Wave Height Estimate for MultiFrequency Flooding Events
Elena Drei-Horgan, PhD, CFM
Darryl Hatheway, CFM
Paul Carroll, PE
May 24, 2012
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 multifrequency depth grids and how
• Multi-Frequency empirical methods
• Summary of investigations & Conclusion
Wave Height Estimate for Multi-Frequency Flooding Events
Page 2
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 nonregulatory 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?
Wave Height Estimate for Multi-Frequency Flooding Events
Page 3
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)
Wave Height Estimate for Multi-Frequency Flooding Events
Page 5
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
Presentation Title
Page 6
Coastal Wave Height Grid
Wave Height Estimate for Multi-Frequency Flooding Events
Page 7
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 Estimate for Multi-Frequency Flooding Events
Page 9
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.”
Wave Height Estimate for Multi-Frequency Flooding Events
Page 10
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
Wave Height Estimate for Multi-Frequency Flooding Events
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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
Wave Height Estimate for Multi-Frequency Flooding Events
Page 13
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
Wave Height Estimate for Multi-Frequency Flooding Events
Page 15
Test Area
Wave Height Estimate for Multi-Frequency Flooding Events
Page 16
Multi-frequency 1D WHAFIS wave profiles
12
WHAFIS Hc (ft)
10
8
Hc_2%event
6
Hc_1%event
Hc_0.2%event
4
2
0
0
500
1000
1500
2000
2500
3000
3500
Distance from shoreline (ft) – Landward to the right
Wave Height Estimate for Multi-Frequency Flooding Events
Page 17
4000
4500
Multi-frequency 2D SWAN wave profiles
12
SWAN Hs (ft)
10
8
Hs_2%event
6
Hs_1%event
Hs_0.2%event
4
2
0
0
500
1000
1500
2000
2500
3000
3500
Distance from shoreline (ft) – Landward to the right
Wave Height Estimate for Multi-Frequency Flooding Events
Page 18
4000
4500
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.
Wave Height Estimate for Multi-Frequency Flooding Events
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Scaling Concept
Hn = TWC/wave height elevation for any given return period
Sn = Stillwater elevation for any given return period
γn = the difference between Sn and Hn at any given location
Wave Height Estimate for Multi-Frequency Flooding Events
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16.0
1.5
14.0
1.0
12.0
0.5
10.0
0.0
8.0
-0.5
Topography
6.0
-1.0
BFE_2% Diff
4.0
-1.5
BFE_0.2% Diff
2.0
-2.0
0.0
0
500
1000
1500
2000
2500
-2.0
3000
3500
4000
-2.5
4500
-3.0
Distance from shoreline (ft)
Wave Height Estimate for Multi-Frequency Flooding Events
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Error Difference (ft)
Ground Elevation (ft)
Scaling Factor Error – WHAFIS Runs (BFE)
16.0
1.5
14.0
1.0
12.0
0.5
10.0
0.0
8.0
-0.5
Topography
6.0
-1.0
Hc_2% Diff
4.0
-1.5
Hc_0.2% Diff
2.0
-2.0
0.0
0
500
1000
1500
2000
2500
3000
-2.0
4000
-2.5
4500
-3.0
Distance from shoreline (ft)
Wave Height Estimate for Multi-Frequency Flooding Events
3500
Page 22
Error Difference (ft)
Ground Elevation (ft)
Scaling Factor Error – WHAFIS Runs (Hc)
Scaling Factor Error – SWAN runs
2.5
Error Difference (ft)
2
2D HS_2% Diff
1.5
2D Hs_0.2% Diff
1
0.5
0
0
500
1000
1500
2000
2500
3000
-0.5
-1
-1.5
Distance from shoreline (ft)
Wave Height Estimate for Multi-Frequency Flooding Events
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3500
4000
4500
Scaling Factor Error – Combined (WHAFIS & SWAN)
3
2D HS_2% Diff
2D Hs_0.2% Diff
2
Error Difference (ft)
WHAFIS Hc_2% Diff
WHAFIS Hc_0.2% Diff
1
0
0
500
1000
1500
2000
2500
3000
-1
-2
-3
-4
Distance from shoreline (ft)
Wave Height Estimate for Multi-Frequency Flooding Events
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3500
4000
4500
Summary of Investigation
Summary
• A scaling factor can be a reasonable approximation for the
determination of wave heights along a transect for nonregulatory 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
Wave Height Estimate for Multi-Frequency Flooding Events
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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
Wave Height Estimate for Multi-Frequency Flooding Events
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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.
Wave Height Estimate for Multi-Frequency Flooding Events
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Conclusion
• Data is readily available from output of the storm surge modeling
frequency analysis
• Available 2D wave height data can be leveraged for nonregulatory 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.
Wave Height Estimate for Multi-Frequency Flooding Events
Page 30
Questions?
Thank You
Elena.Drei-Horgan@AECOM.com