How kilometric sandy shoreline undulations correlate with wave and morphology characteristics: preliminary analysis on the Atlantic coast of Africa

摘要

Sandy coasts are characterized by a number of rhythmic patterns like,amongst others, shoreline undulations or sandwaves at a kilometric scale.One hypothesis for their formation is that high angle waves (large incidenceangle with respect to shore normal) could induce an instability of theshoreline (Ashton et al., 2001). More recently, a scaling for theirwavelength has also been proposed (van den Berg et al., 2014). Theexisting studies rely mainly on modelling but quantitative field tests arelacking. We aim at investigating how both the formation hypothesis of theseshoreline undulations and the theoretical scaling do fit with nature at aglobal scale. The first step, which is the goal of this paper, is to set upthe methodology by analyzing the Atlantic African coast as test site. First,based on global databases, shoreline wavelength L_S, wave characteristics(obliquity θ_W and wavelength λ_W) and mean shoreface slope &beta are determined. Thenthe wave obliquity is confronted with the presence of shorelineundulations. Finally the values of the ratio beta; L_S / λ_W areestimated and discussed in comparison with the estimate ofvan den Berg et al. (2014). It is found that the correlation betweenshoreline sandwave occurrence and wave obliquity isvery good, allowing the identification of 5 new potential unstableshoreline stretches, whereas the results on the scalingare not conclusive and deserve further investigations.