This document summarizes research using the XBeach model to simulate wave processes on coral reef-lined coasts. Key findings include: 1) XBeach can well predict wave transformation over reefs, including separation of incident, infragravity, and very low frequency wave motions. 2) Infragravity waves are important within lagoons and near the shoreline. 3) XBeach tends to underestimate wave runup on reefs, though reef hydrodynamics are well captured. 4) Modeling of future climate change scenarios for coral islands shows that wave-induced flooding will dominate over static sea level rise effects.

1. XBeach application of coral reef-lined coasts
Ap van Dongeren
With Curt Storlazzi, Olivia Cheriton (USGS)
Ryan Lowe, Andrew Pomeroy (U. West Australia),
Ellen Quataert, Matthijs Gawehn, Stuart Pearson, Alessio Giardino,
Kees Nederhoff, Robert McCall and Dano Roelvink

2. How did XBeach go tropical?
• 2001 Delft3D open-source
• Stanford applies Delft3D on
Hawaiian reef with good results for
mean currents
• But: observed IG wave motions
could not be reproduced with D3D
• Shared Xbeach software and
Ningaloo data
• Led to years-long research
interaction on hydrodynamics with
a large group of institutions
17 november 2017Figure from Lowe et al., 2009

3. 17 november 2017
Wave processes on reefs
Incident wavegroups
Outgoing
free long wave
OFLW
Run up
Overwash
Set up, S-S, IG, VLF wavesWave breaking
Deep water
Reef flat Coast
• How are IG waves generated, and how do they decay?
• What is the role of resonance on the reef flat?
• How well can we predict wave runup and flooding?
• What are future climate-change scenarios for coral islands?

4. 17 november 2017
Method: using Xbeach and field and lab data
Ningaloo, WA, AUS
Ebeye

5. Results: Ningaloo Reef, Western Australia
Short waves IG waves water level
C1
C3
C4
C5
C6
Data
XBeach
Wave transformation well predicted by model
IG waves – important in lagoon (40%) and near shoreline
(60%)
Pomeroy et al. (2012), J. Geophysical Res.
Van Dongeren et al. (2013), Coastal Engineering

6. Assessment of flooding: Roi Namur, Marshall Islands
• Small islet on Kwajalein atoll
• Already experiences flooding
• Model validation of two
Xbeach modes
• Climate projections
17 november 2017
fore reef outer mid inner

7. Validation of Xbeach-SB and Xbeach-NH
17 november 2017
Quataert et al, in prep
Offshore fore reef outer mid inner
Non-hydro(NH)Surfbeat(SB)
Blue = MWL Yellow = Sea- Swell (0.04-0.2 Hz)
Red = IG (0.004-0.04 Hz) Purple = VLF (0.001-0.004 Hz)
observed

8. Comparison of vertical runup by XB-SB and -NH
17 november 2017
Vertical runup (blue=XB-NH, yellow=XB-SB, red=data
XB-NH gives better match with observed (marginal) runup
XB-SB consistently underestimates observed runup

9. RCP8.5 + ice-sheet
collapse
RCP8.5 = business as
usual
RCP4.5 = reduced
carbon emissions
by mid-century
17 november 2017
,00
,200
,400
,600
,800
1,00
1,200
1,400
1,600
1,800
2,00
2,200
2,400
2,600
2,800
3,00
2020 2030 2040 2050 2060 2070 2080 2090 2100 2110
SealevelRise
Year
RCP4.5 (+0.5 m)
RCP8.5+ice sheet collapse (+2.0 m)
RCP8.5 (+1.5 m)
Sea level rise estimates for Roi-Namur
Source: DOD (CARSWG, 2016)
Climate change projections

10. RCP 8.5 in 2055 (SLR = 0.8 m)
17 november 2017

11. Application: impact modelling on Ebeye, RMI
17 november 2017
Giardino and Nederhoff, World Bank report
• Run Xbeach for
different climate
scenarios and
time horizons
• Combine with
object locations
and depth-
damage curves
• Obtain Expected
Annual Damages

12. Area-averaged EAD to prioritize intervention
EAD presented for 4 time horizons for RCP 8.5

13. Conclusions
• Sea-Swell, IG and VLF wave transformation well predicted by
Xbeach
• IG generation and dissipation processes are nicely
separated.
• Runup is underestimated, even if reef hydro is predicted well.
• Open research question, see talk by Anne de Beer
• Wave-induced flooding dominates static sea level rise effects in
climate change scenarios
• Model is applicable in practical impact assessment projects
17 november 2017

14. Future work
• Investigation into run-up underprediction
• Alongshore variability in bathymery/topography
• Sediment transport in coral reef environments
• Coupling with ground water models to study salinization
• Remote-sensed inputs of bathymetry and reef properties
17 november 2017