Observation et modélisation des ondes infra-gravitaires et des non-linéarités des vagues en milieu littoral

Abstract : Wave nonlinearities have long been recognised as being among the main drivers of sediment transport in the coastal zone. However, there are still significant errors in the prediction of this transport associated, partially due to inaccurate predictions of the velocity nonlinearities. The infragravity waves, which coexist with the short waves in the coastal zone, are long-period waves (with 20-200 s) associated to the short-wave groups. Their generation, propagation and dissipation mechanisms are already reasonably well understood, but their influence on sediment transport is still very poorly characterised. In order to (i) improve current predictions of velocity nonlinearities and (ii) investigate the role of infragravity waves in sediment transport, new experiments were carried out both in a fixed-bed wave flume and in a light-weight-sediment wave flume. The physical-modelling data set is used in combination with field data and numerical simulations for studying both subjects. Existing parameterizations of velocity nonlinearities account only for the influence of local wave parameters (e.g. wave height, wave length and water depth), which leads to considerable estimation errors, especially of the maximal values of nonlinearity. This work shows that the velocity nonlinearities depend also on non-local wave parameters: (i) offshore wave steepness, (ii) offshore spectral bandwidth and (iii) beach slope. A new parameterization is proposed, which reduces by about 50% the root-mean-square error relatively to former parameterizations. The experimental results in the light-weight-sediment wave flume demonstrate that wave conditions with the same short-wave energy, but different low-frequency modulation, shape different equilibrium beach profiles. The influence of the infragravity waves on the sediment transport is confirmed and depends on two different mechanisms: (i) advection of the short-wave suspended sediment by the infragravity-waves, which is dependent on the infragravity-wave height and phasing with the short-wave groups and (ii) modulation of short-wave nonlinearities by infragravity-wave motion, both directly and indirectly, through water-depth modulation. Changes in the beachface morphology induced by infragravity waves are connected to beach-profile changes in the surf and inner-shoaling zones, highlighting the existent link between the different zones of the cross-shore beach profile.
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Mariana Vieira Lima Matias Da Rocha. Observation et modélisation des ondes infra-gravitaires et des non-linéarités des vagues en milieu littoral. Sciences de la Terre. Université Grenoble Alpes; Universidade de Aveiro (Portugal); Université Grenoble Alpes (2016-2019), 2016. Français. ⟨NNT : 2016GREAU043⟩. ⟨tel-01400356v2⟩

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