Recent morpho-sedimentological change on a “human-dominated” delta: the case of the Arno River (Italy)

The evolution of the Arno River mouth has been tightly connected to human activities in the last 500 years. Two major events drove its development within this timespan, both attributed to anthropic interventions. As the strongest storms typically come from southwest in this sector of the Tuscany coast, in 1606 the southwest-oriented mouth was artificially modified to point towards northwest to avoid devastating floods inland when the storms coincided with river overflows. The new orientation increased the amount of sediments reaching the sea, which led to a huge growth of the delta. The mouth continued to prograde up to the mid-1800s, when several human activities (riverbed dredging, dam construction, slope reforestation) contributed to decreasing the Arno bedload. Towards the end of the century the delta was struck by strong erosive processes: on the left side hard structures were built to protect a seaside village, whereas the right side was left free to erode, which led to the present asymmetrical geometry of the mouth (about 300 m landward offset). The analysis of a series of bathymetric surveys within the 2008-2014 time interval provided insights about the recent evolution of the Arno River mouth. The results showed the significant back and forth mobility of the mouth bar along with swift changes of the shape. At times the mouth bar is located just inside the last tract of the river, and never more than a few tens of meters outside the mouth. On regards to the geometry, the typical profile is frequently replaced by a distinct, incised bar characterized by a 2 m-high crest. All these elements may be related to the asymmetry of the delta determined by the strong human pressure, that may have affected the morpho-sedimentological behavior of the Arno River delta. Lately, the anthropic constraints have likely been more effective than the natural factors in driving the recent evolution of the river mouth and adjusting to a “human-dominated” behavior. These data may be useful to better understand the ongoing processes on such deltas, which are gaining increasing attention especially in the Mediterranean area.