Coastal processes and nearshore hydrodynamics under high contrast wave exposure, Bateau-cassé and Stamboul coasts, Algiers Bay

Abstract

The coastal dynamics along the sandy beaches in the Algiers bay is strongly influenced by the hydrodynamic agents, morphological factors, human activities, societal infrastructure and presence of hard structures, which disrupts their hydro-sedimentary balance. The present work contributes to the knowledge of hydrodynamic and morpho-sedimentary forcing mechanisms at sandy beaches by focusing on the impact of extreme sea-weather events of the two most contrasting periods of the year (winter and summer) on coastal processes. Two beaches in the eastern part of Algiers bay were studied by combining field measurements of currents and sediment flux with numerical models (MIKE 21/3 Coupled Model FM). The model validation was done based on the measured nearshore current and sediment transport during low to moderate energy swell conditions with current meter ‘Global water Fp101’ and ‘Krauss sediment traps’. Time series of wind and wave established by Infoplaza Marine weather database were used as input at the open boundaries of the model domain. The results showed a reversal of the dominant direction of the longshore currents, and a very high prevalence of cross-shore sediment transport over longshore sediment transport (LST) due to the coarse grain size of the sediment (very coarse sand). The results indicated that, waves coming from the west prevail in winter have an average wave height varies from 1.5 to 2.1 m, driven a longshore current of 0.17–0.27 m/s, with an estimated seasonal cross-shore sediment transport between 1530 and 5540 m3/year/m. During the summer season, waves approaching from the northeast dominate with an average of Hs that does not exceed 1 m, these waves generate a rip current equal to 0.24 m/s and this current can generate a cross-shore sediment flux in the order of 4000 m3/year/m. The results also reveal the development of morphological landforms represented in the submerged longshore bars at the shallow water depending largely on the hydrodynamic conditions and the cross-shore flux between the land/sea interfaces.