FRANKLIN COUNTY, FLORIDA

Transcription

1 FRANKLIN COUNTY, FLORIDA AND INCORPORATED AREAS COMMUNITY NAME COMMUNITY NUMBER APALACHICOLA, CITY OF CARRABELLE, CITY OF FRANKLIN COUNTY (UNINCORPORATED AREAS) REVISED: FEBRUARY 5, 2014 Federal Emergency Management Agency FLOOD INSURANCE STUDY NUMBER 12037CV000A

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) may not contain all data available within the repository. It is advisable to contact the community repository for any additional data. Part or all of this FIS may be revised and republished at any time. In addition, part of this FIS may be revised by the Letter of Map Revision process, which does not involve republication or redistribution of the FIS. It is, therefore, the responsibility of the user to consult with community officials and to check the community repository to obtain the most current FIS components. Initial Countywide FIS Effective Date: June 17, 2002 Revised Countywide FIS Date: February 5, 2014

3 TABLE OF CONTENTS Page 1.0 INTRODUCTION Purpose of Study Authority and Acknowledgments Coordination AREA STUDIED Scope of Study Community Description Principal Flood Problems Flood Protection Measures ENGINEERING METHODS Riverine Hydrologic Analyses Riverine Hydraulic Analyses Coastal Hydrologic Analyses Coastal Hydraulic Analyses Vertical Datum FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Floodways INSURANCE APPLICATIONS FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES 50 i

4 TABLE OF CONTENTS continued Page FIGURES Figure 1 Transect Schematic 18 Figure 2 Transect Location Map 39 Figure 3 Floodway Schematic 45 TABLES Table 1 - Summary of Discharges 9 Table 2 Summary of Stillwater Elevations Table 3 Transect Descriptions Table 4 Transect Data Table 5 Floodway Data 44 Table 6 Community Map History 49 EXHIBITS Exhibit 1 - Flood Profiles Apalachicola River Ochlockonee River Panels 01P-05P Panels 06P-07P Exhibit 2 - Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY FRANKLIN COUNTY, FLORIDA AND INCORPORATED AREAS 1.0 INTRODUCTION 1.1 Purpose of Study This countywide Flood Insurance Study (FIS) investigates the existence and severity of flood hazards in, or revises and updates previous FISs/Flood Insurance Rate Maps (FIRMs) for the geographic area of Franklin County, Florida, including: the Cities of Apalachicola and Carrabelle, and the unincorporated areas of Franklin County (hereinafter referred to collectively as Franklin County). This FIS aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of This FIS has developed flood risk data for various areas of the county that will be used to establish actuarial flood insurance rates. This information will also be used by Franklin County to update existing floodplain regulations as part of the Regular Phase of the National Flood Insurance Program (NFIP), and will also be used by local and regional planners to further promote sound land use and floodplain development. Minimum floodplain management requirements for participation in the 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. 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 This FIS was prepared to include the unincorporated areas of, and incorporated communities within, Franklin County in a countywide format. Information on the authority and acknowledgments for each jurisdiction included in this countywide FIS, as compiled from their previously printed FIS reports, is shown below. Apalachicola, City of: the hydrologic and hydraulic analyses for the FIS report dated January 18, 1983, were prepared by Gee & Jensen Engineers, Architects, & Planners (EAP), Inc., for the Federal Emergency Management Agency (FEMA) under Contract No. H That work was completed in November

6 Carrabelle, City of: the hydrologic and hydraulic analyses for the FIS report dated January 18, 1983, were prepared by Gee & Jensen EAP Inc., for FEMA under Contract No. H That work was completed in March Franklin County (Unincorporated Areas): the hydrologic and hydraulic analyses for the FIS report dated January 18, 1983, were prepared by Gee & Jensen EAP Inc., for FEMA under Contract No. H That work was completed in November For the June 17, 2002 countywide FIS, the coastal analyses were revised by Dewberry and Davis LLC. The extent of the revised coastal hydrologic and hydraulic analyses were limited to the barrier island portions of the county: St. Vincent Island, St. George Island, Little St. George Island, and Dog Island, as well as portions of the Alligator Point area, up to the confluence of the Ochlockonee Bay with the Gulf of Mexico. This work was completed in August For this revision to the countywide FIS dated February 5, 2014, the revised coastal analysis for the Gulf of Mexico, including the entire shoreline of Franklin County has been prepared for FEMA by the Northwest Florida Water Management District (NFWMD) under Contract No. EMA-2008-CA Additionally, the floodplain for the Apalachicola River has been redelineated using updated topographic data. Existing data for St. James Bay and the area in the vicinity of Eastpoint has also been incorporated as a part of this revision. This work was completed in Base map information for this FIRM was developed from high resolution digital orthoimagery provided by the Florida Department of Revenue. This information was produced at a scale of 1:200 from photography dated The coordinate system used for the production of this FIRM is Florida State Plane North (FIPS 0903) feet, referenced to the North American Datum of 1983 (NAD83) HARN. Corner coordinates shown on the FIRM are in latitude and longitude referenced to the State Plane projection, NAD 83 HARN. Differences in the datum and spheroid used in the production of FIRMs for adjacent counties may result in slight positional differences in map features at the county boundaries. These differences do not affect the accuracy of information shown on the FIRM. 1.3 Coordination Consultation Coordination Officer s (CCO) meetings may be held for each jurisdiction in this countywide FIS. An initial CCO meeting is held typically with representatives of FEMA, the community, and the study contractor to explain the nature and purpose of a FIS, and to identify the streams to be studied by detailed 2

7 2.0 AREA STUDIED methods. A final CCO meeting is held typically with representatives of FEMA, the community, and the study contractor to review the results of the study. The dates of the initial and final CCO meetings held prior to the countywide FIS for Franklin County and the incorporated communities within its boundaries are in the following tabulation: Community Name Initial CCO Date Final CCO Date Apalachicola, City of March 21, 1978 July 26, 1982 Carrabelle, City of March 21, 1978 July 22, 1982 Franklin County (Unincorporated Areas) March 28, 1978 July 1982 For the June 17, 2002, countywide FIS, Franklin County and the Cities of Apalachicola and Carrabelle were notified of the revision by FEMA in a letter dated July 20, For this revision to the countywide FIS, final CCO meetings were held September 12, These meetings were attended by representatives of the study contractors, the communities, and the State of Florida. 2.1 Scope of Study This FIS covers the geographic area of Franklin County, Florida, located on the Gulf of Mexico in northwest Florida. The following streams were studied by detailed methods: the Carrabelle River, the Ochlockonee River, and the Apalachicola River. For this revision to the countywide FIS, all coastal hazards affecting the county have been revised. The existing detailed study for the Ochlockonee River has been superseded with the revised coastal study. Additionally, the floodplain for the Apalachicola River has been redelineated using updated topographic data. Existing data for St. James Bay and the area in the vicinity of Eastpoint has also been incorporated. Limits of detailed study for riverine flooding sources are indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2). The areas studied by detailed methods were selected with priority given to all known flood hazard areas, and areas of projected development or proposed construction. No Letters of Map Change (LOMCs) were incorporated as part of this revision to the countywide FIS. 3

8 All or portions of numerous flooding sources within the county were studied by approximate methods. Approximate analyses were used to study those areas having a low development potential or minimal flood hazards. The scope and methods of study were proposed to, and agreed upon by, FEMA and Franklin County. 2.2 Community Description Franklin County is located in northwest Florida on the Gulf of Mexico approximately 40 miles southwest of Tallahassee. It is bounded on the west by Gulf County, on the north by Liberty County, and on the east by Wakulla County. Major communities within the county are the City of Apalachicola, located at the mouth of the Apalachicola River, and the City of Carrabelle which is located on St. George Sound. The county includes St. Vincent, St. George and Dog Islands. Chartered in 1832, Franklin County encompasses an area of approximately 1,037 square miles. The 2010 population of Franklin County was reported to be 11,549 (U.S. Department of Commerce, Bureau of the Census, 2010.) The primary east-west artery serving the county is State Route 30 (U.S. Route 98) which provides interconnection to most of the coastal counties in the area. State Routes 65, 67 and 377 provide access to areas north of Franklin County. The Apalachicola Northern Railroad runs north-south through the western portion of the county, and provides service to the City of Apalachicola. The Apalachicola River accommodates barge traffic and is a part of the extensive Flint-Chattahootchee River System with navigable channels as far north as Columbus, Georgia. The Intracoastal Waterway extends from the mouth of the Apalachicola River upstream to the confluence of the Brothers River and then turns westward through Lake Wimico. Residential and commercial development within the county is centered around the coastal cities of Apalachicola and Carrabelle with coastal areas along St. George Sound and Apalachicola Bay also being populated. St. George Island is being increasingly populated, primarily with second home development. Large portions of the county are essentially undeveloped wilderness areas and the Apalachicola National Forest occupies a large area in the northwest quadrant. The climate in Franklin County is relatively mild with mean annual temperatures in the upper sixties and average winter time temperatures about 48 to 50 degrees Fahrenheit ( F). Temperatures in the summer months average in the low 80s F, being moderated by sea breezes and frequent thunderstorms. Rainfall averages about 55 inches per year with the majority of the accumulation occurring in the months of July through September. Winds are generally southerly in summer months and northerly in winter months (U.S. Department of Commerce, 1978). 2.3 Principal Flood Problems General flooding in Franklin County stems from two sources: periods of intense rainfall causing ponding and sheet runoff in the low, poorly-drained areas and coastal flooding associated with hurricanes and tropical storms. The floodplains 4

9 of the Apalachicola River, the New River, the Crooked River, the Carrabelle River, and the Ochlockonee River are also subject to flooding during high river stages. The floodplains of the Apalachicola River are subject to riverine flooding during periods of heavy rainfall. As mentioned previously, the Apalachicola River is part of an extensive river system whose drainage area extends northward about five hundred miles to a point near the northern Georgia border, and encompasses an area over 19,000 square miles. Other rivers in the county have smaller drainage areas and are therefore less significant sources of flooding. These include the New and Crooked Rivers, which flow through the central portion of the county and join to form the Carrabelle River, which then discharges into St. George Sound at Carrabelle. The Ochlockonee River forms a portion of the northeast county boundary and empties into the Gulf of Mexico through Ochlockonee Bay. Low-lying, poorly drained areas of the county are also subject to rainfall ponding. Franklin County is subject to coastal flooding caused by extra tropical cyclones and hurricanes. Extra tropical cyclones can occur at any time of the year but are more prevalent in the winter. The prime hurricane season is from August to October during which time 80 percent of all hurricanes occur. September is the worst month for hurricanes during which 32 percent of the total occur. Hurricanes are of shorter duration than northeasters and generally last through only one tidal cycle. In meteorological terms, a hurricane is defined as a tropical cyclone which has a central barometric pressure of 29 inches or less of mercury, and wind velocities of 75 miles per hour or more. The low barometric pressures and high winds combine to produce abnormally high tides and accompanying tidal flooding. The high winds can generate large waves, provided there are no obstructions or barrier beaches to dissipate wave momentum. The winds of a hurricane in the Northern Hemisphere spiral inward in a counterclockwise direction towards the "eye" or center of low pressure. The eye of the hurricane (where winds are subdued) can vary in diameter. Normally, the "eye" can extend for 15 miles, although the eye of a mature hurricane can reach diameters of 20 to 30 miles or even greater. A hurricane develops as a tropical storm either near the Cape Verde Islands off the African coast or in the western Caribbean Sea. Most hurricanes which reach northwestern Florida approach from a southerly direction after crossing the Florida peninsula, the island of Cuba, or the western Gulf of Mexico. These hurricanes start their journey northward with a forward speed of about 10 miles per hour. The most destructive winds in a hurricane occur east of the eye, where the spiral wind movement and forward motion of the storm combine. Several past hurricanes have tracked over the Florida Panhandle; therefore, Franklin County is prone to experience the full intensity of a major hurricane. In order for Franklin 5

10 County to experience the highest winds and accompanying highest tides of a hurricane, the storm would need to track west of the county. Historical data indicates that several hurricanes have had significant impact on Franklin County, since 1972 on June 19, 1972 (Agnes), on August 31, 1985 (Elena), on November 21, 1985 (Kate), on September 3, 1998 (Earl), and on July 10, 2005 (Dennis). Data provided by the Florida Department of Environmental Protection, regarding these storms, is summarized below: Hurricane Agnes, in 1972, made landfall west of Cape San Blas, in Gulf County, with peak winds reaching 55 mph at Apalachicola. Despite being a Category One hurricane, the storm surge affecting Franklin County is estimated to have been approximately 8 feet at St. Marks. Beach and dune erosion was significant along the entire open coast of Jefferson County, with breaches occurring on the Marsh Islands. Hurricane Elena, in 1985, made two passes offshore of Jefferson County before making landfall in Mississippi. Wind damage associated with Hurricane Elena was limited to shoreline areas of Jefferson County; however, the accompanying storm surge, of approximately 8 to 9 feet at St. Marks, resulted in damage to shorefront protection structures and buildings. Hurricane Kate, in 1985, made landfall at Mexico Beach, in Gulf County, with peak winds reaching 85 mph at Apalachicola, just 2 months after Hurricane Elena. The storm surge affecting Jefferson County is estimated to have been approximately 8.4 feet at Shell Point. Land falling wind and waves, associated with Hurricane Kate, resulted in the destruction of 46 buildings and damage to 15 more. Hurricane Earl, in 1998, made landfall in Panama City Beach in Bay County. In Jefferson County, the storm surge was approximately 8 feet at St. Marks. Shorefront erosion resulted in damage to the Marsh Islands. Hurricane Dennis, in 2005, made landfall on Santa Rosa Island, between Navarre Beach and Pensacola Beach, in Escambia County. Although well westward of Jefferson County, this hurricane produced a storm surge of 6 to 9 feet in Apalachee Bay and 7.5 feet at the mouth of the Aucilla River. High waves, associated with Hurricane Dennis resulted in beach erosion to open coast areas of both Franklin County and Jefferson County, with approximately 37 buildings sustaining damage in Jefferson County. Coastal flooding is not limited to hurricane activity; in fact, extra tropical cyclones, have resulted in significant tidal flooding along the Florida panhandle. Extra tropical cyclones can develop in the Gulf of Mexico and along strong frontal boundaries and can potentially occur at any time of year, but most frequently in the winter and spring months. Typically, these storms have centers that are colder than the surrounding air, with strongest winds in the upper atmosphere, and lower wind velocities and higher central pressures than a major 6

11 hurricane; however, wind velocities associated with an extra tropical cyclone can easily reach tropical storm and Category 1 hurricane levels. In addition, the high winds of an extra tropical cyclone can last for several days, causing repeated flooding and excessive coastal erosion. The long exposure of property to high water, high winds, and pounding wave action can result severe property damage. 2.4 Flood Protection Measures Franklin County does not have any flood protection measures designed and constructed specifically for flood protection. The U.S. Army Corps of Engineers (USACE) designed and built the Jim Woodruff Lock and Dam, which is located north of Franklin County on the Apalachicola River at the Florida/Georgia state line and approximately 108 miles north of the mouth. Construction of this dam was initiated in September 1947, and the impounding of water occurred in May Although the Jim Woodruff Dam was primarily designed for navigation purposes, it does offer a limited amount of flood regulation of the Apalachicola River. Because of the dam s geographical location, it provides minimal flood protection for Franklin County. The Jackson Bluff Dam on Lake Talquin (Ochlockonee River) is a hydroelectric installation operated by the Florida Power Corporation. This project was completed in 1930, and offers no appreciable flood control for properties located downstream. The coastal areas of Franklin County are, for the most part, surrounded by barrier islands. St. George Island and Little St. George Island, for example, offer some protection to the coastal area along St. George Sound and Apalachicola Bay from wave action. It is expected, however, that portions of the barrier islands would be overtopped during the larger storm events. In 1973, the state of Florida established a Coastal Construction Control Line that now includes the coastal beaches of St. George Island, Dog Island, and Alligator Point. The purpose of this line is to control coastal land use and building construction methodology for areas susceptible to direct storm surge, erosion and wave runup. 3.0 ENGINEERING METHODS For the flooding sources studied in detail in the county, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this FIS. Flood events of a magnitude which 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 which equals or exceeds the 100-year flood (1-percent chance of annual exceedence) in any 50-year period is approximately 40 7

12 percent (4 in 10), and, 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 county at the time of completion of this FIS. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Riverine Hydrologic Analyses Initial Countywide Analysis Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for the flooding sources studied in detail in Franklin County. The flows of the required frequencies for the Apalachicola River, in the City of Apalachicola, were based on statistical analyses of discharge records covering the twenty-year period taken from the Bloutstown, Florida gage (No ) on the Apalachicola River. This statistical analysis is the Log-Pearson Type III Method recommended by the Water Resources Council (Water Resources Council, 1976). For locations where no discharge records are available, or where discharge records are not of sufficient length to yield reliable results from statistical analysis, the gage analyses were extrapolated based on increases in drainage area. The extrapolated flows downstream of Gage No were adjusted to account for the Chipola Cutoff and the Intracoastal Waterway-Lake Wimico-Jackson River System. Revised Countywide Analysis Existing data developed by the NFWMD for the area in the vicinity of Eastpoint has been incorporated into this FIS. Hydrologic analysis was performed using the Environmental Protection Agency s Storm Water Management Model (SWMM) version 5 (NFWMD, 2010). Existing data developed by Engineering Methods & Applications, Inc. for St. James Bay has also been incorporated into this FIS. Soil Conservation Service (SCS) curve number-based hydrologic analysis was performed for this study (Engineering Methods & Applications, Inc., 2001). A summary of the drainage area-peak discharge relationships for all the streams studied by detailed methods is shown in Table 1, "Summary of Discharges." 8

13 TABLE 1 - SUMMARY OF DISCHARGES FLOODING SOURCE AND LOCATION DRAINAGE AREA (sq. miles) PEAK DISCHARGES (cfs) 10-YEAR 50-YEAR 100-YEAR 500-YEAR APALACHICOLA RIVER At Apalachicola Bay 18, , , , ,150 At confluence of Brothers River 18, , , , ,485 OCHLOCKONEE RIVER 1 At mouth 2,000 31,000 59,000 74, ,000 1 This information has been superseded by the most recent coastal restudy. 3.2 Riverine Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the source 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 tables in the FIS report. For construction and/or floodplain management purposes, users are encouraged to use the flood elevation data presented in this FIS in conjunction with data shown on the FIRM. Initial Countywide Analysis Cross sections for the water-surface elevation analyses of the Apalachicola River were obtained by aerial survey methods from photography flown in 1979 for upland areas and by field measurement below the water surface. Bridges were field checked to confirm elevation data and structural geometry. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the FIRM (Exhibit 2). Flood profiles were drawn showing computed water-surface elevations for floods of the selected recurrence intervals. Channel roughness factors (the "n" factor for Manning's formula) used in the hydraulic computations were chosen based on field observations of the streams and floodplain areas. This measure of roughness for the main channel of the Apalachicola River ranges from to with floodplain roughness values ranging from to The acceptability of the above hydraulic factors, cross sections, and hydraulic structure data was checked using these computations and comparing the result of known historic storms and the resulting flood elevations. 9

14 Water-surface elevations of floods of the selected recurrence intervals for the Apalachicola River were computed using the USACE HEC-2 step backwater computer program (USACE, 1976). Starting water-surface elevations at the mouth of the Apalachicola River used in these calculations were determined using the slope/area method. Revised Countywide Analysis The hydraulic analyses for this FIS were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. Existing data developed by the NFWMD for the area in the vicinity of Eastpoint has been incorporated into this FIS. One-percent annual chance base flood elevations (BFEs) were established using the Environmental Protection Agency s Storm Water Management Model (SWMM) version 5 (NFWMD, 2010). The 1- percent annual chance floodplain was delineated to be in agreement with the results of the storm surge analysis in this area. Existing data developed by Engineering Methods & Applications, Inc. for St. James Bay has also been incorporated into this FIS. A hydraulic analysis was performed using the ICPR model version 2.x (Engineering Methods & Applications, Inc., 2001). All qualifying bench marks within a given jurisdiction that are cataloged by the National Geodetic Survey (NGS) and entered into the National Spatial Reference System (NSRS) as First or Second Order Vertical and have a vertical stability classification of A, B, or C are shown and labeled on the FIRM with their 6- character NSRS Permanent Identifier. Bench marks cataloged by the NGS and entered into the NSRS vary widely in vertical stability classification. NSRS vertical stability classifications are as follows: Stability A: Monuments of the most reliable nature, expected to hold position/elevation well (e.g., mounted in bedrock) Stability B: Monuments which generally hold their position/elevation well (e.g., concrete bridge abutment) Stability C: Monuments which may be affected by surface ground movements (e.g., concrete monument below frost line) Stability D: Mark of questionable or unknown vertical stability (e.g., concrete monument above frost line, or steel witness post) In addition to NSRS bench marks, the FIRM may also show vertical control monuments established by a local jurisdiction; these monuments will be shown on 10

15 the FIRM with the appropriate designations. Local monuments will only be placed on the FIRM if the community has requested that they be included, and if the monuments meet the aforementioned NSRS inclusion criteria. To obtain current elevation, description, and/or location information for bench marks shown on the FIRM for this jurisdiction, please contact the Information Services Branch of the NGS at (301) , or visit their Web site at It is important to note that temporary vertical monuments are often established during the preparation of a flood hazard analysis for the purpose of establishing local vertical control. Although these monuments are not shown on the FIRM, they may be found in the Technical Support Data Notebook associated with this FIS and FIRM. Interested individuals may contact FEMA to access this data. 3.3 Coastal Hydrologic Analyses For areas subject to tidal inundation, the 10-, 2-, 1-, and 0.2-percent-annualchance stillwater elevations and delineations were taken directly from a detailed storm surge study documented in the Technical Support Data Notebook (TSDN) for the Northwest Florida Water Management District coastal flood hazard study for Franklin, Wakulla, and Jefferson Counties. The Advanced Circulation model for Coastal Ocean Hydrodynamics (ADCIRC), (Luettich, 1992), developed by the USACE was selected to develop the stillwater elevations or storm surge for northwest Florida s Franklin, Wakulla, and Jefferson Counties. ADCIRC uses an unstructured grid and is a finite-element long wave model. ADCIRC has the capability to simulate tidal circulation and storm surge propagation over large areas and is able to provide highly detailed resolution along the shorelines and areas of interest along the open coast and inland bays. It solves three dimensional equations of motion, including tidal potential, Coriolis, and nonlinear terms of the governing equations. The model is formulated from the depth averaged shallow water equations for conservation of mass and momentum which results in the generalized wave continuity equation. The model has the capability to simulate tidal circulation and storm surge propagation over large domains and is able to provide highly detailed resolution along the shoreline and other areas of interest. The coastal wave model Simulating Waves Nearshore (SWAN) developed by Delft University in the Netherlands is used to calculate the nearshore wave fields required for the addition of wave setup effects. This numerical model is a thirdgeneration (phase-averaged) wave model for the simulation of waves in waters of extreme, intermediate, and finite depths. Model characteristics include the capping of the atmospheric drag coefficient, dynamic adjustment of bathymetry for changing water levels, and specification of the required save points. Three nested grids are used to obtain sufficient nearshore resolution to represent the radiation stress gradients required as ADCIRC inputs. Radiation stress fields output from the SWAN inner grids are used by ADCIRC to estimate the 11

16 contribution of breaking waves (wave setup effects) to the total storm surge water level. In order to model storm surge and wave fields using ADCIRC and SWAN, wind and pressure fields are required for input. A model called the Planetary Boundary Layer model (PBL) (Cardone, 1992), uses the parameters from a hurricane or storm to simulate the event and develop wind and pressure fields. The PBL model simulates hurricane induced wind and pressure fields by applying the vertically integrated equations of motion. Oceanweather Inc. provided support to run the PBL model and provide wind and pressure fields for each of the selected storms events. The Joint Probability Method (JPM) was used to develop the stillwater frequency curves for the 10-, 2-, 1-, and 0.2-percent-annual-chance stillwater elevations. The JPM application was not originally named as such (Russell, 1968). The JPM approach is a simulation methodology that relies on the development of statistical distributions of key hurricane input variables such as central pressure, radius to maximum wind speed, maximum wind speed, translation speed, track heading, and sampling from these distributions to develop model hurricanes. The resulting simulation results in a family of modeled storms that preserve the relationships between the various input model components, but provides a means to model the effects and probabilities of storms that historically have not occurred. An ADCIRC finite element mesh was created to determine inundation extents and depths due to hurricane storm surge in northwest Florida s Franklin, Wakulla, and Jefferson Counties. The offshore portion of the mesh covers the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico west of 60 o West Longitude. This offshore portion is adapted from a proven existing mesh (Hagen, 2006). The inland portion of the mesh was extended to floodplain areas of Franklin, Wakulla, and Jefferson Counties and refined with node spacing ranging from meters to meters. The inland bathymetry portion of the ADCIRC mesh was populated with datasets taken from National Ocean Service (NOS) and USACE Surveys, HEC- RAS one-dimensional river cross-sections, NOAA nautical charts, and NWFWMD field knowledge. Bathymetry for most of the bays and northeastern Gulf of Mexico was constructed from the National Geophysical Data Center's (NGDC) Coastal Relief Model and USACE channel surveys. A portion of the northern Apalachee Bay was constructed from NOS Surveys, NOAA nautical chart data, and USACE channel surveys. Further offshore, the mesh restrains its original node elevations as detailed in Hagen, The topographic portion of the ADCIRC mesh was populated with topographic LiDAR (Light Detection and Ranging) data along with five non-lidar terrain datasets. LiDAR data was available for most of the study area with the exception of small portions on the western boundary of Franklin County and the eastern boundary of Jefferson County. LiDAR data for Franklin County was collected between May and August of 2007 as part of the Florida Department of Emergency Management's mapping program. For all other areas, non-lidar terrain datasets were downloaded from the USGS National Map Seamless Server, National Elevation Dataset. A shoreline was manually digitized referencing the 12

17 LiDAR data and 2007 aerial photos to define change between water and land elements. The ADCIRC model mesh includes other features, such as floodplain boundaries, rivers, roads, ridges and valleys. The final mesh includes approximately 2,250 square miles of floodplain area with 869,000 total computational nodes. The horizontal datum for the mesh is North American Datum (NAD) 1983, Geographic Coordinate System. The vertical datum is referenced to the North American Vertical Datum 1988 (NAVD 88) in units of meters. A land cover dataset assembled by the Florida Fish and Wildlife Commission (FWC) specifically to describe the diversity and distribution of vegetation within the state of Florida was used to define Manning s n values for bottom roughness coefficients input at each node in the mesh. Model validation, which tests the model hydraulics and ability to reproduce events, was performed against Hurricanes Agnes (1972), Kate (1985), Opal (1995) and Dennis (2005). Simulated water levels for each event were compared to High Water Mark (HWM) data supplied by FEMA and historic reports and hydrograph data supplied by NWFWMD and NOAA. The SWAN model, used to calculate the wave setup component, uses ocean bathymetry and coastal topography taken from two sources, the National Geophysical Data Center (NGDC GEODAS data set) and the NWFWMD ADCIRC Grid, which incorporated the high resolution LiDAR survey data reported on elsewhere. The coastal bathymetry data merged both the NGDC and ADCIRC Grid data to more accurately represent the topography over the land. The topography data was interpolated from the LiDAR data used to form the ADCIRC grid. At locations farther inland than ADCIRC grid, the NGDC dataset was used. The SWAN model was implemented on a set of nested grids, with resolutions ranging from 10 kilometers down to approximately 160 meters. The model is forced with the same wind and pressure fields from Oceanweather Inc. Hurricanes Kate (1986), Opal (1995), and Dennis (2005) were used to validate the SWAN model. Modeled wave heights were compared to available historic wave data from NOAA wave buoys. Statistical Analyses Due to the excessive number of simulations required for the traditional JPM method, the Joint Probability Method-Optimum Sampling (JPM-OS) was utilized to determine the stillwater elevations associated with tropical events. JPM-OS is a modification of the JPM method developed cooperatively by FEMA and the USACE for Mississippi and Louisiana coastal flood studies that were being performed simultaneously, and is intended to minimize the number of synthetic storms that are needed as input to the ADCIRC model. The methodology entails sampling from a distribution of model storm parameters (e.g., central pressure, radius to maximum wind speed, maximum wind speed, translation speed, and track heading) whose statistical properties are consistent with historical storms impacting the region, but whose detailed tracks differ. 13

18 Production runs were carried out with SWAN and ADCIRC on a set of hypothetical storm tracks and storm parameters in order to obtain the maximum water levels for input to the statistical analysis. A total of 159 individual storms with different tracks and various combinations of the storm parameters were chosen for the production run set of synthetic hurricane simulations. Each storm was run for at least 4 days of simulation and did not include tidal forcing. Wind and pressure fields obtained from the PBL model and wave radiation stress from the SWAN model were input to the ADCIRC model for each production storm. All stillwater results for this study include the effects of wave setup; stillwater. Stillwater Elevations The results of the ADCIRC model, as described above, provided stillwater elevations, including wave setup effects that are statistically analyzed to produce probability curves. The JPM-OS is applied to obtain the return periods associated with tropical storm events. The approach involves assigning statistical weights to each of the simulated storms and generating the flood hazard curves using these statistical weights. The statistical weights are chosen so that the effective probability distributions associated with the selected greater and lesser storm populations reproduce the modeled statistical distributions derived from all historical storms. All of the 869,000 ADCIRC nodes were used as JPM output points. This provided the maximum resolution and provided detailed coverage in Franklin County. At each JPM point, the surge elevations obtained from the standard ADCIRC output files for each of the 159 storms and the annual recurrence rates for each storm were used as input of JPM-OS method. The final result was surge elevations at each JPM point for each recurrence rate. The stillwater elevations have been determined for the 10-, 2-, 1-, and 0.2-percent annual chance floods for the flooding sources studied by detailed methods and are summarized in Table 2, "Summary of Stillwater Elevations." 14

19 FLOODING SOURCE AND LOCATION GULF OF MEXICO South shoreline of St. Vincent Island Shoreline from west end point of St. George Island to Cape St. George Shoreline from Cape St. George to Government Cut South shoreline of St. George Island from Government Cut to St. George Island State Park South shoreline of St. George Island from St. George Island State Park to Dr. Julian G. Bruce St. George Island State Park South shoreline of St. George Island from Dr. Julian G. Bruce St. George Island State Park to east end point of St. George Island TABLE 2 - SUMMARY OF STILLWATER ELEVATIONS ELEVATION (feet 1 NAVD88*) 10-PERCENT 2-PERCENT 1-PERCENT 0.2-PERCENT South shoreline of Dog Island North shoreline of St. George Island from east end point of St. George Island to East Cove North shoreline of St. George Island from East Cove to State Co. Rd 300 Shoreline from Eastpoint to Carrabelle Thompson Airport Shoreline from Ho Hum RV Park to Florida State University Coastal and Marine Laboratory End of Peninsula Point to the shoreline adjacent to the end of Alligator Drive *North American Vertical Datum of Includes wave setup 15

20 TABLE 2 - SUMMARY OF STILLWATER ELEVATIONS - continued FLOODING SOURCE AND LOCATION GULF OF MEXICO, cont. Shoreline adjacent to the intersection of Alligator Drive and Mardi Gras Lane to the shoreline adjacent to the intersection Alligator Drive and Pelican Street Shoreline adjacent to the intersection of Alligator Drive and Pelican Street to the shoreline at the end of Gulf Shore Boulevard Shoreline at the end of Gulf Shore Boulevard to the shoreline at the end of Tarpon Street Shoreline at the end of Tarpon Street to Bald Point Beach (Ochlockonee Bay) ELEVATION (feet 1 NAVD88*) 10-PERCENT 2-PERCENT 1-PERCENT 0.2-PERCENT SAINT GEORGE SOUND North shoreline of Dog Island Shoreline from Carrabelle Thompson Airport to Ho Hum RV Park St. Teresa shoreline from Turkey Point (St. George Sound) to 2.6 miles ENE of Turkey Point (St. George Sound) St. Teresa shoreline from 2.6 miles ENE of Turkey Point (St. George Sound) to Alligator Harbor APALACHICOLA BAY North shoreline of St. George Island from State Co. Rd 300 to Government Cut *North American Vertical Datum of Includes wave setup 16

21 TABLE 2 - SUMMARY OF STILLWATER ELEVATIONS - continued FLOODING SOURCE AND LOCATION APALACHICOLA BAY, cont. North shoreline of St. George Island from Government Cut to west end point of St. George Island Shoreline from Apalachicola Municipal Airport to entrance of Little Bay Shoreline from east end of John Corrie Memorial Bridge to State Co. Rd 300 ELEVATION (feet 1 NAVD88*) 10-PERCENT 2-PERCENT 1-PERCENT 0.2-PERCENT SAINT VINCENT SOUND North shoreline of St. Vincent Island Shoreline from County boundary to Apalachicola Municipal Airport EAST BAY Shoreline from entrance of Little Bay to east end of John Corrie Memorial Bridge ALLIGATOR HARBOR Entire WSW facing shoreline on the Franklin County mainland Entire north facing shoreline along the peninsula OCKLOCKONEE BAY Bald Point Beach (Gulf of Mexico) to Ocklockonee Bay Bridge Shoreline from Ocklockonee Bay Bridge to Wakulla County Line *North American Vertical Datum of Includes wave setup 17

22 3.4 Coastal Hydraulic Analyses Areas of coastline subject to significant wave attack are referred to as coastal high hazard zones. The USACE has established the 3.0-foot breaking wave as the criterion for identifying the limit of coastal high hazard zones (USACE, 1975). The 3.0-foot wave has been determined as the minimum size wave capable of causing major damage to conventional wood frame and brick veneer structures. Figure 1, Transect Schematic, illustrates a profile for a typical transect along with the effects of energy dissipation and regeneration on a wave as it moves inland. This figure shows the wave crest elevations being decreased by obstructions, such as buildings, vegetation, and rising ground elevations, and being increased by open, unobstructed wind fetches. The figure also illustrates the relationship between the local still water elevation, the ground profile and the location of the V/A boundary. This inland limit of the coastal high hazard area is delineated to ensure that adequate insurance rates apply and appropriate construction standards are imposed, should local agencies permit building in this coastal high hazard area. Figure 1: Transect Schematic For Franklin County the deepwater wave conditions associated with the 1-percent annual chance storm were developed using the Simulating WAves Nearshore (SWAN) model results. The outputs from the model production runs provided wave heights and periods to determine the wave heights associated with the 1- percent annual chance flood level. For each of the production runs, the maximum wave heights achieved at each grid point were put into files, as well as the average wave periods associated with the time when the maximum waves occurred. Then the wave heights at each of 596,000 coastal wave grid points were rank ordered. Using the probability of each storm, the 1-percent annual chance flood thresholds were determined, so the wave periods associated with the wave heights were determined afterwards. This technique gave a least squares best fit linear relationship between the flood levels from each storm and the wave heights for each storm. 18

23 FEMA guidelines for V Zone mapping define H as the significant wave height or s the average over the highest one third of waves and T as the significant wave s period associated with the significant wave height. Mean wave conditions are described as: H = H s T = T 0.85 s where H is the average wave height of all waves and T is the average wave period. 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, transects 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. Transects are shown on the FIRM panels for Franklin County. The transect profiles were obtained using bathymetric and topographic data from various sources. The topographic dataset was comprised of LiDAR data provided by the NWFWMD. LiDAR data was collected in July 2007 in leaf-off conditions, and delivered in ESRI multipoint format in November Data, as delivered, was in the North American Datum (NAD) of 1983, projected to Florida HARN State Plane coordinates, North Zone, in units of feet. The vertical datum was relative to North American Vertical Datum (NAVD) of 1988 in units of feet. The LiDAR mass point dataset had a nominal point spacing of 0.7 meters (2.3 feet), with a horizontal accuracy of 3.8-foot, and a vertical accuracy of 9.14 cm RMSEz. This data fully meets and exceeds the accuracy standards of FEMA specifications, and should meet the expectations for an accurate, high quality digital terrain product. The bathymetric dataset for Franklin County was processed and provided by the University of Central Florida during April Bathymetry for most of the bays and northeastern Gulf of Mexico consisted of NOAA National Ocean Service (NOS) hydrographic surveys, NOAA National Geophysical Data Center (NGDC) Coastal Relief Model, NOAA nautical chart data, and USACE navigation channel surveys. Data, as delivered, were in grid format with various grid spacing, in the NAD of 1983, projected to Florida State Plane coordinates, North Zone, in units of feet. The vertical datum was relative to NAVD of 1988 in units of meters. The inland bathymetry in the Apalachicola and Carabelle/Ochlockonee areas consisted of NOS Surveys, USACE navigation channel surveys, HEC-RAS onedimensional river cross-sections, NOAA nautical charts, and NWFWMD field knowledge. Data, as delivered, were in grid format with various grid spacing, in the NAD of 1983, projected to Florida State Plane coordinates, North Zone, in 19

24 units of feet. The vertical datum was relative to NAVD of 1988 in units of meters. The bathymetric dataset s depths were converted from meters to feet using a factor of Data were then reprojected to the NAD83 FL HARN State Plane North zone coordinate system in units of feet, in agreement with the topographic dataset. Where surveys overlapped, the older survey data were removed. To facilitate use of the bathymetric data to build a seamless digital elevation model, the ESRI shapefile-format point data were converted to three-dimensional feature class, and then to ASCII format dataset. Finally, the bathymetric data were ready to be merged with the topographic data-multipoint feature class. To facilitate floodplain analysis, the provided datasets were processed into a digital elevation model (DEM). The recently developed ESRI Terrain modeling framework is considered to be the most efficient data format to create terrain, and was utilized for this study. First, a file geodatabase was created to contain the topographic and bathymetric dataset and allow generation of the terrain model. A pre-determined coverage shapefile was loaded into the database to serve as the study area boundary. Next, a shoreline vector was also loaded as a hard line feature class with an assigned zero-elevation, in order to enforce the shoreline feature in the terrain dataset. The terrain was then created by combining the topographic and bathymetric multipoint files, zero-elevation shoreline vector and study-area boundary. The completed terrain dataset was generated with an average point spacing of 10 feet. The terrain was then converted directly to the final seamless DEM in order to support the overland wave modeling and coastal hazard mapping. Storm-induced beach erosion is well documented along the Gulf of Mexico coastlines of Franklin County. Review of the literature showed that the standard FEMA (2003 and 2007) Guidelines and Specifications for Flood Hazard Mapping Partners methodology were applicable for the Gulf coast of Franklin County. Where dunes were identified and delineated, the VE Zone was mapped up to the extent of the Primary Frontal Dune (PFD). Nearshore wave-induced processes, such as wave setup and wave runup, constitute a greater part of the combined wave envelope than storm surge due to the coast exposure to ocean waves. For this study the wave setup was included in the storm surge modeling results. RUNUP 2.0 was used to predict wave runup value on natural shore and then adjusted to follow the FEMA (2005) Procedure Memorandum No. 37 that recommends the use of the 2% wave runup for determining base flood elevations. For wave run-up at the crest of a slope that transitions to a plateau or downslope, run-up values were determined using the Methodology for wave run-up on a hypothetical slope as described in the FEMA (2007) Guidelines and Specifications for Flood Hazard Mapping Partners. Wave height calculation used in this study follows the methodology described in FEMA s Guidelines and Specifications for Flood Hazard Mapping Partners (2003 and 2007). The Wave Height Analysis for Flood Insurance Studies 20