Channel adjustments, bedload transport and sediment sources in a gravel-bed river, Brenta River, Italy

This paper deals with the Brenta River (the Alps, Italy), a gravel-bed river strongly affected by human impact. The aims of this work are (a) to reconstruct the channel adjustments that have occurred during the last decades and (b) to analyse the present condition of the river in terms of bedload transport and sediment sources. The study reach, which is 23 km long, was investigated using historical maps, aerial photographs, 12 monumented cross-sections, laser altimetry data and field survey. Bedload transport rates were estimated by a morphological approach, which allows the construction of a sediment budget. Dramatic channel adjustments have occurred in response to an alteration of sediment fluxes caused by different human interventions (gravel mining, dams, torrent-control works). Major channel adjustments, specifically incision (up to 8\u20139 m) and narrowing, took place between the 1950s and the 1980s. On the other hand, widening has become the most important process, and incision has continued only in the upper part of the study reach in more recent times (about the last 20\u201325 years). The estimate of bedload transport rates for the period 1984\u20131997 was carried out starting from the erosion and deposition volumes of the single sub-reaches. Such an estimate has shown that (a) a high spatial variability of gravel transport rates exists (from 0 up to 73 200\u2013 85 400 m3/year) and (b) most of the bedload is supplied by local sources. Bank erosion is the main source of sediments for bedload transport and its contribution can be estimated as from 9 up to 20 times higher than the upstream contribution. Finally, from a practical point of view, results from this study should be taken into account in addressing maintenance, sediment management and restoration along a dynamic and unstable river such as the Brenta