Non-Linear modelling of Extreme High-Angle Waves Instability

Peer ReviewedPostprint (published version

MORFO70: Un nuevo modelo numérico para estudiar la interacción entre patrones morfológicos en playas y la dinámica de la línea de costa.

Peer ReviewedPostprint (published version

A new instability mechanism related to high-angle waves

Waves with a large incidence angle in deep water can drive a morphodynamic instability on a sandy coast whereby shoreline sand waves, cuspate forelands, and spits can emerge. This instability is related to bathymetric perturbations extending offshore in the shoaling zone. Here, we explore a different mechanism where the large incidence angle is supposed to occur at breaking and the bathymetric perturbations occur only in the surf zone. For wave incidence angles at breaking above ˜¿45°, the one-line approximation of coastal dynamics predicts an unstable shoreline. This instability (EHAWI) is scale-free and the growth rate increases without bound for decreasing wavelength. Here we use a 2DH morphodynamic model resolving surf zone instabilities to investigate whether EHAWI could approximate a real instability in nature with a characteristic length scale. Assuming very idealized conditions on the bathymetric profile and sediment transport, we find a 2DH instability mode consisting of shore-oblique up-current bars coupled to a meandering of the longshore current. This mode grows for high-angle waves, above about 30° (offshore) and the maximum growth rate occurs for the angle maximizing the angle at breaking, about 70° (offshore). The dominant wavelength is of the order of the surf zone width. Interestingly, for long sand waves, the growth rate never becomes negative and it matches very well the anti-diffusive behavior of EHAWI. This distinguishes the present instability mode from other modes found in previous studies for other bathymetric and sediment transport conditions. Thus, we conclude that EHAWI approximates a real morphodynamic instability only for quite particular conditions. In such case, a characteristic length scale of the instability emerges thanks to surf zone processes that damp short wavelengths.Postprint (author's final draft

A new shoreline instability mechanism related to high-angle waves

A new shoreline instability triggered by high-angle waves and leading to sand wave formation is investigated. In contrast with the well-known high angle wave instability, which involves both the surf and shoaling zones and develops at km-scale wavelengths, the present mechanism involves only the surf zone. The emerging morphology features up-current oriented bars coupled to a meandering of the longshore current. The dominant wavelengths scale with the surf zone width, Xb= O(102 m) and the characteristic growth times are O(1 day). The instability occurs only above a critical angle ¿30° in deep water and its maximum intensity is for ¿70°. It is associated to the gradients in longshore transport and, for wavelengths larger than Xb, the growth rate matches that of the instability predicted by the one-line approximation for an angle at breaking which is above the angle maximizing alongshore transport.Postprint (published version

On the morphodynamic stability of intertidal environments and the role of vegetation

We describe the coupled biotic and abiotic dynamics in intertidal environments using a point model that includes suspended sediment deposition, wave-and current-driven erosion, biofilm sediment stabilization, and sediment production and stabilization by vegetation. We explore the effects of two widely different types of vegetation: salt-marsh vegetation and mangroves. These two types of vegetation, which colonize distinct geographical areas, are characterized by different biomass productivities and stabilization mechanisms. We show that changing vegetation and biofilm properties result in differing stable states, both in their type and number. The presence of the biofilm exerts a dominant control on the tidal flat (lower intertidal) equilibrium elevation and stability. Vegetation controls the elevation of the marsh platform (i.e., the upper intertidal equilibrium). The two types of vegetation considered lead to similar effects on the stability of the system despite their distinct biophysical interactions, they ultimately lead to similar e¿ects on the stability of the system.Peer Reviewe

Non-Linear modelling of Extreme High-Angle Waves Instability

Peer Reviewe