NILE DELTA PROFILES AND MIGRATING SAND BLANKETS

Abstract

Before construction of the various Nile River dams, the position of the Nile Delta shoreline was in equilibrium between the sediment supplied by the river and the transport of sediment along the coast. Following dam construction sediment yield from the river has virtually ceased. In the absence of sediment from the river, currents, waves and winds are actively eroding the delta. Extensive beach profiling shows that the primary locus of erosion, in excess of 10 x 106 m3 yr, is from the Rosetta promontory. This material is carried eastward in part by wave action but predominantly by currents of the east Mediterranean gyre which sweep across the shallow delta shelf with speeds up to 100 cm/sec. Divergence of the current downcoast from Rosetta and Burullus promontories results in formation of accretionary blankets of sand that episodically impinge on the shoreline. Individual blankets of sand form 1.5 m thick covers over the residual profile in depths of 6 to 4 m, and extend for 2 to 4 km along the shore. The sand blankets move progressively downcoast at rates of 0.5 to 1 km/yr, generating series of accretion/erosion waves along the shoreline. The erosion/accretion/erosion shoreline change is commonly 50 to 100 m and has a periodicity of 3 to 8 years. When the accretionary phase of the wave has passed downcoast, the shoreline returns to its previous form and exhibits a "residual" equilibrium profile. In its residual form, the beach profile goes through characteristic adjustments to seasonal wave climate, exhibiting an equilibrium winter profile with a pronounced bar and a summer profile when the bar tends to migrate onshore. When the nearshore is blanketed with sand during the accretionary part of the cycle, the profile is in disequilibrium with wave forcing and frequently shows several, irregularly spaced bars and troughs. Although the year to year shoreline changes associated with the accretion/erosion waves are large, 0(50 m/yr), the long-term (decadal) changes based on successive residual profiles from the erosion portion of the cycle are relatively moderate, say 0(2 m/yr).