Coastal Flood Hazard Mapping

Transcription

1 Coastal Flood Hazard Mapping Current Status and Update April 4, 2017

2 NCFMP Program Objectives Purpose: Develop, Maintain, and Disseminate current, accurate, digital flood hazard data for all of NC, to include: Update Flood hazard data, models and displays (approximately 40,000 miles / Review every 3 ½ years); Calculate the financial loss at the structure level; Calculate anticipated flood insurance rates / ranges; and, Provided real-time and forecasted inundation, impact and alerting (FIMAN). Flood hazard data and studies (BFEs and Impact): support: Determination of Required Flood Insurance Purchase; Calculation of Insurance Rating; and, County and municipal floodplain management. Since its inception, NCFMP has: Acquired additional accurate data (i.e. LiDAR-derived topography, building footprints, first floor elevations); Transitioned to database-derived, digital display of flood data, models and maps; Studied approximately 32,000 stream / coastal miles, and replaced all unnumbered A Zones with AE Zones (that have BFEs) Calculating impact / financial loss to properties from different flood elevations; and, Integrated data with FIMAN.

3 Coastal Flood Study Updates Summary All coastal flood studies and mapping were performed in accordance with FEMA standard coastal guidelines and specifications for flood hazard mapping partners for the Atlantic Basin and sealed by Licensed Professional Engineers and Land Surveyors in the State of North Carolina. Apply the latest state-of-the-art models capable of providing better resolution of coastal hydrodynamic processes, based on the latest FEMA methodologies and standards developed following Hurricane Katrina. Current effective / legacy coastal storm surge along the North Carolina coastline was determined by a FEMA study developed in 1981 (using JPM; the same as the updated modeling methodology). Study and mapping was applied to the existing shoreline characteristics, and wave and storm weather data available at the time of study, and in accordance with current Code of Federal Regulations (CFRs); updated mapping and flood hazards did not consider or analyze anticipated future sea levels or climate change. We Evaluate and update flood studies where appropriate every 3 1/2-years, subject to need and funding

4 Coastal Flood Study Updates Summary The surge model is one of many components used for developing the digital flood insurance rate maps (DFIRMs) for coastal areas. Other integral components: o Field transect survey and reconnaissance o Evaluation of the regulatory primary frontal dune (PFD) o Storm included erosion processes o Wave hazard modeling o Coastal flood hazard mapping All modeling and mapping for the updated studies are reviewed for compliance with standard engineering practices and FEMA specifications and standards by Licensed Professional Engineers (PE) in the State of North Carolina, including: o NCFMP o FEMA o Independent quality assurance/quality control (QA/QC) contractor PE s

5 FEMA Coastal Flood Hazards BFE on FIRM based on 4 components: Storm surge Wave setup Overland waves Wave runup Surge

6 Coastal Modeling and Mapping Storm Surge Modeling Overland Wave Hazard Modeling Mapping

7 Storm Surge Modeling Storm Surge Modeling Overland Wave Hazard Modeling Mapping

8 Storm Surge Modeling Process Collect Topo/Bathy Create Seamless DEM & Mesh Surge Model Tidal Calibration Select Validation Storms Surge Suite Validation Select Hurricanes for Water Levels Runs Compute Surge Suite Water Levels Return Period Analysis (Water levels statistics) Stillwater/Wave Setup Surface

9 Entire coastline restudied o Avoid discontinuities, maximize efficiency Integrated 2-D hydrodynamic and wave model o o NC Surge Study Approach Surge ADCIRC o Tidally and wind driven circulation o Hurricane surge and flooding Nearshore Waves - SWAN Combined extratropical and tropical surge o o Joint Probability Method - statistical method for tropical Empirical Simulation Technique (EST) statistical method for extratropical storm

10 NC Surge Modeling Team TECHNICAL ADVISORS Don Resio USACOE, ERDC Jessie Feyen NOAA Enrique Reyes ECU Dave Divokey WSC Spencer Rogers NC Sea Grant NCFMP John Dorman, Director Ken Ashe, Assistant Director Tom Cadwallader, Lead Engineer/PM Tom Langan, Engineer VA INTERFACE REGION III David Gunn VA DCR SC INTERFACE REGION IV Lisa Jones SC DNR FEMA REGION IV L. Algeo, Engineer Tucker Mahoney, Coastal Eng. Christina Lindemer, Coastal Eng. RENAISSANCE COMPUTING CENTER (RENCI) TEAM Ken Galuppi Team Leader COMPUTER SERVICES Sallie Wright Brian Blanton Lisa Stillwell Kevin Gamiel DATABASE/DEM DEVELOPMENT Scott Madry Hong Yi Helena Mitasonova - NCSU MODELING Rick Leuttich - UNC Crystal Fulcher Craig Mattock CONSULTANTS Peter Vickery ARA John McCormick CRT Jeff Hanson USACOE, Duck QA/QC PROGRAM MANAGEMENT Tara Aims Tim McCormick DEWBERY Chris Mack Team Leader Mike Hanson Assistant Team Leader QUALITY ASSURANCE/ QUALITY CONTROL Jeff Gangai Elena Drei-Horgan CONSULTANT Andrew Cox - OCEANWEATHER TECHNICAL SUPPORT Kevin Slover Jeff Sample

11 Need for Upgrade - Effective / Legacy Surge Model Limitations Surge modeling performed in 1981 Last FEMA Surge software update in 1988 Code is not being maintained or updated Surge model did not include wave setup Model resolution of 1-mile (1 point per nautical mile to represent land features and movement of water) Model did not extend far enough inland to reflect coastal influence from coastal surge (e.g. Jacksonville) Wind and pressure field data is low resolution (very few points to represent wind and pressure fields in model)

12 Need for Upgrade - Effective / Legacy Surge Model Limitations Topography and bathymetry are low / coarse resolution The legacy model can not simulate water piling up in inlets from wind-driven surge More comprehensive, higher resolution data Advances in coastal surge modeling methods and functions Significant Surge Events since 1980 (Emily, Fran, Isabel, Ophelia) New FEMA Coastal Mapping Guidelines (LIMWA and 2% Runup)

13 Coastal Flood Study Resulting Changes Overall, the coastal flood studies result in the following impacts to regulatory flood zones, buildings and National Flood Insurance Program insurance policies in North Carolina: o A seaward shift of the mapped regulatory PFD, and thus a subsequent reduction in the width of VE zones based on more detailed and updated topographic information o On average, regulatory base flood elevations (BFEs) are lower for the majority of coastal counties

14 Changes relative to effective study: Decrease Blue Increase Yellow/Red No Change - Grey

15 BFE Changes Building Impacts in Effective SFHA Net change in the regulatory BFE for structures in the effective coastal regulatory flood boundary (for the total of 94,332 buildings in the SFHA): o 58,154 (a lower BFE for 62% of total buildings) - average 2.0-ft lower o 23,682 (a higher BFE for 25% of total buildings) - average 1.5-ft higher o 12,496 (13% of total buildings) no change in BFE

16 BFE Changes - Flood Zones A net reduction in the number of buildings and NFIP policies in the regulatory coastal AE and VE flood zones including: o 14,882 buildings and 10,179 NFIP policies removed from the VE zone Statewide o 17,005 buildings and 8,681 NFIP policies removed from the AE zone Statewide

17 NC Coastal Flood Mapping Timeline

18 NCFMP: 6 Years of Coastal Outreach Coastal Regional Updates Meetings (April 2011): Three regional coastal meeting held prior to preliminary issuance to provide background and details on coastal modeling methods and draft storm surge modeling results, and training on coastal construction/development standards. (All 20 Coastal Counties) Flood Risk Review Meeting (Dec. 2015): six months prior to Preliminary issuance, share draft flood hazard risk data with local officials and staff, for the purpose of building awareness of the upcoming changes in flood risk, as well as provide communities extra time to develop their outreach strategies for areas showing an increased flood risk. (June 2016 Preliminary Issuance 6 Counties) Development Stakeholders Meetings (May/June 2016): prior to Preliminary Issuance, meet with local development community stakeholders (home builders, developers, bankers, insurance agents, etc.) to inform them of changing flood risk. (Carteret, Craven and Onslow) Pre-release Officials Meetings (May/June 2016): prior to Preliminary Issuance, meet with community officials and staff (elected officials, floodplain administrators, development review staff) to inform them of changing flood risk, and explain the post-preliminary and appeals process. (Carteret, Craven, Onslow and New Hanover) Preliminary DFIRM Public Officials Meetings (Sept. 2016): present the preliminary flood map and flood study to CEOs, floodplain administrators, and other officials for their review, discuss the results of the study/mapping project, and solicit community officials feedback on the information provided. (This is the only FEMA-required meeting 18 Counties Completed) Public Open House Meetings (Oct. 2016): provide an opportunity for citizens to learn about the Preliminary flood map and flood study, FEMA s expanded appeals process, floodplain management regulations and flood insurance, as well as ask questions and provide comments. (18 Counties Completed)

19 NCFMP Current Mapping Status Appeal period has ended Preliminary Fall 2017 Appeal period has not started

20 NCFMP Current Coastal Adjustments Sensitivity Analysis NCFMP is updating new riverine flood studies based on recently published new empirical regression equations from the United States Geological Survey for urban areas in the City of Jacksonville. NCFMP is performing sensitivity analyses of the tropical storm surge model to mesh resolution and more recent LiDAR-derived topographic data for the City of Jacksonville, Morehead City (Newport River area), and Surf City. The NCFMP has been actively sharing data, coordinating and meeting with Carteret County s consultant to assist in their review and possible revisions to the preliminary DFIRM maps.

21 Key Points of Discussion

22 Factors Resulting in BFE Changes New Coastal Surge Study; first update since early 1980 s Substantially more detailed terrain (NC LiDAR vs. USGS Quads ) and bathymetric data Higher-resolution storm surge model grid/mesh and increased inland extents 21 more actual strong NC Hurricanes in the model More detailed wind fields; significant advancements in wind field modeling and measurements Topographic constrictions factored in modeling; missing in the effective model Wave set up in model; missing in the effective model for many counties More overland wave transects and reverse transects Refinement of primary frontal dune Enhanced wave runup modeling and new guidelines Added coastal/riverine combined probability

23 Surge Model GRID/MESH Resolution ADCIRC MESH (Yellow) ft FEMA SURGE GRID (1980) (Blue) 6,067 x 6,067-ft

24 Surge Model GRID/MESH Extents ADCIRC MESH (Yellow) 45-ft Elevation FEMA SURGE GRID (1980) (Blue) Effective Coastal Flood Extents (Blue Area)

25 21 Additional Hurricanes (Start of new surge study) Updates the JPM Storm Parameters and Probabilities (Central pressure, forward speed and heading, wind speed and direction, and radius of maximum winds) JPM Storms Effective 300 New Studies - 675

26 Topographic Constrictions Highest surge water levels in inlets as water builds from winds and wave setup. FEMA Surge 1 NM Grid could not simulate this process

27 Wave Setup ADCIRC/SWAN Wave Setup ( ft, Avg 0.4 ft) (Yellow) Effective Empirical wave setup computed ( ft) Setup for Open Coast Only Pender - Topsail Beach and Surf City Onslow and Carteret

28 Overland Wave Analyses: Statewide No Effective Overland Wave Analyses (Yellow) Restudy Transects (Red Lines) Effective 783 Transects New Studies 2,739 Transects 9 New Counties (New FEMA Sheltered Water Guidelines and Specifications)

29 Reverse Transects: Statewide County With Effective Reverse Transects (Yellow) Restudy Reverse Transects (Red Lines) Transects Effective - 67 New Studies 192 (4 Counties)

30 Primary Frontal Dune Refinement Effective VE Zone (Yellow) Restudy PFD and VE Zone Extent (Black/White Line)

31 Wave Runup Runup Transects (Yellow) Runup Based VE Zone (Red) Effective No BFEs based on Runup

32 Coastal/Riverine Combined Probability Effective Combined Probability (Yellow) 9 Counties

33 Is the survey data used for the flood study accurate? All survey data used for the study has been sealed by NC Licensed Professional Land Surveyors and meets all FEMA vertical and horizontal accuracy requirements Overland Wave Model Transect Survey Data Sources Dune transects surveys field collected Primary frontal dune and overland wave transects near shoreline NC LIDAR (Vertical accuracy +/- 0.7-ft) Mapped PFD Limit Field Survey Points Overland Wave Model Transect

34 What type of storm is creating the 1-percent flood in my area? Winds are most important component of storm surge (Approximately 95% of surge elevation) Wind probabilities derived from historical storm record between 1940 and 2007 Approximate 1-Percent Annual Chance Flood Hurricane Saffir-Simpson Winds

35 What type of storm is creating the 1-percent flood in my area? ACIRC run closest to 1% at Morehead City (Cat 3 Strength Similar to Max from Fran) 2 other tracks produced similar surge elevations for Morehead City area

36 Why have I never encountered flooding at the proposed levels? P Time (years) P flood % % % % % % % % % % Percent chance of seeing one 1-pct annual chance flood over certain time period 37 percent chance you will not have one 1-pct annual chance event in 100-years Need a very long period of record to accurately derive 1- percent annual chance flood potential from measured water levels

37 Why ADCIRC for Surge? There were only three FEMA-authorized 2-D models available for Storm Surge in 2006 ADCIRC, MIKE-11 and FEMA Surge (Last update 1988) ADCIRC Advantages o Existing Mesh/Grid entire Atlantic seaboard and Gulf of Mexico o Ability to have variable resolution flexible mesh (FEMA Surge is a GRID) o Able to combine/integrate with 2-D offshore and nearshore wave and wind field models o Local NC experts on model o No software fees

38 Why ADCIRC for Surge? o Methodology and application of ADCIRC are well-established in the coastal engineering community and in the published literature. o ADCIRC has been used and validated extensively for storm surge and flooding predictions and forensic and academic studies o ADCIRC has been used in all coastal FEMA NFIP studies conducted in the last decade from Texas to New York and the Great Lakes. ADCIRC was recommended as the model to use by the FEMA, the NCFMP, various academic advisors and NCFMP coastal storm surge professional engineering firms.

39 How Accurate is ADCIRC? Hurricane Fran Track Cat mph (Black line) ADCIRC/SWAN Water Level (Red circle) High Water Mark (Blue triangle)

40 Hurricane Matthew Existing NCFMP, USGS, NOAA and USGS Rapid Deployment Gages

41 Hurricane Matthew ADCIRC Comparison to NOAA Beaufort Gage

42 Hurricane Matthew ADCIRC Comparison to Swansboro USGS Rapid Deployment Gage

43 Hurricane Matthew ADCIRC Comparison to New River USGS Rapid Deployment Gage

44 Why use historically derived statistical hurricanes? Methodology used for all new coastal flood studies for the Atlantic Basin Statistical historical storms are required for FEMA coastal flood frequency analyses where: o Insufficient period of record and gage coverage to analyze flood frequency water based on gages alone NC only 7 active NOAA coastal 3,250 miles of open and inland coastline at risk from coastal flooding Only 3 gages have adequate period of record to compute flood frequencies (30-years) Could over or underestimate risk depending on hurricane activity during period Larger statistical errors at greater recurrence intervals NOAA Tidal Gages (7 Gages)

45 Why use historically derived statistical hurricanes? Flood frequencies analyses based solely on gages and high water marks are lacking: o Only relevant for a small geographic area o Does not capture spatial variability of water levels during hurricanes Historical statistical storms developed to capture risk for the entire coastline are based on historical tracks and data Relying on historical water levels alone would significantly underestimate risk of NC coastal flooding Risk is based on meteorological data, not gages alone, since hurricanes tracks are random NOAA Historical Hurricane Tracks Since 1851 Tropical Model Hurricane Tracks

46 Why use historically derived statistical hurricanes? Wind Weather Stations Wind Model is validated at 29 weather locations all over the state (ASOS, mesonet, buoy)

47 Areas For Further Improvement Coastal Gages Bathymetry Terrain (New LIDAR) Risk Analysis Continued Improvement to ADIRC Model

48 Current Coastal Tidal Gages Current NOAA Tidal Gages (7) 2014 NCFMP Tidal Gages (7)

49 Current and Proposed Coastal Tidal Gages Current NOAA Tidal Gages (7) Proposed Tidal Gages (13) 2014 NCFMP Tidal Gages (7)

50 Questions?