This article presents a methodology for simulating the Algarve coastal circulation using realistic forcing (e.g. low-frequency circulation, tide, high-resolution atmospheric forcing). Low-frequency open boundary conditions are defined via a downscaling of the HYCOM-US operational solution for the Gulf of Cadiz. Atmospheric forcing is imposed using the MM5 high-resolution mesoscale model (9 km resolution near the coast). A 3-level nesting system based on the MOHID numerical system is implemented. The higher nesting level has a horizontal resolution of 0.02° along the Algarve coast. The methodology is first validated qualitatively. A comparison between the numerical results and the conceptual model of the circulation described in the literature is presented. A quantitative validation is also performed, based mainly on remote sensing data (sea surface temperature and altimetry) available for July 2004. The numerical system is able to reproduce many of the circulation features described in the literature (e.g. Azores current recirculation in the Gulf of Cadiz, the upwelling jet, Mediterranean Water undercurrent, Mediterranean Outflow splitting, generation of meddies) and observed with remote-sensing data (e.g. the signature in sea surface temperature (SST) during a regime of upwelling relaxation).

Coastal defence structures are often constructed to prevent beach erosion. However, poorly designed structures may cause serious erosion problems in the downdrift direction. Morphological models are useful tools to predict such impacts and assess the efficiency of defence structures for different scenarios. Nevertheless, morphological modelling is still a topic under intense research effort. The processes simulated by a morphological model depend on model complexity. For instance, undertow currents are neglected in coastal area models (2DH), which is a limitation for simulating the evolution of beach profiles for long periods. Model limitations are generally overcome by predefining invariant equilibrium profiles that are allowed to shift offshore or onshore. A more flexible approach is described in this paper, which can be generalised to 3-D models. The present work is based on the coupling of the MOHID modelling system and the SWAN wave model. The impacts of different designs of detached breakwaters and groynes were simulated in a schematic beach configuration following a 2DH approach. The results of bathymetry evolution are in agreement with the patterns found in the literature for several existing structures. The model was also tested in a 3-D test case to simulate the formation of sandbars by undertow currents. The findings of this work confirmed the applicability of the MOHID modelling system to study sediment transport and morphological changes in coastal zones under the combined action of waves and currents. The same modelling methodology was applied to a coastal zone (Costa da Caparica) located at the mouth of a mesotidal estuary (Tagus Estuary, Portugal) to evaluate the hydrodynamics and sediment transport both in calm water conditions and during events of highly energetic waves. The MOHID code is available in the GitHub repository.