Summary
210
Algeria possesses a coastline of 1622 km whose band represents 1.9% of the national territory
on which 37% of the population is concentrated. However, the Algiers coast, and particularly
the Algiers bay beaches, being highly anthropized and at the same time highly coveted, suffer
from the erosive phenomenon that has affected nearly 80% of sandy beaches worldwide. The
coastal dynamics along the Algiers bay beaches is strongly influenced by hydrodynamic agents,
morphological factors, human activities, societal infrastructures and the presence of rigid
structures, which perturbs their hydro-sedimentary balance. For this purpose, and in order to
better understand the hydrosedimentary functioning of the area in calm and stormy periods and
to assess the morphological response of the beaches to such forcing, hydrographic (Simrad
Cruise7), currentological (Global water Fp101 and Valeport 106) and sedimentological (Benne
Ven-Veen, unidirectional and multidirectional sediment traps) surveys have been carried out.
In parallel to these measurements, the INFOPLAZA sea state database between 1992 and 2020
and the DATWELL wave buoy records between 1998 and 1999 were extensively exploited. In
addition, a statistical analysis of the climate of offshore waves/winds, a statistical modeling of
extreme values, the action of hydrodynamic agents on sediments (estimation of the shear stress),
a thresholding of minor and major storms, as well as a numerical modeling of coastal processes
using the model (Mike 21/3 FM) have been performed. In a context of coastline evolution, the
study of the morphological response of the shoreline has shown a notable retreat of 40% of the
Algiers bay beaches during the last 63 years. This study also highlights the prevalence of a sea
state similar to that of a wind waves than a swell while emphasizing that coastal hydrodynamics
(especially the longshore current), the rate of sediment transport and shallow water
morphological changes are deeply impacted by the very strong contrast between winter and
summer periods in terms of significant wave heights (very energetic waves in winter), storm
surge occurrence (low in summer) and dominant wave direction (the dominance of the WNW
sector in winter and the NNE sector in summer). However, it is valuable to clarify the influence
of d50 on the dominant pattern of sediment transport in the surf zone, since winter cross-shore
sediment fluxes play a major role in natural coastal equilibrium processes within very coarse
sand beaches characterized by limited longitudinal flow. In addition, minor storm threshold
values were found to be strongly and uniquely influenced by d50, leading us to set the Hs
threshold at 01 m (≈ Hs, 50%) for fine to medium sediment beaches and 1.5 m (≈ Hs, 75%) for
coarse to very coarse sediment beaches while storms exceeding a height of 2.7 m (≈ Hs, 95%) are
considered major and values of Hs < Hs, 50% are considered dominant only on seasonal beach
dynamics. By gathering all the information and the results obtained, the collaboration between
the different actors of the coastal management should allow a concerted, global and efficient
management of this space.
Keywords : Algiers Bay, erosion, hydrodynamics, extreme value theory, shear stress, numerical
modeling, sediment transport, surf zone.
210
Algeria possesses a coastline of 1622 km whose band represents 1.9% of the national territory
on which 37% of the population is concentrated. However, the Algiers coast, and particularly
the Algiers bay beaches, being highly anthropized and at the same time highly coveted, suffer
from the erosive phenomenon that has affected nearly 80% of sandy beaches worldwide. The
coastal dynamics along the Algiers bay beaches is strongly influenced by hydrodynamic agents,
morphological factors, human activities, societal infrastructures and the presence of rigid
structures, which perturbs their hydro-sedimentary balance. For this purpose, and in order to
better understand the hydrosedimentary functioning of the area in calm and stormy periods and
to assess the morphological response of the beaches to such forcing, hydrographic (Simrad
Cruise7), currentological (Global water Fp101 and Valeport 106) and sedimentological (Benne
Ven-Veen, unidirectional and multidirectional sediment traps) surveys have been carried out.
In parallel to these measurements, the INFOPLAZA sea state database between 1992 and 2020
and the DATWELL wave buoy records between 1998 and 1999 were extensively exploited. In
addition, a statistical analysis of the climate of offshore waves/winds, a statistical modeling of
extreme values, the action of hydrodynamic agents on sediments (estimation of the shear stress),
a thresholding of minor and major storms, as well as a numerical modeling of coastal processes
using the model (Mike 21/3 FM) have been performed. In a context of coastline evolution, the
study of the morphological response of the shoreline has shown a notable retreat of 40% of the
Algiers bay beaches during the last 63 years. This study also highlights the prevalence of a sea
state similar to that of a wind waves than a swell while emphasizing that coastal hydrodynamics
(especially the longshore current), the rate of sediment transport and shallow water
morphological changes are deeply impacted by the very strong contrast between winter and
summer periods in terms of significant wave heights (very energetic waves in winter), storm
surge occurrence (low in summer) and dominant wave direction (the dominance of the WNW
sector in winter and the NNE sector in summer). However, it is valuable to clarify the influence
of d50 on the dominant pattern of sediment transport in the surf zone, since winter cross-shore
sediment fluxes play a major role in natural coastal equilibrium processes within very coarse
sand beaches characterized by limited longitudinal flow. In addition, minor storm threshold
values were found to be strongly and uniquely influenced by d50, leading us to set the Hs
threshold at 01 m (≈ Hs, 50%) for fine to medium sediment beaches and 1.5 m (≈ Hs, 75%) for
coarse to very coarse sediment beaches while storms exceeding a height of 2.7 m (≈ Hs, 95%) are
considered major and values of Hs < Hs, 50% are considered dominant only on seasonal beach
dynamics. By gathering all the information and the results obtained, the collaboration between
the different actors of the coastal management should allow a concerted, global and efficient
management of this space.
Keywords : Algiers Bay, erosion, hydrodynamics, extreme value theory, shear stress, numerical
modeling, sediment transport, surf zone.