ON THE IMPACT OF A SERIES OF SEVERE STORMS ON A DOUBLE-BARRED SANDY COAST: DUNE EROSION AND MEGACUPS EMBAYMENTS

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1 1 ON THE IMPACT OF A SERIES OF SEVERE STORMS ON A DOUBLE-BARRED SANDY COAST: DUNE EROSION AND MEGACUPS EMBAYMENTS BRUNO CASTELLE 1, VINCENT MARIEU 1, STEPHANE BUJAN 1, KRISTEN D. SPLINTER 2, ARTHUR ROBINET 1,3, NADIA SENECHAL 1, SOPHIE FERREIRA 1,4 1. UMR EPOC, CNRS / Université de Bordeaux, 33615, Pessac Cedex, France. b.castelle@epoc.u-bordeaux1.fr, v.marieu@epoc.u-bordeaux1.fr, s.bujan@epoc.ubordeaux1.fr, a.robinet@epoc.u-bordeaux1.fr, n.senechal@epoc.u-bordeaux1.fr. 2. Water Research Laboratory, School of Civil and Environmental Engineering, UNSW Australia, Sydney, NSW, Australia. k.splinter@unsw.edu.au.. 3. BRGM, Service RNSC, BP 6009, Orléans Cedex 2, France. a.robinet@brgm.fr. 4. UMS OASU, BP 49, Floirac Cedex, France. s.ferreira@epoc.u-bordeaux1.fr. Abstract: The winter of 2013/2014 was outstanding in terms of the available energy to move sediment and caused large-scale unprecedented erosion of the Gironde coast (France), comprising the onset of megacusp embayments cutting the dune with a typical alongshore and cross-shore lengthscale of O (100 m) and O(10 m), and a dune erosion scarp height often exceeding 10 m. Megacusps are found to be coupled to the outer crescentic sandbar and triggered by the outstandingly high-energy, long-period and normally-incident storm wave event Hercules on January 6-7. Our observations indicate that both the antecedent outer sandbar morphology and storm wave characteristics, including period and angle of incidence, govern patterns of beach and dune erosion along open multiple-barred sandy coasts during severe storms. Introduction Sandy beaches buffer wave energy, with periods of accretion and erosion alternating over time coupled to low- and high-energy wave conditions (Stive et al., 2002). Short-term erosion occurs during severe storms that usually cause foredune erosion and scarping (e.g. Hesp, 2002). On alongshore lengthscales from tens of meters to a few kilometers, variability in beach erosion can appear along the coast with sometimes a striking alongshore rhythmicity. These shoreline rhythms are observed on rip-channeled beaches associated with surfzone crescentic sandbars or transverse bars (e.g. Aagaard, 1988) with the megacusp embayments, where maximum erosion occurs, in the alignment of the rip channels (e.g. Dalon et al., 2007; Thornton et al., 2007; Birrien et al., 2013). Megacusp embayment studies were essentially performed on single-barred beaches. On multiple-barred beaches it is unclear to which sandbar (e.g. inner or outer bar) the megacusps couple during storms as well as which storm wave conditions promote such a coupling.

2 2 Western Europe coastlines are exposed to severe winter storms which are strong extra-tropical cyclones associated with low pressure systems that track westward across the North Atlantic Ocean, and are associated with high-energy ocean waves from the W-NW. The winter of 2013/2014 was outstanding in terms of the available energy to move sediment and caused large-scale unprecedented erosion in SW France with foredune erosion and scarping as well as the formation of megacusp embayments. Here we analyze beach and dune erosion along a 110-km stretch of sandy coast with a focus on Truc Vert beach and shoreline-sandbar coupling. Study site and data Gironde coast and Truc Vert beach The Gironde open sandy coast, located in south west France, extends approximately 110 km from the Cap Ferret sand spit in the south to the Medoc peninsula in the north, the latter separating the open ocean and the Gironde estuary, and is bordered by high Aeolian dunes (Fig. 1a). The Gironde coast is a meso-macrotidal environment with an annual mean spring tidal range of approximately 3.7 m and maximum tidal range reaching up to 5 m during spring tides. The wave climate is energetic, strongly seasonally modulated, with a dominant W to NW incidence. The Gironde coast beaches are mostly intermediate double-barred, with rip spacing of the inner and outer bars of about 400 m and 700 m in the south of the Gironde coast, respectively, and progressively decreases northwards (Castelle et al., 2007). Fig. 1. (a) Map of SW France showing locations of Truc Vert beach and that of (red circle) the Candhis directional wave buoy and (black) the WW3 virtual buoy. The thick black solid line indicates the section of coast studied here. (b) Aerial photograph of Truc Vert beach.

3 3 Truc Vert is a remote and conserved beach (Fig. 1b) that has been studied over the last 15 years. It is representative of most of the Gironde coast beaches. A notable difference is that the shoreline is rather stable at Truc Vert beach, while beaches tend to erode from approximately Le Porge northwards. Data 18.3 years of WWIII significant wave height data was further corrected via linear regression fit with approximately 6 years of interspersed Candhis buoy wave data. After the very last 2013/2014 winter storm, the shoreline was surveyed with an ATV on April 3-4, 2014 along the 110-km Gironde coast (thick black line in Fig. 1a). Following Dalon et al. (2007), a running average filter was applied to the raw shoreline data with a window size of 2000 m to get the mean shoreline position. The shoreline data was computed with another running average with a window size of 100 m to remove short-scale features such as beach cusps. The mean shoreline was subtracted from the shoreline data resulting in a band-passed signal between 100 m and 2000 m. LANDSAT satellite images, with a 30-m resolution from spring 2013 to spring 2014 were combined with available aerial photographs of both the nearshore zone and the dune field during the winter 2013/2014 to address the link between surfzone morphology and megacusp embayments. From April 8, 2005 to April 30, 2014, topographic surveys of Truc Vert beach were sampled every 2-4 weeks, with a 1-year gap in To address mean shoreline change, the beach topography was alongshore averaged over the extent of the available survey data to minimize the local influence of mega cusps coupled to the inner-bar rip channels. For a detailed description of the methodology used to perform and further analyze the beach surveys at Truc Vert beach, the reader is referred to Castelle et al. (2014). Results Gironde coast post-winter state: dune erosion and megacusp embayments Cuspate-type rhythmic dune erosion scarps, i.e. megacusp embayments, were striking along the Gironde coast (Fig. 2). Fig. 3a shows the cross-shore deviation from the mean shoreline position (dune foot proxy) as a function of alongshore distance along the 90-km long section shown by the thick solid black line in Fig. 1a. Shoreline rhythm alongshore spacing was irregular and varied between approximately 300 m and 2000 m from L'Horizon to Le Pin Sec, with larger alongshore spacing up to 4000 m between Le Pin Sec and Montalivet. The average cross-shore amplitude was m with a maximum exceeding 35 m between Truc Vert and Le Grand Crohot. The local wavelet spectrum of the

4 4 erosion scarp foot deviation from the mean shoreline shows a continuous decrease in the alongshore lengthscales from approximately 1500 m in the south to 400 m in the north at Le Pin Sec (not shown, see Castelle et al., in press). Fig. 2. Example of rhythmic cuspate erosion scarp north of Lacanau on March 7, 2014 (Ph. Julien Lestage). The progressively decreasing alongshore lengthscale of the shoreline rhythms from south to north between L'Horizon and Le Pin Sec is reminiscent of a similar trend in outer-bar and inner-bar spacing along the Gironde coast (Castelle et al., 2007), suggesting a potential coupling. To further test this hypothesis, LANDSAT satellite images pre and post 2013/2014 winter storms were gathered to address the inner- and outer-bar geometries. This coupling is evidenced in Fig. 3b, c that zooms onto the Lacanau area. Interestingly enough, there was no outer-bar reset during the winter of 2013/2014 (see the breaker patterns that are highly discontinuous alongshore in Fig. 3c). The outer-bar bays even further developed with deepened bays and intense depth-induced wave breaking across the horns. The superimposed deviation from mean shoreline in meters measured on April 3-4, 2014 clearly shows that the megacusp embayments face the outer bar bays where strong rip currents (orange arrows in Fig. 3c) are expected to occur during storms. The inner bar flattened during the winter of 2013/2014 to become a poorly pronounced terrace with the notable absence of inner-bar rip channels. This analysis strongly suggests that the megacusp embayments that developed during the winter 2013/2014 from approximately L'Horizon to Le Pin Sec were enforced by the outer-bar morphology. This is also corroborated by aerial photographs encompassing both the surf zone and the dune field (not shown). In the next subsection, the storm that triggered the megacusp embayments is identified and the time evolution of the beach throughout the winter of 2013/2014 is analyzed.

5 5 Fig. 3. (a) Deviation from mean shoreline (dune foot proxy) as a function of alongshore distance (positive northwards) with indication of the most popular beaches (except Truc Vert beach which is remote with no direct access). LANDSAT satellite images zoomed into the Lacanau area (b) on July 10, 2013 and (c) on March 23, In (b,c) the shoreline (dune foot proxy) measured with the ATV on April 3-4, 2014 is superimposed with color bar indicating the deviation from mean shoreline in meters. The orange arrows in (c) show the location of rip current occurrence during storm waves clearly facing megacusp embayments. Truc Vert beach response to storm waves The detailed wave analysis is given in Castelle et al. (in press). During the winter of 2013/2014, the Gironde coast was exposed to the most energetic wave conditions over the last 18.3 years (Fig. 4a). The 2-month average significant wave height H s peaked at 3.63 m in late January, just below H s,95% (0.95 quantile). 22 individual storms with H s > H s,99% were recorded during the winter 2013/2014, with 4 storms with H s exceeding 9 m. Using the definition of storm events by Masselink et al. (2014), the two longest storm events (26 and days) identified over the 18.3-year period occurred during the winter of 2013/2014. This, combined with persistent shore-normal wave incidence and large swell peak wave periods makes the winter 2013/2014 outstanding in terms of the available energy to move sediment and cause large-scale erosion. Fig. 4b shows the time series of measured shoreline cross-shore position for the proxies z = 0, 2, 4 and 6 m (above MSL). Seasonal cycles are clearly seen for

6 6 proxies z = 2 m and 4 m with typical amplitude of about 50 m and 30 m, respectively. For the first time in this 9-year dataset, a severe dune retreat was observed at Truc Vert during the winter 2013/2014. The alongshore-averaged iso-contour z = 6 m retreated by approximately 20 m over 3 months (grey area in Fig. 4b), that is, by only a slightly lower amount than the iso-contours z = 2 and 4 m (approximately 30 m). Fig. 4. (a) Time series of 3-hourly H s, T p (blue) and 2-month moving average of H s (thick green) using WWIII data further corrected via linear regression fit with Candhis wave buoy data. Based on Massselink et al. (2014), the red bubbles are storms events (H s>h s,95%), whereby the size of the bubbles is proportional to storm duration based on H s,75% cut-off either size of storm maximum. H s,99%, H s,95% and H s,75% are indicated by the horizontal yellow dashed lines. (b) Measured mean shoreline cross-shore position for the proxies z = 0, 2 m, 4 m and 6 m with ±1 cross-shore standard deviation indicated by the vertical error bars. In (a,b), the grey zone indicates the period studied in detail here. Truc Vert beach response was not uniform alongshore. Prior to the first winter storms, the subaerial beach at Truc Vert was mostly uniform alongshore with a well-developed berm and an untouched foredune (Fig. 5a), while the intertidal morphology was alongshore variable due to the presence of inner-bar rip channels. The first storms occurred in late October with H s peaking 4 times at more than 4.5 m within approximately 3 weeks. These storms eroded the berm and lowered the beach but did not affect the foredune (Fig. 5b, d). Between December 20 and January 10, the beach was exposed to storm events coinciding with spring tides in early January. The very last storm (Hercules) of the sequence was severe, with measured significant wave height and peak wave

7 7 period exceeding 9.6 m and 22 s, respectively, with a shore-normal incidence. While the first storms, coinciding with gusty westerly winds, eroded the beach with only limited dune erosion, the Hercules long-period swell induced severe and localized beach and dune erosion with the notable formation of a megacusp embayment in the center of the surveyed domain. This localized dune erosion extended about 400 m alongshore, with an approximately 6-m high erosion scarp in the embayment (not shown). The subsequent survey performed on January 31 (Fig. 5c) shows that this megacusp embayment was preserved and even further enlarged by about 200 m in the alongshore during the subsequent storm(s) in January. The resulting erosion/accretion patterns between December 19 and January 31 show 3 distinct patches of large erosion along the beach (Fig. 5e). The most intense erosion area was located in the center of the domain with a beach lowering of approximately 2-3 m, and exceeding 4 m where the dune was cut. Fig. 5. Left-hand panels: beach surveys at Truc Vert beach with color bar indicating elevation in meters. Right-hand panels: corresponding difference plots with the blue and red areas showing a decrease and an increase in beach elevation, respectively. The color bar indicates bed elevation change in meters.

8 8 This dune erosional hot-spot, initially with a 30-m cross-shore amplitude, remained throughout the subsequent storm events in February and March (not shown), although the cross-shore amplitude of the erosion scarp decreased to about 20 m by the end of March mostly through erosion of the megacusp horns. Conclusion The winter storms of 2013/2014 drove massive and unprecedented beach and dune erosion along the Gironde coast, comprising the onset of megacusp embayments enforced by the outer bar cutting the dune with a typical alongshore and cross-shore lengthscale of O(100 m) and O(10 m), and a dune erosion scarp height often exceeding 10 m. These megacusp embayments were triggered by the outstandingly high-energy, long-period and normally-incident storm wave event Hercules on January 6-7. Results indicate that both the antecedent outer sandbar morphology and storm wave characteristics, including period and angle of incidence, govern patterns of beach and dune erosion along open multiplebarred sandy coasts during severe storms. The detailed analysis of the storm wave, and beach and dune response are given in Castelle et al. (in press). Acknowledgements This work was done within the framework of the project BARBEC (ANR N2010 JCJC ) and CHIPO (ANR ASTRID) both financially supported by the Agence Nationale de la Recherche (ANR), as well as the France-Australia Science Innovation Collaboration (FASIC) program Early Career Fellowships and INSU/EC2CO (Project DECA). Additional support from SOLAQUI (OASU), Region Aquitaine and SNO DYNALIT is acknowledged. We thank Xavier Bertin and Julien Lestage for providing the WW3 wave outputs and photographs, respectively, as well as the Candhis buoy network operated by CEREMA. References Aagaard, T., (1988). Rhythmic beach and nearshore topography: examples from Denmark, Geografisk Tidsskrift, 88, Birrien, F., Castelle, B., Dailloux, D., Marieu, V., Rihouey, D., Price, T. D., (2013). Video observation of megacusp evolution along a high-energy engineered sandy beach: Anglet, SW France, Journal of Coastal Research, Special Issue No 65,

9 9 Castelle, B., Bonneton, P., Dupuis, H., Sénéchal, N., (2007). Double bar beach dynamics on the high-energy meso-macrotidal French Aquitanian Coast: a review, Marine Geology, 245, Castelle, B., Marieu, V., Bujan, S., Ferreira, S., Parisot, J. P., Capo, S., Senechal, N., Chouzenoux, T., (2014). Equilibrium shoreline modelling of a high-energy meso-macrotidal multiple-barred beach, Marine Geology, 347, Castelle, B., Marieu, V., Bujan, S., Splinter, K.D., Robinet, A., Sénéchal, N., Ferreira, S. (in press). Impact of the winter series of severe Western Europe storms on a double-barred sandy coast: beach and dune erosion and megacusp embayments, Geomorphology. Dalon, M. M., Haller, M. C., Allan, J., (2007). Morphological characteristics of rip current embayments on the Oregon Coast, Proceedings of Coastal Sediments 07, ASCE Press, Hesp, P. A., (2002). Foredunes and blowouts: initiation, geomorphology and dynamics, Geomorphology, 48, Masselink, G., Austin, M., Scott, T., Poate, T., Russell, P., (2014). Role of wave forcing, storms and NAO in outer bar dynamics on a high-energy, macro-tidal beach, Geomorphology, 226, Stive, M. J. F., Aarninkhof, S. G. J., Hamm, L., Hanson, H., Larson, M., Wijnberg, K. M., Nicholls, R. J., Capobianco, M., (2002). Variability of shore and shoreline evolution, Coastal Engineering, 47, Thornton, E. B., MacMahan, J. H., Sallenger Jr, A. H., (2007). Rip currents, mega-cusps, and eroding dunes, Marine Geology, 240,