DAWSON COUNTY, GEORGIA

Transcription

1 DAWSON COUNTY, GEORGIA AND INCORPORATED AREAS Dawson County COMMUNITY NAME COMMUNITY NUMBER DAWSON COUNTY (UNINCORPORATED AREAS) DAWSONVILLE, CITY OF SEPTEMBER 26, 2008 FLOOD INSURANCE STUDY NUMBER 13085CV000A 42125CV001A

2 NOTICE TO FLOOD INSURANCE STUDY USERS Communities participating in the National Flood Insurance Program have established repositories of flood hazard data for floodplain management and flood insurance purposes. This Flood Insurance Study (FIS) report may not contain all data available within the Community Map Repository. Please contact the Community Map Repository for any additional data. The Federal Emergency Management Agency (FEMA) may revise and republish part or all of this FIS report at any time. In addition, FEMA may revise part of this FIS report by the Letter of Map Revision process, which does not involve republication or redistribution of the FIS report. Therefore, users should consult with community officials and check the Community Map Repository to obtain the most current FIS report components. Selected Flood Insurance Rate Map (FIRM) panels for this community contain information that was previously shown separately on the corresponding Flood Boundary and Floodway Map (FBFM) panels (e.g., floodways, cross sections). In addition, former flood hazard zone designations have been changed as follows: Old Zone(s) Al through A30 B C New Zone AE X X 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 i

4 TABLE OF CONTENTS (Continued) TABLES Table 1 Initial and Final CCO Meetings... 2 Table 2 Areas Studied by Detailed Methods... 3 Table 3 Streams Restudied by Approximate Methods... 3 Table 4 Summary of Discharges... 8 Table 5 Manning s n Value Table... 9 Table 6 Vertical Datum Conversion Table 7 Community Map History EXHIBITS Exhibit 1 Flood Profiles Black Mill Creek Panels 01P-04P Etowah River Panels 05P-09P Mill Creek Panel 10P Exhibit 2 Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY DAWSON COUNTY, GEORGIA AND INCORPORATED AREAS 1.0 INTRODUCTION 1.1 Purpose of Study This Flood Insurance Study (FIS) revises and updates the information on the existence and severity of flood hazards in the geographic area of Dawson County, including the City of Dawsonville; and the unincorporated areas of Dawson County (referred to collectively herein as Dawson 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 Dawson County (Unincorporated Areas): In the December 15, 1990 study (Reference 1), the hydrologic and hydraulic analyses were prepared by Post, Buckley, Schuh & Jernigan, Inc., (the Study Contractor) for the Federal Emergency Management Agency (FEMA), under Contract No. EMA-86-C That work was completed in December For this countywide FIS revision, 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 September No previous reports were prepared for the City of Dawsonville. 1

6 Also, detailed study information for Black Mill Creek, Etowah River, and Little Mill Creek was taken from the FIS for Dawson County, Georgia and Unincorporated Areas (Reference 1). Base map information shown on the Flood Insurance Rate Map (FIRM) was derived from U.S. Geological Survey (USGS) Digital Orthophoto Quadrangles produced at a scale of 1:12,000, from National Aerial Photography Program black and white photography dated 2001 or later. The projection used in the preparation of this map is North American Datum of 1983 StatePlane Georgia West, and the horizontal datum used is 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. The initial and final meeting dates for previous FIS reports for Dawson County and its communities are listed in the following table: Table 1 Initial and Final CCO Meetings Community Name FIS Date Initial Meeting Final Meeting Dawson County, December 15, 1990 October 14, 1986 January 29, 1990 (Unincorporated Areas) Dawsonville, City of None * * * Data not available For this countywide FIS, a scoping meeting was held on January 18, 2006, and attended by representatives of Dawson County, the Georgia Department of Natural Resources, FEMA, Georgia Environmental Protection Division and PBS&J. The initial meetings were held to discuss the nature and purpose of the FIS here. 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Dawson County, Georgia, including incorporate communities listed in Table 2, Areas Studied by Detailed Methods lists the streams that were studied by detailed methods. The areas studied by detailed methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction. 2

7 Stream Black Mill Creek Table 2 Areas Studied by Detailed Methods Limits of Detailed Study From mouth of Etowah River to 9,350 feet upstream of Blacks Mill Road Etowah River From State Route 9 to 1,900 feet upstream of State Route 136 Mill Creek From mouth of Etowah River to 3,700 feet upstream of Thompson Road 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 May The streams restudied by approximate methods are presented in Table 3, Streams Restudies by Approximate Methods. Table 3 Streams Restudied by Approximate Methods Stream Amicalola Creek Flat Creek Reach Description From Dawson and Forsyth Counties boundary to State Route 136 From the confluence of Shoal Creek to approximately 1200 feet West of State Route 9. Holly Creek From confluence of Amicalola Creek to Approximately 4600 feet Northeast of State Router 183 Pigeon Creek Pigeon Creek Tributary 5 Pigeon Creek Tributary 6 Pigeon Creek Tributary 6.1 Pigeon Creek Tributary 7 From the confluence of Shoal Creek to approximately 2300 feet West of Dawson and Lumpkin counties boundary. From the confluence of Pigeon Creek to approximately 500 feet East of State Route 183 From the confluence of Pigeon Creek to approximately 360 feet Southwest of State Route 136 From the confluence of Pigeon Creek Tributary 6 to approximately 1200 feet East of State Route 136 From the confluence of Pigeon Creek to approximately 800 feet West of Dawson and Lumpkin counties boundary. 3

8 Table 3 Streams Restudied by Approximate Methods Stream Pigeon Creek Tributary 7.1 Pigeon Creek Tributary 8 Pigeon Creek Tributary 8.1 Pigeon Creek Tributary 9 Pigeon Creek Tributary 10 Reach Description From the confluence of Pigeon Creek Tributary 7 to approximately 700 feet Little Mountain Road From the confluence of Pigeon Creek to approximately 3800 feet Southwest of Dawson and Lumpkin counties boundary. From the confluence of Pigeon Creek Tributary 8 to approximately 3200 feet Southwest of Dawson and Lumpkin counties boundary. From the confluence of Pigeon Creek to approximately 3100 feet Southwest of Dawson and Lumpkin counties boundary. From the confluence of Pigeon Creek to approximately 1200 feet Southwest of Dawson and Lumpkin counties boundary. Shoal Creek Tributary 6 From the confluence of Shoal Creek to approximately 1200 feet west of the intersection of Perimeter Road and State Route 9 From Dawson and Forsyth Counties boundary to State Route 136 All other streams studied by approximate methods were redelineated to better match existing 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 the North American Vertical Datum of 1988 (NAVD). In addition, the Transverse Mercator, State Plane coordinates, previously referenced to the North American Datum of 1927, are now referenced to the North American Datum of Approximate analyses were used to study those areas having low development potential or minimal flood hazards. The scope and methods of study were proposed to and agreed upon by FEMA and the communities. 2.2 Community Description Dawson County, encompassing 210 square miles, is in north-central Georgia, approximately 31 miles north of Atlanta. It is bordered by Gilmer, Fannin and Lumpkin Counties, Georgia, on the north; Hall County, Georgia, on the east; Forsyth County, Georgia, on the south; and 4

9 Cherokee and Pickens Counties, Georgia, on the west. State Routes 9, 53, 136, and 400 provide Dawson County with major transportation links to the City of Atlanta and surrounding communities. The 2000 population of Dawson County was reported as 15,999 as updated based on 2000 Census (Reference 2). Dawson County was established in 1857; formed from parts of Forsyth, Gilmer and Lumpkin Counties. The City of Dawsonville is the County Seat. The economy is supported by fabricating, finishing, forming and machining. The county's topography is typified by steep terrain. Elevations vary from approximately 1,200 feet to 3,000 feet National Geodetic Vertical Datum of 1929 update to NAVD 1988 (NGVD). Of the more than 22 soil types in the county, four pose severe limitations to development: gullied land with severe erosion, rocky terrain, soils with excessive slope, and floodplain soils. Located along the southern slopes of the Blue Ridge Mountains, Dawson County has mild winters and warm summers. The average annual temperature is 70 degrees Fahrenheit ( F). Summer temperatures average 90 F and winter temperatures average 32 F. Rainfall averages inches annually (Reference 3). A small portion of the county's watershed, located in the southeastern corner drains east into Lake Sidney Lanier and enters the Chattahoochee River drainage basin. The majority of the tributaries flow into the Etowah River which eventually flows into the Coosa River drainage basin. 2.3 Principal Flood Problems Most of the flood problems existing in the study area are in low lying agriculture areas. Localized flooding in certain areas not identified in this study may exist. 2.4 Flood Protection Measures There are eleven watershed control structures in Dawson County which fall under jurisdiction of the U.S. Department of Agriculture Soil Conservation Service (USDA SCS). Of these eleven structures, four occur within the upstream drainage basin of the study area. Although it is believed that these structures may provide some measure of downstream flood attenuation for the higher frequency events (lower floods), they are not believed to provide significant attenuation for the 1- and 0.2-percent-annual-chance floods studied herein. Therefore, no routings or detailed analysis of storage effects in these reservoirs was performed in conjunction with this technical analysis. Congress authorized the Lake Lanier-Buford Dam project in the Rivers and Harbors Act approved July 24, This legislation targeted developing the nation s rivers systems for national defense, flood control, power production, navigation and water supply. The scope of developing the nation s waterways was a massive and unprecedented undertaking. The Lake Lanier-Buford Dam construction began in 1950 and was completed in 1955 at a cost of approximately 46 million dollars. Initially the project was authorized to provide hydropower, flood control and downstream navigation. Later, additional purposes were added to the project for water supply, water quality, recreation and fish and wildlife management. The 5

10 Lake Lanier-Buford Dam project encompasses approximately 39,000 acres of water and 18,000 acres of land. It is among top three most-visited corps recreation sites (Reference 4). 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-percentannual-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 the peak discharge-frequency relationships for the flooding source studied in detail affecting the community. A gaging station (No ) located on the Etowah River near Dawsonville, Georgia, intercepts a drainage area of approximately 107 square miles and has a period of record from The Log Pearson Type III distribution was used to determine peak discharges in accordance with the procedures in Bulletin 17B (Reference 5). Peak discharges determined in this manner were then weighted with peak discharges computed to this site using the regression equations. In applying the regression equation an accounting was made for the fact that the Etowah River forms the division line between hydrologic regions (Region 1 lies to the north and Region 2 to the south of Etowah River). After determining the weighted discharges at the gage, these values were transposed downstream to the ungaged sites (nodes) in the study in accordance with the procedures in Water Resources Investigation (WRI) (Reference 6). No gaging stations are located on Black Mill Creek or Mill Creek. Discharge-frequency relationships on these two streams were determined using the USGS regional regression equations (Reference 6). For areas where significant urbanization has occurred, these rural discharges were adjusted (Reference 7). The three-parameter estimating equations were used in this flood study. The threeparameters used in the regression equations are the previously determined rural discharges, the drainage area, and the basin development factor (BDF) which is a measure of the efficiency of the drainage system. Discharges were computed at specific locations or nodes by determining the contributing drainage area at that point and applying the techniques 6

11 outlined above. Nodes were established at points of major inflow. Drainage areas were measured from topographic maps (Reference 8). BDFs were assigned based on aerial photos and information gathered during field reconnaissance. This Countywide FIS Report Peak flood discharges for Black Mill Creek, Etowah River, and Little Mill Creek were estimated using the 1976 regional regression equations for Georgia (Reference 9). The single drainage area regression parameter was measured using topographic maps and compared to the USGS drainage area data (Reference 10). The USGS data was adopted where the data corresponded to the flow change locations in the hydraulic models. For the approximate study streams listed in Table 3, Streams Restudied by Approximate Methods, peak flows were determined using the rural regression equations for Georgia (Reference 11). Peak discharge-drainage area relationships for the 10-, 2-, 1-, and 0.2-percent-annual-chance floods for each stream studied by detailed methods are presented in Table 4, Summary of Discharges. 7

12 FLOODING SOURCE AND LOCATION BLACK MILL CREEK DRAINAGE AREA (Sq. Miles) Table 4 Summary of Discharges 10%- ANNUAL- CHANCE PEAK DISCHARGES (cfs) 2%- ANNUAL- CHANCE 1%- ANNUAL- CHANCE 0.2%- ANNUAL- CHANCE At mouth 7.8 * * 2,677 3,683 Approximately 2,500 feet 3.8 * * 1,845 2,555 downstream of confluence of Route 115 Approximately 1.53 miles upstream of confluence of Route * * 1,386 1,929 ETOWAH RIVER At State Route * * 12,573 16,287 Approximately 0.70 mile * * 11,263 14,401 upstream of confluence of Black Mill Creek Approximately 800 feet * * 10,393 13,160 upstream of confluence of Mill Creek Approximately 2,100 feet * * 9,813 12,332 downstream of County Route 248 Approximately 2,800 feet * * 9,276 11,567 downstream of State Route 53 Approximately 0.66 mile downstream of confluence of Palmer Creek * * 8,403 10,330 MILL CREEK At mouth 2.8 * * 1,229 1,652 * 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. Overbank cross sections for the backwater analyses of the streams studied in detailed methods were obtained from aerial photography (Reference 12). The below-water sections were obtained by field measurement at bridge structures and interpolated or estimated 8

13 between structures. All bridges, dams and culverts were field surveyed to obtain elevation data and structural geometry. 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 tables 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 in conjunction with the data shown on the FIRM. The USACE HEC-2 computer program (Reference 13) was used to calculate the WSELs for Black Mill Creek, Etowah River, and Little Mill Creek. Starting WSELs for Black Mill Creek, Etowah River, and Little Mill Creek were determined by the slope/area method. For the approximate study streams listed in Table 3, cross section data was obtained from the USGS 7.5 Minute Series topographic quadrangles. 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 the road elevations were estimated from the topography. The studied streams listed in Table 3 were modeled using HEC-RAS Version (Reference 14). Channel and overbank roughness coefficients (Manning s n ) used in the hydraulic computations were estimated by engineering judgment and based on field observation at each cross-section and adjusted with known high-water marks and stream gage rating curves where possible. Table 5, Manning s n Values, shows the channel and overbank n values for the streams studied by detailed methods. Table 5 Manning s n Value Table STREAM CHANNEL OVERBANK Black Mill Creek Etowah River Mill Creek Water-surface elevations of floods of the selected recurrence intervals were computed using HEC-2 step back-water computer program (Reference 15). Flood profiles were drawn showing the computed water-surface elevations for floods of the selected recurrence intervals. Starting water-surface elevations for the streams studied were calculated using the slope-area method. 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). 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. 9

14 The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the Flood Profiles (Exhibit 1) are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. 3.3 Vertical Datum All FIS reports and FIRMs are referenced to a specific vertical datum. The vertical datum provides a starting point against which flood, ground, and structure elevations can be referenced and compared. Until recently, the standard vertical datum used for newly created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of 1929 (NGVD). With the completion of the North American Vertical Datum of 1988 (NAVD), many FIS reports and FIRMs are now prepared using NAVD as the referenced vertical datum. Flood elevations shown in this FIS report and on the FIRM are referenced to NAVD. These flood elevations must be compared to structure and ground elevations referenced to the same vertical datum. Some of the data used in this revision were taken from the prior effective FIS reports and FIRMs and adjusted to NAVD. The datum conversion factor from NGVD to NAVD in Dawson County is The data points used to determine the conversion are listed in Table 6, Vertical Datum Conversion. 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 (Reference 16). The data points used to determine the conversion are listed in Table 6. Table 6 Vertical Datum Conversion Quad Name Corner Longitude Latitude Conversion from NGVD to NAVD Amicalola SE Dawsonville SE Juno SE Nelson SE Nimblewill SE Tickanetley SE 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: 10

15 NGS Information Services NOAA, N/NGS12 National Geodetic Survey, SSMC-3, # East-West Highway Silver Spring, Maryland (301) Temporary vertical monuments are often established during the preparation of a flood hazard analysis for the purpose of establishing local vertical control. Although these monuments are not shown on the FIRM, they may be found in the Technical Support Data Notebook associated with the FIS report and FIRM for this 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 and Floodway Data 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-percent-annual-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-annualchance 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 1:24,000 with a contour interval of 20 feet (Reference 8). For streams restudied by approximate methods presented in Table 3, and for redelineated areas studied by approximate methods, the 1-percent-annual-chance floodplain boundaries were delineated using data with a 20-foot contour interval (Reference 10). 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-annualchance 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 11

16 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 streams studied by approximate methods, only the 1-percent-annual-chance floodplain boundary is shown on the FIRM. 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. Floodways have not been shown or computed for this community. Along streams where floodways have not been computed, the community must ensure that the cumulative effect of development in the floodplain will not cause more than a 1.0-foot increase in the base flood elevations at any point within the community. 5.0 INSURANCE APPLICATIONS Zone A Zone A is the flood insurance rate zone that corresponds to the 1-percent-annual-chance floodplains that are determined in the FIS report by approximate methods. Because detailed hydraulic analyses are not performed for such areas, no base (1-percent-annual-chance) flood elevations (BFEs) or 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 report by detailed methods. Whole-foot BFEs derived from the detailed hydraulic analyses are shown at selected intervals within this zone. Zone AH Zone AH is the flood insurance rate zone that corresponds to areas of 1-percent-annual-chance shallow flooding (usually areas of ponding) where average depths are between 1 and 3 feet. Whole-foot BFEs derived from the detailed hydraulic analyses are shown at selected intervals within this zone. 12

17 Zone AO Zone AO is the flood insurance rate zone that corresponds to areas of 1-percent-annual-chance shallow flooding (usually sheet flow on sloping terrain) where average depths are between 1 and 3 feet. Average whole-foot depths derived from the detailed hydraulic analyses are shown within this zone. Zone X Zone X is the flood insurance rate 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 (sq. mi.), and areas protected from the base flood by levees. No BFEs or 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 geographic area of Dawson County. Previously, FIRMs were prepared for each incorporated community of the County identified as flood-prone. This countywide FIRM also includes flood hazard information that was presented separately on Flood Boundary and Floodway Maps (FBFMs), where applicable. Historical data relating to the maps prepared for each community are presented in Table 6, Community Map History. 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 Korger Center, Rutgers Building, 3003 Chamblee Rucker Road, Atlanta, Georgia

18 Table 7 Community Map History COMMUNITY NAME INITIAL IDENTIFICATION FLOOD HAZARD BOUNDARY MAP REVISIONS DATE FIRM EFFECTIVE DATE FIRM REVISIONS DATE Dawsonville, City of January 17, 1975 None May 21, 1982 None Dawson, County (Unincorporated Areas) June 18, 1976 None December 15, 1990 None TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY DAWSON COUNTY, GA (AND INCORPORATED AREAS) COMMUNITY MAP HISTORY

19 9.0 BIBLIOGRAPHY AND REFERENCES 1. Federal Emergency Agency, Flood Insurance Study, Dawson County, Georgia, (Unincorporated Areas), Washington, D.C. December 15, United States Department of Commerce, Bureau of the Census, 1980 Census of Population, Number of Inhabitants, Georgia, Washington, D.C., January The Weather Channel, Monthly Averages for Dawson, GA. Retrieved September 12, 2007, from 4. U.S. Army Corps of Engineers, Mobile District, Buford Dam. Retrieved September 20, 2007 from 5. U.S. Department of the Interior, U.S. Geological Survey, Log-Pearson Type Ill Method, Water Resources Council's Bulletin No. 17, revised June U.S. Geological Survey, Water Resources Investigation , Floods in Georgia, Magnitude and Frequency: Techniques for Estimating the Magnitude and Frequency of Floods in Georgia with Compilation of Flood Data through 1974, McGlone Price, October U.S. Geological Survey, Water Supply Paper 2207, Flood Characteristics of Urban Watersheds in the United States Techniques for Estimating Magnitude and Frequency of Urban Floods, U.B. Sauer, W.O. Dawson, Jr., V.A. Stricker, and K.V. Wilson, U.S. Geological Survey, 7.5 Minute Series Topographic Maps, Scale 1:24000, Contour Interval 20 feet, Chestatee, Georgia, 1973; Coal Mountain, Georgia, 1985; Dawsonville, Georgia, Georgia Department of Natural Resources, Flood Frequency Analysis for Small Streams in Georgia, U.S. Geological Survey, 7.5-Minute Series Topographic Maps, Scale 1:24,000, Contour Interval 20 feet: Dawson, Georgia, U.S. Department of the Interior, 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, Woolpert Consultants, Aerial Photographs, Dawson County, Georgia, Scale 1:14400: Mobile, Alabama,

20 13. U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-2 Water Surface Profiles, Davis, California, September 1984b. 14. Hydrologic Engineering Center, HEC-RAS River Analysis System, Version 3.1.3, U.S. Army Corps of Engineers, Davis, California, April U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-2 Water Surface Profiles, Generalized Computer Program, Davis, California, April National Geodetic Survey, VERTCON-North American Vertical Datum Conversion Utility. Retrieved January 9, 2007, from 16

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