Field measurement of sediment transport on a few rivers of the northern Apennines

During the last decades, many river deltas and the beaches associated with them underwent remarkable erosion and retreated at a very high and worrying rate. In response to such a situation a few River Authorities of Tuscany and Emilia Romagan Regions set up a sediment transport monitoring programme in order to quantify the sediment flux to the beaches fed by rivers draining both sides of the northern Apennines. Bed and suspended loads, flow velocity and discharge and changes in the cross section geometry have been measured at downstream sections during floods of different intensity. Bedload transport rates measured in the field resulted very low also during floods larger than bankfull and even those equations, known in the literature to underpredict bedload transport, overestimate the actual sediment transport. The study rivers transport capacity always resulted by far larger than sediment supply and no significant relation was found between simple flow parameters and the bedload transport rates measured. The bedload field data have been compared with results provided by a few of the most common sediment transport formulas reported in the literature but none of them is able to predict at an acceptable degree of accuracy the actual values. Only a modified version (sensu Wang and Parker, 2005) of Meyer-Peter and Muller equation (1948), incorporating the flow resistance effects of dune bedforms and Martin (2003) equation, based on a simple power relation using the unit stream power and field data reported by Gomez and Church (1989), resulted in a good fitting with the data measured on the study rivers. The occurrence of cyclic peaks and lows of bedload rate observed during floods has been interpreted as produced by the transit of dune bedforms. This finding can explain the good performance of the modified Mayer-Peter and Muller equation. Moreover, the possibility to use the dune geometry and migration rate as a complementary method to asses bedlod trasnport is investigated as well