CAMDEN COUNTY, GEORGIA AND INCORPORATED AREAS COMMUNITY NAME COMMUNITY NUMBER CAMDEN COUNTY 130262 (UNINCORPORATED AREAS) KINGSLAND, CITY OF 130238 ST. MARYS, CITY OF 130027 WOODBINE, CITY OF 130241 CAMDEN COUNTY Preliminary: FLOOD INSURANCE STUDY NUMBER 13039CV000A
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 30, 1988 Revised Dates: To Be Determined
TABLE OF CONTENTS 1.0 INTRODUCTION...1 1.1 Purpose of Study...1 1.2 Authority and Acknowledgments...1 1.3 Coordination...3 2.0 AREA STUDIED...3 2.1 Scope of Study...3 2.2 Community Description...5 2.3 Principal Flood Problems...6 2.4 Flood Protection Measures...6 3.0 ENGINEERING METHODS...6 3.1 Hydrologic Analyses...6 3.2 Hydraulic Analyses...11 3.3 Wave Height Analysis...14 3.4 Vertical Datum...17 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS...26 4.1 Floodplain Boundaries...26 4.2 Floodways...27 4.3 Base Flood Elevations...28 4.4 Velocity Zones...28 5.0 INSURANCE APPLICATIONS...29 6.0 FLOOD INSURANCE RATE MAP...31 7.0 OTHER STUDIES...31 8.0 LOCATION OF DATA...31 9.0 BIBLIOGRAPHY AND REFERENCES...33 i
TABLE OF CONTENTS (Continued) FIGURES Figure 1 - Transect Location Map... 16 Figure 2 - Transect Schematic... 17 Figure 3 - Floodway Schematic... 28 TABLES Table 1 - Streams Restudied by Approximate Methods... 4 Table 2 - Storm Track Data... 7 Table 3 - Representative Tides... 9 Table 4 - Summary of Discharges... 10 Table 5 - Parameter Values for Surge Elevation Computations... 12 Table 6 - Transect Data... 18 Table 7 - Vertical Datum Conversion... 25 Table 8 - Community Map History... 32 Exhibit 1 - Flood Profiles EXHIBITS St. Marys River Panel 01P Exhibit 2 - Flood Insurance Rate Map Index Flood Insurance Rate Map ii
FLOOD INSURANCE STUDY CAMDEN 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 Camden County, including the Cities of Kingsland, St. Marys, and Woodbine; and the unincorporated areas of Camden County (referred to collectively herein as Camden County), and aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973. 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, 60.3. 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 1973. Precountywide Analyses Information on the authority and acknowledgements for each jurisdiction included in this countywide FIS, as compiled from their previously printed FIS reports, is shown below: 1
Camden County (Unincorporated Areas): Kingsland, City of: St. Marys, City of: Woodbine, City of: The hydrologic and hydraulic analyses for this study were performed by Camp Dresser and McKee, Inc. (CDM), for FEMA, under Contract No. H-4052. The work was completed in August 1981 (FEMA, 1983a). The hydrologic and hydraulic analyses for this study were performed by CDM, for FEMA, under Contract No. H-4052. The work was completed in August 1981 (FEMA, 1983b). The hydrologic and hydraulic analyses for this study were performed by CDM, for FEMA, under Contract No. H-4052. The work was completed in August 1981 (FEMA, 1983c). The hydrologic and hydraulic analyses for this study were performed by CDM, for FEMA, under Contract No. H-4052. The work was completed in August 1981 (FEMA, 1983d). Initial Countywide FIS Report For the initial countywide FIS, the hydrologic and hydraulic analyses were performed by FEMA and completed in July 1987 (FEMA, 1988b). This Countywide FIS Revision For this countywide FIS revision, streams restudied by approximate methods and redelineation of floodplain boundaries for streams studied by approximate methods was performed PBS&J, for the Georgia Department of Natural Resources (DNR), under Contract No. EMA-2006-CA-5615, with FEMA. The work was completed in June 2007. Detailed flooding information for the St. Marys River was taken from the FIS for Charlton County, Georgia and Incorporated Areas (FEMA, 1988c). Base map information shown on the Flood Insurance Rate Map (FIRM) was derived from digital orthophotography produced at a scale of 1:6,000, and published February 1, 2005, by Camden County. The projection used in the preparation of this map is State Plane Georgia East, and the horizontal datum used is the North American Datum 1983. 2
1.3 Coordination Precountywide Analyses 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 Camden County and its communities are listed in the following table: Community FIS Date Initial Meeting Final Meeting Camden County December 1, 1983 1978 June 28, 1983 (Unincorporated Areas) City of Kingsland December 1, 1983 1978 June 30, 1983 City of St. Marys December 1, 1983 1978 June 28, 1983 City of Woodbine December 1, 1983 1978 June 28, 1983 Initial Countywide FIS Report For the initial countywide FIS, FEMA initiated a restudy for Camden County in May 1987. The final meeting was held on November 3, 1987, and attended by representatives of FEMA, and the communities. This Countywide Revision For this countywide FIS revision, the initial meeting was held on December 8, 2005, and attended by representatives of FEMA, Georgia DNR, PBS&J, and the communities. 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Camden County, Georgia, including the incorporated communities listed in Section 1.1. The areas studied by detailed methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction through the time of the study. 3
The Atlantic Ocean, Cumberland Sound, Satilla River, and St. Marys River were studied by detailed methods. The limits of detailed study are indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2). Precountywide Analyses The potential for flooding due to open coast surge wave was studied, and the added effect of wind induced waves was also examined. The effects of flooding due to ponding were not considered. Initial Countywide Analyses For the initial countywide FIS, the areas studied by detailed methods were selected based on the extent and validity of available existing hydrologic and hydraulic data. A detailed coastal flooding analysis of the Atlantic Ocean was performed on the complete coastline of Camden County. This Countywide Revision For this countywide revision, the vertical datum was converted from the National Geodetic Vertical Datum (NGVD) to the North American Vertical Datum (NAVD). Also, the Universal Transverse Mercator coordinates, previously referenced to the North American Datum of 1927, are now referenced to the North American Datum of 1983. Detailed flooding for the St. Marys River was added from the county boundary to a point approximately 460 feet downstream of the county boundary. 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 2007. The streams studied by approximate methods are presented in Table 1. Table 1 - Streams Restudied by Approximate Methods Stream Bullhead Creek Bullhead Creek Tributary No. 1 Reach Description From the confluence with Satilla River to approximately 9,320 feet upstream of Cabbage Ford Road From the confluence with Bullhead Creek to approximately 10,980 feet upstream of Springhill Road North 4
Table 1 - Streams Restudied by Approximate Methods (Continued) Stream Crooked River Satilla River White Oak Creek Reach Description From approximately 4,010 feet downstream of Old Still Road to approximately 44,410 feet upstream of CSX Railroad From just downstream of Interstate Highway 17 to the County Boundary From just downstream of Interstate Highway 17 to the 7,200 feet upstream of Old State Highway 259 Existing areas studied by approximate methods were redelineated as part of this countywide revision. U.S. Geological Survey (USGS) 30 meter resolution Digital Elevation Models (DEMs) were obtained from the National Elevation Dataset (NED) (USGS, 1999). Priority was given to areas studied by approximate methods where it was evident the flooding source followed the DEM, and areas that did not follow the DEM were not redelineated. 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 Camden County. 2.2 Community Description Camden County is located in the southeast portion of Georgia and is the southernmost county in Georgia adjacent to the Atlantic Ocean. The county is bordered by Glynn County, Georgia, to the north, Brantley County, Georgia, to the northwest, Charlton County, Georgia, to the west, Nassau County, Florida, to the south, and the Atlantic Ocean to the east. The total land area within Camden County is 629 square miles (U.S. Census Bureau, 2007). Major transportation routes to serve Camden County include Interstate Highways 17 and 95, and Georgia State Routes 40 and 110. According to the U.S. Census Bureau, in 2006, the population of Camden County was estimated to be 45,118 (U.S. Census Bureau, 2007). The climate in southeast Georgia is warm and temperate to subtropical. The average temperature in January is 63 degrees Fahrenheit ( F), and is 90 F in July. The average annual precipitation is 50.4 inches, with the maximum average monthly precipitation occurring in September (The Weather Channel, 2007). The county is primarily drained by the Little Satilla River, which is the northern county boundary, the Satilla River in the central portion of the county, and the St. Marys River, which is the southern county boundary. The mainland coastline is protected by Cumberland Island, a barrier island adjacent to the Atlantic Ocean. 5
2.3 Principal Flood Problems The principal flooding threat to Camden County is the coastal storm surge tides that are augmented by wind induced waves. Camden County may also be subjected to flooding due to rains induced by hurricanes, tropical storms, and other storms. Hurricanes and tropical storms normally occur in the summer and early fall months of the year. 2.4 Flood Protection Measures Specific flood protection measures for Camden County have not been identified. Cumberland Island serves as a natural barrier island and reduces the impact from coastal storm surge tides. 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 In order to analyze open coast flooding, information was obtained on tropical storms affecting the study area (U.S. Department of Commerce, 1967; American Meteorology Society, date unknown; U.S. Department of Commerce, 1957a; U.S. Department of Commerce, 1957b). For the Joint Probability Method, which was used to analyze flooding due to storm surge waves, the storm parameters of interest include direction of travel, forward velocity, radius to maximum winds, 6
and central pressure depression. The last three parameters are basic descriptors required to generate synthetic storms. Information on each of the storms considered in the analysis is presented in Table 2. Forward Velocity (KT) Table 2 - Storm Track Data Radius Max Winds (NM) Pressure Depression Longitude (in Hg) Type 1 Storm ID Year No. Compass Bearing (Degrees W) 1 1871 3 340.0 7.5 * * 9 81.7 30.7 2 1871 4 313.0 18.4 * * 9 81.1 30.8 3 1871 5 49.0 18.3 * * 9 80.2 31.4 4 1871 6 59.0 10.5 * * 9 80.9 31.7 5 1872 5 43.0 9.5 * * 9 80.8 31.4 6 1873 1 311.0 16.0 * * 9 81.5 31.1 7 1873 3 54.0 26.2 * * 9 80.6 32.0 8 1873 4 45.0 24.9 * * 9 78.5 31.6 9 1873 5 48.0 8.9 * * 9 78.7 29.6 10 1874 6 43.0 17.8 * * 9 80.9 30.9 11 1876 1 5.0 10.0 * * 9 79.5 30.5 12 1877 2 75.0 9.3 * * 9 80.4 31.0 13 1877 7 78.0 12.5 * * 9 82.7 29.5 14 1878 3 8.0 6.6 * * 9 80.6 31.1 15 1878 6 58.0 25.3 * * 9 83.7 30.6 16 1879 7 51.0 32.4 * * 9 75.6 33.8 17 1880 8 57.0 21.3 * * 9 80.5 29.9 18 1881 4 313.0 8.2 * * 9 80.6 31.4 19 1881 6 52.0 7.3 * * 9 81.9 30.0 20 1884 2 339.0 5.9 * * 9 81.2 31.7 21 1885 2 353.0 10.6 * * 9 80.8 31.1 22 1885 3 74.0 16.1 * * 9 81.5 31.4 23 1885 4 71.0 32.2 * * 9 78.6 32.1 24 1885 6 81.0 10.1 * * 9 80.6 30.4 25 1885 8 13.0 11.3 * * 9 82.1 31.0 26 1886 3 32.0 16.0 * * 8 81.9 32.5 27 1887 4 60.0 12.0 * * 8 80.3 30.5 28 1887 4 6.0 16.6 * * 8 78.9 31.4 29 1888 5 16.0 15.1 * * 7 81.8 32.9 30 1888 7 43.0 19.9 * * 8 79.3 32.8 31 1889 2 49.0 16.1 * * 7 80.7 30.7 32 1891 8 41.0 9.9 * * 7 80.1 29.9 33 1893 1 53.0 20.8 * * 8 83.0 30.0 34 1893 6 346.0 13.9 4.0 1.33 8 81.1 31.8 35 1893 9 335.0 13.3 3.0 1.28 8 80.2 30.1 36 1894 3 10.0 10.2 * * 8 80.8 31.3 37 1897 3 25.0 11.6 * * 7 80.1 30.3 38 1897 4 33.0 24.3 * * 7 80.0 31.0 39 1898 2 337.0 7.6 * * 8 80.1 31.5 *Data not available 1 Type 9 = Unclassified 8 = Hurricane 7 = Tropical Storm, no other distinction made in storm tracks 6 = Tropical Storm in area, formerly a hurricane 5 = Depression, dissipation stage 4 = Depression, developing stage Lattitude (Degrees N) 7
Forward Velocity (KT) Table 2 - Storm Track Data (Continued) Radius Max Winds (NM) 8 Pressure Depression Longitude (in Hg) Type 1 Storm ID Year No. Compass Bearing (Degrees W) 40 1898 7 291.0 14.3 * * 8 81.0 31.0 41 1899 2 13.0 5.7 * * 8 79.6 30.8 42 1899 5 46.0 23.2 * * 7 79.1 32.2 43 1900 6 52.0 16.4 * * 7 81.0 31.0 44 1906 8 223.0 7.5 35.0 0.98 7 80.2 30.7 45 1906 9 243.0 14.5 * * 7 81.0 29.7 46 1907 1 68.0 20.2 * * 7 82.6 30.3 47 1907 3 50.0 30.3 * * 7 84.0 31.3 48 1909 6 348.0 6.2 * * 7 81.0 29.9 49 1910 4 15.0 10.3 48.0 1.44 6 81.7 31.0 50 1912 2 288.0 9.5 * * 7 80.5 31.3 51 1914 1 286.0 11.1 * * 7 83.0 31.6 52 1915 1 356.0 11.0 * * 7 82.3 32.4 53 1916 8 303.0 18.4 * * 7 81.6 29.2 54 1916 11 303.0 8.3 * * 7 78.7 31.2 55 1920 4 63.0 18.6 * * 6 81.0 30.3 56 1924 4 69.0 6.0 * * 6 82.8 31.3 57 1924 5 41.0 31.0 * * 5 82.0 31.0 58 1926 1 317.0 10.9 14.0 1.58 6 82.8 30.5 59 1927 5 347.0 18.5 * * 7 81.0 33.4 60 1928 4 26.0 11.7 28.0 2.76 6 80.8 32.5 61 1930 2 48.0 5.3 * * 6 79.9 30.2 62 1932 5 62.0 23.1 * * 7 80.1 31.4 63 1934 1 357.0 14.0 * * 7 80.2 31.2 64 1934 3 217.0 13.7 * * 7 81.0 29.8 65 1936 9 301.0 15.8 * * 7 81.7 29.9 66 1937 1 50.0 13.2 * * 7 80.1 30.1 67 1937 3 291.0 11.3 * * 7 82.9 29.6 68 1938 7 55.0 40.8 * * 7 79.9 32.1 69 1940 2 235.0 7.0 * * 7 81.2 29.6 70 1944 11 17.0 14.7 27.0 1.78 6 81.1 31.2 71 1945 1 48.0 14.1 * * 6 80.0 30.7 72 1945 9 22.0 12.4 12.0 1.91 6 80.8 32.2 73 1946 2 20.0 8.5 * * 7 78.9 33.4 74 1946 5 6.0 16.1 * * 6 82.4 30.3 75 1946 6 13.0 15.9 * * 5 81.0 31.6 76 1947 6 20.0 18.7 * * 7 81.2 32.9 77 1947 7 280.0 14.4 * * 7 83.5 30.8 78 1947 8 275.0 11.9 13.0 1.06 8 81.0 32.0 79 1949 2 4.0 13.5 22.0 1.96 6 82.7 33.0 80 1950 5 325.0 13.5 * * 6 83.0 31.2 81 1950 11 335.0 16.6 6.0 1.72 7 82.4 30.0 82 1952 2 342.0 10.0 * * 8 80.5 31.8 83 1953 3 304.0 10.8 * * 7 82.0 32.4 84 1953 7 59.0 23.1 * * 5 80.1 32.0 85 1953 8 67.0 12.6 * * 6 82.3 31.7 *Data not available 1 Type 9 = Unclassified 8 = Hurricane 7 = Tropical Storm, no other distinction made in storm tracks 6 = Tropical Storm in area, formerly a hurricane 5 = Depression, dissipation stage 4 = Depression, developing stage Lattitude (Degrees N)
Forward Velocity (KT) Table 2 - Storm Track Data (Continued) Radius Max Winds (NM) Pressure Depression Longitude (in Hg) Type 1 Storm ID Year No. Compass Bearing (Degrees W) 86 1957 1 53.0 21.8 * * 7 80.2 32.6 87 1959 8 332.0 13.6 10.0 1.84 8 81.1 33.7 88 1960 3 26.0 15.6 * * 4 81.6 31.8 89 1960 5 20.0 13.8 20.0 2.37 8 80.8 29.9 90 1962 1 9.0 14.7 * * 7 79.7 30.6 91 1963 8 360.0 8.0 * * 8 79.6 31.9 92 1964 6 100.0 7.0 20.0 1.39 8 81.4 29.9 93 1968 1 337.0 8.8 * 0.57 7 81.6 30.8 94 1968 2 50.0 8.5 * * 7 79.6 30.8 95 1968 4 8.0 12.6 * 0.63 7 80.2 29.5 96 1968 7 46.0 12.8 * 0.43 8 81.1 30.2 97 1969 10 249.0 2.8 * * 7 81.2 28.7 98 1970 1 21.0 9.6 * * 5 81.3 32.5 99 1972 4 287.0 5.1 * 0.48 7 81.1 36.1 *Data not available 1 Type 9 = Unclassified 8 = Hurricane 7 = Tropical Storm, no other distinction made in storm tracks 6 = Tropical Storm in area, formerly a hurricane 5 = Depression, dissipation stage 4 = Depression, developing stage Lattitude (Degrees N) An additional parameter diurnal tide was also included in the Joint Probability Method in order to establish the relationship between flood magnitude (comprising storm surge and astronomic tide) and frequency. To this end, the National Oceanic and Atmospheric Administration Astronomic Tide Prediction Program was modified to develop hurricane season weighted high and low water probability distributions at two control stations in the study area for a full, nineteen-year, tidal epoch. The tidal ranges selected for use in the analysis, and their associated probabilities, are shown in Table 3. Table 3 - Representative Tides Low Tide (feet NAVD)* High Tide (feet NAVD)* Range (Feet) Probability (%) Savannah River entrance, Georgia -3.04 1.66 4.70 0.20-3.64 2.36 6.00 0.30-4.24 3.06 7.30 0.30-4.94 3.76 8.70 0.20 Mayport, Florida -2.02 0.78 2.80 0.20-2.52 1.38 3.90 0.30-2.92 1.88 4.80 0.30-3.42 2.38 5.80 0.20 *North American Vertical Datum of 1988 9
Initial Countywide FIS Report For the initial countywide study, the hydrologic analysis for Camden County was modified to reflect a reevaluation of local storm statistics and hydrodynamic methodology. The statistical approach was based on meteorological data developed by the National Weather Service (U.S. Department of Commerce, 1986). These data were used to adjust the hurricane parameter statistics. The adjusted parameter statistics include storm frequency, pressure deficit distribution, and radius to maximum wind-pressure deficit distribution. The hydrodynamic adjustments inland of the open coast reflect methodologies used in the 1986 version of the SURGE program (FEMA, 1987). The adjustments include better resolution of bathymetry and the incorporation of a moving boundary. This Countywide Revision For this countywide revision, for the St. Marys River, the U.S. Army Corps of Engineer s (USACE s) HEC-1 computer program was used to determine the peak discharges of the 10-, 2-, 1-, and 0.2-percent-annual-chance floods (HEC, 1973). For streams restudied by approximate methods presented in Table 1, peak discharges were calculated using a regression equation (Stamey and Hess, 1993). For the portion of the Satilla River that was restudied by approximate methods, peak discharges were extrapolated from data collected at the USGS Gage No. 02228000, Satilla River at Atkinson, Georgia (Stamey and Hess, 1993). Peak discharge-drainage area relationships for 10-, 2-, 1-, and 0.2-percent-annualchance floods for the St. Marys River are presented in Table 4. Table 4 - 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 St. Marys River At the County Boundary * 8,095 11,490 12,945 16,270 *Data not available 10
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. Users of the FIRM should also be aware that coastal flood elevations are provided in the Transect Data table in this report. If the elevation on the FIRM is higher than the elevation shown in this table, a wave height, wave runup and/or wave setup component likely exists, in which case, the higher elevation should be used for construction and/or floodplain management purposes. Precountywide Analyses Hydraulic analyses of the shoreline characteristics of the flooding sources studied in detail were carried out to provide estimates of the elevations of floods of the selected recurrence intervals. The analysis of the susceptibility of Camden County to flooding due to coastal storm surge waves was accomplished as part of a larger analysis of Glynn and Camden Counties, Georgia and Nassau County, Florida. The extent of coastal flooding due to hurricanes and other tropical storms is determined by three factors: 1) the nature of the storm with respect to intensity, duration, and path; 2) tide elevations when the storm surge wave reaches shore; and 3) the physical geometry and bathymetry of particular areas which affect the timing and passage of the surge wave. The relationships of recurrence interval and tidal surge elevation developed for this study area are based on a statistical analysis of historical storm data (Table 2) and a frequency analysis of regional tidal variation (Table 3). Discretized values of the tropical storm parameters, shown in Table 5, were used to generate a large number of synthetic storms using the coastal surge model TTSURGE (Tetra Tech, 1980). The surge model was calibrated and run on a grid system representation of the physical features of the study area. 11
Table 5 - Parameter Values for Surge Elevation Computations P PP F PF R PR A PA Exiting Storms FN=0.276397 Parallel Storms FN=0.1439565 29.49 0.775 18.7 1.0 27.0 1.0 55.0 1.0 29.23 0.045 28.43 0.110 27.93 0.550 27.16 0.015 29.49 0.775 14.2 1.0 27.0 1.0 11.0 1.0 29.23 0.045 28.43 0.110 27.93 0.550 27.16 0.015 29.49 0.775 32.3 0.11 35.0 0.46-33.0 0.46 Entering Storms 29.23 0.045 15.2 0.89 14.0 0.54-77.0 0.38 FN=0.1439565 28.43 0.110-121.0 0.16 27.93 0.550 27.16 0.015 P = Central Pressure (in Hg) PP = Probability of storm with P Value F = Forward velocity of storm (KTS) PF = Probability of storm with F Value R = Radius to maximum winds (NM) PR = Probability of storm with R Value A = Direction of storm measured counterclockwise from coast (North) (degrees) PA = Probability of storm with A Value D = Distance from shore (NM) FN = Frequency of storm occurrence/year/nm Hydraulic Simulation Stillwater elevations due to coastal flooding were obtained using FEMA s coastal flooding storm surge model TTSURGE (Tetra Tech, 1980) for both offcoast and inland surge routing. The study area in the coastal flooding analysis consisted of a large scale offcoast grid made up of 34 north-south and 30 east-west square cells, 5.14 nautical miles on a side. The grid spanned 174 miles of southern Georgia and northern Florida coastline from 29 to 32 degrees approximate north latitude, and 82 to 79 degrees approximate west longitude. Contained in the large scale grid was a fine scale moving boundary inland grid made up of cells two statute miles on a side. The inland grid ranged from Altamaha Sound in Georgia to the Nassau River in northern Florida. The TTSURGE model was calibrated to mean tide and verified to three other tidal periods. This was done to set the hydrodynamic conditions of the study area. 12
Thirty-six control points were examined during tidal calibration and thirty-four points were predicted by the model to within the ± 0.5-foot criteria. The model was then calibrated to surge conditions by predicting three historical hurricanes, including one of each type of storm encountered in the area an exiting, a parallel, and an entering storm. There are few detailed gage readings available in the study area and only three gages had sufficient records against which to calibrate the model. The model sufficiently predicted the observed surge plus tide levels for the three storms chosen. There were numerous points selected in the three-county study area at which a tide-surge frequency relationship was developed. The relationship was developed by considering 16 combinations of tidal amplitude and phase, 5 central pressure depressions, and 124 synthetic storm surge waves (entering, exiting, and parallel storms) resulting in a product of 9,920 elevations at each point of interest. Using the Joint Probability Method, a probability of occurrence was calculated for each of the 9,920 elevations at each point of interest. The total probability for a given elevation is the product of probabilities for the combination of tidal phase and amplitude as well as storm direction, velocity, pressure, and radius to maximum winds which generate the particular elevation. These elevations may then be used to determine the surge-tidal frequency relationship. Initial Countywide FIS For the initial countywide FIS, the statistical and hydrodynamic adjustments were combined to obtain revised 1-percent-annual-chance flood stillwater elevations. The adjusted stillwater elevations decreased by as much as 3 feet from the stillwater elevations presented in Table 6. This Countywide FIS Revision Cross sections for St. Marys River were obtained from field surveys. All bridges, dams, and culverts were surveyed to obtain elevation and structural geometry data. 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). Water surface elevations (WSELs) of floods of the selected recurrence intervals were computed using the USACE s HEC-2 step-backwater computer program (HEC, 1984). Channel roughness factors (Mannings n ) used in the hydraulic computations were chosen by field observations. 13
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. For the approximate study streams, cross section data was obtained from the topography. Low flow channels were added to the cross section data, based on the estimated depth of the 50-percent-annual-chance flow. Roads appearing on the topographic maps were modeled as weirs; top of road elevations were estimated from the topography. The studied streams were modeled using HEC- RAS version 3.1.3 (HEC, 2004). 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 Wave Height Analysis Precountywide Analyses The methodology for analyzing the effects of wave heights associated with coastal storm surge flooding was developed by the National Academy of Sciences (NAS) (NAS, 1977). This method is based on the following three major concepts. First, depth-limited waves in shallow water reach a maximum breaking height that is equal to 0.78 times the stillwater depth. The wave crest elevation is 70-percent of the total wave height plus the stillwater elevation. The second major concept is that wave height may be diminished by dissipation of energy due to the presence of obstructions such as sand dunes, dikes and seawalls, buildings, and vegetation. The amount of energy dissipation is a function of the physical characteristics of the obstruction and is determined by procedures described in the NAS report. The third major concept is that wave height can be regenerated in open fetch areas due to the transfer of wind energy to the water. This added energy is related to the fetch length and depth. As described in Procedures for Applying Marsh Grass Methodology (FEMA, 1984), a modification to the NAS Methodology (NAS, 1977) has been developed to analyze in detail the attenuating effect of marsh grass on waves. The rate of wave energy dissipation is dependent on the wave characteristics (e.g. height and period), and the species of marsh grass. Two conditions result from this modification depending on the initial wave height at the beginning of the marsh segment: 1) if the initial wave is relatively small, wave growth will occur but at a significantly lower rate as compared to the NAS methodology, and 2) if the initial wave is sufficiently large, a wave height reduction will occur over the marsh. 14
Wave heights were computed along transects (cross section lines) that were located along the coastal areas, as illustrated in the Transect Location Map (Figure 1), in accordance with the Users Manual for Wave Height Analysis (FEMA, 1981). These transects are also shown on the FIRM. The transects were located with consideration given to the physical and cultural characteristics of the land so that they would closely represent conditions in their locality. Transects were spaced close together in areas of complex topography and dense development. In areas having more uniform characteristics, they were spaced at larger intervals. It was also necessary to locate transects in areas where unique flooding existed and in areas where computed wave heights varied significantly between adjacent transects. The transects were continued inland until the wave was dissipated or until flooding from another source with equal or greater elevation was reached. Along each transect, wave heights and elevations were computed considering the combined effects of changes in ground elevation, vegetation, and physical features. The stillwater elevations for the 1-percent-annual-chance flood were used as the starting elevations for these computations. Wave heights were calculated to the nearest 0.1 foot, and wave elevations were determined at wholefoot increments along the transects. Areas with a wave height component 3-feet or greater were designated as velocity zones (VE). Other areas subject to wave action were designated as AE Zones with Base Flood Elevations (BFEs) adjusted to include wave crest elevations. Figure 2 is a profile for a hypothetical transect showing the effects of energy dissipation on a wave as it moves inland. This figure shows the wave elevations being decreased by obstructions, such as buildings, vegetation, and rising ground elevations and being increased by open, unobstructed wind fetches. Actual wave conditions may not necessarily include all of the situations shown in Figure 2. Initial Countywide FIS Report For the initial countywide FIS, the wave height analysis incorporated the adjusted 1-percent-annual-chance flood stillwater elevations and the May 1985 version of the WHAFIS program (FEMA, 1985), which included the marsh grass methodology (FEMA, 1984). The original transects were modified so that marshy areas were treated as marsh rather than inland fetches with adjusted ground elevations. 15
Figure 2 - Transect Schematic Results from the wave height analysis are incorporated into the information presented on the FIRM and summarized in Table 6. Computed wave elevations were based on existing topography, vegetation, and development patterns. 3.4 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 the NGVD. With the finalization of the NAVD, many FIS reports and FIRMs are being prepared using NAVD as the referenced vertical datum. 17
FLOODING SOURCE TRANSECTS 10-PERCENT- STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- 1-PERCENT- 0.2-PERCENT- ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN 4, 5 5.1 8.4 9.4 12.0 VE 15-12 5.1 8.4 9.4 12.0 AE 11-10 5.1 8.4 9.4 12.0 AE 10-11 5.3 9.1 9.8 12.6 VE 12-14 5.3 9.1 9.8 12.6 VE 14-13 5.5 9.4 10.3 12.8 AE 12-10 5.6 9.5 10.3 13.2 AE 10-12 6.0 10.0 10.9 13.7 VE 13-14 6.0 10.0 10.9 13.7 VE 14 4, 6 5.1 * 9.6 * VE 16-12 5.1 * 9.6 * AE 11-10 5.1 * 9.6 * AE 10-11 5.1 * 9.6 * VE 12-14 5.6 9.2 10.2 12.4 VE 14-13 5.6 9.2 10.2 12.4 AE 12-11 5.6 9.4 10.3 12.8 AE 11-12 5.6 9.4 10.3 12.8 VE 13 5.6 9.4 10.3 12.8 VE 13 1 Includes the effects of wave action, where applicable 2 Due to map scale limitations, BFEs shown on the FIRM may represent average elevation for the zone depicted *Data not available TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY CAMDEN COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN
FLOODING SOURCE TRANSECTS 10-PERCENT- STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- 1-PERCENT- 0.2-PERCENT- ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN 4, 6 5.6 9.4 10.3 12.8 AE 12-10 (CONTINUED) (CONTINUED) 5.6 9.5 10.3 13.2 AE 10 5.8 * 10.6 * AE 10-12 5.8 * 10.6 * AE 11 5.8 * 10.6 * VE 14 5.8 * 10.6 * AE 13-11 5.9 * 10.2 * AE 10 11 5.6 * 10.3 * VE 17-13 5.6 * 10.3 * AE 12-10 5.6 * 10.3 * AE 10-12 5.6 * 10.3 * VE 13-16 6.3 10.0 11.9 14.1 VE 16-15 6.3 10.0 11.9 14.1 AE 14-12 1 Includes the effects of wave action, where applicable 2 Due to map scale limitations, BFEs shown on the FIRM may represent average elevation for the zone depicted *Data not available TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY CAMDEN COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN
FLOODING SOURCE TRANSECTS 10-PERCENT- STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- 1-PERCENT- 0.2-PERCENT- ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN 12 5.8 9.1 10.7 13.2 VE 17-13 (CONTINUED) 5.8 9.1 10.7 13.2 AE 12-11 5.8 9.1 10.7 13.2 AE 11-13 5.8 9.1 10.7 13.2 AE 13-12 6.1 * 11.2 * VE 13-15 6.1 * 11.2 * VE 15-16 6.3 10.0 11.7 14.2 VE 16-14 6.3 10.0 11.7 14.2 AE 13-12 7.0 * 12.4 * AE 13 15 6.5 * 12.0 * VE 15-17 6.5 * 12.0 * VE 17-15 7.0 10.6 12.7 15.3 AE 14-13 7.3 * 13.1 * AE 13-14 7.3 * 13.1 * VE 15-17 7.3 * 13.2 * VE 17-15 7.3 * 13.2 * AE 14-13 1 Includes the effects of wave action, where applicable 2 Due to map scale limitations, BFEs shown on the FIRM may represent average elevation for the zone depicted *Data not available TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY CAMDEN COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN
FLOODING SOURCE TRANSECTS 10-PERCENT- STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- 1-PERCENT- 0.2-PERCENT- ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN 16 6.6 * 12.1 * VE 17-15 (CONTINUED) 7.1 10.7 13.2 15.5 AE 14-13 17 7.7 11.5 12.8 16.0 VE 17-15 7.7 11.5 12.8 16.0 AE 14-13 18 7.3 11.0 12.7 15.7 VE 15 7.6 11.2 12.8 16.1 AE 14-13 ATLANTIC OCEAN/ 2, 3, 9 5.8 8.9 10.3 12.8 VE 17-13 CUMBERLAND 5.8 8.9 10.3 12.8 AE 12-10 SOUND 5.9 8.9 10.8 12.8 AE 10-12 5.9 8.9 10.8 12.8 VE 13 6.4 10.3 11.6 13.7 VE 14-16 6.4 * 11.6 * VE 16-14 6.4 * 11.6 * AE 13-12 6.7 10.8 11.8 14.4 AE 12-13 6.7 10.8 11.8 14.4 VE 14-16 7.2 11.2 12.3 15.1 VE 16-14 1 Includes the effects of wave action, where applicable 2 Due to map scale limitations, BFEs shown on the FIRM may represent average elevation for the zone depicted *Data not available TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY CAMDEN COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN ATLANTIC OCEAN/CUMBERLAND SOUND
FLOODING SOURCE TRANSECTS 10-PERCENT- STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- 1-PERCENT- 0.2-PERCENT- ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN/ 2, 3, 9 7.2 11.2 12.3 15.1 AE 13-12 CUMBERLAND (CONTINUED) 7.0 * 11.9 * AE 12-14 SOUND 7.0 * 11.9 * AE 14-12 (CONTINUED) 2, 3, 10 5.4 8.6 9.9 12.8 VE 17-13 5.4 8.6 9.9 12.8 AE 12-10 5.4 8.6 9.9 12.8 AE 10-12 5.8 * 10.8 * VE 13 5.8 * 10.8 * AE 12-11 5.8 * 10.8 * AE 11-12 6.1 9.9 11.6 13.8 VE 13-16 6.1 9.9 11.6 13.8 VE 16-14 6.5 10.4 11.7 14.1 AE 13-12 6.5 10.4 11.7 14.1 AE 12-13 6.8 10.8 11.7 14.5 AE 13-12 1 Includes the effects of wave action, where applicable 2 Due to map scale limitations, BFEs shown on the FIRM may represent average elevation for the zone depicted *Data not available TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY CAMDEN COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN/CUMBERLAND SOUND
FLOODING SOURCE TRANSECTS 10-PERCENT- STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- 1-PERCENT- 0.2-PERCENT- ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN/ 3, 7 5.1 8.5 9.8 12.7 VE 16-13 CUMBERLAND 5.1 8.5 9.8 12.7 AE 12-10 SOUND 5.4 9.4 10.3 13.0 AE 10-12 (CONTINUED) 5.4 9.4 10.3 13.0 VE 13-14 5.8 9.5 10.7 12.6 VE 14-13 5.8 9.5 10.7 12.6 AE 12-11 5.8 9.5 10.7 12.6 AE 11-12 5.7 * 10.5 * AE 12-11 3, 8 5.4 * 10.1 * VE 16-13 5.4 * 10.1 * AE 12-10 5.8 * 10.6 * AE 11-12 6.1 9.7 11.1 13.1 VE 13-15 6.1 9.7 11.1 13.1 VE 16 6.2 10.2 11.2 13.6 VE 15-14 6.2 10.2 11.2 13.6 AE 13-11 6.5 * 11.6 * AE 11-14 6.5 * 11.6 * VE 15 7.0 10.8 12.2 14.9 VE 15 7.0 * 11.9 * AE 14-12 1 Includes the effects of wave action, where applicable 2 Due to map scale limitations, BFEs shown on the FIRM may represent average elevation for the zone depicted *Data not available TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY CAMDEN COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN/CUMBERLAND SOUND
FLOODING SOURCE TRANSECTS 10-PERCENT- STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- 1-PERCENT- 0.2-PERCENT- ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN/ 1, 13 5.7 9.2 10.8 13.4 VE 17-13 SATILLA RIVER 5.7 9.2 10.8 13.4 AE 12-11 5.7 9.2 10.8 13.4 AE 11-12 6.4 * 11.7 * VE 13-19 6.4 * 11.7 * VE 19 7.0 10.6 12.7 15.2 VE 19-16 7.1 11.0 13.2 15.6 AE 15-13 14 6.4 * 11.9 * VE 19 7.0 * 12.7 * VE 19 7.1 10.9 13.2 15.6 VE 19 7.4 11.0 13.2 15.8 VE 19 7.3 * 13.2 * VE 19 7.3 * 13.2 * VE 19-16 7.3 * 13.2 * AE 15-13 7.1 * 12.5 * AE 13 1 Includes the effects of wave action, where applicable 2 Due to map scale limitations, BFEs shown on the FIRM may represent average elevation for the zone depicted *Data not available TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY CAMDEN COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN/SATILLA RIVER
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 BFEs across the corporate limits between the communities. The average conversion factor that was used to convert the data in this FIS report to NAVD was calculated using the National Geodetic Survey s VERTCON online utility (NGS, 2007). The data points used to determine the conversion are listed in Table 7. Table 7 - Vertical Datum Conversion Conversion from Quad Name Corner Latitude Longitude NGVD to NAVD Tarboro NW 31.125-81.875-1.096 Tarboro NE 31.125-81.750-1.093 Dover Bluff NW 31.125-81.625-1.083 Jerusalem NW 31.000-81.875-1.099 Jerusalem NE 31.000-81.750-1.125 Kingsland NE NW 31.000-81.625-1.125 Kingsland NE NE 31.000-81.500-1.083 Cumberland Island North NE 31.000-81.375-1.050 Jerusalem SW 30.875-81.875-1.050 Kingsland NW 30.875-81.750-1.142 Kingsland NE 30.875-81.625-1.161 Cumberland Island South NW 30.875-81.500-1.155 Cumberland Island South NE 30.875-81.375-1.109 Kingsland SW 30.750-81.750-1.152 Kingsland SE 30.750-81.625-1.155 Cumberland Island South SW 30.750-81.500-1.181 Average: -1.116 For additional information regarding conversion between NGVD and NAVD, visit the National Geodetic Survey website at www.ngs.noaa.gov, or contact the National Geodetic Survey at the following address: Vertical Network Branch, N/CG13 National Geodetic Survey, NOAA Silver Spring Metro Center 3 1315 East-West Highway Silver Spring, Maryland 20910 (301) 713-3191 Temporary vertical monuments are often established during the preparation of a flood hazard analysis for the purpose of establishing local vertical control. 25
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) 713-3242, or visit their website at www.ngs.noaa.gov. 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, the 1- and 0.2-percent-annual-chance floodplain boundaries have been delineated using the flood elevations determined at each cross section. For the Atlantic Ocean between transects, the boundaries were interpolated using topographic maps at a scale of 1:24,000, with a contour interval of 5 feet and 1.5 meters (USGS, various dates), 1:1,200 with a contour interval of 2 feet (Clarson & Associates, Inc., 1980), 1:600 with a contour interval of 1 foot (Clarson & Associates, Inc., 1982), 1:600 with a contour interval of 1 foot (Privett & Associates, Inc., 1985), 1:600 with a contour interval of 1 foot (Privett & Associates, Inc., 1986a), 1:1,200 with a contour interval of 1 foot (Privett & Associates, Inc., 1986b), and 1:480 with a contour interval of 1 foot (Privett & Associates, Inc., 1986c). For the St. Marys River between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:24,000, with a contour interval of 5 feet (USGS, 1999). 26
For streams restudied by approximate methods presented in Table 1, and for redelineated areas studied by approximate methods, the 1-percent-annualchance floodplain boundaries were delineated using NED data with a 5-foot contour interval (USGS, 1999). 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, AE, and VE), and the 0.2-percent-annual-chance floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 1- and 0.2-percent-annual-chance floodplain boundaries are close together, only the 1-percent-annual-chance floodplain boundary has been shown. Small areas within the floodplain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. For the streams studied by approximate methods, only the 1-percent-annualchance floodplain boundary is shown on the FIRM (Exhibit 2). 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the 1-percent-annual-chance floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the 1-percent-annual-chance flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to 1 foot, provided that hazardous velocities are not produced. The floodways in this study are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The area between the floodway and 1-percent-annual-chance floodplain boundaries is termed the floodway fringe. The floodway fringe encompasses the portion of the floodplain that could be completely obstructed without increasing the WSEL of the 1-percent-annual-chance flood more than 1 foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in Figure 3. 27
Figure 3 - Floodway Schematic No floodway was computed for St. Marys River. 4.3 Base Flood Elevations Areas within the community studied by detailed engineering methods have BFEs established in AE and VE Zones. These are the elevations of the 1-percentannual-chance (base flood) relative to NAVD88. In coastal areas affected by wave action, BFEs are generally maximum at the normal open shoreline. These elevations generally decrease in a landward direction at a rate dependent on the presence of obstructions capable of dissipating the wave energy. Where possible, changes in BFEs have been shown in 1-foot increments on the FIRM. However, where the scale did not permit, 2- or 3-foot increments were sometimes used. BFEs shown in the wave action areas represent the average elevation within the zone. Current program regulations generally require that all new construction be elevated such that the first floor, including basement, is elevated to or above the BFE in AE and VE Zones. 4.4 Velocity Zones The USACE has established the 3-foot wave height as the criterion for identifying coastal high hazard zones (USACE, 1975). This was based on a study of wave 28
action effects on structures. This criterion has been adopted by FEMA for the determination of VE zones. Because of the additional hazards associated with high-energy waves, the NFIP regulations require much more stringent floodplain management measures in these areas, such as elevating structures on piles or piers. In addition, insurance rates in VE zones are higher than those in AE zones. The location of the VE zone is determined by the 3-foot wave as discussed previously. The detailed analysis of wave heights performed in this study allowed a much more accurate location of the VE zone to be established. The VE zone generally extends inland to the point where the 1-percent-annual-chance stillwater flood depth is insufficient to support a 3-foot wave. 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 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 VE Zone VE is the flood insurance risk zone that corresponds to the 1-percent-annual-chance coastal floodplains that have additional hazards associated with storm waves. Whole-foot 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 29