Channel adjustments and alterations of sediment fluxes in gravel-bed rivers of north-eastern Italy: potentials and limitations for channel recovery

The aim of this paper is to explore possibilities and limitations of restoring physical processes in five gravel-bed rivers (Brenta, Piave, Cellina, Tagliamento and Torre Rivers) in north-eastern Italy. The selected rivers were analysed through a range of techniques, specifically analysis of historical maps and aerial photographs with geographical information systems (GIS), comparison of topographic surveys and geomorphological surveys. After illustrating channel adjustments and sediment fluxes, we discuss how the understanding of physical processes can be used for channel restoration. The studied river channels have undergone notable adjustments in the last 100 years, specifically narrowing by up to 76%, incision by up to 8.5 m, and changes in channel configuration. Alteration of sediment fluxes, mainly due to in-channel mining, has been the main factor driving such channel adjustments. Evolutionary trends show that channel recovery is on-going in several of the selected reaches, since widening and aggradation have occurred over the last 15–20 years. This channel recovery has been possible because sediment mining has significantly decreased or ceased along the study reaches. However, several constraints still exist on sediment fluxes (e.g. dams). Four categories of river channel were defined, taking into account the recent evolution of the studied channels (from ‘A’, high channel recovery, to ‘D’, no channel recovery). The impact of different sediment management strategies on channel dynamics over the next 40–50 years was then analysed.Without any intervention, channel recovery would only be possible in those reaches that have a relatively high degree of connectivity with upstream sediment sources or tributaries, while further incision and narrowing would be expected in those reaches where connectivity is low. A more substantial channel recovery could be obtained through interventions at reach (e.g. removal of bank protection) and basin (e.g. sediment transfer downstream of dams) scales. Notwithstanding such actions, it is likely that channels will not recover in the next few decades to the morphology they exhibited in the first half of the 20th century, when bed-load yield and connectivity were higher.