Wave Height Estimate for Multi- Frequency Flooding Events Elena Drei-Horgan, PhD, CFM Darryl Hatheway, CFM Paul Carroll, PE May 24, 2012

Wave Height Estimate for Multi-Frequency Flooding Events

Elena Drei-Horgan, PhD, CFM

Darryl Hatheway, CFM

Paul Carroll, PE

May 24, 2012

Wave Height Estimate for Multi-Frequency Flooding Events Page 2

Introduction

• Background

• Non-regulatory coastal Risk Map products overview– Coastal Risk Map and Products

• Wave height depth grid development background– How to use it– Operating guidance

• Available wave height data to be used for developing multi-frequency depth grids and how

• Multi-Frequency empirical methods

• Summary of investigations & Conclusion


Background

• FEMA & PTS Contractors’ brainstorming for identification of best non-regulatory products identified the availability of a wave height depth grid as a useful dataset to assess the risk from wave hazard in the floodplain

• Operating guidance and procedures for coastal non-regulatory products have been developed for the determination of a wave height depth grid associated with the 1% annual chance event

• Would it be possible to develop wave height depth grids for all regulatory returned periods?

Non-Regulatory Coastal Risk Map Products


Guidelines and Standards for Flood Risk Analysis and Mapping

• Appendix N & O define guidelines, standards and formatting specifications for the non-regulatory Risk Map products

• Flood Risk Datasets and Products

• Final Draft Coastal and Dam Procedure Memorandum and Operating Guidance (2012)

Presentation Title Page 6

Coastal Flood Risk Map & Products

• Coastal Depth Grid

• Coastal Increased Inundation Areas

• Coastal Wave Height Grid

• Coastal Wave Hazard Severity Areas

• Primary Frontal Dune Erosion Area

• Erosion Dune Peak

• Coastal Flood Risk Assessment


Coastal Wave Height Grid

Wave Height Depth Grid Development Background


Why to use it and how to use it?

• SFHAs identification on a FIRM based on a Zone type (VE vs. AE) allows for an immediate determination of areas with wave height above or below 3ft

• PM 50 defines the Limit of Moderate Wave Height (LiMWA) as the area where wave heights are between 3 ft and <1.5 ft and recommends the enforcement in this area of VE Zone building standards

• Within an VE/AE Zone it is not possible to assess from the FIRM the height of the overland wave

• The wave height grid allows users to quickly determine wave heights depth and assess wave hazard risks


Wave Height Grid Operating Guidelines

• Input data to raster:– Created using results from the WHAFIS model (Part 2)– Wave heights are available at each station along each modeled

transect.– Raster is created using the controlling wave height, not just the

portion of the wave crest that lies above the SWEL– Raster resolution as low as 10 ft– Raster is obtained by interpolation of results between transects

across the modeled areas– Interpolated areas between transects may be subjected to

“engineering judgment”. Accuracy of the wave height grid may be lower in these areas and wave magnitude may not fully represent ground/land use conditions

• Operating Draft Guidance (2012) states “ … If WHAFIS results are not available for a referenced event, approximate methods may be used to estimate the corresponding wave heights.”

Available Data for Wave Height Depth Grid Determination


1D Wave Model - WHAFIS

• WHAFIS wave height for the 1% annual-chance event (some FIS studies are funded to determine the wave crest profile for the 0.2% annual-chance event)

• Fine resolution of data along the transects, often down to a 10 ft horizontal spacing (based on typical terrain raster resolution)

• Takes into account wave dissipation due to obstructions and wave regeneration due to open fetches

• Models one event and assumes waves propagate inland at 90° (perpendicular) to shoreline


2D Wave Model – SWAN, STWAVE, MIKE 21 SW

• A 2D wave model is run to compute more accurate wave setup (increase of water elevation due to wave breaking)

• To better capture wave setup variation (e.g. variability in coastal morphology) more detail is added to the storm surge mesh, increasing modeling time, length and cost of studies

• Accounts for friction and dissipation due to land use

• Wave height, period and direction are computed throughout the modeling domain for all synthetic storms run during production.

• Generally speaking the 1% wave is a statistical value result of a frequency analysis over hundreds of storms

• A frequency analysis holds wave data at different return periods but this data is usually utilized only for the determination of WHAFIS starting wave conditions

Investigation


Approximate Methods: What are the alternatives?

• Available Methods for determining wave heights for different return periods, based on above data (and at low cost): – Empirical approach:

1. Depth-limited condition (based on surge depth)– Wave conditions do not compare to WHAFIS because the depth-limited approach

does not account for obstructions/regeneration.

2. Scaling factor


Test Area


Multi-frequency 1D WHAFIS wave profiles

0 500 1000 1500 2000 2500 3000 3500 4000 45000

2

4

6

8

10

12

Hc_2%eventHc_1%eventHc_0.2%event

Distance from shoreline (ft) – Landward to the right

WH

AF

IS H

c (f

t)


Multi-frequency 2D SWAN wave profiles

0 500 1000 1500 2000 2500 3000 3500 4000 45000

2

4

6

8

10

12

Hs_2%eventHs_1%eventHs_0.2%event

Distance from shoreline (ft) – Landward to the right

SW

AN

Hs

(ft)


Where are the “main” differences coming from?

• 2D vs. 1D resolution of the wave processes– Model resolution: minimum spacing = 10 ft in WHAFIS; 300/500 ft in SWAN.

• Different resolution of topography at the model scale

• Winds are treated differently

• x% annual chance event wave in WHAFIS travels inland from one direction (90 degree from shoreline)

• x% annual chance event wave in SWAN is the result of a frequency analysis that accounts for hundred of storms with different directions. The wave envelope is a statistical surface.

• Wave period stays constant in WHAFIS until first AS card is reached. Wave period is fully resolved in SWAN.


Scaling Concept

Hn = TWC/wave height elevation for any given return periodSn = Stillwater elevation for any given return periodγn = the difference between Sn and Hn at any given location


Scaling Factor Error – WHAFIS Runs (BFE)

0 500 1000 1500 2000 2500 3000 3500 4000 4500

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Topography

BFE_2% Diff

BFE_0.2% Diff

Distance from shoreline (ft)

Gro

un

d E

lev

ati

on

(ft

)

Err

or

Dif

fere

nc

e (

ft)


Scaling Factor Error – WHAFIS Runs (Hc)

0 500 1000 1500 2000 2500 3000 3500 4000 4500

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Topography

Hc_2% Diff

Hc_0.2% Diff


Gro

un

d E

lev

ati

on

(ft

)

Err

or

Dif

fere

nc

e (

ft)


Scaling Factor Error – SWAN runs

0 500 1000 1500 2000 2500 3000 3500 4000 4500

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

2D HS_2% Diff

2D Hs_0.2% Diff


Err

or

Dif

fere

nc

e (

ft)


Scaling Factor Error – Combined (WHAFIS & SWAN)

0 500 1000 1500 2000 2500 3000 3500 4000 4500

-4

-3

-2

-1

0

1

2

2D HS_2% Diff

2D Hs_0.2% Diff

WHAFIS Hc_2% Diff

WHAFIS Hc_0.2% Diff


Err

or

Dif

fere

nc

e (

ft)

Summary of Investigation


Summary

• A scaling factor can be a reasonable approximation for the determination of wave heights along a transect for non-regulatory products when only the 1% WHAFIS modeling is available

• The scaling factor approach holds true when applied to a different dataset such as 2D wave data

• The error associated to the re-computed (scaled) wave is larger around AS areas.

• The scaling still allows accounting for wave dissipation and regeneration, aspects not possible when computing a depth-limited wave


Why not leverage 2D wave data for non-regulatory products?

• At this current time, SWAN cannot substitute WHAFIS for the determination of regulatory Base Flood Elevations along a transect.

• Ongoing research is looking at the implementation of 2D models such as SWAN for overland wave modeling (Slinn, 2010)

• Yet, the NFIP relies for the determination of the BFEs on a model based on simple linear wave equations developed upon the NAS 1977 recommendation

• 2D wave modeling data could be leveraged for non-regulatory products to get a better return of FEMA’s $$$ spent for coastal studies

Conclusion


Conclusion

• Wave heights can be determined using a scaling approach for the development of non-regulatory depth grids for return periods different than the 1% annual chance event.

• Minor manipulation of the data is needed to reduce errors in proximity of AS or to match the extent of the floodplain for each appropriate return period. Or, can we accept this error?

On the other hand …

• While WHAFIS transects are spaced on average from 1000 ft to ½ mile, the SWAN model provides output at an even point coverage throughout the floodplain (300-500ft).

• Less interpolation between transects is needed if we use the 2D data.


Conclusion

• Data is readily available from output of the storm surge modeling frequency analysis

• Available 2D wave height data can be leveraged for non-regulatory products helping with :– Identification of wave hazard risks– Identification of vulnerability areas– Risk Assessment (HAZUS)– Wave damage estimates – Mitigation strategies planning

• Require minimum GIS processing

• Can be packaged easily in the Flood Risk Database at a very low cost.

Questions?

Thank You

[email protected]

Documents

Wave Height Estimate for Multi- Frequency Flooding Events Elena Drei-Horgan, PhD, CFM Darryl Hatheway, CFM Paul Carroll, PE May 24, 2012