Morphodynamics of a microtidal sandbar beach under storm condition: case of the Mahdia nearshore, Tunisia

L?effet du changement climatique s?illustre dans l?élévation du niveau de mer et l?agressivité ascendante des évènements météo-marins sur les milieux côtiers, voué à une accélération extrêmement forte (IPCC 2007). L?action des tempêtes marines va entraîner des réponses morphologiques rapides et des impacts importants sur les littoraux sableux particulièrement l?érosion des plages et le recul du trait de côte. Certains rivages tunisiens sont déjà touchés par cet aléa notamment la baie de Ras Dimas-Cap Africa (Mahdia), objet de cette étude. L?objectif de ce travail consiste à suivre la réponse de deux profils de plage sableuse dissipative à barres subtidales sous des conditions de forçages hydrodynamiques de tempête au niveau de Mahdia au Golfe de Hammamet en Tunisie. La méthodologie adoptée s?est basée sur des mesures bathymétriques de l?avant côte acquises lors de deux campagnes de mesures en 2004 et 2006, complétées par la simulation numérique grâce à un nouvel modèle morphodynamique: Système de Modélisation côtière (SMC). Cet outil a été développé et validé en Espagne par la Direction Générale des Côtes et le Groupe d?Ingénierie Océanographique et Côtière (GIOC) de l?Université de Cantabrie. La modélisation côtière appliquée fournit une estimation de l?évolution spatiale de la hauteur de la houle en fonction de la bathymétrie, un suivi de la surélévation de la masse d?eau ainsi que le déplacement des barres sédimentaires engendré par les courants de fond (courant de retour). Soumises à des conditions de haute énergie d?une houle de tempête, les barres sableuses migrent vers le large puis reviennent vers la côte. Le rapprochement des barres du rivage est associé à un engraissement de la plage sous des conditions de forte houle au niveau du centre de la baie où la plage est à faible pente et à deux barres sableuses. Néanmoins au sud de la baie, où une seule barre est présente, le bilan sédimentaire s?oriente vers une érosion et un recul de la ligne de rivage lors d?événements météo-marins de forte énergie et il est marqué par une migration vers le large de la barre subtidale.The effects of global climate change are illustrated by rising sea level and the increase of intensity of storm events. The highest waves generated attack the upper beach. This combination of the two factors would produce severe erosion, threatening the coastline. The Ras Dimas-Cape Africa (Mahdia, Tunisia) coastline and adjacent areas are particularly exposed to such impact. The aim of this study is to assess the response of wave-dominated sandbar beach under storm events in two different cross shore cases in the Mahdia beaches. The methodology adopted is based on the surveyed bathymetry of the nearshore (2004-2006), completed with the morphodynamic numerical model SMC (Tunisian SMC software, Hispano- Tunisian project of coastal rehabilitation, TUNEROSION). The model has been developed and validated by the Spanish Government and the Ocean and Coastal research Group (GIOC) (University of Cantabria, Spain). Coastal modelling provides the wave high and wave induced set-up created by breaking wave and the sediment transport volume calculated as a function of local conditions. The response of sandbars to storm condition is moving onshore and migrates seaward. The morphodynamic result is however an accretion of the nearshore under the storm wave at the center of the bay (two sandbars with gentle slope). Nevertheless, in the southeast beach, where we find single sandbar, the sediment budget is characterized by shoreline retreat during high wave energy and the migration offshore of this sanbar beach

Alarming coastal vulnerability of the deltaic and sandy beaches of North Africa

International audienceThe arid coasts of North Africa, extending over 4633 km from the Gulf of Tunis to the Nile Delta, are undergoing pronounced shoreline retreats and coastal floodings that are reported as a consequence of the ongoing sea level rise resulting from global warming. Of particular interest are the abnormal shoreline dynamics for deltaic and sandy beaches, which are severely impacted by abrupt decadal variabilities in both climatic and anthropogenic drivers resulting in their increased vulnerability to disturbances from coastal hazards. Unfortunately, the evolution, distribution and impacts of these drivers remain largely unquantified, let alone understood, for these extensive arid coasts that harbor the major portion of North Africa's population as well as unique and fragile marine ecosystems. To address this deficiency, we use GIS-based multi-criteria approaches combined with analytic hierarchy process to map the Coastal Vulnerability Index and the Socioeconomic Vulnerability Index along these coasts to investigate the amplitude and extent of shoreline deterioration resulting from sudden fluctuations in sediment transport to the coastline. We use the western bay of the Gulf of Tunis, the coasts of Tripoli and the Nile Delta as three validation sites for our vulnerability assessment. The statistical Integrated Coastal Vulnerability Index map reveals that 47% of arid North African coasts are characterized by high to very high vulnerability. In particular, we observe that the densely populated deltaic coasts in both Tunisia and Egypt are 70% more vulnerable than any others coast in the eastern Mediterranean Basin. These abnormally high-vulnerability extensive areas are also correlated with significant deterioration of coastal aquifers and hence in crop production, compromising local food security and resulting in increasing outflow migration trends. Both Tunisia and Egypt observed dramatic increases in the net population outflow migration by respectively 62% and 248% between 2000 and 2016, mostly from coastal areas. Our source analysis of the amplitude and extent of these high coastal vulnerabilities suggests that they result from the anthropogenic drivers of damming and rapid urban growth over the last few decades rather than the effects of global warming