HUMAN IMPACT ON CHANNEL MORPHOLOGY AND SEDIMENT FLUX OF THE RENO RIVER IN NORTHERN ITALY

The Reno River is the eleventh largest river in Italy. It has been extensively affected by man activity for a very long span of time. The first relevant impacts date back to the Romans time and were reiterated with more or less intensity until present. During the last five centuries, the lowland portion of the river was subjected to remarkable channel modifications, diversion, levee construction, reclamation of the this portion of the Po plain. In the recent decades, mainly after World War II, significant changes in land use and management in the mountain portion of the catchment, extensive exploitation of the streambed material, the construction of a sluice gate for irrigation purposes a few kilometres upstream of the river mouth and large withdrawal of fluids from the underground caused important channel and sediment budget changes. Three main effects of such human impacts are evident: a remarkable streambed degradation (as much as 5 m during the last 60 years), the reduction to a hard to detect quantity of bedload flux and, consequently, a worrying beach erosion that is rapidly retreating along a vast portion of the Adriatic coast fed by the Reno R., that is an important tourist resort and economic resource for the region. In order to understand such morphological changes, their causes and, possibly, to envisage some solutions a field campaign of sediment transport measurement was started in 2003 and it is still in progress. To get a clearer picture of the present situation also bed material samples were taken along the alluvial plain, lower reach of the Reno R. and the hydraulic geometry data of hundreds of cross sections, surveyed at different times, were analysed to quantify the streambed degradation and the channel morphology changes. Bedload transport rate, in particular, resulted very low also during floods larger than bankfull and even those equations renown in the literature to underpredict bedload transport overestimate the sediment transport of the Reno R.. The river transport capacity always resulted by far larger than sediment supply. However, no significant relation was found between the flow main hydraulic parameters and the bedload transport rates measured. The occurrence of cyclic peaks and lows of bedload rate suggests the transit of dune bedforms across the measuring site. The data gathered indicate that the only reliable way to assess bed material flux in a river with an impoverished sediment supply and transport is through the analysis of dune morphology and migration rate, though seasonal variations in sediment supply may require to take into account also this aspect in adapting such a methodology.