Florida Watershed Journal Florida Coastal DFIRMs : Page 1

SPRING 2011 Volume 4 • Issue 2 1 An Overview of a FEMA Coastal Inundation Study for the Big Bend Region of Florida Jeff Gangai, CFM Scott C. Hagen, Ph.D., P.E., D. CE, D. WRE Ron Bartel 4 A Digital Elevation Model for Franklin, Wakulla, and Jefferson Counties Florida David Coggin Scott C. Hagen, Ph.D., P.E., D. CE, D. WRE Michael B. Salisbury, P.E. 11 Unstructured Mesh Development for the Big Bend Region (Florida) Michael B. Salisbury, P.E. Scott C. Hagen, Ph.D., P.E., D. CE, D. WRE David Coggin Peter Bacopoulos, Ph.D. John Atkinson, Ph.D. Hugh Roberts, P.E. An Overview of a FEMA Coastal Inundation Study for the Big Bend Region of Florida Jeff Gangai, CFM Management & Consulting Services Division Dewberry Fairfax, VA GANGAI 15 Wave Modeling for the East Florida Panhandle Flood Study Donald N. Slinn, Ph.D. M. C. Robinson Go Fujita Lynda C. Charles Scott C. Hagen, Ph.D., P.E., D. CE, D. WRE Civil, Environmental, and Construction Engineering Department University of Central Florida Orlando, FL Ron Bartel HAGEN 22 Deriving Frictional Parameters and Performing Historical Validation for an ADCIRC Storm Surge Model of the Florida Gulf Coast John Atkinson, Ph.D. Hugh Roberts, P.E. Scott C. Hagen, Ph.D., P.E., D. CE, D. WRE Shan Zou, Ph.D. Peter Bacopoulos, Ph.D. Stephen Medeiros, P.E. John Weishampel, Ph.D. Zach Cobell Resource Management Division Northwest Florida Water Management District Havana, FL 28 Production Runs for the Big Bend Region of Florida Gabriel R. Toro, Ph.D. Scott C. Hagen, Ph.D., P.E., D. CE, D. WRE John Atkinson, Ph.D. Chris Reed, Ph.D. he Federal Emer-gency Management Agency (FEMA) has embarked on an ambitious effort to re-evaluate all of BARTEL the nation’s coastal flood hazards for the purpose of updating Flood In-surance Rate Maps (FIRMs). These coastal study updates will include storm-surge modeling, overland-wave-height propagation analysis, and coastal-hazard mapping. Northwest Florida Water Management District (NWFWMD) is a Corporative Technical Partner (CTP) for FEMA and is one of the first to perform a new coastal study. The overall process is divided into two phases: the storm surge analysis with overland wave height analyses and mapping of the FIRMs. The following issue will focus on the entire process of coastal inundation assessment for the Florida Big Bend region, namely Franklin, Wakulla, and Jefferson Counties. T BACKGROUND The National Flood Insurance Program (NFIP) depicts the flood risk, in the form of a FIRM, which is provided by FEMA to communities participating in the program. Local officials use the FIRM for permitting development and insurance personnel use the FIRM for developing a premium rating for insured property. The dual functions served by the maps for regulating floodplain development and determining insurance rates merit periodic reviews of their precision in depicting flood hazards. Often this review takes place in the wake of a major flood event, when primary evidence of flooding patterns is readily available and economic resources may be available to update the maps. Hurricane Ivan and Katrina were two such events along the Gulf coast that indicated the real need to update the coastal hazard mapping and FIRMs along the Gulf region as well as the nation. Throughout the last 30 years FEMA has made many improvements and added additional components to a coastal flood hazard analysis. With the inclusion of wave heights in 1979, Flood Insurance Studies (FISs) were updated between 1981 and 1984 to include additional flooding hazards from Continued on page 4

An Overview Of A FEMA Coastal Inundation Study For The Big Bend Region Of Florida

The Federal Emergency Management Agency (FEMA) has embarked on an ambitious effort to re-evaluate all of the nation’s coastal flood hazards for the purpose of updating Flood Insurance Rate Maps (FIRMs). These coastal study updates will include storm-surge modeling, overland-wave-height propagation analysis, and coastal-hazard mapping. Northwest Florida Water Management District (NWFWMD) is a Corporative Technical Partner (CTP) for FEMA and is one of the first to perform a new coastal study. The overall process is divided into two phases: the storm surge analysis with overland wave height analyses and mapping of the FIRMs. The following issue will focus on the entire process of coastal inundation assessment for the Florida Big Bend region, namely Franklin, Wakulla, and Jefferson Counties.<br /> <br /> BACKGROUND <br /> <br /> The National Flood Insurance Program (NFIP) depicts the flood risk, in the form of a FIRM, which is provided by FEMA to communities participating in the program. Local officials use the FIRM for permitting development and insurance personnel use the FIRM for developing a premium rating for insured property. The dual functions served by the maps for regulating floodplain development and determining insurance rates merit periodic reviews of their precision in depicting flood hazards. Often this review takes place in the wake of a major flood event, when primary evidence of flooding patterns is readily available and economic resources may be available to update the maps. Hurricane Ivan and Katrina were two such events along the Gulf coast that indicated the real need to update the coastal hazard mapping and FIRMs along the Gulf region as well as the nation.<br /> <br /> Throughout the last 30 years FEMA has made many improvements and added additional components to a coastal flood hazard analysis. With the inclusion of wave heights in 1979, Flood Insurance Studies (FISs) were updated between 1981 and 1984 to include additional flooding hazards from Waves, including overland wave propagation and wave runup. It was not until around 1988 that FEMA started adding dune erosion and the definition of the primary frontal dune (the requirement that the coastal high hazard area or VE Zone extends landward to the inland limit of the primary frontal dune) to coastal flood hazard studies. There have also been some revisions throughout the years on how wave runup is calculated and mapped; presently the 2% wave runup (runup exceeded by 2% of the runup crests during a storm) is being calculated and mapped (a change from the 1991 FEMA approach to map the mean wave runup value). Technology and models to simulate storm surge have improved greatly over the last 30 years and as a result of Hurricanes Katrina and Ivan, FEMA has adopted new models and procedures to simulate storm surge. These hurricanes have been one of the main forces behind FEMA’s present effort to update the nation’s coastal flood hazard mapping. The increases in coastal population and the number of people at risk in coastal areas over the last 30 years also contributed to the need for updated storm surge modeling and coastal hazard mapping.<br /> <br /> The predicted storm surge elevations and numerical modeling for Franklin and Wakulla Counties were computed during the 1980 study by Gee & Jensen, EAP, Inc. (FEMA 1983 and 1986). The storm surge elevations and modeling for Jefferson County were computed during a 1987 study by Gee & Jenson, EAP, Inc. (FEMA 1991). The coastal analysis has not been updated since this original study in Wakulla and Jefferson Counties; hence, no erosion analysis or determination of wave setup are included in the present FIRMs (FEMA 1986 and 1991).<br /> Dewberry and Davis, LLC revised the WHAFIS modeling in 1999 for Franklin County; however, this coastal revision did not include any updates to previously computed storm surge elevations and models.Also, this revision only included the barrier islands and not the inland bay areas. Wave setup, which was not calculated in the 1980 study, was estimated in the revision and varied by transect from 1.5 ft to 2.0 ft.<br /> Primary frontal dune mapping was also included in the revision to Franklin County (FEMA 1983).<br /> <br /> The 1980 surge study was conducted using the FEMA Coastal Surge Model based on the Joint Probability Method (JPM). FEMA’s recent report “Stillwater: FEMA Coastal Flood Hazard Analysis and Mapping Guidelines Focused Study Report” (FEMA 2005) recognizes the need to update storm-surge studies using models capable of providing better resolution of hydrodynamic processes and more accurate statistic results. Further, over 25 years of storm history have occurred since the original storm-surge statistics were established. An update is necessary taking into account that some of the most active and intense hurricane seasons in Florida have occurred over the past two decades, which would change statistical distributions with the benefit of a longer period of record. The Big Bend area of Florida, namely Franklin, Wakulla, and Jefferson have the potential for the highest storm surge values in the nation due to local geography. As a result these counties have some of the highest 1% (100-year return period) annual chance flood surge elevation mapping by FEMA.<br /> <br /> Additionally, updated bathymetric and topographic data, as well as aerial photography, are available for the project area. In 2006, the Florida Department of Emergency Management (FDEM) funded a project to update the coastal plain topographic data using LiDAR.These LiDAR data have been collected to the Category 5 inundation limit as computed by the Sea, Lake, and Overland Surges from Hurricane (SLOSH) model and will meet the specifications as required by FEMA for inclusion in the FIRMs.<br /> <br /> PROJECT TEAM <br /> <br /> The NWFWMD has put together a collaborative team made up of government agencies, academia, and private consulting firms to complete a coastal flood hazard study for Franklin, Wakulla, and Jefferson Counties in Florida. NWFWMD is the lead agency for these counties in managing and administrating the coastal floodplains for the NFIP. NWFWMD has partnered with FEMA to provide the resources to fund the study. The consultant firms of URS/Dewberry are providing technical oversight and helping to complete various components of the coastal study and are primarily responsible for producing the updated FIRMs for the counties. The University of Central Florida and its industry partners are responsible for all phases of the storm surge modeling. Slinn Engineering is conducting twodimensional wave modeling. Fugro defined the storm statistics. Finally, Oceanweather Inc. provided the metrological forcing, namely wind and pressure fields, for the modeling efforts.<br /> <br /> ISSUE DESCRIPTION <br /> <br /> The following special issue of the Florida Watershed Journal contains five articles that describe all phases of the coastal inundation modeling effort for the Big Bend region of Florida. The first article describes the assemblage of the digital elevation model and presents a unique vertical feature extraction routine. The second paper describes how the DEM and vertical features are used in constructing a high-resolution, finite-element mesh. A third article presents the wave calculation procedures and results. In the fourth paper the process of deriving frictional parameters is presented along with model validation. The issue is concluded with a fifth article describing the determining of synthetic storms and the subsequent implementing of the coastal inundation model in production runs.<br /> <br /> ACKNOWLEDGEMENTS <br /> <br /> This work was funded through the Federal Emergency Management Agency and the Northwest Florida Water Management District. Any views or opinions expressed in this introduction or any of the following articles are those of the authors and do not necessarily represent the views and/or opinions of the Federal Emergency Management Agency and the Northwest Florida Water Management District or any of their affiliates.<br /> <br /> REFERENCES<br /> <br /> • Federal Emergency Management Agency. (June 1986). Flood Insurance Study, Wakulla County, Florida, Unincorporated Areas.<br /> <br /> Washington, D.C.<br /> <br /> • Federal Emergency Management Agency. (July 1991). Flood Insurance Study, Jefferson County, Florida, Unincorporated Areas.<br /> <br /> Washington, D.C.<br /> <br /> • Federal Emergency Management Agency. (January 1983). Flood Insurance Study, Franklin County, Florida, Unincorporated Areas.<br /> <br /> Washington, D.C.<br /> <br /> • Federal Emergency Management Agency. (February 2005). Stillwater: FEMA Coastal Flood Hazard Analysis and Mapping Guidelines Focus Study Report, Phase 1 Summary Report. <br /> <br /> Washington, D.C.

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