Abstract
The central part of Platier d'Oye beach, Northern France, has been steadily eroding and several groynes were constructed on this beach in order to stop the erosion. Data from one field experiment combining hydrodynamic (wave and mean currents) and topographic measurements on this macrotidal beach with intertidal bar morphology, were combined with results of monthly beach profile surveys (five months) in order to highlight the morphological characteristics and morphodynamics behaviour of a ridge and runnel beach located in Northern France. To compare with short-term measurements, medium-term temporal analysis of shoreline recession and accretion were carried out between 2000 and 2005. The aim of this article is to evaluate the influence of groynes on the rate of shoreline change, particularly on ridges and runnels beach morphodynamics. Our results suggest that perpendicular structures disturb longshore currents and wave direction under various hydro-meteorological conditions. Our observations show a destabilisation of the bar/trough system (reduction of the number of bars and foreshore lowering) related to the groynes. Several hypotheses are proposed to explain the poor efficiency of these structures on a medium time scale.