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: xbertin@univ-lr.fr

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.