Bedload transport processes in a coastal sand-bed river: the study case of Fiumi Uniti river in the northern Adriatic

Over the last decades, most of the Emilia-Romagna (Italy) beaches have been affected by marked erosion that is still progressing, which is primarily due to the reduction of sediment supply by the local rivers. In addition to larger fluvial systems, the role of small rivers has been recognized as important in contributing to both beach stability and changes. Bedload measurements were carried out by previous authors in 2005–2006 but, in order to widen the data set of river sediment supply for the whole region, a new bedload measurement campaign was performed in 2017 on the Fiumi Uniti River, a typical example of a small river system whose sediment supply variations substantially influence the stability of the beaches adjoining its mouth. Marked changes were observed in the bedload transport rates and bed material grain size between the two sampling campaigns of 2005/2006 and 2017. The threshold conditions for bed particle entrainment are analysed by means of the Shields classical criterion and other approaches. The results indicate that all these criteria are unable to predict the actual value, except for Carling and Bagnold criteria. This result has large implications in the assessment of bedload yield in sand-bed rivers.