Shelf Sediments of Alexandria Region, Egypt: Explorations and Evaluation of Offshore Sand Sources for Beach Nourishment and Transport Dispersion

Abstract

Textural and compositional characteristics of beach and bottom sediments collected from the beach-continental shelf of Alexandria region on the Mediterranean coast of Egypt were examined to search for quality nourishment borrow material, and to initially assess its compatibility with the existing native beaches. Sediment characteristics and carbonate content combined with analyses of coastal processes (waves and currents) and seabed morphology are also explored to interpret sediment transport pathways. Three sediment types composed of carbonate material are identified on the Alexandria shelf based on their texture and compositional characteristics. These are: Late Holocene to modern fine to medium sand (average Mz = 0.23 mm; range of 0.063–0.4 mm), a relict late Pleistocene to mid-Holocene shelly coarser material, coarse to very coarse sand (average Mz = 0.7 mm; range of 0.41–3.3 mm), and relict mud to very fine sand (average Mz = < 0.051 mm; < 0.063 mm). Textural and compositional evaluations indicate that the coarse-grained relict sand (54 km2) is locally found just seaward of the Alexandria Metropolitan area between water depths of ∼10 and 70 m; corresponds to 1 to 10 km distance from the coastline, and could be used as a nourishment and borrow material for eroded beaches of Alexandria that have experienced prolonged sediment deficiency combined with anthropogenic influences. Compatibility analysis indicates that the characteristics of the defined borrow material and those of the native beach are generally compatible with each other. With respect to the quality standards, the borrow material is similar to and slightly coarser than the native beach sand and also matches the same range of sorting (0.25–2.37 Ø) and carbonate content (8.0 to 99.9%); therefore it is compatible to use as a sand source for beach nourishment. Moreover, estimated overfill factor (RA) implies that eroded beaches would maintain a stable state following the sand-filling operation with a re-nourishment factor (Rj) of 0.39. Our observations indicate that shelf sediments, both relict and modern, are displaced northeasterly in the alongshore and offshore directions by the N and NW wave-induced currents in conjunction with the counter-clockwise geostrophic East Mediterranean current. These sediments are reworked and deposited as mixed carbonate and biogenic sands on the irregular rocky seafloor of the submerged ridges that characterize the inner shelf and are considered as the major source of sediment in the study region.