Reliable calculation of the anthropogenic sediment budget along the Northern Tuscany coast.

The algebraic sum of sediment inputs, outputs and transfer within a 40 year time interval (1980–2020) represents a study case of a possible way to calculate the anthropogenic sediment budget. This sediment budget is defined as “anthropogenic” because it only involves the interventions related to human activities, which implies that natural inputs (i.d., bedload river discharge) and outputs (i.e., sediment loss to the offshore) are not taken into consideration. A simple and highly replicable methodology to address such a calculation is here described. The methodology has been applied within a littoral cell system along the Northern Tuscany coast (Italy). This area has been selected because it is characterized by either natural and highly urbanized sectors of coast, which are currently subjected to erosion and accretion processes. As in many other coastal settings along the Italian coast, here sediment management affected the sediment budget. The evaluation of this aspect is paramount for a conscious planning of any coastal protection, but it is very hard to quantify in a reliable fashion. Every coastal management policy would be improved should this feature be known. In the last 40 years, a long series of different protection schemes has been implemented along the Northern Tuscany coast, often financed by a variety of competent authorities (Port Authorities, Municipalities, Marinas, Region). Also for this reason, a dependable accountability of the sediment budget is not possible. In general terms, sediment deposition is concentrated in drift convergent zones or updrift of port structures, which have caused actions such as offshore dumping and sediment disposal into confined facilities: these interventions represent the sediment output in this study. At the same time, bypassing and redistribution processes contributed to the sediment transfer, while the replenishments that have been carried out in the areas most subjected to erosion effects have been considered as sediment input. Thanks to a thorough collaboration with the main authority of the area, the Region of Tuscany, a detailed assessment of the anthropogenic sediment budget over the last 40 years (1980–2020) has been calculated for the first time. Most coastal protection interventions were carried out to redistribute the sediments from one site to another within the study area (2,949,800 m3), while the sediment input (1,011,000 m3) almost matched the sediment output (1,254,900 m3) in the considered time interval. Summarizing, a negative anthropogenic sediment budget (243,900 m3) has been documented.