Bilan des tempêtes de l’hiver 2013-2014 sur la dynamique de recul du trait de côte en Bretagne

Abstract

International audienceBetween December 2013 and March 2014, a cluster of about 12 storm events hit the coast of Brittany with an exceptional frequency. It was in February that these storm events were the most frequent and particularly virulent. The significant wave heights measured off Finistère reached respectively 12.3 m and 12.4 m during Petra and Ulla storms on February 5 and 14. However, analysis of hydrodynamic conditions shows that only three episodes promoted extreme morphogenetic conditions because they were combined with high spring tide level. The first one occurred on January 1rst to 4, it was followed by events from February 1rst to 3, and March 2-3. During these three extreme events observed tide levels were above highest astronomical tide level (HAT). Maximum surge level (0.97m) was reached during Ulla storm of February 14-15. For comparison, we must go back in the winter of 1989-90 to find such extreme storm frequency. High frequency topomorphological measurements were achieved on more than ten coastal zones distributed around Brittany peninsula to assess the effects of these storms on shoreline erosion. They show that during the first phase (December-January), meeting it's climax from 1rst to 4th January 2014, shoreline erosion has been limited, with the exception of southern Brittany. This is due to the SW orientation of waves. For all monitoring sites, it has averaged -2.7m, the averaged minimum equal to 0.6m, and the averaged maximum at -6.20m. During the second phase from mid-January to mid-February, reaching it's climax on 1-2 of February storm corresponding to the most morphogenetic event of the winter, the average of shoreline retreat reached -4.2m, the averaged minimum reached approximately -1.5m, the averaged maximum -9.5m. It is essentially the Northern and Western coast of Brittany that experienced largest shoreline retreat due to W-NW storm wave orientation. During the third and last phase, running from mid-February to mid-March, and characterized by the March 2-3 extreme morphogenetic event, shoreline retreat was very low. It reaches -1m on average, for an averaged minimum of -0.6m and an averaged minimum of -1.9m. Considering the whole winter 2013-14 period, shoreline erosion for all monitoring sites reached -6.3m on average, with a minimum of about -0.2 m and a maximum of -30.1m. Depending on the type of environment, it appears that the dunes have retreated the most, followed by gravel or sandy barriers; the lowest erosion rates concern beaches backed by low cliffs cut in highly consistent materials such as periglacial deposits (head). Considering the three morphogenous episodes, the morphological response in terms of shoreline retreat of beaches and barriers was different. Storm occurring at the beginning of February induced the largest erosive rates partly explained by the large morphological sensitivity of beaches and barriers which were weakened by the previous storm events in the beginning of January. Conversely, the storm of March induced very few impacts. These elements show...

Observations and Predictions of Wave Runup, Extreme Water Levels, and Medium-Term Dune Erosion during Storm Conditions

“…The third and last phase that began in the winter of 2012-2013 is characterized by a positive NAO index. It accompanied warm and stormy winters, especially the winter of 2013-2014 [38,46]. Between December 2013 and March 2014, a cluster of about 12 storm events hit the coast of Brittany with an exceptional frequency.…”

“…This results in mild and wet winters characterized by active storms that hit Western Europe at the latitude of England and Brittany. The winter of 1989-1990 is a good example of this weather pattern [44,45], as well as the winter of 2013-2014, during which a cluster of a dozen storms hit the Brittany coast [38,46]. In contrast, the second phase, from 2008 to 2012, is characterized by a negative index (NAO−).…”

“…During these four years, the sediment budget of the dune increased considerably, inducing an elevation of the foot of the dune up to 2 m. Therefore, extreme water levels never hit the foot of the dune during this entire period except during the short stage mentioned above. The two major dune erosion stages which were recorded are related to the big storm event of 10 March 2008 [36,37], and to a cluster of storms occurring during the winter of 2013-2014 [38]. …”

“…It was characterized by dune progradation reaching +12 m on the zones with the most accretion [37]. Finally, during the winter of 2013-2014, a cluster of about 12 storm events hit the coast of Brittany with an exceptional frequency [38]. [33] to calculate beach slope for the runup calculation.…”

“…As shown in Figure 10b, the foot of the dune was situated at 15.5 m from the head profile mark at the beginning of the survey. It retreated over more than 3 m during the big storm of 10 March 2008 [36,37], and retreated again over 3 m during the storm of 1 February 2014 [38]. These three reference distances (15.5 m, 12 m, and 9.5 m) were used for the calculation of the dune foot height.…”