Presentation by Camila Gaido Lasserre (University of California, Santa Cruz (UCSC), USA) at the Delft3D User Days, during the Delft Software Days - Edition 2023 (DSD-INT 2023). Wednesday, 15 November 2023, Delft.

1. Efficient approach to model
regional flooding on coral
reef-lined coasts
Camila Gaido*, Kees Nederhoff , Curt D. Storlazzi, Borja G. Reguero, Michael W. Beck
*cgaido@ucsc.edu

2. Agenda
• Motivation
• Methodology
• Case study
1. Coupling XBeach-1D with SFINCS
2. XBeach-1D + SFINCS compared to XBeach-2D (benchmark)
• Discussion & Conclusion
2

3. Motivation
3
• 3 storms: 1- (annual), 20-, and
100-year storm return periods
• 6 sea levels: Current sea level
+ 5 SLR (0.5 – 3.0 m)
• Coral reef environments
• Wave-driven coastal flooding

4. Previous Methodology
4
SWAN
XBeach - 1D
ArcGIS approach
Bottleneck
NO- physics
Static flooding

5. Previous Methodology
5
SWAN
XBeach - 1D
ArcGIS approach
SFINCS

6. New Methodology
6
SFINCS input
XBeach-1D output Flood map
XBeach-1D
Extraction
Total water level
(+ U velocity) IG wave time series
Water level
SFINCS
Flooding

7. Case study
7
High relief area
Low relief area

8. Case study
Settings
• XBeach on Surfbeat mode.
• XBeach varying cross-shore
resolution from 2-10 m.
• SFINCS constant 2 m cross-shore
resolution.
• Both models have a 10 m
alongshore resolution.
• XBeach-1D every 100 m
alongshore distance.
• Keyword wbcevarreduce = 0.5
(XBeach-1D).
• SFINCS with advection (SFINCS-
SSWE).
Forcing
• Storms: daily, 1-, 10-, 50-, 100- and
500-years return period.
• Sea levels: current, +0.5 m SLR ,
+1.0 m SLR.
• JONSWAP spectrum.
• Spatially varying forcing.
• Waves were imposed shore
normal.
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9. Coupling XBeach - 1D with SFINCS
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10. Coupling XBeach - 1D with SFINCS
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SFINCS input
XBeach-1D output Flood map
XBeach-1D
Total water level
(+ U velocity) IG wave time series
Water level
SFINCS
Flooding
Extraction

11. Finding the optimal coupling method
11
• Directly using the
incoming water level.
• Irregular phase
difference between
consecutive boundary
conditions.
1. Direct method 2. Indirect method
• Spectra-based.
• Consistent alongshore
phasing.
3. Indirect method +
• Spectra-based.
• Consistent
alongshore phasing.
• Added setup
correction.

12. Finding the optimal coupling method
13

13. XBeach-1D + SFINCS
vs
XBeach-2D
14

14. Low relief area
15
• Similar flood patterns.
• Water levels are overestimated
in the nearshore → wave setup
correction.
• Maximum surface level is
overestimated by 12 cm on
average (~20% of the average
water depth of flooding).

15. High relief area
16
• Similar flood patterns.
• Water levels are
overestimated in the
nearshore → wave setup
correction.
• Flooding underestimation
near inland waterways.
• Maximum surface level is
overestimated by 3 cm
on average (~8% of the
average water depth of
flooding).

16. General results
17
Low relief area High relief area
• Flood extents are underestimated
for more regular storms and
overestimated for rare storms.
• Water depths of flooding are
overestimated across scenarios.
• Greater variance in the results for
the low relief area.
Success: 78% Success: 85%
RMSD: 22 cm RMSD: 8 cm

17. What can explain the differences?
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• Even when reducing the short-
wave group variance by 50%
XBeach-1D water levels and wave
heights are larger than XBeach-
2D.
• In SFINCS the maximum water
level decreases inland due to
friction.
• XBeach-1D and XBeach-2D can
still add setup after the extraction
location due to the presence of
short waves.

18. Discussion & Conclusion
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19. Discussion
20
• Flooding differences between models can be explained by simplifications
applied to the SFINCS approach:
o Using 1D models assumes alongshore-uniform hydrodynamics and does not
include wave directional spreading.
o Applying a setup correction to compensate for the use of a simplified
physics model such as SFINCS.
• The simplifications make the SFINCS flooding approach ~100 times faster than
XBeach-2D.
• Flooding differences between the modeling approaches can be reduced
when numerical parameters are calibrated per site.

20. Conclusion
• Coupling XBeach-1D with SFINCS using the indirect method
and adding a setup correction gives the best runup signal
reproduction.
• When extracting the XBeach-1D data shallower than 5 m of
water depth, the scatter index for wave runup is contained
under 20% across modeled wave climate scenarios and
geographies.
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21. Conclusion
• XBeach-1D coupled with SFINCS produces similar flooding extents to
XBeach-2D flooding.
• High-frequency storm flood extents tend to be underpredicted, and
low-frequency storm flood extents are overpredicted.
• The predicted water depth of flooding generally exhibits an
overestimation across scenarios.
• Due to its computational efficiency, XBeach-1D coupled with
SFINCS is a suitable choice for large-scale wave-driven flood
modeling or when involving numerous scenarios.
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22. Questions?
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