CHANNEL ADJUSTMENTS IN ITALIAN RIVERS: EVOLUTIONARY TRENDS, CAUSES AND MANAGEMENT IMPLICATIONS

During the last centuries, and particularly in the second half of the 20th century, most Italian rivers have been dramatically affected by human interventions, specifically sediment mining, dam construction, channelization works and land use changes (e.g. reforestation). Such interventions have had effects on flow regime, channel boundary conditions and, particularly, on sediment regime, causing channel adjustments that are generally much larger than those expected from natural channel dynamics.

The results of an ongoing national project on channel adjustments in Italian rivers are discussed. The aims of the project are (a) to reconstruct the evolutionary trends (e.g. trends of channel width, bed elevation, braiding intensity) for several rivers, increasing the relatively small number of case studies that was available, (b) to understand the relationship between channel adjustments and the different human interventions, (c) to improve the existing conceptual models of channel evolution, and (d) to analyse the implications of channel adjustments in terms of river management and restoration.

The selected rivers have experienced almost the same processes as for temporal trends, whereas the magnitude of adjustments varies according to several factors (e.g. original channel morphology). Initially, river channels have undergone a long phase of narrowing (up to 85-90 %) and incision (up to 10 m or more), which started in the 19th century and was very intense from the 1950s to the 1980s. Then, in the last 15-20 years, channel widening and aggradation have become the dominant processes, though some reaches are still showing a decrease of channel width. This recent phase of channel evolution is associated with a significant reduction of the mining activity.

Besides recognition that most alluvial channels in Italy were and are still in an unstable condition, it has been documented how far many systems are from a natural or semi-natural condition. Sediment management is a key issue in management and restoration of such fluvial systems. For instance, preservation of bank erosion seems to be a good strategy in those reaches where this process represents the main source for bedload transport.