Abstract
Abstract. 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 incidence angle with respect to shore normal) could induce an instability of the shoreline (Ashton et al., 2001). More recently, a scaling for their wavelength has also been proposed (van den Berg et al., 2014). The existing studies rely mainly on modelling but quantitative field tests are lacking. We aim at investigating how both the formation hypothesis of these shoreline undulations and the theoretical scaling do fit with nature at a global scale. The first step, which is the goal of this paper, is to set up the methodology by analyzing the Atlantic African coast as test site. First, based on global databases, shoreline wavelength LS, wave characteristics (obliquity θW and wavelength λW) and mean shoreface slope β are determined. Then the wave obliquity is confronted with the presence of shoreline undulations. Finally the values of the ratio β LS / λW are estimated and discussed in comparison with the estimate of van den Berg et al. (2014). It is found that the correlation between shoreline sandwave occurrence and wave obliquity is very good, allowing the identification of 5 new potential unstable shoreline stretches, whereas the results on the scaling are not conclusive and deserve further investigations.
Highlights
IntroductionSandy coasts are characterized by a number of rhythmic patterns like, for instance, cusps (metric scale), megacusps (hundreds of meters) and shoreline sandwaves (kilometric scale)
Sandy coasts are characterized by a number of rhythmic patterns like, for instance, cusps, megacusps and shoreline sandwaves
A first research question (Q1) is : how does the hypothesis of HAWI origin of shoreline undulations fit with nature? Presently, it is still not clear if the observed shoreline sandwaves result from HAWI
Summary
Sandy coasts are characterized by a number of rhythmic patterns like, for instance, cusps (metric scale), megacusps (hundreds of meters) and shoreline sandwaves (kilometric scale). On one hand it is theoretically clear that a rectilinear shoreline can be unstable in case of high wave incidence angle (High Angle Wave Instability: HAWI) and that from such instability a number of large scale shoreline features may appear (Ashton et al, 2001), including these kilometric scale shoreline sandwaves This is widely supported by a number of modelling studies, e.g., Ashton et al (2006a), van den Berg et al (2012), Kaergaard and Fredsoe (2013a), showing that the instability develops for deepwater wave incidence θW larger than about 42◦, with respect to shoreline normal. Kaergaard et al (2012) investigated the cross-shore extent of coastline undulations on a site located on the West coast of Denmark, based on specific bathymetric surveys providing temporal and spatial data This field approach allows a better understanding, but cannot be used for a global analysis, because of the lack of such bathymetric surveys.
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