McINTOSH COUNTY, GEORGIA

Transcription

1 McINTOSH COUNTY, GEORGIA AND INCORPORATED AREAS Community Name Community Number Darien, City of McIntosh County (Unincorporated Areas) Preliminary: McIntosh County FLOOD INSURANCE STUDY NUMBER 13191CV000A

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 panels for this community contain information that was previously shown separately on the corresponding Flood Boundary and Floodway Map panels (e.g., floodways, cross sections). In addition, former flood hazard zone designations have been changed as follows: Old Zone(s) Al through A30 VI through V30 B C New Zone AE VE X X Initial Countywide FIS Effective Date: To Be Determined

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 Wave Height Analysis Vertical Datum FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Base Flood Elevations Velocity Zones INSURANCE APPLICATIONS FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES i

4 TABLE OF CONTENTS (Continued) FIGURES Figure 1 - Transect Location Map Figure 2 - Transect Schematic TABLES Table 1 - Streams Studied by Approximate Methods... 4 Table 2 - Summary of Discharges... 9 Table 3 - Parameter Values for Surge Elevation Computations... 9 Table 4 - Transect Descriptions Table 5 - Transect Data Table 6 - Vertical Datum Conversion Table 7 - Community Map History Exhibit 1 - Flood Insurance Rate Map Index Flood Insurance Rate Map EXHIBITS ii

5 FLOOD INSURANCE STUDY McINTOSH 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 McIntosh County, including the City of Darien, and the unincorporated areas of McIntosh County (referred to collectively herein as McIntosh County), and aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of This study has developed flood-risk data for various areas of the community that will be used to establish actuarial flood insurance rates and to assist the community in its efforts to promote sound floodplain management. Minimum floodplain management requirements for participation in the National Flood Insurance Program (NFIP) are set forth in the Code of Federal Regulations at 44 CFR, In some states or communities, floodplain management criteria or regulations may exist that are more restrictive or comprehensive than the minimum Federal requirements. In such cases, the more restrictive criteria take precedence and the State (or other jurisdictional agency) will be able to explain them. The Digital Flood Insurance Rate Map (DFIRM) and FIS report for this countywide study have been produced in digital format. Flood hazard information was converted to meet the Federal Emergency Management Agency (FEMA) DFIRM database specifications and Geographic Information System (GIS) format requirements. The flood hazard information was created and is provided in a digital format so that it can be incorporated into a local GIS and be accessed more easily by the community. 1.2 Authority and Acknowledgments The sources of authority for this FIS are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of Precountywide Analyses Information on the authority and acknowledgements for each jurisdiction included in this countywide FIS report, as compiled from their previously printed FIS reports, are shown below: 1

6 Darien, City of: The hydrologic and hydraulic analyses for the original FIS report dated January 2, 1981 (FIA, 1981), were prepared by Post, Buckley, Schuh & Jernigan, Inc., for the Federal Insurance Administration, under Contract No. H The work was completed in January The revised hydrologic and hydraulic analyses for the first revision, dated June 3, 1988 (FEMA, 1988a), were taken from the FIS for the unincorporated areas of McIntosh County (FEMA, 1983b). McIntosh County (Unincorporated Areas): The hydrologic and hydraulic analyses for the original FIS report dated November 15, 1983 (FEMA, 1983), were completed by Post, Buckley, Schuh & Jernigan, Inc., for FEMA, under Contract No. H The work was completed in April This Countywide FIS Report For this countywide FIS, streams studied by approximate methods and redelineation of floodplain boundaries for streams studied by detailed and 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 October Base map information shown on the Flood Insurance Rate Map (FIRM) was provided by Geofiny Technologies LLC, dated 2005, and captured at a resolution of 1 foot per pixel. The projection used in the preparation of the map is State Plane Georgia East (FIPS Zone 1001), and the horizontal datum used is NAD 83, GRS 80 spheroid. 1.3 Coordination An initial meeting is held with representatives from FEMA, the community, and the study contractor to explain the nature and purpose of a FIS, and to identify the streams to be studied or restudied. A final meeting is held with representatives from FEMA, the community, and the study contractor to review the results of the study. 2

7 The initial and final meeting dates for the previous FIS reports for the City of Darien and McIntosh County are listed in the following table: Community FIS Date Initial Meeting Final Meeting Darien, City of January 2, 1981 May 1978 August 29, 1980 June 3, 1988 * * McIntosh County (Unincorporated Areas) November 15, 1983 May 1978 May 25, 1983 * Data not available For this countywide FIS, a scoping meeting was held on December 8, 2005, at the Coastal Georgia Regional Development Center, and attended by representatives of McIntosh County, the Georgia DNR, FEMA, and PBS&J. The purpose of this meeting was to discuss the scope of the countywide FIS. The results of the study were reviewed at the final meeting held on, and attended by representatives of, Georgia DNR, and PBS&J. All problems raised at the meeting have been addressed. 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of McIntosh 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. Precountywide FIS Reports For the January 2, 1981, FIS report for the City of Darien, the Darien River/Cathead Creek was studied by detailed methods. For the November 15, 1983, FIS report for McIntosh County (Unincorporated Areas), Darien River/Cathead Creek, Sapelo River, and South Newport River were studied by detailed methods. The coastline flooding effects from the Atlantic Ocean due to coastal storm surge tides and the added effects of wave heights were studied by detailed methods. 3

8 For the June 3, 1988, revision to the FIS report of the City of Darien, the Darien River/Cathead Creek was revised to reflect the coastal flooding effects from the Atlantic Ocean. This Countywide FIS Report For this countywide FIS, reaches that have been studied by approximate methods are listed in Table 1. Table 1 - Streams Studied by Approximate Methods Stream Althamaha River Cathead Creek Tributary No. 4 Cathead Creek Tributary No. 35 Reach Description From approximately 23,160 feet upstream of Seaboard Coastline Railway to county boundary From River Road/State Highway 251 to approximately 1,160 feet upstream of North Way/State Highway 25/U.S. Highway 17 From River Road/State Highway 251 to approximately 15,150 feet upstream of North Way/State Highway 25/U.S. Highway 17 Cathead Creek Tributary No From confluence with Cathead Creek Tributary 35 to approximately 8,410 feet upstream of confluence with Cathead Creek Tributary No. 35 Cathead Creek Tributary No From confluence with Cathead Creek Tributary No. 35 to approximately 11,100 feet upstream of North Way/State Highway 25/U.S. Highway 17 Cathead Creek Tributary No From confluence with Cathead Creek Tributary No. 35 to approximately 10,800 feet upstream of confluence with Cathead Creek Tributary No. 35 McClendon Creek From Cox Road to approximately 12,170 feet upstream of Townsend Cox Road 4

9 Table 1 Streams Studied by Approximate Methods (Continued) Stream McClendon Creek Tributary No. 1 McClendon Creek Tributary No. 2 McClendon Creek Tributary No. 3 McClendon Creek Tributary No. 4 McClendon Creek Tributary No. 4.1 McClendon Creek Tributary No. 6 Reach Description From confluence with McClendon Creek to approximately 8,000 feet upstream of confluence with McClendon Creek From confluence with McClendon Creek to approximately 2,930 feet upstream of confluence with McClendon Creek From confluence with McClendon Creek to approximately 9,500 feet upstream of confluence with McClendon Creek From confluence with McClendon Creek to approximately 14,450 feet upstream of Cox Road From confluence with McClendon Creek Tributary No. 4 to approximately 4,020 feet upstream of Cox Road From confluence with McClendon Creek to approximately to approximately 8,730 feet upstream of confluence with McClendon Creek Sapelo River Tributary No. 50 From Bell Bluff Road to approximately 8,400 feet upstream of Bell Bluff Road South Newport River South Newport River Tributary No. 1 South Newport River Tributary No. 2 South Newport River Tributary No. 2.1 From North Way/State Highway 25/U.S. Highway 17 to county boundary From confluence with South Newport River to Interstate Highway 95/State Highway 401 From confluence with South Newport River to approximately 2,740 feet upstream of Brickston Road From confluence with South Newport River Tributary No. 2 to approximately 1,190 feet upstream of Horsepen Road 5

10 Table 1 Streams Studied by Approximate Methods (Continued) Stream South Newport River Tributary No. 2.2 White Chimney River Tributary No. 10 White Chimney River Tributary No White Chimney River Tributary No Reach Description From confluence with South Newport River Tributary No. 2 to approximately 5,530 feet upstream of Coker Place Road From confluence with White Chimney River to approximately 2,280 feet upstream of Tram Road From confluence with White Chimney River Tributary No. 10 to North Way/State Highway 25/U.S. Highway 17 From confluence with White Chimney River Tributary No. 10 to Tram Road Also for this countywide FIS, all streams, detailed and approximate, were redelineated to better match existing topography, excluding areas of coastal and wetland flooding. 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 McIntosh County. 2.2 Community Description McIntosh County is located in the southeast portion of Georgia. The county is bordered by Liberty County to the north, Long County to the northwest, Wayne County to the west, Glynn County to the south, and the Atlantic Ocean to the east. The total land area contained within the county is approximately 433 square miles (U.S. Census Bureau, 2007). The City of Darien, the county seat for McIntosh County and the second oldest planned city in the state of Georgia, is located in the southeast corner of the county. According to the U.S. Census Bureau, in 2000, the population of McIntosh County was 10,847 (U.S. Census, 2007). Major transportation routes to serving McIntosh County include Interstate Highways 17 and 95. 6

11 The climate in southeast Georgia is warm and temperate to subtropical. The average high temperature in January is 60 degrees Fahrenheit ( F), and is 90 F in July. The average annual precipitation is 51.8 inches, with significantly higher monthly averages occurring in the late summer months of August and September (The Weather Channel, 2007). The South Newport River, which delimits the county boundary with Liberty County, has a drainage area of approximately 160 square miles and flows generally southeasterly. The Altamaha River also flows generally southeasterly and delimits the county boundary with Glynn County. Other significant flooding sources within McIntosh County include the Darien River/Cathead Creek system with a drainage area of 98.3 square miles, and the Sapelo River with a drainage area of 34.6 square miles. The downstream reaches of these flooding sources cut across relatively flat tidal marshlands, creating a series of sounds and a network of tributaries prior to the confluence with the Atlantic Ocean. 2.3 Principal Flood Problems McIntosh County is subject to flooding from coastal storm surge tides in addition 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. The area was affected by major hurricanes in 1824, October 1898, August 1911, July 1916, September 1927, September 1928, September 1929, August 1940, October 1944, September 1945, October 1947, September and October 1950, September 1953, August and September 1964 (Environmental Science Services Administration, 1970a). Tropical Storm Tammy also caused minor flooding in McIntosh County during landfall on October 6, Flood Protection Measures The City of Darien Subdivision Regulations, dated August 1969 (Coastal Area Planning and Development Commission, 1969), indicates that developers shall obtain drainage easements and install drainage facilities such that the drainage water does not materially affect the adjacent property. The coastal Area Planning and Development Commission has developed a document titled Areawide Land Use Plan Non-Metropolitan Coastal Area (Coastal Area Planning and Development Commission, 1978). Its coverage is McIntosh County and seven other counties. It defines flood-prone areas as those inundated by the 1-percent-annual-chance flood. The document states that only new development that can sustain periodic flooding or that will not create public burdens should be encouraged to locate in flood-prone areas. Examples given of activities that are well suited for floodplain locations include recreation and agriculture, with necessary incidental structures. 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 For Darien River/Cathead Creek, Sapelo River, and South Newport River, the discharge-frequency relationships for riverine flooding were obtained from the U.S. Geological Survey (USGS) regional regression equations for Georgia (USGS, 1976). The regional analysis is based on statistical computations of discharge records at various sites in Georgia, regressed against basin characteristics. The 0.2-percent-annual-chance flood discharge values were extrapolated from the lower frequency flood discharges. Peak discharge-drainage area relationships for 10-, 2-, 1-, and 0.2-percentannual-chance floods for each flooding source studied in detail in the county are presented in Table 2. 8

13 Table 2 - Summary of Discharges Peak Discharges (cubic feet per second) Flooding Source and Location Drainage Area (square miles) 10-Percent- Annual-Chance 2-Percent- Annual-Chance 1-Percent- Annual-Chance 0.2-Percent- Annual-Chance Darien River/Cathead Creek At the confluence with Rockdedundy River At U.S. Highway 17/State Highway 25 Approximately 6,920 feet upstream of the confluence of Darien Creek Just downstream of State Highway 251 Sapelo River At the confluence with Sapelo Sound Just downstream of U.S. Highway 17/ State Highway 25 South Newport River At the confluence with Sapelo Sound Just downstream of U.S. Highway 17/ State Highway ,240 5,210 6,310 9, ,210 5,160 6,250 8, ,170 5,100 6,170 8, ,480 3,970 4,780 6, ,750 2,780 3,340 4, ,200 1,900 2,260 3, ,310 6,980 8,490 12, ,870 6,240 7,580 10,600 All of the flooding sources studied by detailed methods are also subject to flooding due to coastal storm surge tides. The Joint Probability Method (Environmental Science Services Administration, 1970b) 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, and central pressure depression. A summary of the parameters used for the McIntosh County area is presented in Table 3. Table 3 - Parameter Values for Surge Elevation Computations P PP F PF R PR A PA D FN Exiting Storms Parallel Storms

14 Table 3 - Parameter Values for Surge Elevation Computations (Continued) P PP F PF R PR A PA D FN Entering Storms Distance storm crossing point from the City of Savannah in Nautical Miles 2 Distance from shore in Nautical Miles P = Central Pressure (millibars) 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 from coast (North) (degrees) PA = Probability of storm with A Value D = Distance from shore (NM) FN = Frequency of storm occurrence/year This Countywide FIS Report For the streams studied by approximate methods listed in Table 1, peak flows were determined using the rural regression equations for Georgia (Stamey and Hess, 1993). 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. 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 sections for backwater analyses were obtained from aerial photographs (Abrams Aerial Survey Corporation, 1979). All bridges, dams, and culverts were field checked to obtain elevation data and structural geometry. For streams studied by detailed methods, water surface elevations (WSELs) of floods of the selected recurrence intervals were computed using the U.S. Army 10

15 Corps of Engineer s (USACE) Hydrologic Engineering Center (HEC) HEC-2 step-backwater computer program (HEC, 1984). Starting WSELs for Darien River/Cathead Creek were determined to be the estimate for a 1-percent-annual-chance tide (NOAA, 1979). Frequency of coastal storm surge tides at the open coast were computed from a statistical analysis of the results of the TTSURGE computer program (FIA, 1978). Discretized values of the tropical storm parameters used to generate a large number of synthetic storms using the TTSURGE computer program are presented in Table 3. Performing simulations for a large number of storm events, each of known total probability, established the frequency distributions of surge height as a function of coastal location. These distributions incorporate the large scale tide surge behavior, but does not include an analysis of the added effects associated with much finer scale wave phenomenon such as wave height, setup, and runup. The astronomic tide for the region is then statistically combined with the computed storm surge tide to yield recurrence intervals of total water level. Routing of coastal storm surge tides inland through the Altamaha, Doboy, and Sapelo Sounds was conducted using an adaptation of an existing Inland Routing Model to the specific conditions along the Georgia coastline (Harleman, 1976). The model was calibrated against WSELs recorded as a result of two hurricanes affecting City of Savannah, Georgia. This Countywide FIS Report For the streams studied by approximate methods listed in Table 1, 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). The hydraulic analyses for this study were based on unobstructed flow. 3.3 Wave Height Analysis The methodology for analyzing the effects of wave heights associated with coastal storm surge flooding is described in the National Academy of Sciences (NAS) report (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 11

16 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) 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. 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. 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 my not necessarily include all of the situations shown in Figure 2. 12

17 FIGURE 1 FEDERAL EMERGENCY MANAGEMENT AGENCY McINTOSH COUNTY, GA AND INCORPORATED AREAS TRANSECT LOCATION MAP ATLANTIC OCEAN

18 Figure 2 Transect Schematic The transects were coded in reference to USGS quadrangle maps at a scale of 1:24,000, and a contour interval of 5 feet (USGS, various dates). Additional detailed topographic maps, at a scale of 1:4,800, and a contour interval of 2 feet (Abrams Aerial Survey Corporation., 1979), were used in conjunction with the USGS quadrangle maps. Aerial photographs were used to determine the type, size, and density of vegetation and physical features (Abrams Aural Survey Corporation, 1979). Descriptions of the locations of each transect, and the 1- percent-annual-chance starting stillwater and maximum 1-percent-annualchance wave crest elevations are presented in Table 4. Table 4 - Transect Descriptions Transect Location Water Surface Elevations (Feet NAVD 1 ) 1-percent-annualchance 1-percent-annual- Stillwater chance Wave Crest 1 At mouth of Altamaha Sound Approximately 2,640 feet north of Altamaha Sound Approximately 7,920 feet north of Altamaha Sound at Wolf Island 4 At the confluences of South River and Beacon Creek At the confluence of Doboy Sound Approximately 2,640 feet north of Doboy Sound, at Sapelo Island 7 Approximately 5,280 feet north of Doboy Sound, at Town of Sapelo 8 Approximately 13,200 feet north of Doboy Sound, at Saplo Island 9 Approximately 14,785 feet north of Doboy Sound, at Saplo Island, near Hog Hammock

19 Table 4 Transect Description (Continued) Transect Location Water Surface Elevations (Feet NAVD 1 ) 1-percent-annualchance 1-percent-annual- Stillwater chance Wave Crest 10 Approximately 5,280 feet south of Cabretta Inlet, at Sapelo Island 11 At the Cabretta Inlet at Sapelo Island Approximately 2,640 feet north of Cabretta Inlet, at Sapelo Island 13 Approximately 7,920 feet north of Cabretta Inlet, at Blackbeard Island 14 Approximately 16,500 feet south of Sapelo Sound, near Nelsons Bluff, at Blackbeard Island 15 Approximately 13,200 feet south of Sapelo Sound, at Blackbeard Island 16 Approximately 7,920 feet south of Sapelo Sound, at Blackbeard Island 17 Approximately 2,640 feet south of Sapelo Sound, near Northeast Point, at Blackbeard Island 18 At mouth of Sapelo Sound Approximately 2,640 feet north of Sapelo Sound, at Oldnor Island 20 Approximately 13,200 feet north of Sapelo Sound, at Wahoo Island North American Vertical Datum of 1988 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 whole-foot 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. Results from the wave height analysis are incorporated into the information presented on the FIRM and summarized in Table 5. Computed wave elevations were based on existing topography, vegetation, and development patterns. Users of the FIRM should be aware that coastal flood elevations are provided in the Transect Data Table (Table 5). 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. 15

20 FLOODING SOURCE TRANSECTS 10-PERCENT- ANNUAL-CHANCE STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- ANNUAL-CHANCE 1-PERCENT- ANNUAL-CHANCE 0.2-PERCENT- ANNUAL-CHANCE ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN VE AE VE AE VE AE VE AE VE * * * * AE * * * * VE AE 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 5 FEDERAL EMERGENCY MANAGEMENT AGENCY McINTOSH COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN Table 5 - Transect Data 15

21 FLOODING SOURCE TRANSECTS 10-PERCENT- ANNUAL-CHANCE STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- ANNUAL-CHANCE 1-PERCENT- ANNUAL-CHANCE 0.2-PERCENT- ANNUAL-CHANCE ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN VE AE VE AE VE AE VE AE VE AE VE AE VE AE VE AE 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 TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY McINTOSH COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN 16

22 FLOODING SOURCE TRANSECTS 10-PERCENT- ANNUAL-CHANCE STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- ANNUAL-CHANCE 1-PERCENT- ANNUAL-CHANCE 0.2-PERCENT- ANNUAL-CHANCE ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN VE AE VE AE VE AE VE AE VE AE VE AE VE AE VE AE 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 TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY McINTOSH COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN 17

23 FLOODING SOURCE TRANSECTS 10-PERCENT- ANNUAL-CHANCE STILLWATER ELEVATION (FEET NAVD) 2-PERCENT- ANNUAL-CHANCE 1-PERCENT- ANNUAL-CHANCE 0.2-PERCENT- ANNUAL-CHANCE ZONE 1 BASE FLOOD ELEVATION (FEET NAVD) 2 ATLANTIC OCEAN VE AE VE AE VE AE VE AE VE AE VE VE AE VE 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 TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY McINTOSH COUNTY, GA AND INCORPORATED AREAS TRANSECT DATA ATLANTIC OCEAN 18

24 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 NGVD. With the finalization of NAVD, many FIS reports and FIRMs are being prepared using NAVD as the referenced vertical datum. All flood elevations shown in this FIS report and on the FIRM are referenced to NAVD. Structure and ground elevations in the community must, therefore, be referenced to NAVD. It is important to note that adjacent communities may be referenced to NGVD. This may result in differences in Base Flood Elevations (BFEs) across the corporate limits between the communities. The average conversion factor that was used to convert the data in this FIS report to NAVD was calculated using the National Geodetic Survey s VERTCON online utility (NGS, 2007). The data points used to determine the conversion are listed in Table 6. Table 6 - Vertical Datum Conversion Conversion from Quad Name Corner Latitude Longitude NGVD to NAVD Townsend SW Eulonia NW Eulonia NE Eulonia SW Eulonia SE Shellman Bluff NE Shellman Bluff SE Ridgeville SW Doboy Sound SE Doboy Sound SW Average:

25 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 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 areas studied by detailed and approximate methods comprised of coastal flooding and wetlands, the 1- and 0.2-percent-annual-chance floodplain boundaries were delineated using topographic maps at a scale of 1:4,800, 21

26 1:9,600, and 1:24,000, all with a contour interval of 2 feet (Abrams Aerial Survey Corporation, 1979). 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 topographic maps at a scale of 1:24,000 with a contour interval of 5 feet (USGS, various dates). The 1- and 0.2-percent-annual-chance floodplain boundaries are shown on the FIRM (Exhibit 2). On this map, the 1-percent-annual-chance floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones A, 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 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 NAVD. 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. 4.3 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 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. 22

27 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 mile, and areas protected from the 1-percent-annual-chance flood by levees. No BFEs or base flood depths are shown within this zone. The FIRM for McIntosh County includes areas designated by Congress as units of the Coastal Barrier Resources System (CBRS), where federally backed flood insurance is not available. 23

28 The Coastal Barrier Resources Act of 1982 and the Coastal Barrier Improvement Act of 1990 define and establish a system of protected coastal areas (including the Great Lakes) known as the CBRS. The Acts define areas within the CBRS as depositional geologic features consisting of unconsolidated sedimentary materials; subject to wave, tidal, and wind energies; and protecting landward aquatic habitats from direct wave attack. The Acts further define coastal barriers as all associated aquatic habitats, including the adjacent wetlands, marshes, estuaries, inlets and nearshore waters, but only if such features and associated habitats contain few manmade structures and these structures and man s activities on such features, and within such habitats do not significantly impede geomorphic and ecological processes. The Acts provide protection to CBRS areas by prohibiting most expenditures of Federal funds within them. These prohibitions refer to any form of loan, grant, guarantee, insurance, payment, rebate, subsidy or any other form of direct or indirect Federal assistance, with specific and limited exceptions. The CBRS boundaries depicted on the FIRM for Glynn County were adopted into public law by Acts of Congress and are, therefore, considered final and not subject to appeal. In addition to the CBRS, the Coastal Barrier Improvement Act of 1990 established Otherwise Protected Areas (OPAs). OPAs are undeveloped coastal barriers within the boundaries of an area established under Federal, State, or local law, or held by a qualifying organization, primarily for wildlife refuge, sanctuary, recreational, or natural resource conservation purposes. Congress designated the initial CBRS areas in Subsequent modifications of the CBRS are introduced as legislation to be acted on by Congress, and originate from State and local requests, as well as recommendations made by the U.S. Fish and Wildlife Service. After Congress approves additions to the CBRS, the new areas are assigned a unique effective date, after which Federal assistance prohibitions apply. In cooperation with the U.S. Department of the Interior, FEMA transfers CBRS boundaries to FIRMs using Congressionally adopted source maps titled Coastal Barrier Resources System. FIRMs clearly depict the different CBRS areas and their effective dates with special map notes and symbols. It should be noted that although FEMA shows CBRS areas on FIRMs, only Congress may authorize a revision of CBRS boundaries. Within CBRS boundaries, Federal flood insurance is not available for structures built or substantially improved on or after the date that the subject area was added to the CBRS. To assist map users in determining the correct insurance prohibition date in CBRS areas, each separate CBRS unit is clearly identified on the FIRM. It is important to note that insurance for structures in OPAs may be obtained if written documentation is provided, which certifies that the structures are used in a manner consistent with the purpose for which the area is protected. 6.0 FLOOD INSURANCE RATE MAP The FIRM is designed for flood insurance and floodplain management applications. 24

29 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 McIntosh 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 Previous FIS reports have been prepared for Glynn County, Georgia and Incorporated Areas (FEMA, 2006), and Wayne County, Georgia (Unincorporated Areas) (FEMA, 1988b). This report either supersedes or is compatible with all previous studies on streams studied in this report and should be considered authoritative for purposes of the NFIP. 8.0 LOCATION OF DATA Information concerning the pertinent data used in the preparation of this study can be obtained by contacting FEMA, Federal Insurance and Mitigation Division, Koger Center - Rutgers Building, 3003 Chamblee Tucker Road, Atlanta, Georgia BIBLIOGRAPHY AND REFERENCES Abrams Aerial Survey Corporation, Aerial Surveys, Scale 1:4,800, 1:9,600, and 1:24,000, Contour Interval 2 foot, March Coastal Area Planning and Development Commission, Subdivision Regulations City of Darien, Georgia, August Coastal Area Planning and Development Commission, Areawide Land Use Plan, Non- Metropolitan Coastal Area, March

30 COMMUNITY NAME INITIAL IDENTIFICATION FLOOD HAZARD BOUNDARY MAP REVISION DATE FIRM EFFECTIVE DATE FIRM REVISION DATE Darien, City of May 10, 1974 February 13, 1976 July 2, 1981 June 3, 1988 To Be Determined McIntosh County (Unincorporated Areas) April 9, 1976 None May 15, 1984 October 16, 1992 To Be Determined TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY McINTOSH COUNTY, GA AND INCORPORATED AREAS COMMUNITY MAP HISTORY Table 7 - Community Map History

31 Environmental Science Services Administration, Georgia Tropical Cyclones and Their Effect on the State, ESSA Technical Memorandum EDSTM 14, Silver Spring, Maryland, January 1970a. Environmental Science Services Administration, Joint Probability Methods of Tide Frequency Analysis Applied to Atlantic City and Long Beach Island, New Jersey, Vance A. Meyers, ESSA Technical Memorandum WBPM, Hydro 11, April 1970b. Federal Emergency Management Agency, Users Manual for Wave Height Analysis, Washington, D.C., Revised February Federal Emergency Management Agency, Flood Insurance Study, McIntosh County, Georgia (Unincorporated Areas), Flood Insurance Study Report, November 15, 1983; Flood Insurance Rate Map, May 15, Federal Emergency Management Agency, Procedures for Applying Marsh Grass Methodology, Washington, D.C., July Federal Emergency Management Agency, Flood Insurance Study, City of Darien, McIntosh County, Georgia, June 3, 1988a. Federal Emergency Management Agency, Flood Insurance Study, Wayne County, Georgia (Unincorporated Areas), September 30, 1988b. Federal Emergency Management Agency, Flood Insurance Study, Glynn County, Georgia and Incorporated Areas, September 6, Federal Insurance Administration, Coastal Floodway Storm Surge Model, Parts 1 and 2, Washington, D.C., May Federal Insurance Administration, Flood Insurance Study, City of Darien, McIntosh County, Georgia, Flood Insurance Study Report, January 2, 1981; Flood Insurance Rate Map, July 2, Harleman, D.R.F. et. al., User Manual for the MIT Transient Water Quality Network Model Including Nitrogen-Cycle Dynamics for Rivers and Estuaries, Report No. 216, Ralph M. Parsons Laboratory for Water Resources and Hydrodynamics, Massachusetts Institute of Technology, August Hydrologic Engineering Center, HEC-2 Water Surface Profiles, 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