Dinamica recente ed attuale degli alvei fluviali in Italia: stato dell'arte e prospettive

During the last centuries, and particularly in the second half of the 20th century, many fluvial systems have been significantly affected by human interventions (e.g. dams, sediment mining and channelization). Such interventions have effects on flows, sediment regime and channel morphology, causing channel adjustments that are generally much larger than those that could be expected from natural channel evolution. For instance, most Italian rivers have experienced considerable channel adjustments in the last decades, e.g. channel incision up to 10 m or more and channel narrowing up to 85-90 %. The aim of this work is to review the present state of knowledge on channel adjustments in Italian rivers and to analyse research needs and perspectives. The work is based on a review of published studies (seventy papers have been collected and analysed) and on the research carried out within a national project (PRIN 2005) over the last two years. The first studies on channel adjustments in Italian rivers date back to the beginning of the 1960s. Considering the distribution of publications from such a period to the present (2007), an increase can be observed in the number of publications through time, however the number of publications remains quite low in each year (the highest value, 7 papers, was reached in 1994). Concerning the methodologies used in these studies, a qualitative approach was dominant up to the end of the 1990s, then a more quantitative approach. The latter has implied, for instance, the reconstruction of temporal trends for channel parameters such as channel width and bed elevation. Most Italian rivers have experienced almost the same processes as for temporal trends, though the magnitude of adjustments varies according to several factors (e.g. initial channel morphology). Initially, river channels underwent a long phase of narrowing and incision, which started in the late 19th century and was very intense after the 1950s. Then, during the last 15-20 years, channel widening has become the dominant process in several cases. Such channel changes have been induced mainly by a range of human interventions, specifically sediment mining, dam construction, channelization works and land use changes (e.g. reforestation). A significant advance in the understanding of channel adjustments was obtained recently as for the reconstruction of evolutionary trends (in particular trends of channel width) and causes of channel adjustments, but several issues will require more efforts in the future. One crucial point will be, for instance, the analysis of past and present bed level adjustments. Reconstruction of past trends, in the absence or scarcity of topographic surveys, will need specific geomorphological field surveys, whereas the analysis of present trends should be supported by recent topographic technologies, such as GPS, LiDAR and digital photogrammetry. Last, but not least, there should be an increasing transfer and application of the results to river management. In fact, it is becoming more and more established among fluvial scientists and practitioners that knowledge of channel evolution in the medium-short term is a key ingredient for the sustainable management of fluvial systems.