Numerical modeling of processes controlling the migration of tidal inlets

The migration of tidal inlets is the cause of drastic changes in the adjacent barrier beaches. These changes in the coastline have effects in a range of temporal and spatial scales, from the short-term stability of these systems to the geological evolution of the lagoon/barrier system. The morphodynamic processes at inlet systems are closely related to the combined action of a variety of driving forces that create unique hydrodynamic conditions. Tidal currents, waves and river discharge interacting at different levels result in complicated sediment transport patterns which control the stability of its channels. In order to assess the relative influence of the main controlling forces in the lateral inlet migration, two main numerical modeling experiments have been designed based on idealized lagoon/inlet/barrier systems. The numerical model used in this study is the well-known MIKE21 model developed by DHI Water & Environment. Three modules of the MIKE21 Flexible Mesh (FM) modelling system have been applied: the hydrodynamic module (HD), the wave propagation module (SW) and the sediment transport module (ST), with the MIKE21 FM morphological model including the feedback on hydrodynamic, wave and sand transport calculations. Several runs have been carried out for each experiment set up. The experiments include the main variables that control the migration of inlet channels, such as varying wave directions and channel orientation. In experiment 1, with fixed wave incidence direction and varying channel orientation it is possible to assess the influence of the channel orientation in the inlet migration due to the meandering channel effect. In experiment 2, the effect of waves on the exposed channel margin is assessed, since it has important effects on the sediment resuspension and transport which can enhance the channel migration. Summarizing the results of the numerical experiments, it is shown that the channel migration is controlled by the longshore drift, but also strongly influenced by its orientation in relation to the coastline. Tidal currents flowing through the inlet channel during flood or ebb tide have its effects on sediment transport at the exposed channel margin increased as the main axis orientation angle is increased. Considering a tidal range of 1.5 m in the simulations, the differences across the channel can be of up to 1 m/s, with maximum flow velocity reaching 2.2 m/s close to the exposed margin and 1.1 m/s close to the protected margin. This effect can overcome the influence of the longshore drift, resulting in updrift channel migration. Considering an oblique channel, wave action on its exposed margin also increases its erosion, enhancing its migration independently of the longshore drift direction. The numerical experiments show quantitatively that it is the interaction of those variables at different time scales that control the migration of inlet channels.