iahr 2015 - coastal flooding and dune breaching in the central part of the bay of biscay, bertin,...
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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]
Introduction
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
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
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?
Analysis of the storm
surge associated with
Xynthia
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
Modelling the storm surge during Xynthia
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?
Modelling storm-induced
coastal flooding
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
Modelling the flooding associated with Xynthia
The impact of marine flooding on coastal water levels
Limite d’inondation observée
Dune breaching during
winter 2013-2014
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)
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
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.
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.