PATTERNS OF BEACH-SAND SORTING AND SHORELINE EROSION ON THE NILE DELTA

Rapid erosion has occurred on the beaches of the Nile Delta in response to dam construction that has cut off the river sediment supply. This erosion is greatest adjacent to the two distributary mouths, with shoreline retreat rates up to 60 m per year. The eroded sand is carried alongshore away from the river mouths, and deposited in areas of shoreline accretion. These patterns of shoreline erosion and deposition are reflected in the mineralogy of the beach sands. On a delta-wide scale, high-density opaques, garnet and zircon are concentrated in the erosion areas to form black-sand placers. Lower-density hornblende, as well as quartz and feldspars, are preferentially transported out of the erosion areas and deposited in the stretches of accreting shoreline. A detailed analysis of these sorting patterns has been undertaken with beach samples from Abu Quir Bay at the western end of the delta. Concentrations of opaques, zircon, rutile and garnet decrease exponentially with longshore distance from the river mouth, while augite and hornblende increase exponentially. The coefficients in the exponential equations are found to be a function of mineral density, demonstrating the importance of this grain property in mineral sorting leading to the observed compositional variations along the shores of the Nile Delta.