HomeMy Public PortalAbout20140417 - Tybee Island BTF Meeting_V05aTybee Island Beach Task Force Meeting
Georgia - Northeast Florida
Coastal Study Overview
April 17, 2014
1
Agenda
Introductions (BTF, GA-DNR, FEMA Region IV, and BakerAECOM).
Overview of the coastal study process and phases for GA-NEFL.
Overview of the Beach Nourishment Fact Sheet.
Overview of the
WHAFIS model (Wave Height Analysis for Flood Insurance Studies) and Erosion Treatment.
Overview of WHAFIS tests of Tybee Island beach profiles .
Q & A.
Nassau County, FL
2
Introductions
Risk MAP Project Team
Tybee Island BTF
FEMA Region IV
BakerAECOM, FEMA Mapping Partner
Project Stakeholders
Community CEOs
Community Floodplain Administrators
Political Representatives
Other
State and Federal Agencies
Partnership between FEMA and the contractors
3
Coastal Flood Risk Study Phases
Surge
Waves
Maps
4
Study Project Area
5
Basic Elements of a Coastal
Flood Risk Study
Base Flood Elevation (BFE) on FIRM includes 4 components:
Storm surge stillwater elevation (SWEL)
Amount of wave setup
Wave height above storm surge (SWEL) elevation
Wave runup above
storm surge elevation (where present)
Determined from storm surge model
6
Storm Surge Modeling – ADCIRC & JPM
.
7
Overland Wave Modeling - WHAFIS
The model simulates the response of the still water surface to atmospheric forcing (water interacts with elevation features)
8
Flood Hazard Mapping
9
FEMA PFD Definition
FEMA defined the PFD in 44 CFR, Section 59.1 (October 1,1988):
Primary frontal dune means a continuous or nearly continuous mound or ridge of sand with relatively steep seaward and
landward slopes immediately landward and adjacent to the beach and subject to erosion and overtopping from high tides and waves during major coastal storms. The inland limit of the primary
frontal dune occurs at the point where there is a distinct change from a relatively steep slope to a relatively mild slope.
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PFD Identification
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Dune Reservoir – The “540 Square Foot Rule”
FEMA also included a new section in Part 65 of the NFIP regulations, identifying a cross-sectional area of 540 square feet as the basic criterion to be used in evaluating whether a Primary
Frontal Dune (PFD) will act as an effective barrier during the 1-percent-annual-chance flood.
Two Cases:
1. Dune Retreated?
2. Dune Removed?
12
Dune Reservoir (cont’d)
13
Erosion Treatment
14
Removal – PFD / WHAFIS
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Removal – Mapped Flood Zones
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PFD Data Sources
Topography (from LIDAR)
Beach Profiles (from profile/transect surveys)
Aerial Photographs
Field Notes
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Land Cover
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Land Cover
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Waterfront Features
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Field Investigations & Verification
formatted
GIS
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Analysis & Mapping Tools
22
Beach Nourishment & Dune Construction – Fact Sheet
23
Tybee Island WHAFIS Tests
24
Study Data — Topography & Bathymetry
2009 LiDAR
REUSE
25
2008 Tybee Nourishment
November 2007 Pre-construction Survey By Arc Surveying & Mapping
Between October and November 2008, 1.2 million CY of sediments placed between monitoring stations 6 and 13A
Berm Design
Elevations +7-ft NAVD88 (+11-ft MLW)
No clear dune construction in profiles
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Tybee Beach Topo Analysis
**These Tybee Beach Analysis Transects mimic the annual monitoring transects collected by the Town and do not reflect the same location as the BakerAECOM Modeling transects that will
be used for the FEMA flood study.
*PFD line still in draft form
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Tybee Beach Topo Analysis
**These Tybee Beach Analysis Transects mimic the annual monitoring transects collected by the Town and do not reflect the same location as the BakerAECOM Modeling transects that will
be used for the FEMA flood study.
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Test Profile Analysis – T1
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Test Profile Analysis – T2
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Test Profile Analysis – T3
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Test Profile Analysis – T4
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Survey Mapping (Draft for testing)
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LIDAR Mapping (Draft for testing)
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Comparison (Draft for testing)
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Conclusion
At the open coast, the water depths controls the size of the waves.
For many beach nourishment projects, the PFD v-zone requirement will supersede WHAFIS computed water surface elevations
(and flood zones).
Although beach nourishment berms will reduce wave energy at the open coast (due to shoaling and wave dissipation), changes to computed flood zone elevations upland
of the PFD are negligible.
REUSE??
36
Q & A – Study Contacts
Michael DelCharco, PE
904.472.0082
mdelcharco@TaylorEngineering.com
Chris Mack, PE
(843) 740-7328
chris.mack@aecom.com
Mark Vieira, PE
770.220.5450
mark.vieira@fema.dhs.gov
Christina Lindemer
770-220-5424
christina.lindemer@fema.dhs.gov
REUSE??
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