Shoreline sand waves along the catalan coast

The beach of Calella, north of Barcelona, in the Catalan coast, features a series of shoreline sand waves with wavelengths ranging from 700 to 1400 m that match with similar undulations in the -5 m bathymetric line. Historical satellite images from 2002 till 2010 show that these undulations slightly change in time. The wave climate on that stretch of the Catalan coast has a large proportion of waves from the E-NE and from the SW, i.e., with high angles with respect to shore normal rending the shoreline potentially unstable. Here we show that those sand waves might be due to that instability. Model results, both Linear Stability Analysis and nonlinear time evolution, show that the shoreline is nearly at the threshold for instability and that the emergent wavelengths are roughly consistent with the observed ones.Postprint (published version

Self-organized morphological patterns in coastal dynamics

the meaning and significance of self-organization processes in coastal morphodynamics is presented. Two types of coastal self-organized patterns are discussed: crescentic bars in the surf zone and free shoreline sand waves emerging along coastlines where the wave incidence is very oblique to the shore. The coupling between wave breaking and bathymetry in the surf zone is presented as the mechanism which is responsible for crescentic bar formation. Numerical modelling illustrating this process is presented. Shoreline sand waves are described and numerical model results showing their growth and propagation are shown

Potential instabilities of Catalan coastline induced by high-angle waves

The relatively long sandy beaches of El Maresme, and the bimodal wave climate of the Catalan coast, with dominant E-ENE and SSW waves that leads to high angle of incidence, propitiate good conditions for high angle of wave instability. Subtle undulations of the shoreline and the more intense undulations of the bathymetric contour line of 5 meters depth, with wavelengths of about 1300 m, can be found at that stretch of the coast. A morphodynamic model has been used to test if such undulations could be generated by high angle wave instability. Model results, for wave period of 4 s, show that at El Maresme coast high angle wave instability may develop with time rate of about 1 year and with dominant wavelength that ranges from 600 to 1400 m. For the wave climate of the Catalan coast, wave heights of 0.5 – 1 m and a mean peak period of 5.6 s, and at the steep beaches of El Mareme, it has been found that the wavelength of the instability is in good agreement with the observed undulations, and depends in a sensitive way of the mean slope of the bathymetric profile, and on the length of the bathymetric perturbation.Postprint (author’s final draft

Potential instabilities of Catalan coastline induced by high-angle waves

The relatively long sandy beaches of El Maresme, and the bimodal wave climate of the Catalan coast, with dominant E-ENE and SSW waves that leads to high angle of incidence, propitiate good conditions for high angle of wave instability. Subtle undulations of the shoreline and the more intense undulations of the bathymetric contour line of 5 meters depth, with wavelengths of about 1300 m, can be found at that stretch of the coast. A morphodynamic model has been used to test if such undulations could be generated by high angle wave instability. Model results, for wave period of 4 s, show that at El Maresme coast high angle wave instability may develop with time rate of about 1 year and with dominant wavelength that ranges from 600 to 1400 m. For the wave climate of the Catalan coast, wave heights of 0.5 – 1 m and a mean peak period of 5.6 s, and at the steep beaches of El Mareme, it has been found that the wavelength of the instability is in good agreement with the observed undulations, and depends in a sensitive way of the mean slope of the bathymetric profile, and on the length of the bathymetric perturbation

Potential instabilities of Catalan coastline induced by high-angle waves

The relatively long sandy beaches of El Maresme, and the bimodal wave climate of the Catalan coast, with dominant E-ENE and SSW waves that leads to high angle of incidence, propitiate good conditions for high angle of wave instability. Subtle undulations of the shoreline and the more intense undulations of the bathymetric contour line of 5 meters depth, with wavelengths of about 1300 m, can be found at that stretch of the coast. A morphodynamic model has been used to test if such undulations could be generated by high angle wave instability. Model results, for wave period of 4 s, show that at El Maresme coast high angle wave instability may develop with time rate of about 1 year and with dominant wavelength that ranges from 600 to 1400 m. For the wave climate of the Catalan coast, wave heights of 0.5 – 1 m and a mean peak period of 5.6 s, and at the steep beaches of El Mareme, it has been found that the wavelength of the instability is in good agreement with the observed undulations, and depends in a sensitive way of the mean slope of the bathymetric profile, and on the length of the bathymetric perturbation

Generation and nonlinear evolution os shore-oblique/transverse sand bars

The coupling between topography, waves and currents in the surf zone may self-organize to produce the formation of shore-transverse or shore-oblique sand bars on an otherwise alongshore uniform beach. In the absence of shore-parallel bars, this has been shown by previous studies of linear stability analysis, but is now extended to the finite-amplitude regime. To this end, a nonlinear model coupling wave transformation and breaking, a shallow-water equations solver, sediment transport and bed updating is developed. The sediment flux consists of a stirring factor multiplied by the depth-averaged current plus a downslope correction. It is found that the cross-shore profile of the ratio of stirring factor to water depth together with the wave incidence angle primarily determine the shape and the type of bars, either transverse or oblique to the shore. In the latter case, they can open an acute angle against the current (up-current oriented) or with the current (down-current oriented). At the initial stages of development, both the intensity of the instability which is responsible for the formation of the bars and the damping due to downslope transport grow at a similar rate with bar amplitude, the former being somewhat stronger. As bars keep on growing, their finite-amplitude shape either enhances downslope transport or weakens the instability mechanism so that an equilibrium between both opposing tendencies occurs, leading to a final saturated amplitude. The overall shape of the saturated bars in plan view is similar to that of the small-amplitude ones. However, the final spacings may be up to a factor of 2 larger and final celerities can also be about a factor of 2 smaller or larger. In the case of alongshore migrating bars, the asymmetry of the longshore sections, the lee being steeper than the stoss, is well reproduced. Complex dynamics with merging and splitting of individual bars sometimes occur. Finally, in the case of shore-normal incidence the rip currents in the troughs between the bars are jet-like while the onshore return flow is wider and weaker as is observed in nature

Modelling shoreline sand waves: application to the coast of Namibia

The SW coast of Africa (Namibia and part of Angola) features very long sandy beaches and a wave climate dominated by energetic swells from the SSW, therefore approaching the coast with a very high obliquity. Satellite images reveal that along that coast there are many shoreline sand waves with wavelengths ranging from 2 to 8 km. A more detailed study, including a Fourier analysis of the shoreline position, confirms a high spectral density concentration at these lengths scales. Also, it becomes apparent that at least some of the sand waves are dynamically active rather than being controlled by the geological setting. A morphodynamic model is used to test the hypothesis that these sand waves could emerge as free morphodynamic instabilities of the coastline due to the obliquity in wave incidence. It is found that the wave period, Tp, is crucial to establish the tendency to stability or instability, instability increasing for decreasing period, whilst there is some discrepancy in the observed periods. Model results for Tp = 7 s clearly show the tendency for the coast to develop free sand waves at 2 km wavelength within a few years, which migrate to the north at rates of 0.6-0.7 km/yr. For Tp = 8 s, instability is weaker and rather sensitive to other factors as the underlying bathymetry. In this case, the coast seems to be nearly at neutral stability so that sand waves originated from other mechanisms can propagate downdrift with little decay.Postprint (author’s final draft

Modelling shoreline sand waves: application to the coast of Namibia

The SW coast of Africa (Namibia and part of Angola) features very long sandy beaches and a wave climate dominated by energetic swells from the SSW, therefore approaching the coast with a very high obliquity. Satellite images reveal that along that coast there are many shoreline sand waves with wavelengths ranging from 2 to 8 km. A more detailed study, including a Fourier analysis of the shoreline position, confirms a high spectral density concentration at these lengths scales. Also, it becomes apparent that at least some of the sand waves are dynamically active rather than being controlled by the geological setting. A morphodynamic model is used to test the hypothesis that these sand waves could emerge as free morphodynamic instabilities of the coastline due to the obliquity in wave incidence. It is found that the wave period, Tp, is crucial to establish the tendency to stability or instability, instability increasing for decreasing period, whilst there is some discrepancy in the observed periods. Model results for Tp = 7 s clearly show the tendency for the coast to develop free sand waves at 2 km wavelength within a few years, which migrate to the north at rates of 0.6-0.7 km/yr. For Tp = 8 s, instability is weaker and rather sensitive to other factors as the underlying bathymetry. In this case, the coast seems to be nearly at neutral stability so that sand waves originated from other mechanisms can propagate downdrift with little decay