Analysis of morphodynamic beach states along the South Brittany coast

Abstract

This study presents a classification of morphodynamic beach states monitored along the South Brittany coast, on the western part of France. The investigated area faces the Atlantic Ocean and it is exposed to prevailing wind and waves mainly coming from the NW to SW and experience similar waves and wind climates as most beaches located on the French Atlantic coast. However, morphological settings of Brittany coast display more specific characteristics which contribute to make the morphodynamic beach behaviours more complex. To characterise the morphodynamic beach states, a beach survey programme was carried out from February 2008 to June 2009. Twenty five beaches were surveyed. Beach profiles were levelled from the toe of the dune to the water level line using a TRIMBLE electronic theodolite. Wave data were obtained from two offshore buoys, named l’Ile d’Yeu Nord (8503) and Le plateau du Four (04403), and owned by CETMEF (Centre d'Etudes Techniques Maritimes Et Fluviales). Wave characteristics used include significant wave height (Hs) and its associated period (Ts). Morphodynamic beach states were determined by the surf scaling and the surf similarity parameters using breaker wave’s height values derived from offshore wave height. Temporal changes of morphodynamic beach states were assessed in relation with beaches profiles and hydrodynamic factors variations. Six different morphodynamic beach behaviours were identified, mainly varying between reflective and intermediate states. Multivariate statistical analyses were then carried out to assess the main factors driving morphologic and morphodynamic beach disparities. Obtained results are related to both coastline orientation and nearshore bedrocks distribution, enhancing the role of these specific morphologic constraints in the complex behaviour of South Brittany coast.