconee County, Georgia, Contour Interval 2 feet, Soil Conservation Service, Urban Hydrology for Small Watersheds, Technical Release No. 55, U.S. Department of Agriculture, January Stamey, T.C. and C.W. Hess, Techniques for Estimating Magnitude and Frequency of Floods in Rural Basins of Georgia, USGS Water Resources Investigations Report , U.S. Army Corps of Engineers, Expanded Flood Plain Information Report, Upper Oconee River Basin, Georgia, May U.S. Census Bureau, Quickfacts, Oconee County, Georgia, , Retrieved January 5, 2007, from U.S. Geological Survey, Water Resources Investigations Report , Floods in Georgia, Magnitude and Frequency, McGlone Price, October Weather Channel, Monthly Averages for Watkinsville, Georgia, Retrieved January 5, 2007, from 24

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