Serge Suanez

Extreme wave activity during 2013/2014 winter and morphological impacts along the Atlantic coast of Europe

Studies of coastal vulnerability due to climate change tend to focus on the consequences of sea level rise, rather than the complex coastal responses resulting from changes to the extreme wave climate. Here we investigate the 2013/2014 winter wave conditions that severely impacted the Atlantic coast of Europe and demonstrate that this winter was the most energetic along most of the Atlantic coast of Europe since at least 1948. Along exposed open-coast sites, extensive beach and dune erosion occurred due to offshore sediment transport. More sheltered sites experienced less erosion and one of the sites even experienced accretion due to beach rotation induced by alongshore sediment transport. Storm wave conditions such as were encountered during the 2013/2014 winter have the potential to dramatically change the equilibrium state (beach gradient, coastal alignment, and nearshore bar position) of beaches along the Atlantic coast of Europe.

Observations of large infragravity wave runup at Banneg Island, France

On Banneg Island, France, very high water-level events (6.5 m above the astronomical tide) have been observed on the western cliff, exposed to large swells from the North Atlantic. The analysis of hydrodynamic measurements collected during the storm of 10 February 2009 shows unusually high (over 2 m) infragravity wave runup events. By comparing runup observations to measurements in approximately 7 m of water and numerical simulations with a simplified nonlinear model, two distinct infragravity bands may be identified: an 80 s infragravity wave, produced by nonlinear shoaling of the storm swell; and a 300 s wave, trapped on the intertidal platform of the island and generating intermittent, low-frequency inundation. Our analysis shows that the 300 s waves are a key component of the extreme water levels recorded on the island.

Morphosedimentary behaviour of the deltaic fringe in comparison to the relative sea-level rise on the Rhone delta

Abstract For the first time, mareographic data obtained by the ‘Compagnie du Salin du Midi’ have been used to estimate the relative sea-level rise (RSLR) in the Rhone delta. The results were compared with the data obtained in the Marseille region which is known to be a tectonically stable zone, at least, on the secular time scale. The magnitude of relative sea-level rise is equal to 2.1 mm/year, from which 1 mm/year could be attributed to subsidence. The morphosedimentary behaviour of the deltaic fringe has been analyzed in order to see if the amount of sediment input is sufficient to allow the wetlands to continue to exist with RSLR. The horizontal variations (erosion and progradation) have been quantified from land surface changes obtained by shoreline variation analysis. The vertical accretion is measured using isotopic dating ( 137 Cs). It appears that the eastern part of the mouth of the Rhone is characterized by high sediment input, sufficient enough to offset the recent rise of sea level. The western part remains vulnerable due to the lack of sediment in this area.

Long-, Mid- and Short-Term Evolution of Coastal Gravel Spits of Brittany, France

Gravel spits of Brittany have experimented a long morphosedimentary evolution over the last millenia. Based on analysis of several back-barrier holocene sediment stratigraphies, distinct phases of construction and barrier breakdown were recognized, indicating the role played by storminess and sediment supply during the late-holocene period. Over the last centuries and decades, a deficit of sediment bubget affecting several gravel spits is highlighted. Therefore, actual coastal evolution of most of them is mainly dominated by cannibalization, landward retreat by rollover and complete destruction of the spits in some places. This coastal erosion is related to the lack of significant offshore sediment input or from the erosion of unconsolided cliffs. Locally, anthropogenic forcing have axacerbated the erosion processes by sediment minings and/or the construction of hard defense structures. For the swash-aligned gravel spits, frequency and magnitude of overwash processes is controlling the rate of landward retreat by rollover. This morphodynamic behaviour is illustrated by topo-morphological surveys realised between 2002 and 2012 on Sillon de Talbert spit which has experienced a complete crestal removal during the 10 March 2008 Johanna storm. Although this event have a 50-100yrs occurence, the barrier has exhibited a rapid crestal rebuilding by overtopping processes, illustrating the great resilience of the spit. Thus, coastal erosion management strategies mainly based on hard defense structures are gradually abandoned for new management policies based on soft operations. Nowadays, gravel spits of Brittany are also considered as a geological heritage and management plans are establishing by local authorities. 1. GENERAL SETTING In Brittany, gravel beaches are located mainly on the northern and western coast (fig. 1). Their construction is mainly due to the shoreward removing of periglacial deposits initially accumulated on the coastal shelf during the post-glacial marine transgression. The southern part of Brittany is characterized by sandy beaches and coastal dunes, except locally where a coarse material is provided by erosion of cliffs formed by Pleistocene deposits (head). Nowadays, unconsolidated cliffs are considered as the most significant source of coarse sediments in Brittany (Guilcher et al. 1957, 1990). In the Bay of Brest, the highly weathered of shale cliffs also contribute locally to feed the gravel barriers. In the Bay of Brest, the indentations of the jagged coastline were favorable to the construction of a numerous smale-scale barriers and spits with a high morphological diversity (fig. 2A to 2D). The lenght and the volume of gravel spits in the bay of Brest never exceed 700 m and 100.000 m 3 respectively. Nevertheless, the Sillon de Talbert spit studied in the north of Brittany forms the bigger accumulation reaching 3.2 km long and sediment volume estimated at 1.23 x 10 6 m

Effect of Tropical Cyclones on Short-Term Evolution of Carbonate Sandy Beaches on Reunion Island, Indian Ocean

ABSTRACT Mahabot, M.-M.; Pennober, G.; Suanez, S.; Troadec, R., and Delacourt, C., 2017. Effect of tropical cyclones on short-term evolution of carbonate sandy beaches on Reunion Island, Indian Ocean. Carbonate sandy beaches in Reunion Island show various forms of evidence of erosion. Extreme waves associated with tropical cyclones (TCs) play a major role in beach dynamics. The present study analyzes and quantifies back-reef beach response and recovery from forcing generated by TCs Dumile, Felleng, and Bejisa, which occurred in 2013 and 2014. The study focuses on carbonate beaches of Reunion Island from Cap Champagne to the Passe de Trois-Bassins. Morphological and volumetric changes on beaches were analyzed by comparing 19 beach profiles. The results show that TCs are able to cause significant morphosedimentary change on the back-reef beaches of Reunion Island. These changes affect beach topography and involve longshore and cross-shore sediment transport. An alongshore variation in beach response is observed, which varies according to tropical storm intensity and coastal morphology. The intensity of impact seems to be related to reef width. The most severe erosion occurred at Boucan Canot, where reef is absent with a loss of −24 ± 2 m3 after TC Dumile, −38.7 ± 1.2 m3 after Felleng, and −42.5 ± 1.6 m3 after Bejisa. Elsewhere, the volumetric changes is less than 5 m3 under TC Dumile and vary between 2 and 11 m3 under TC Felleng and between 2 and 23 m3 under TC Bejisa. No significant impact occurred at La Saline where the reef flat is large and provides good protection for the beach; however, relative coastline orientation and prestorm beach-profile morphology also play an essential role in storm impact. Wave height and water level are also determinant factors of storm erosion potential. After storms, the beaches show a relative capacity for recovering because of calm conditions; however, different behaviours are observed along the same beach compartment. This suggests local influence of coastal structure and/or reef geomorphology in sediment transport processes.

Equilibrium modeling of the beach profile on a macrotidal embayed low tide terrace beach

Eleven-year long time series of monthly beach profile surveys and hourly incident wave conditions are analyzed for a macrotidal Low Tide Terrace beach. The lower intertidal zone of the beach has a pluriannual cycle, whereas the upper beach profile has a predominantly seasonal cycle. An equilibrium model is applied to study the variation of the contour elevation positions in the intertidal zone as a function of the wave energy, wave power, and water level. When forcing the model with wave energy, the predictive ability of the equilibrium model is around 60% in the upper intertidal zone but decreases to 40% in the lower intertidal zone. Using wave power increases the predictive ability up to 70% in both the upper and lower intertidal zones. However, changes around the inflection point are not well predicted. The equilibrium model is then extended to take into account the effects of the tide level. The initial results do not show an increase in the predictive capacity of the model, but do allow the model free parameters to represent more accurately the values expected in a macrotidal environment. This allows comparing the empirical model calibration in different tidal environment. The interpretation of the model free parameter variation across the intertidal zone highlights the behavior of the different zones along the intertidal beach profile. This contributes to a global interpretation of the four model parameters for beaches with different tidal ranges, and therefore to a global model applicable at a wide variety sites.

Beach recovery from extreme storm activity during the 2013-14 winter along the Atlantic coast of Europe: BEACH RECOVERY FROM EXTREME STORM ACTIVITY

The storm sequence of the 2013/14 winter left many beaches along the Atlantic coast of Europe in their most eroded state for decades. Understanding how beaches recover from such extreme events is essential for coastal managers, especially in light of potential regional increases in storminess due to climate change. Here we analyze a unique dataset of decadal beach morphological changes along the west coast of Europe to investigate the post‐2013/14‐winter recovery. We show that the recovery signatureis site‐specific and multi‐annual, with one studied beach fully recovered after two years, and the others only partially recovered after four years. During the recovery phase, winter waves primarily control the timescales of beach recovery, as energetic winter conditions stall the recovery process while moderate winter conditions accelerate it. This inter‐annual variability is well correlated with climate indices. On exposed beaches, an equilibrium model showed significant skill in reproducing the post‐storm recovery and thus can be used to investigate the recovery process in more details.