Presentation by Anouk de Bakker (CNRS-Université de La Rochelle) at the XBeach X (10th Year Anniversary) Conference, during Delft Software Days - Edition 2017. Wednesday, 1 November 2017, Delft.

1. Energy transfers and reflection of
infragravity waves at a dissipative beach
under extreme storm conditions
Xavier Bertin, Anouk de Bakker, Teddy
Chataigner and Thomas Guérin
UMR 7266 LIENSs, CNRS/Université de La Rochelle, 2 rue Olympe de
Gouges, 17000 La Rochelle France
E-mail: xbertin@univ-lr.fr

2. On gently sloping beaches: generated by groups of short waves
Infragravity wave period = 20-200 s
Bound-wave mechanism

3. Study area
 Semi-diurnal tides, ranging from 1.5 to 5.5 m
 Wave regime dominated by Hs ~1.5 m, Tp= 8-12s, Dir = 260-290°
 Winter storms can produce offshore Hs > 10 m and Tp > 20 s

4. Why are we studying this beach?
 Dissipative beach, 2D-uniform  Large shoreline retreat
 Development of massive IG waves?

5. The field experiment in February 2017
30/01/2017 to 09/02/2017 01/02/2017 to 03/02/2017

6. XBeach simulations (Roelvink et al., 2009)
 XBeach surfbeat mode used in 2DH, using a curvilinear grid with ∆x = 5-25 m
 XBeach was forced by measured water levels at the ADCP and time series of
directional wave spectra computed with a regional application of WWIII

7. Very energetic offshore wave conditions
Frequency (Hz)
Wave periods (s)
Wave height (m)
Water depth (m)
Waveheight(m)

8. Development of very large infragravity waves
Time (s)

9. Development of very large infragravity waves

10. Can XBeach reproduce such large IG waves?

11. Can XBeach reproduce such large IG waves?
Measured Hm0 (m) Measured Hm0 (m)
ModeledHm0(m)
ModeledHm0(m)
Short waves Infragravity waves

12. Why such large IG waves?
 First, very energetic incident short waves (Hm0~6 m, Tp~22 s).
 Presence of a phase lag between the wave energy envelop and the bound wave.
Waterlevel(m)

13. Why such large IG waves?
 XBeach captures phase lag between the wave energy envelop and the bound wave.

14. Energy transfers in frequency

15. XBeachObservations
 Subharmonic IG waves: interactions IG/IG?
 Shift towards higher frequencies:
interactions between incident and reflected
IG waves?
Energy transfers in frequency

16. Reflection of IG waves
 Strong tidal modulation of IG wave reflection, well captured by XBeach.
 Strong and unexpected reflection, related to subharmonic IG waves?

17. Conclusions and perspectives:
 We collected an exceptional dataset at the SW of Oléron Island,
showing some of the largest IG waves ever recorded.
 Such massive IG waves are reasonably captured by XBeach, and are
related to the presence of a phase lag between the BW and the WEE
over a large distance.
 Due to the development of very low frequency IG waves, the reflection
is quite high for a dissipative beach, and is tidally modulated.
 Perspectives: subsequent data analysis (Radon Transform, bispectra,
etc…) and XBeach modeling to further analyse these mechanisms.
 Across the surfzone, IG waves develop super- and sub-harmonics,
with an energy shift towards higher frequencies.

18. Thanks for your attention!
Xavier Bertin, Anouk de Bakker, Teddy
Chataigner et Thomas Guérin
UMR 7266 LIENSs, CNRS/Université de La Rochelle, 2 rue Olympe de
Gouges, 17000 La Rochelle France
E-mail: xbertin@univ-lr.fr