Tsunami-induced morphological change - A model-based impact assessment of the 1755 tsunami in NE Atlantic from the Morocco coast

Tsunamis can be responsible for coastal geomorphic crises. Extensively inundated coasts can suffer widespread and dramatic changes in their morphology due to sediment erosion, transport, and deposition processes. In this paper, we investigate the tsunami-induced morphological changes on coasts prone to inundation and how sediment transport is controlled by both tsunami and sediment characteristics. We focus on a tsunami-prone coast in the NE Atlantic, Tangier-Morocco, that experienced the 1755 Lisbon tsunami. We use a coupled hydrodynamic and morphodynamic numerical model, together with high-resolution digital elevation and sedimentary models, to simulate the impact of tsunamis on the morphology and sediment distribution of Tangier Bay. Due to the uncertainty on the source and effects of the 1755 event, we consider two 1755-like tsunamigenic scenarios that have the potential to cause different levels of inundation. Results show that the coastal zone of Tangier can undergo substantial changes in morphology and sediment distribution following the tsunami impact. For the selected scenarios, the volume of sediments mobilized, at both the offshore and onshore zones of the Tangier site, ranges from 30,000 to 200,000 m3. Moreover, the presence of mobile sediments and conditions favoring tsunami shoaling lead to an increase in wave impact on the coastal configuration. Conversely, the presence of the breakwaters helps to protect the Tangier coast by limiting the extent of the affected area. The results of this study suggest that the assessment of tsunami-induced coastal impact should include the effect on morphology.