Presentation by Floor Roelvink, Deltares, at the Delft3D and XBeach User Day: Coastal morphodynamics, during Delft Software Days - Edition 2019. Wednesday, 13 November 2019, Delft.

1. Coral Restoration for Coastal
Hazard Risk Reduction
Floortje Roelvink
A.R. van Dongeren
C.D. Storlazzi
S.G. Pearson
M.F.S. Tissier
A.J.H.M. Reniers

2. Coral Restoration for Coastal
Hazard Risk Reduction
Cluster analysis of reef bathymetric profiles
XBeach non-
hydrostatic
The effect of coral restoration on wave
transformation over various reef
morphologies and the resulting runup

3. Deltares–CoralRestoration
3
Content
Conceptual modelXBeach modellingProfile ClassificationMethodIntroduction Conclusion

4. 4
Introduction
Functions of a reef
• Ecological value
• Economic value
• Shoreline protection
Threat to the shoreline protection function
• Coral Degradation
• Sea level rise
Before
After
What to do to protect the vulnerable
reef lined coasts?
Coral Restoration?
Deltares–CoralRestoration

5. Objective
• Large knowledge gap on effect of restoration
• Operational constraints & limited funds ask for efficient coral restorations
➢ What are the hydrodynamic effects of coral restorations?
➢ Which reef morphologies are worth restoration efforts?
➢ Where along the cross-shore is restoration most efficient in reducing the runup?
5
Deltares–CoralRestoration

6. Method
• Classification reef morphologies using a cluster algorithm
• Reef geometry control on nearshore hydrodynamics using XBeach non-hydrostatic
• Effects of coral restoration on reef hydrodynamics and coastal hazards using XBeach non-hydrostatic
• Aggregration results in Conceptual model
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Deltares–CoralRestoration

7. 7
Classification Reef Morphologies
• 30.000 reef profiles
• Depth measurements at 2 m interval, between -30 and + 20 m
• Locations: Hawaii, Florida, Guam, Puerto Rico, US Virgin Islands, American Samoa, and the Northern Mariana Islands
• Cluster algorithm
• Testing of different Cluster Variables and Clustering techniques
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8. 8
Classification Reef Morphologies
• Clustering Techniques
• K-means algorithm
• Gaussian Mixture Model
• Maximum Dissimilarity Algorithm
• Clustering variables
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9. 9
Classification Reef Morphologies
Deltares–CoralRestoration
• Evaluation of methods on performance in grouping into
geometrically distinct profiles with relatively high frequency of
occurrence
• Best results with k-means algorithm and depth at regular
cross-shore intervals

10. 10
XBeach model setup
• XBeach non-hydrostatic
• Validated for reef lined coasts (e.g. Quataert (2015), Pomeroy (2012))
• Short-wave resolving
• 1D
• Fast, efficient, easy to setup
• Wave driven circulation not captured
• Coral restoration modeled as an impermeable bed level elevation with enhanced friction
• First indication of the effect of coral restorations
• Effects such as in-canopy flow not taken into account, possibly overestimating setup and reflection
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11. 11
XBeach model setup
• No calibration
• Relative effect of different restoration configurations
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12. 12
XBeach model setup
• Input conditions
Hydrodynamic forcing Profile parameters Restoration
parameters
Water level
[m]
Wave
height
[m]
Wave
steepness
[-]
Slope [-] Width [m] Height
[m]
Width [m]
0.5 2 0.01 0.025 100 0.25 5
1 4 0.05 0.1 250 1.25 10
2 6 0.5 25
3
4
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13. 13
XBeach results
• Reef geometry control
• Steep slope (narrow surfzone) -> large setup
and IG wave generation
• Reef flat width and submergence determine
beach toe short wave heights
• Convex and typical fringing profile sheltered
from waves by reef flat in contrast to
straight and three-slope profile
Deltares–CoralRestoration

14. 14
XBeach results
• Effect of coral restoration
• +/- 15.000 model runs
• At steep interface of restoration, waves are
reflected
• Across restoration, short and IG waves are
dissipated, setup across restoration
• Reduction of short wave heights decreases
setup near the shore due to smaller
radiation stress gradients
• Diminished transfer of energy from short
waves to infragravity waves.
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15. 15
XBeach results
• Effect of coral restoration
• Location determines effect on hydrodynamic
processes
• Restoration potential distinctly different
across different reef morphologies
• Typical fringing reef profile
0-10 % reduction
• Convex profile
0-10 % reduction
• Straight profile
up to 20 % reduction
• Three-slope profile
up to 40 % reduction
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16. 16
Conceptual model aggregration
• Large variety of stakeholders involved with coral
restoration
• Aggregation results into simple model could be
highly beneficial
• Pinpoint where restoration efforts are most
efficient
• Conceptual model synthesis
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17. 17
Conclusion
Take home
• There is potential for coral restoration!
• First guideline on how to optimize the effect
of coral restoration
• Promising example of green infrastructure
• XBeach simulations as input for empirical
parametrization
• Possibility for simple reef hydrodynamic
model
Not there yet
• 1D schematization
• Schematization of restoration
• Limited variation forcing conditions, profile
parameters etc
• Include restoration in conceptual model
• Validation conceptual model
So much to do and learn!
• Ecological aspects of coral restoration
• Calibration and validation of restoration
schematization (field and lab experiments)
• Convert knowledge into simple tool for
stakeholders working with coral restoration
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18. • Thank you very much for your attention!
Questions?
floortje.roevink@deltares.nl