Impacted fluvial and coastal sediment connectivity in the Mediterranean: a brief review and implications in the context of global environmental change

Abstract

Sediment connectivity refers to the continuity of the flux of sediment along transport pathways that may be fluvial as well as coastal. Clastic coasts built essentially from sediments supplied by rivers are abundant in the Mediterranean. This supply has been modulated by river catchment characteristics and human influence. Many pocket beaches in small bays in the Mediterranean directly trap bedload supplied by small streams or eroded from nearby bounding headlands, whereas fluvial bedload supply to more or less long open shores has been conditioned by longshore current redistribution from river-mouth bar deposits, many associated with deltas that have built up from fine-grained and organic sedimentation behind coarse-grained barriers. Sediment has also been derived from nearby abandoned delta lobes, older relict or actively formed nearshore carbonate deposits, and from shoreline reworking, but connectivity between shore and lower shoreface has always been limited in the Mediterranean because of the steep continental shelf. Significant sediment deficits along many of the Mediterranean’s coasts have resulted from anthropogenic fragmentation of rivers that has generated loss of sediment connectivity. The most important human interventions are flow regulation by dams, sediment trapping by reservoirs, fluvial channel engineering and bank engineering, and sand and gravel extractions. These activities were largely preceded in many river catchments by multi-millennial climate and land-use changes. Because of the strong wave influence and low tidal ranges, longshore sediment transport from river mouths operates within the framework of one or several sediment cells. Many such cells are now characterized by artificial boundaries that block bedload transport and impair alongshore sediment connectivity. These include harbours and terminal groynes, products of coastal urbanization and economic development, especially over the last two centuries. Climate change and sea-level rise spell further increasing vulnerability of the Mediterranean’s fragmented rivers and coasts and call for the urgent need to foster efforts aimed at re-establishing sediment connectivity.