coastal flood hazard mapping - ncbiwa floodplain mapping program.pdf · study and mapping was...

North Carolina Emergency Management

Coastal Flood Hazard MappingCurrent Status and Update

April 4, 2017


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.

NCFMP Program Objectives


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

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 reconnaissanceo Evaluation of the regulatory primary frontal dune (PFD)o Storm included erosion processeso 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

Coastal Flood Study Updates Summary


BFE on FIRM based on 4 components: Storm surge Wave setup Overland waves Wave runup

FEMA Coastal Flood Hazards

Surge


Coastal Modeling and Mapping

Storm Surge Modeling

Overland Wave Hazard

ModelingMapping




Overland Wave Hazard

ModelingMapping


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


NC Surge Study Approach

Entire coastline restudiedo Avoid discontinuities, maximize

efficiency

Integrated 2-D hydro-dynamic and wave model

o Surge – ADCIRCo Tidally and wind driven

circulationo Hurricane surge and flooding

o Nearshore Waves - SWAN

Combined extratropical and tropical surge

o Joint Probability Method - statistical method for tropical

o Empirical Simulation Technique (EST) – statistical method for extratropical storm


NC Surge Modeling TeamNCFMP

John Dorman, DirectorKen Ashe, Assistant Director

Tom Cadwallader, Lead Engineer/PMTom Langan, Engineer

DEWBERYChris Mack – Team Leader

Mike Hanson – Assistant Team Leader

TECHNICAL ADVISORSDon Resio – USACOE, ERDC

Jessie Feyen – NOAAEnrique Reyes – ECUDave Divokey – WSC

Spencer Rogers – NC Sea Grant

RENAISSANCE COMPUTING CENTER (RENCI) TEAMKen Galuppi – Team Leader

CONSULTANTAndrew Cox - OCEANWEATHER

FEMA – REGION IV

L. Algeo, EngineerTucker Mahoney, Coastal Eng.

Christina Lindemer, Coastal Eng.

VA INTERFACE – REGION III

David Gunn – VA DCR

SC INTERFACE – REGION IV

Lisa Jones – SC DNR

COMPUTER SERVICESSallie WrightBrian BlantonLisa StillwellKevin Gamiel

DATABASE/DEM DEVELOPMENT

Scott MadryHong Yi

Helena Mitasonova -NCSU

PROGRAM MANAGEMENT

Tara AimsTim McCormick

QUALITY ASSURANCE/

QUALITY CONTROLJeff Gangai

Elena Drei-Horgan

TECHNICAL SUPPORT

Kevin SloverJeff Sample

MODELINGRick Leuttich - UNC

Crystal FulcherCraig Mattock

CONSULTANTSPeter Vickery – ARA John McCormick – CRT

Jeff Hanson – USACOE, Duck

QA/QC


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)


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)


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


Changes relative to effective study:Decrease – BlueIncrease – Yellow/RedNo Change - Grey


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


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


NC Coastal Flood Mapping Timeline


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 draftflood 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)


NCFMP Current Mapping Status

Appeal period has ended

Appeal period has not started

Preliminary Fall 2017


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.


Key Points of Discussion


Factors Resulting in BFE ChangesNew 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


Surge Model GRID/MESH Resolution

ADCIRC MESH(Yellow) - 150-800-ft

FEMA SURGE GRID (1980)(Blue) – 6,067 x 6,067-ft


Surge Model GRID/MESH Extents

ADCIRC MESH(Yellow) – 45-ft Elevation

FEMA SURGE GRID (1980)(Blue)

Effective Coastal Flood Extents (Blue Area)


21 Additional Hurricanes 1980-2006 (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


Topographic Constrictions

Highest surge water levels in inlets as water builds from winds and wave setup.FEMA Surge 1 NM Gridcould not simulate this process


Wave Setup

ADCIRC/SWAN Wave Setup (0.1-3.8 ft,Avg 0.4 ft) (Yellow)

Effective – Empirical wave setup computed (0.6-2.6 ft)

Setup for Open Coast Only• Pender - Topsail Beach

and Surf City• Onslow and Carteret


Overland Wave Analyses: Statewide

No Effective Overland Wave Analyses(Yellow)

Restudy Transects(Red Lines)

Effective – 783 Transects

New Studies – 2,739 Transects9 New Counties (New FEMA Sheltered Water Guidelines and Specifications)


Reverse Transects: Statewide

County With Effective Reverse Transects(Yellow)

Restudy Reverse Transects(Red Lines)

Transects

Effective - 67New Studies – 192 (4 Counties)


Primary Frontal Dune Refinement

Effective VE Zone(Yellow)

Restudy PFD and VE Zone Extent(Black/White Line)


Wave Runup

Runup Transects (Yellow)

Runup Based VE Zone (Red)

Effective – No BFEs based on Runup


Coastal/Riverine Combined Probability

Effective Combined Probability (Yellow)9 Counties


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 2009-2011

Primary frontal dune and overland wave transects near shoreline NC LIDAR 2001-2005 (Vertical accuracy +/- 0.7-ft)

Is the survey data used for the flood study accurate?

Mapped PFD Limit

Field Survey Points

Overland Wave Model Transect


What type of storm is creating the 1-percent flood in my area?

Approximate 1-Percent Annual Chance Flood Hurricane Saffir-Simpson Winds

Winds are most important component of storm surge (Approximately 95% of surge elevation)

Wind probabilities derived from historical storm record between 1940 and 2007


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


Why have I never encountered flooding at the proposed levels?

PTime (years) Pflood

0.99 1 1%0.99 10 10%0.99 30 26%0.99 50 39%0.99 100 63%0.99 150 78%0.99 200 87%0.99 300 95%0.99 460 99%0.99 530 100%

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


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 Advantageso Existing Mesh/Grid – entire Atlantic

seaboard and Gulf of Mexicoo 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 modelo No software fees


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.


How Accurate is ADCIRC?

High Water Mark (Blue triangle)

ADCIRC/SWAN Water Level (Red circle)

Hurricane Fran Track – Cat 3 115 mph(Black line)


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


Hurricane Matthew ADCIRC Comparison to NOAA Beaufort Gage


Hurricane Matthew ADCIRC Comparison to Swansboro USGS Rapid Deployment Gage


Hurricane Matthew ADCIRC Comparison to New River USGS Rapid Deployment Gage


NOAA Tidal Gages (7 Gages)

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

Why use historically derived statistical hurricanes?


NOAA Historical Hurricane Tracks Since 1851

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

Tropical Model Hurricane Tracks



Wind Weather Stations

Wind Model is validated at 29 weather locations all over the state (ASOS, mesonet, buoy)



Coastal Gages Bathymetry Terrain (New LIDAR)

Risk Analysis Continued Improvement to

ADIRC Model

Areas For Further Improvement


Current Coastal Tidal Gages

2014 NCFMP Tidal Gages (7)

Current NOAA Tidal Gages (7)


Current and Proposed Coastal Tidal Gages

2014 NCFMP Tidal Gages (7)

Current NOAA Tidal Gages (7)

Proposed Tidal Gages (13)


Questions?

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