Channel adjustments and alteration of sediment fluxes in gravel‐bed rivers of North‐Eastern Italy: potentials and limitations for channel recovery

Abstract

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.

Geomorphological Approaches for River Management and Restoration in Italian and French Rivers

“…Few braided rivers still exist in northeastern Italy (for example the Tagliamento River) and in southeastern France , and there is a need to promote their preservation. In a recent study, Surian et al [2009b] have made an attempt to discuss systematically the possible future scenarios of channel changes, starting from past and present trends and considering potential sediment yield and connectivity. The five selected gravel bed rivers in northeastern Italy have undergone notable channel adjustments in the last 100 years, specifically narrowing by up to 76%, incision by up to 8.5 m, and changing from braided to wandering or single-threaded rivers.…”

“…To assess the potentials and limitations of channel recovery, the analysis proceeded in two steps [Surian et al, 2009b]: (1) the identification of the recent trajectory of channel change and (2) the definition of restoration goals for three different scenarios, basin-and reach-scale interventions, reach scale only, and no interventions. Four categories of channel were defined taking into account recent channel evolution (Figure 8a): A, high recovery; B, moderate recovery; C, slight recovery or no significant changes in channel morphology; and D, no channel recovery.…”

Variabilidad espacial de los cambios de cauce en una rambla mediterránea en las seis últimas décadas (1946-2006)

“…In contrast, channel reduction in the second half of the twentieth century is considered a human-induced fluvial adjustment (Bravard et al, 1997;Liébault and Piégay, 2002;Arnaud-Fassetta, 2003;Liébault et al, 2005). In Italy, Surian et al (2009) detected small width changes during the nineteenth century, but with no significant trend. From the end of the nineteenth century to the 1980s/1990s, channel narrowing and incision occurred, with particular intensity after the 1950s.…”

“…Causal interpretation has often been addressed through retrospective analyses of anthropogenic actions such as dam construction, reforestation, torrent control works, or river channelization (Bravard et al, 1997;Rinaldi, 2003;Surian and Rinaldi, 2003;Surian and Cisotto, 2007;García-Ruíz and Lana-Renault, 2011). Gravel extraction has also had a profound impact on rivers around the world (Kondolf et al, 2002;Liébault and Piégay, 2002;Surian and Rinaldi, 2003;Piégay et al, 2004;Rinaldi et al, 2005;Wishart et al, 2008;Surian et al, 2009).…”

“…Mean incision values have been estimated in the Italian perennial rivers (3-4 m) (Surian and Rinaldi, 2003;Surian and Cisotto, 2007;Rinaldi et al, 2009;Surian et al, 2009;Preciso et al, 2012;Gurnell et al, 2012), French rivers (1-5 m) (Bravard et al, 1997;Liébault and Piégay, 2002;Arnaud-Fassetta, 2003;Liébault et al, 2005), and Spanish rivers (0.6-5.5 m) (Boix-Fayos et al, 2007;Martín-Vide et al, 2010;SeguraBeltrán and Sanchis-Ibor, 2013). These values range in the same order of magnitude as those observed in ephemeral streams in the USA (Rinaldi et al, 2005;Cadol et al, 2011) and Israel (Rozin and Schick, 1996).…”