Storm Induced Coastal Erosion for Flood Insurance Studies and Forecasting Coastal Flood Damage Impacts: Erosion, Runup & Overtopping

Transcription

1 Storm Induced Coastal Erosion for Flood Insurance Studies and Forecasting Coastal Flood Damage Impacts: Erosion, Runup & Overtopping Jeff Gangai- Dewberry Tucker Mahoney FEMA HQ

2 Introduction Background FEMA Storm Induced Erosion for FIS Short-term and long-term needs Disaster Response Forecasting erosion, runup, and overtopping. 1

3 Background Early 1980 s used WHAFIS but no erosion was applied Storm surge study concerns in NC in early 1980 s identified the need for erosion. Studies in 1985 and 1986 justified the need for erosion as well as other aspects Reports in 1987 and 1989 Identified procedure sq ft rule and Primary Frontal Dune (PFD) put in regulations FEMA guidelines and studies updated in 1990s 2004 Pacific Guidelines 2014 Great Lakes Guidelines 2

4 Background - Documentation History 1985_Review of Coastal Surge Model and Dune Erosion NC FIS 1986_Assessment_Of_Current_Procedures_Used_for_Identification_of_Coastal _High_Hazard_Areas_compiled 1987_Relationship Between Frontal Dune Erosion And Storm Recurrence Interval 1988_VZone-fr_finalrule 1988_CFR44_Definitions_and_Sec _Basis of Assessment Procedures for Dune Erosion in Coastal Flood Insurance Studies 1999_bellomo-pajak-sparks 2004_D.4.6_Coastal_Erosion_Pacific 2005_Focus Study_ Erosion 2007_D.2.9_Coastal_Erosion_Atlantic_GC 2014_D.3.7_Coastal_Erosion_GtLakes 3

5 Erosion Recommendations Description and Assessment of Coastal Dune Erosion Include, at a minimum, the primary frontal dune in the V Zone 2. For purposes of the wave analysis, consider that the primary frontal dune does not exist (unless its cross-sectional area above the flood level and seaward of the dune crest exceeds 540 square feet) 3. Incorporate an erosion model into the wave analysis model for use in areas of appropriate applicability 4. Incorporate a routine in the wave height model to compute the effects of wave runup 5. Maintain the 3-foot breaking wave criteria for defining V zone inland of the primary frontal dune 6. FIA should issue a comprehensive set of guidelines and specifications for wave analysis and V zone mapping 7. Refine Coastal High Hazard Area for publication in the Federal Register 8. Offer insurance rate incentives to communities that have adopted and are enforcing construction restrictions more stringent than NFIP minimum requirements 4

6 Erosion Recommendations

7 Erosion Recommendations

8 Erosion Recommendations

9 Regulations Final Rule 8

10 NFIP Regulations Definitions in Section 59.1 Coastal high hazard area means an area of special flood hazard extending from offshore to the inland limit of a primary frontal dune along an open coast and any other area subject to high velocity wave action from storms or seismic sources. Primary frontal dune (PFD) 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 PFD occurs as the point where there is a distinct change from a relatively steep slope to a relatively mild slope. 60.3(e)(7) Prohibit man-made alteration of sand dunes and mangrove strands within Zones V1-30, VE, and V on the community s FIRM which would increase potential Flood damage. 9

11 Present Methods Atlantic & Gulf Guidelines 540 Sq ft Dune Methods Standardized Profile slopes Primary Frontal Dune Bluff Retreat Pacific Guidelines Historic Data Geometric profiles Bluff Retreat Great Lake Guidelines CSHORE Model Historic Data Lacks guidance 10

12 TMAC Recommendation #9 Scientific review and revision, as warranted, of the 540 sq ft criterion would help ensure accurate identification of coastal food and erosion hazard areas and facilitate community management and protection of dunes (including Primary Frontal Dunes, as required by NFIP regulations). 11

13 Moving Forward Short Term How to use Historic Erosion guidance Update 540 with current local data, as well as profile shapes Guidance on altering profile shape Process based Model - CSHORE, SBEACH, XBEACH Long Term 2D Modeling incorporation of erosion to Surge and Waves Removal of VE extension to PFD in regulations 12

14 Use of Historic Data USGS Coastal Research Center The Richard Stockton College of New Jersey 13

15 Update 540 sq ft with Local Data 14

16 CSHORE Processes 1D cross-shore transect model Models near-shore processes simultaneously Sediment transport / erosion Wave setup Wave transformation and breaking Wave runup & overtopping 15

17 CSHORE Erosion USACE CSHORE model Applies real physics Near-shore wave processes Cross-shore and along shore sediment transport Requires sediment grain size 16

18 USACE CSHORE Model Applies real physics Near-shore wave processes Cross-shore and along shore sediment transport Time series input from forecasted storm data Gives storm impacts to: Erosion Waves Runup Overtopping rate 17

19 Hurricanes on the Approach Hurricane Hermine Hurricane Matthew 18

20 Hurricane Hermine Landfall as a Category 1 near St. Marks, Florida First landfall hurricane since 2005 Strong storm surge and damaging winds RFP received 3pm Friday 9/2 (Labor Day) Signed NTP by 7pm 1941 Alma 1966 Dora King

21 Surge Gages National Hurricane Center SLOSH RENCI (Renaissance Computing Institute) ADCIRC Surge and Waves 20

22 Hurricane Hermine 21

23 Hurricane Matthew Flood Depth Categories: Affected < 2 feet Minor 2 5 feet Major 5 8 feet Destroyed > 8 feet 22

24 Coastal Erosion and Overtopping 2015 & 2016 Winter Storms Sandy 23

25 CSHORE for Forecasting? 24

26 CSHORE for Forecasting? Ability to answer critical EM-related questions: How much erosion will occur? When will overtopping occur? Potential breaching areas? When will roads be impassable? When do homeowners need to be out? Forecasting & Analytics Potential EM/planning products Severity of beach erosion areas Potential overtopping and breaching areas High Velocity potential Wave damage potential areas Scenarios with probabilities 25

27 Coastal Storm Erosion Questions? 26