Presentation by Guillaume Dodet (Université de Brest) at the XBeach X (10th Year Anniversary) Conference, during Delft Software Days - Edition 2017. Thursday, 2 November 2017, Delft.

1. XbeachX Conference 1-3 November 2017
Infragravity wave modelling
over a steep rocky bathymetry
Guillaume Dodet

2. Motivation
Testing xbeach skills in new environments
● convincing 2D applications over coral reefs and rocky platforms
● what about steep rocky shoreface ?
Improving our knowledge on IG wave dynamics over a complex
rocky bathymetry
● few field cases (Pomeroy et al., Van Dongeren et al., Mc Call et al.)
● many open questions :
➢ Impact of longshore varying bathymetry on IG wave propagation?
➢ generation mechanisms over steep slopes ?
➢ impact of bottom friction on IG wave dissipation ?
Exploring a large data set acquired by the French Navy from 2012 to 2016
● ~ 4 winters / ~16 months of data !
● a broad range of conditions, including extreme storm waves (Hm0 > 10 m
and IG waves up to 2 m) ! (Sheremet et al. 2014, Dodet et al., 2016)

3. Banneg Island

4. Channel
France
England
Brittany
Banneg Island
Ushant Island
M
olèneArchipelago
Storm path
Where is Banneg Island ?
Iroise
Sea
Fromveur channel

5. Ushant Island
Banneg Island
SWELL
Fromveur Channel
North

6. South
SWELL

7. South
SWELL
Frequent activation of cliff-top storm deposits
Autret et al., 2016

8. SWELL
● 1 directionnal waverider (DW5)
measuring offshore wave
conditions continuously
● 1 pressure sensor (PT0)
measuring nearshore wave
condition once a day (20 min)
● 3 pressure sensors (PT1, PT2,
PT3) measuring intertidal wave
conditions continuously
Measurements
DW5
PT0
PT1
PT2
PT3
Field campaign during winter 2012-2013

9. Model setup

10. Computational grid
● 1800 m x 1800 m curvilinear grid
● 2 m < dx < 10 m ; 5 m < dy < 10 m
● ~80 000 nodes
● Bathymetry smoothing and linear
interpolation at the boundaries

11. Forcing conditions
● 160 events during winter 2012-2013
● 0.4 m < offshore Hm0 < 5 m
● 5 s < offshore Tp < 20 s
● measured water level and wave spectra
Parameterization
● surf-beat mode
● 36 directions
● 30-min runs with 5-min spin-up
● Cf = 0.003, fw = 0.0 ; gamma = 0.55

12. Validation
On average, for Hm0 : BIAS = 0.03 m NRMSE = 40 % R2
= 0.8
On average, for Hm0 IG : BIAS = -0.03 m NRMSE = 55 % R2
= 0.7

13. Infragravity wave
dynamics

14. Propagation
30-min simulation during energetic wave conditions (March 10 2013):
Hm0 = 3.0 m ; Tp = 14 s ; Mwd = 245°

15. Propagation

16. Coupling between the wave enveloppe and the IG wave
R data R model
PT0 -0.56 -0.47
PT1 0.07 -0.21
PT2 0.34 0.13
PT3 0.51 0.25 PT0
PT1
PT2
PT3
Bound-wave release vs. Breakpoint shifting mechanism ?

17. Generation mechanism

18. Generation mechanism

19. Some preliminary conclusions...
● XBEACH reproduces hydrodynamic processes fairly well at Banneg Island ;
● IG wave energy is slightly underestimated by the model despite Cf = 0.003 ;
● The release of bound IG waves seems to be the dominant mechanism for the generation
of IG waves on Banneg's shores;
● The complex patterns of incident short and long waves over the rocky bathymetry require
an appropriate 2D analysis for estimating incident vs. reflected ig wave ;
● Calibration / sensitivity analysis to improve model results and assess the impact of friction
and breaking parameterization
● PCA analysis of the current field to retrieve dominant IG wave direction
● Investigating IG wave refletion and tanding wave patterns
● Testing more energetic cases (eg. Petra storm on 14/02/2014)
● Non-hydrostatic runs to study the extreme wave runup and overwash
…. and perspectives :

20. THANK YOU
Banneg Island after a flooding episode during Petra storm (February 14 2014)