Virtual Velocity Approach for Estimating Bed Material Transport in Gravel-Bed Rivers: Key Factors and Significance

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 17months 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.