Alongshore sediment transport on curvilinear coasts and implications on coastal evolution

Résumé

Coastlines around the world are frequently rectilinear or weakly curved; they exhibit a wide range of shoreline morphological features of different dimensiones superimposed. This Thesis studies the morphodynamics of weakly curvilinear coasts (radius of curvature greater that 1000 m) and the physical processes involved in the development of rhythmical shoreline features. Among these features, this work focus on shoreline undulations, which are defined as medium to large spatial-scale shoreline features that have alongshore dimensions ranging from hundreds to thousands of meters and cross-shore widths from tens to hundreds of meters. This is accomplished using a sequential methodology. First, the locations where these shoreline undulations develop are identified and characterized. For that, these features were identified along 50.000 kms and 15 countries, and different fields of data were collected and included into an European database. Results show that shoreline undulations are strongly related to the presence of at least one of the following peculiarities: (1) a curvilinear mean alignment of the coast; (2) a sudden change in the beach alignment; and (3) the presence of artificial or natural obstacles nearby. The wave climate that favors their appearance is characterized by mild waves and micro or mesotidal conditions. The second step is the identification and quantification of the effects that these types of coasts have on the wave propagation patterns. This is accomplished through field evidences from video imagery and results of a numerical wave propagation model. After the analysis of the results, it is concluded that the presence of curvilinear bathymetries trigger significant alongshore variations in wave energy and wave angle. Both variables are of major importance for the alongshore sediment transport. The third step is the update of an expression for the alongshore sediment transport to include the main characteristic processes involved on curvilinear coasts, as the effect of a curvilinear bathymetry on wave propagation or the alongshore variations on coast characteristics, i.e. beach slope, shoreline angle or sediment size. This approach was conceived as a simple coastal engineering tool to understand how the different environmental, hydrodynamical and morphological variables affect and interact with the shoreline morphology evolution. An effort was made to avoid restrictions on the variables analyzed. To develop the analytical formulation for the alongshore sediment transport, two different simple models for wave refraction were chosen obtaining two final expressions, but any other solution for wave refraction could be used. The following step is the analysis of the implications of the alongshore sediment transport expression previously obtained. Numerical experiments with different beach and wave climate characteristics are performed, and the hydrodynamical consequences are also examined. Three different shoreline tendencies were obtained for undulating shorelines: migration for changes in deep-water wave angle; amplitude growth for increases in wave height or wave period and changes in shoreline geometry; and wavelength changes and asymmetries for coasts with alongshore changes in beach slope. The results also indicate that wave setup may enhance the formation of undulating shoreline features. The fifth step is the definition of a morphological evolution model based on the alongshore sediment transport expression: a one-line type model. An analytical analysis of this model is used to compare the results derived with the presented alongshore sediment transport expression with other recent works. This analysis shows that, unlike these works, with our approach shoreline features develop on curvilinear coasts without the presence of waves with great obliquity respect to the shore. Finally, the last step of the methodology is the application of a numerical solution of the one-line model developed in this Thesis to different specific locations exhibiting shoreline undulations. The results of the model are in good agreement with nature in terms of location, amplitude and wavelength of the undulations for two prograding spit fronts in the Guadalquivir Estuary (Souther Spain) and the Santander Bay (Northern Spain). The asymmetries of a horn-embayment system in Carchuna Beach (Southern Spain) are also reproduced. Moreover, preliminary results on a track of coast near a river mouth indicate that both deficits in the available sediment and irregularities in the bathymetry may trigger the formation of shoreline undulations.