Updrift river mouth migration on cuspate deltas: two examples from the coast of Tuscany (Italy)

The configuration of the Arno and Ombrone deltas (Italy) is evaluated. The terminal courses of the rivers forming these deltas face the dominant wave orthogonal. This configuration came about during rapid delta progradation as a consequence of increased river sediment input due to widespread deforestation within the catchment area, a prevailing phenomenon from the Early Middle Ages to the XVIII century. The rivers attained and maintained their directions as the result of the increased accretion rate on the less exposed downdrift side of the delta. On the more exposed updrift side, delta growth caused the shoreline to gradually rotate so that it directly faced the waves approaching the coast. Here, due to the lower refraction, wave energy per shore unit increased whilst accretion rate decreased. This rotation of the shoreline-induced longshore inversion on the updrift side, whereas today cusp erosion is restoring the original drift direction.