Variazioni morfologiche ed instabilità di alvei fluviali: metodi ed attuali conoscenze sui fiumi italiani

Channel adjustments and instability have several consequences on the human activity in the adjacent alluvial plain and on natural resources, and management of such problems requires a geomorphological approach, taking into account the processes and trends of natural river adjustments. It is commonly accepted that a river can be defined as unstable when it varies its size, geometry and shape during an intermediate time scale (decades or a hundred of years). Instability is considered as the result of an alteration of the dynamic equilibrium between available stream energy (power) and sediment load supplied from the upstream catchment. Three main categories of adjustments in an unstable channel can be distinguished: a) bed-level adjustments (incision or aggradation); b) width adjustments (widening or narrowing); c) changes in morphological pattern. Bed- level changes can be interpreted by using the Lane equation, while a series of equations proposed by Schumm include changes in channel width, and express the proportionality among driving variables (water and sediment discharge) and channel form variables. Conceptual geomorphological channel evolution models (CEMs), describing the morphological adjustments through a temporal sequence of stages of evolution, have been proposed for some areas of USA. Channel adjustments and instability may be studied by the following main methods: 1) historical research; 2) comparison of maps and aerial photos; 3) comparison of longitudinal profiles; 4) comparison of cross-sections; 5) specific gage analysis; 6) temporal trend of bed elevation; 7) field evidences. In response to various types of human disturbances, most of the Italian rivers have experienced considerable channel adjustments during the last centuries and in particular in the last decades. From a bibliographic review on channel adjustments of Italian rivers, two main types of channel adjustments have been recognised: (a) incision, which is commonly on the order of 3-4 m, but in some cases even more than 10 m; (b) narrowing, with channel width reduction up to 50 per cent or more. In some reaches these adjustments have led to changes in channel pattern in particular from braided to wandering. Such channel adjustments are due to several types of human interventions, particularly sediment extraction, dams and channelization. The results of this review are synthesised in a general classification scheme that summarises the main styles of adjustment observed in Italian rivers. According to the scheme, braided rivers adjust through prevalent narrowing with varying rates of incision, whereas single-thread rivers adjust mainly through a more pronounced incision accompanied by various amount of narrowing.