Modelling of sediment transport and morphological evolution under the combined action of waves and currents

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

Widespread inundation of Pacific islands triggered by distant-source wind-waves

It is essential to understand the causes of sea level extremes in order to anticipate and respond to coastal flooding (inundation), and to adapt to sea level rise. We investigate a series of inundation events which occurred across the western Pacific over several consecutive days during December 2008, causing severe impacts to five Pacific Island nations. These events were not associated with commonly identified causes: tropical cyclones or unusually large astronomical tides. Instead, the dissipation of wind-waves generated by distant extra-tropical cyclones (swell) was the main cause, although regional sea level variability, including recent accelerated rise, significantly contributed to the severity of impact experienced at many locations. The implication of recent sea level rise in the severity of these events suggests that episodic swell will increasingly cause major impacts of the nature described herein, although such impacts will continue to be modulated by El Niño/Southern Oscillation (ENSO) variability in the region. Significantly, tide gauges recorded little evidence of extreme sea levels during the event, implying that causes of extreme sea levels inferred from tide gauge analysis are unlikely to include this important cause of inundation. Therefore, any assessment of inundation risk predicated on tide gauge information (as well as larger scale sea level information such as satellite altimetry) may fail at many locations in the Pacific. To be accurate, such efforts must include information on the relationship between wave climate, wave forecasts and local extreme water levels. Further development of related early warning systems will become more pertinent as modern SLR continues to add to the magnitude of extremes