Multi-decadal shoreline change, inherited coastal morphology and sediment supply in the Souss-Massa littoral cell (Morocco), and a prognosis with sea-level rise

Abstract

Knowledge on controls involved in the long-term (multi-decadal to secular) evolution of shorelines is important in terms of fundamental science and coastal management, especially with sea-level rise associated with climate change. The wave-exposed Atlantic coast of Morocco south of Agadir is dominated by 5–10 m-high lithified aeolianite cliffs and shore platforms interrupted by mobile aeolian dunes in the vicinity of the mouths of two rivers (wadis), the Souss and the Massa, which are part of a RAMSAR site of significant biodiversity. We examined the multi-decadal geomorphic evolution of the Massa coastal sector and the main drivers of this evolution, based on an analysis of aerial photographs and satellite images from 1970 to 2020. We also monitored beach topography from January 2019 to July 2020 to determine short-term shoreline response to seasonal variations in wave conditions, and how this ties up with the longer time frame. The long-term pattern is essentially controlled by a balance, in the shoreline position, between the erosional cliff line, aeolian dune sand accumulation, and longshore sediment transport. The beaches show a seasonal regime that conforms to the classical cut in winter and accretion in summer. Sediment supply from the Massa has been curtailed by the Youssef Ben Tachfine dam constructed in 1972, but limited sand supplies from wave erosion of the aeolianite cliffs and reworking of the aeolian dunes assure overall shoreline stability. These sources are very likely complemented by carbonate sand from the wave-dominated shoreface. Periodic sand trapping and release occur within the Massa mouth, often sand-blocked but reopened during floods. The future sediment-budget equilibrium of the Massa littoral cell will depend on the continuity of these sources of sediment supply. The aeolianite cliffs and large aeolian dunes are significant sources of sediment that could mitigate, as along much of the Atlantic coast of Morocco, the expected shoreline retreat from sea-level rise.