2.1.3. Hydraulic Works

Among interventions carried out on rivers, it is not only deforestation that increases fluvial input; modifications of water courses and embankments construction have a similar effect. Cutting meanders, carried out to prevent alluvial plain flooding, imposes a shorter path, and therefore a greater slope, favoring sediment transport to the sea. From 1338 to 1771, the River Arno course from Pisa to the sea was shortened by approximately 5 km (from 12 to 7 km) by cutting three meanders, which almost halved its slope. On the same river, upstream and downstream from the town, embankments were built by the Romans together with reclamation works, in order to transform the marshy area into an agricultural area, as centurion footprints still show [44]. Together with the watershed deforestation this helped the river to build its delta and extend its course for 7.5 km (which reduces the slope). However, most anthropogenic interventions carried out on river channels has led to a reduction in sedimentary input: the construction of "diversions" to introduce the "slurries" into reclamation basins [41] and that of the bridles along river channels [45] have often caused a drastic reduction in the amount of sediment transported to the sea.

After reclamation, rivers can be redirected to the sea, enriching the sediment input, but the same can be done by diverting river course previously emptying into a lagoon in a natural way. This has been carried out with rivers on the western Veneto coast to prevent

Venice Lagoon siltation and expanding beaches near the new river mouth [46]. Additionally, weirs can be removed, releasing sediments they trapped [47], but more important, and well studied, is the effect of dams on riverine input to the coast [48].

From large dams, e.g., the Nile River at Aswan [49] and that of the Seven Gorges on the Yangtze River [50], to smaller ones, this impact has been well documented, e.g., rivers emptying into the Catania Gulf, Italy [51]. More than 100 × 109 tons of sediment has been sequestered in reservoirs constructed largely within the past 50 years [52]. Against this, several dam removals have been performed during the last few decades and channel evolution documented [53,54], but none had the tremendous impact on the coast as that of the Elwha River [55]. Here, a two years post-removal monitoring showed that 2.2 million m3 of sand and gravel were deposited on the seafloor offshore at the river mouth [56]. However, each intervention does not have an instantaneous impact on the coast and its effect increases and reduces over time, never maintaining the same effect.
