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The investigation of coarse sediment transport in wide mountain streams is very challenging because of limitations that affect the adoption of direct and indirect field monitoring techniques. Such river type represents then a stream typology for which quantitative knowledge of coarse sediment mobility, dynamics and fluxes is poor. This study uses tracer particles and a virtual velocity approach to gain insights about sediment mobility and coarse bed material transport in a wide mountain stream. Monitoring at the study area was conducted by tagging two types of tracer clasts, installing scour chains, and collecting data on water levels, transport processes, particle travel distances, and active layers over a period of 19 months and five transporting floods in the Tegnas Torrent (the Dolomites, Italy). Bed sediment was mobilized during a limited number of hydrological events (i.e. 6 to 9 per year), taking place in 50 to 400 h and inducing a total coarse sediment flux of about 1200 to 1400 m3 yr−1. The mobilization of coarse particles forming the streambed begins in response to a dimensionless shear stress (τ* c = 0.018) that is lower than thresholds commonly reported in the literature for mountain rivers and the estimated unit bed material transport was remarkably higher compared with those obtained in other mountain streams or gravel-bed rivers. The high mobility of streambed material can be related to the availability of fresh and unstructured sediment transported and deposited in the active channel by a high-magnitude event that occurred a few months before monitoring was conducted in this work, suggesting that sediment fluxes in a stream recently affected by a large flood could be considerably higher than those normally expected in a specific context. Finally, despite the existence of some application constraints, the experiment generated satisfactory results as regards the applicability of the virtual velocity approach in investigations of this stream type. This new quantitative knowledge of sediment fluxes and dynamics is also crucial for management of wide mountain streams. • Quantification of coarse sediment fluxes in wide mountain streams is very challenging. • The virtual velocity approach provides new knowledge of sediment fluxes and dynamics. • High bed material mobility was observed in the Tegnas Torrent after the Vaia storm. [ABSTRACT FROM AUTHOR]

• A high magnitude/low frequency event in an alpine basin was comprehensive analyzed. • The flood exhibited ω = 3865 W m−2 and an unit peak discharge = 3.3 m3 s−1 km−2. • A moderate response was shown by hillslopes, while the main stream was deeply altered. • Boulder mobility and massive bedload volume proved the outstanding flood magnitude. • Two large and infrequent floods occurred in the study basin were compared. To achieve a reliable analysis of the impacts induced in mountain basins by large and infrequent floods, all their main components, from the spatial-temporal distribution of meteorological agents to the hydrological, geomorphic and sedimentological response should be considered. Comprehensive study of the hydro-geomorphic responses is extremely valuable to increase the awareness of large floods, especially, in highly populated mountain areas. Such type of investigation requires a solid and wide dataset, which is why only few studies had the chance to describe the response in such a holistic way. This work comprehensively analyzed the high magnitude/low frequency Vaia event, a severe storm that affected northeastern Italy in October 2018 and thus the Rio Cordon study basin. The 80 h precipitation registered in the basin showed a total rainfall equal to 29.8% of the mean annual precipitation. The temporal distribution of rainfall presented two phases, i.e., a first characterized by moderate but persistent precipitation and a second more intense, exhibiting recurrence intervals over 50 years. A combination of indirect methods permitted the reconstruction of the hydraulic forcing acted in the Rio Cordon. Despite the implicit uncertainty, these methods clearly highlighted the high magnitude expressed by October 2018 flood, which generated a unit peak discharge equal to 3.3 m3 s−1 km−2 and a peak of unit stream power of 3865 W m−2, i.e., the highest hydraulic forcing conditions ever observed in 34 years. In terms of geomorphic changes, the use of pre- and post-event LiDAR data stressed out a moderate response of the hillslopes, where the (re)activation of the sediment sources was limited. Only few of these acted as sediment supplier to the main channel. The channel network instead, exhibited an evident response, with the Rio Cordon severely altered by wide lateral widening, deep streambed incision and armouring removal. The hydraulic and geomorphic forcing generated by October 2018 flood caused extensive streambed remobilization and boulder mobility that, in turn, induced the transport of a massive sediment volume. In this sense, the match between bedload observed and bedload predicted suggested the outstanding hydraulic forcing occurred and the quasi-unlimited supply conditions acted during the flood. The long-lasting monitoring program maintained in the Rio Cordon basin provided the rare opportunity to compare two high magnitude/low frequency floods and their induced effects. The September 1994 and October 2018 events were caused by different rainfall conditions, which resulted in different hydrological- and, especially, geomorphic- and sedimentological-responses. Particularly, the October 2018 flood induced an unprecedented alteration on the fluvial system, the effects of which could persist over the long-term. [ABSTRACT FROM AUTHOR]

The alteration of sediment transport regime represents one of the key factors controlling the response of fluvial systems to human impacts. Additional research efforts are required to define sound cause-effect relationships between the human pressures and the morphodynamic responses of sediment-starved gravel-bed rivers. This work deals with the evolution of the Parma River, a large Italian gravel-bed river affected over the last 160 year by multiple anthropogenic disturbances (i.e., land-use changes, in-channel mining and, more recently, the construction of a detention basin), which strongly limited the sediment availability to the high-plain portion of its course. The data collected allowed us to reconstruct the history of the impacts, the river morphological evolutionary trajectory, the sedimentological characteristics of the bed, and to estimate, leveraging a morphological approach, the variation in coarse sediment regime over the most recent period. After the cessation of intense in-channel mining in 1990s, the study sector located upstream from the detention basin shifted from a degrading (i.e., narrowing and incising) to a recovery phase, characterized by a mean coarse transport of 15,000–31,000 m3yr−1. These results demonstrate that the in-channel mining was the most impactful historical pressure acting on the river, and that a channel preserving some natural characteristics and passing from a condition of sediment-starvation to one of increasing sediment availability can recover an active morphodynamics within a short time period. The downstream sector was greatly affected by the reduction of sediment transfer that occurred in the 2000s caused by the construction of a detention basin. Its degradation continued over the most recent period and the coarse transport is currently very low (up to about 3000 m3yr−1 4 km downstream from the basin). This sector is characterized at the present by a morphodynamics defined as "slack", a condition induced by the complete evacuation of the mobile channel sediments, leading to extremely low bed material mobility and transport, and, consequently, to morphological degradation up to the achievement of a new quasi-equilibrium. We hypothesized that such an extreme state of alteration is relatively limited in space, affecting a gravel-bed river only at the reach scale in response to multiple overlapped impacts or because of an extremely severe perturbation (i.e., reduction) of the sediment availability and dynamics. • Human-induced sediment starvation causes alteration of gravel-bed river morphodynamics. • High armouring and low bed material load characterize a sediment-starved channel. • Increasing sediment availability can lead to a recovery of channel morphodynamics. • Multiple overlapped impacts can induce extreme morphodynamic alterations. [ABSTRACT FROM AUTHOR]