![Page 1: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/1.jpg)
Coastal flooding and dune breaching
in the central part of the Bay of Biscay
Xavier Bertin and Gael Arnaud
UMR 7266 LIENSs , CNRS/Univ. La Rochelle
2 rue Olympe de Gouges, 17000 La Rochelle
E-mail: [email protected]
![Page 2: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/2.jpg)
Introduction
![Page 3: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/3.jpg)
The storm Xynthia
→ Minimum atmospheric pressure of 969 mbar
→ Hourly-mean wind ranging from 100 to 130 km/h, gusts up to 150 km/h
![Page 4: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/4.jpg)
This surge was in phase with a high spring tide.
Associated return periods:
- Storm surge : > 50 years (Hamdi et al., NHESS 2015)
- Total water level: > 200 ans (Bulteau et al., NHESS 2015)
Xynthia induced a storm surge >1.5 m in La Rochelle.
The storm Xynthia
![Page 5: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/5.jpg)
The flooding associated with Xynthia
→ 47 peoples died
→ More than 2.5 billions € damage
Why Xynthia induced such a large storm surge?
Is that possible to hindcast the associated flooding?
What is the contribution of the breaches in the flooding?
![Page 6: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/6.jpg)
Analysis of the storm
surge associated with
Xynthia
![Page 7: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/7.jpg)
The numerical modelling system
→ SELFE is forced along its open boundaries by the 18 main harmonic
constituents obtained from TUGO (Pairaud et al., 2006)
→ The atmospheric forcing originates from ECMWF (1/8°/1h)
-1,700,000 element
unstructured grid
-Resolution ranging
from 30000 m to 5 m
→ WWMII is also forced with wave spectral originating from a WWIII regional model
Circulation model
SELFE (Zhang et Baptista, 2008)
Wave model
WWMII (Roland et al., 2008)
-Water levels
-Currents
-Radiation stresse
-Surface stress
![Page 8: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/8.jpg)
Modelling the storm surge during Xynthia
![Page 9: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/9.jpg)
The origin of the exceptional surge during Xynthia
2
winddas UC
Bulk formula approach
wsas U 2
*
,sin,cos
2
0
max
min
in
f
f
ws Sk
where
Wave-dependent approach
Why such a particular sea-state?
![Page 10: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/10.jpg)
Modelling storm-induced
coastal flooding
![Page 11: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/11.jpg)
Modelling storm-induced coastal flooding
Storm surges are nowadays reproduced with a good accuracy (e.g. 10-
20 % RMSE), mostly due to:
- Increase computationnal powed, allowing for higher resolutions
- Improved knowledge regarding wave-circulation-atmosphere interactions
- Improved representation of atmospheric forcing
On the opposite, modelling of storm-induced flooding is scarce in the literature:
- Challenging multiscale problem, implying large grids with locally very HR
- Steep dikes and barriers cause very strong gradients
- The large variability of CFL conditions implies very robust numerical methods
Unstructured grids appear more and more appealing to adress this
multi-scale challenging problems
![Page 12: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/12.jpg)
Modelling the flooding associated with Xynthia
![Page 13: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/13.jpg)
The impact of marine flooding on coastal water levels
Limite d’inondation observée
![Page 14: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/14.jpg)
Dune breaching during
winter 2013-2014
![Page 15: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/15.jpg)
Winter 2013-2014: an endless succession of storms
→ The coastline suffered a huge erosion in
the BoB (e.g. Castelle et al., GEOM 2015).
→ Hs exceeded 10 m six times at Biscay.
→ The return period associated with
Hs=10 m is ~1 yr. (Lerma et al., OE 2015)
![Page 16: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/16.jpg)
Development of a washover at La Faute-sur-Mer
X-beach 2DH non-hydrostatic, forced with water level and velocities
ADCP
PT
AWAC 01/02/2014
02/02/2014
![Page 17: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/17.jpg)
Conclusions
→ The analysis of model results revealed that the large storm
surge during Xynthia was strongly enhanced by the presence of
young waves, associated with the particular track of the storm.
→ Our simulations of flooding are quite relalistic and the
analysis of the results suggest that massive flooding can impact
coastal water levels significantly.
→ We collected a nice dataset during a washover development
and its hindcast with Xbeach will improve understanding such
phenomena.
![Page 18: IAHR 2015 - Coastal flooding and dune breaching in the central part of the bay of Biscay, Bertin, 30062015](https://reader031.vdocuments.us/reader031/viewer/2022020307/55c24296bb61eb884a8b4723/html5/thumbnails/18.jpg)
Thank you for your attention!
Acknowledgements -CG 17/Région Poitou Charente
-Virginia Institutte for Marine Science (USA)
-S.H.O.M./REFMAR
-C.E.T.M.E.F.
-ECMWF
Bertin, X., Bruneau, N., Breilh, J.F., Fortunato, A.B. and Karpytchev M., 2012. Importance of wave
age and resonance in storm surges: the case Xynthia, Bay of Biscay. Ocean Modelling 42, 16-30.
.
Bertin, X., Li, K., Roland, A., and Bidlot, J.R., 2015. The contributions of short-waves in storm
surges: two case studies in the Bay of Biscay. In press to Continental Shelf Research.
Bertin, X., Li, K., Roland, A., Zhang, Y., Breilh, J.F. and Chaumillon, E., 2014. A modelling-based
analysis of the flooding associated with Xynthia. Coastal Engineering 94, 80-89.