Comment on “High‐angle wave instability and emergent shoreline shapes: 1. Modeling of sand waves, flying spits, and capes” by Andrew D. Ashton and A. Brad Murray

Abstract

[1] Ashton and Murray [2006a, 2006b] (henceforth referred to as AM06) showed that coastal evolution can be strongly affected by wave angle when it is formulated in terms of deepwater wave quantities. Most alongshore sediment transport formulae (i.e., CERC formulation) are expressed in terms of the breaking-wave height and the breaking-wave angle. In such formulae, if the breakingwave height is held constant, the alongshore sediment transport is maximized when waves are breaking at an angle of 45°. For breaking-wave angles greater than 45°, the shoreline is unstable [Wang and Le Mehaute, 1980], although it is normally argued that due to wave refraction angles are typically much smaller. AM06 demonstrated that shoreline instability should occur whenever waves approach a shoreline with deep water wave angles greater than approximately 45°, even if breaking-wave angles are much smaller. This process may lead to the self-organization of cuspate shoreline features. We would like to explore the influence of this mechanism in the formation and preservation of the irregular cuspate features along Carchuna beach (southern Spain), as suggested by Ashton and Murray [2004].[2] Carchuna is a 4 km long semireflective beach exhibiting large-scale shoreline features (Figure 1), with a mean alignment W-E [Ortega-Sánchez et al., 2003]. The sediment is very heterogeneous, varying from fine to coarse, and the most energetic wave climate approaches are W, WSW, SW, ESE and E, thus they are strongly oblique to the initially straight shoreline (Figure 1c). AM06 simulations suggest four types of shoreline response depending on the high angle wave climate: migrating alongshore sand waves, cuspate bumps, flying spits and reconnecting spits. The authors controlled the wave climate using two variables (other variables such as wave height are held constant): the fraction of waves approaching from high versus low angles (U) and the wave climate asymmetry (A). Detailed offshore wave climate analysis for Carchuna yields U = 0.92 and A $ 0.5, thus indicating strong obliquity. According to Figure 9 of AM06a the Carchuna shoreline morphology should be characterized by the existence of pronounced cuspate bumps. These authors showed that such features extend in the cross-shore direction with a roughly similar spacing when wave climate is relatively symmetric. Higher U values increase the cross-shore extent of the features and make them more pointed.[3] As a first approach, there is general good agreement between Carchuna large-scale features and the results of the AM06 model, and five cuspate bumps can be found, which extend cross-shore at different distances. Nevertheless, there are some important aspects than are not reproduced by their model: (1) the protruding horns do not have a symmetric shape (Figure 1b, H1 to H5); (2) there is a general alongshore variation of the features shape: from H1 to H5 they change from a generally pointed to curvy shape; (3) the relative orientation of the coastline changes between features (between H1 and H2 the ori...

The influence of shelf-indenting canyons and infralittoral prograding wedges on coastal morphology: The Carchuna system in Southern Spain

“…As a first approximation, Ortega-Sánchez et al (2008b) applied the model developed by Ashton and Murray (2006a,b) and found good general agreement between the large-scale features of Carchuna Beach and the results obtained using the Ashton and Murray (2006a) model. According to this model, instabilities and rhythmic features tend to appear on coastlines where the waves impinge with an obliquity greater than 45°with respect to the shore-normal orientation.…”

“…Nevertheless, there were some aspects that were not reproduced by the model: 1) the protruding horns do not have a symmetric shape, 2) there is a general alongshore variation in the shape of the features, and 3) the relative orientation of the shoreline changes between features. Ortega-Sánchez et al (2008b) concluded that nearshore wave energy modulation should be considered to fully explain the morphology of Carchuna Beach.…”

Atmospheric‐hydrodynamic coupling in the nearshore

“…The beach presents a complex shoreline morphology, characterized by four large scale irregularly spaced cuspate features limited by seaward protruding horns and a peculiar bathymetry comprised of fluvial valleys to the west of the Cape. The beach experiences the co‐existence of a number of physical processes (wave refraction patterns over submarine fluvial canyons, progressive or quasi‐stationary edge waves trapped between the horns, offshore high angle waves instabilities, wind‐wave coupling) that may define its morphology [ Ortega‐Sánchez et al , 2003, 2008; Quevedo et al , 2008].…”

Reply to comment by M. Ortega‐Sánchez et al. on “High‐angle wave instability and emergent shoreline shapes: 1. Modeling of sand waves, flying spits, and capes”

“… Ashton and Murray [2006b] present a methodology for more rigorously analyzing and characterizing a wave climate, introducing a stability parameter, Γ. Approximate values of this stability parameter can be computed using wave energy roses such as those presented in the comment by Ortega‐Sánchez et al [2008], and its computation should be more straightforward than approximations of the model parameter H .…”

“…A and U are model-specific parameters used to determine the wave climate in the numerical experiments [e.g., Ashton and Murray, 2006a, Figure 8], and should not be directly related to a natural wave climate with the expectation of a quantitatively accurate comparison. Ashton and Murray [2006b] present a methodology for more rigorously analyzing and characterizing a wave climate, introducing a stability parameter, G. Approximate values of this stability parameter can be computed using wave energy roses such as those presented in the comment by Ortega-Sánchez et al [2008], and its computation should be more straightforward than approximations of the model parameter H.…”