Analisi della dinamica passata ed attuale del fiume Piave nel Vallone Bellunese finalizzata ad una gestione integrata del suo corridoio fluviale

The present research aims to define the timing and the extent of the morphological changes occurred in a large gravel bed river, the Piave River in the “Vallone Bellunese” (Eastern Italian Alps) that was heavily impacted by human activities (training structures, hydropower schemes, and gravel mining) and to develop innovative strategies and tools for river management. The Piave River basin (drainage area 3899 km2) lies in the eastern Italian Alps, and the main channel flows south for 220 km from its headwaters (at ~2000 m asl near the Italy–Austria border) to the outlet in the Adriatic Sea NE of Venice. The study rich is 13,5 km long and is located between Soverzene and Belluno in the intermediate course of river within the mountain district. The morphology of the river in the study reach is dominated by braided and wandering channel patterns, but narrower reaches display an alternate bars channel pattern. To achieve the aims of the research the analysis was divided into two phases, in the first phase we collected data about recent and historical morphological evolution (historical maps, aerial photos, topographic surveys and historical photos) and produced thematic maps, in the second phase we produced maps and guidelines useful for river and fluvial corridor management. In the first phase planform changes of river features and bed planform were analyzed on six aerial photos (1960, 1970, 1991, 2000, 2003 and 2006). Aerial photos of 2006, produced by flight specially commissioned by TESAF department, were rectified and coregistered to a common mapping base at 1:5000 by GIS software (Esri ArcGIS 9.1). Significant planform features were digitized on rectified photos in order to derive planform characteristics for each image. Aerial photos of six years analized allowed the identification of these classes: unvegetated active channel, islands with arboreal vegetation, islands with shrubby vegetation, arboreal marginal vegetation, shrubby marginal vegetation, and herbaceous marginal vegetation. Furthermore, three additional classes related to human use of the river corridor were adopted: urban areas, cultivated areas, and gravel mines. The aerial photos of 2006 were also used to distinguish morphological units of the active channel: main channel, secondary channel, lateral low bar, longitudinal low bar, lateral high bar, longitudinal high bar, backwater channel, islands with arboreal vegetation and islands with shrubby vegetation, The objectives of this first phase are to quantify morphological changes in bed planform, to quantify the variation of vegetation cover, with particular emphasis on islands dynamics and to identify the driving factors of channel evolution and vegetation cover changes and thus to envisage the most likely future trends. In the second phase of analysis we produced maps and guidelines useful for river and fluvial corridor management. We analyzed the changes in bed planform due to the occurance of two flood events in 1966 and 2002. The flood event occurred in 1966, with an estimated peak discharge of ~ 3300 m3/s-1 and a RI = ~200 years, is the largest flood event of the twentieth century. The flood event occurred in 2002 with a RI = ~12 years is a more frequent flood event but that had a relevant impact on river morphology. Then we produced a series of maps, starting from the concepts of river corridor and streamway, wich aim to describe and quantify natural processes occurring within the fluvial corridor, such as bank erosion and wood input. The analysis of the historical maps and aerial photographs shows that substantial changes took place in the Piave River within the investigated time interval. The results indicates a marked tendency to the reduction in the active channel, but associated to the main flood events (RI > about 10-15 yr) an increase in the active corridor extensions is apparent. As to the morphological pattern of the entire study reach, this shifted from braided (still dominant until the 1960s) to single thread/wandering in the 1990s. The expansion phase of the last decade is associated with a general recovery of at least a wandering style, with occasional braiding morphology. Reduction of active channel has led a subsequent expansion of the portion of river corridor covered by vegetation. Similarly, the increase in active channel area that took place between 1991 and 2003 occurred mostly at the expense of vegetated areas located at the channel margins. The island dynamics were found to be strictly associated to the occurrence of major floods (RI >10–15 years), which are the only ones able to determine substantial island erosion.