Dams & coastal erosion: problem and resource in California and in Campania, Italy

Dams and river barrages are considered causes of local coastal erosion, together with other factors of regional and global scale. It is believed that dams have beneficial effects such as large reservoir of freshwater for agricultural and industrial purposes, for the production of hydroelectric power and the decapitation of floods. But dams also have adverse ecogeomorphic effects on the environment, both upstream and downstream the barrier: strong decrease in sediment bedload and suspended load increase, changes in riparian vegetation and aquatic populations, landslides and microclimatic variation. Consequently, this results in diminishing of fluvial sediment supplies over 30%, then in intense focus of erosion and littoral retreat in the coastal physiographic units. Erosion rate is higher especially if there are several dams on the same river or along nearby rivers, and if the dams are located less than 50 km from the river mouth. However, a dam also represents a significant source of sediment, such as sand, which remains trapped in the reservoir, reducing its functionality after 50 years up to 18% for the 20% of dams. Pre-dam and post-dam case studies are presented and compared: pre-1850 to post-1960 in California (e.g., Russian River, Dry Creek), from the 50s to the 90s in Italy (e.g., Po and Magra rivers, Gulf of Taranto rivers) and particularly those of Campania region (Garigliano, Volturno, Sele and Alento rivers). Finally, some mitigation techniques of fluvial-coastal erosion and dam rehabilitation are shown, such as dam desilting and sediment bypassing, environmental engineering interventions in the reservoir and restoration of facing littorals.