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In large gravel-bed rivers, bed material transport estimation is challenging since theoretically based formulas often fail to accurately predict sediment fluxes, and it is difficult to carry out field measurements. A viable alternative to direct measurement is provided by the virtual velocity approach representing a hybrid solution to calculate the bed material transport based on a theoretical framework and use of tracers. This work aims to improve the methodology and to assess the role of input factors through a case-study application carried out in the Parma River (Italy). Two tracer types and scour chains were deployed at four sections. Data on water level, transport processes, particle travel distances, and active layers were collected over 17 months and 6 events. The transport that occurred during two events was calculated applying different configurations taking in account for several input factors (i.e., grain size, water stage, and topography). Applying simple or more complex configurations led to significant differences in transport estimates: In relation to channel morphodynamics, different factors (e.g., variability of water level within the cross section in multithread channels) play a key role on transport processes. Results indicate that it is crucial to collect and process field data developing reach-specific transport rating curves and to combine different type of tracers for monitoring the clast displacement lengths. Based on the methodological improvements and sensitivity analysis addressed in this study, we developed a decision tree in order to design future applications of the virtual velocity approach for estimating the bed material load in different gravel-bed river contexts.

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This work deals with the impacts of dams on large gravel ‐bed rivers in terms of altering coarse transport regimes and the relationship with river morphodynamics. Using data collected by a tracer ‐based monitoring programme carried out in a 4 ‐km ‐long study sector of the Parma River (Italy), located downstream from a relatively recently established dam, we applied a virtual velocity approach to estimate the coarse bed material load at four river cross ‐sections. Monitoring and calculation results provided new insights into the impacts of the dam on streambed material mobility and the sediment regime over the 17 ‐month calculation period. A longitudinal gradient of effects was observed along the study sector. Sections located closer to the dam are characterized by more evident impacts due to deficits in coarse sediment input from upstream. Sediment mobility here is strongly altered, especially in the highly armoured main channel, and the overall bed material load is extremely low. A partial recovery of sediment dynamics was observed at the sections located further from the dam, where estimates indicate higher sediment yield. The observed longitudinal trend in the coarse sediment transport regime matches the morphology, as the river shifts downstream from a sinuous configuration with alternate bars to a wandering one. The novel insights into alteration of coarse sediment dynamics and the relationship with river morphodynamics are potentially applicable to many other fluvial contexts affected by similar impoundments. © 2019 John Wiley & Sons, Ltd.

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