Storm Induced Coastal Erosion for Flood Insurance Studies and Forecasting Coastal Flood Damage Impacts: Erosion, Runup & OvertoppingJeff Gangai- Dewberry

Tucker Mahoney – FEMA HQ

1

Introduction

▸Background

▸FEMA Storm Induced Erosion for FIS

▸Short-term and long-term needs

▸Disaster Response Forecasting erosion, runup,

and overtopping.

2

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

• 1988 540 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

3

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_Sec65.11

• 1989_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

4

Erosion Recommendations

▸ Description and Assessment of Coastal Dune Erosion – 1986

1. 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

5

Erosion Recommendations 1986

6

Erosion Recommendations 1986

7

Erosion Recommendations 1986

8

Regulations

▸ Final Rule

9

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.

10

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

11

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).”

12

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

13

Use of Historic Data

Coastal Research Center

The Richard Stockton College of New Jersey

USGS

14

Update 540 sq ft with Local Data

15

▸1D cross-shore transect model

▸ Models near-shore processes simultaneously

• Sediment transport / erosion

• Wave setup

• Wave transformation and breaking

• Wave runup & overtopping

CSHORE Processes

16

CSHORE Erosion

▸ USACE CSHORE model

• Applies real physics

• Near-shore wave processes

• Cross-shore and along shore

sediment transport

• Requires sediment grain size

17

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

18

Hurricane Hermine

Hurricanes on the Approach

Hurricane Matthew

19

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

Dora 1964

King 19501949

1935

Alma 1966

1941

20

▸ Gages

▸ National Hurricane Center – SLOSH

▸ RENCI (Renaissance Computing Institute) – ADCIRC – Surge and Waves

Surge

21

Hurricane Hermine

22

Hurricane Matthew

Flood Depth Categories:• Affected < 2 feet

• Minor 2 – 5 feet

• Major 5 – 8 feet

• Destroyed > 8 feet

23

Coastal Erosion and Overtopping

Sandy2015 & 2016 Winter

Storms

24

CSHORE for Forecasting?

25

▸ 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?

▸ Potential EM/planning products

• Severity of beach erosion areas

• Potential overtopping and breaching areas

• High Velocity potential

• Wave damage potential areas

• Scenarios with probabilities

CSHORE for Forecasting?

Forecasting

& Analytics

26

Questions?

Coastal Storm Erosion