Effects of shoreline and bedrock irregularities on the morphodynamics of the Alexandria coast littoral cell,Egypt

Beach-nearshore profiles combined with beach and surficial sediment samples were analyzed in conjunction with wave, current, littoral drift and sea-level data to determine the effect of bedrock on morphodynamic processes within the littoral zone of Alexandria on the Mediterranean coast of Egypt. This 14.5-km-long littoral cell is bounded by pronounced embayments and pocket beaches separated by headlands which prevent bypassing of beach sands, in effect making this cell a large, semi-closed basin. The compartmented nature of this cell acts together with the rough irregularity of the rocky seafloor to trap a thin veneer of sediment (<3 m thick), showing proportional mixing between two sedimentary provinces. A modern fine-grained sediment facies consisting of mixed carbonate/siliciclastic sand flanks most of the nearshore zone down to a depth of 8–10 m. Beyond this depth, considered to be the depth of closure, a relict late Pleistocene to mid-Holocene coarse-grained facies composed of biogenic carbonate sand is found. Along a short section of the coastline (km 3–6), the coarser sediment also occupies the nearshore zone. Over most of the study area the two sediment types are mixed in various proportions, largest mixing coinciding with poorest sorting. Profile analyses revealed seasonal changes in sediment volume along the coast which closely follow the cyclicity of seasonal changes in wave climate. The present shoreline orientation, headlands and rough, irregular rocky seabed are reflected in the erosion/accretion pattern, sediment characteristics, and the reversibility of longshore currents and littoral drift. Although there is a marked deficiency in the sediment balance, the sand budget for this cell, including artificial material (2.339*10⁶ m³) has increased slightly by 0.041*10⁶ m³ year^–1as a result of engineering works carried out to widen the coastal road (Corniche). In addition to the physical properties of the bedrock (degree of induration), the accelerating sea-level rise during the Holocene and human influences, the modern morphology of the coast, the erosional seabed features in the nearshore zone, and the texture of seabed sediments are all controlled by the original geometry of the coast which consisted of an elevated subaerial ridge.