The effects of low-magnitude flow conditions on bedload mobility in a steep mountain stream

Abstract

The transport of coarse material strongly controls the stability and evolution of mountain fluvial systems but, despite this, bedload dynamics are not yet fully understood especially in mountain streams. In this sense, particular attention was paid on the bedload magnitude (volume) expressed at event-scale and on the long-term, while few studies were focused on when and how the transport of the coarse material occurs. The aim of this work was to investigate the bedload mobility in the Rio Cordon, a mountain stream characterized by cascade and step-pool morphologies with a rough streambed. Here, the critical conditions for initiation of motion, transport distance and virtual velocity expressed by the coarse streambed material were assessed and their relationships with hydraulic forcing conditions and grain size were analyzed. To this end, a monitoring program based on bedload tracing was maintained over 7 years, allowing to analyze the bedload mobility during persistent high frequency/low magnitude flow conditions. To investigate the bedload mobility, 250 tracers were released between 2011 and 2012 and their propagation along study site was monitored until 2018. Overall, 14 tracer inventories were realized, determining 1697 tracer localizations. During the study period, the bedload dispersion resulted well described by the peak discharge magnitude (Qp, qp and ωp − ωc), while no significant relationships were observed with duration of competent flow (tover) and effective runoff volume (ER). Transport distance and tracer grain size were negatively correlated, whereas virtual velocity increased with increasing particle size. In this sense, the propagation velocity seems to be affected by the high frequency flows that, on the one hand triggered mobilization of the coarser tracers only through limited and impulsive events and, on the other hand favored a slowdown on the finer particles due to bedforms disturbance. Compared to other study sites, the Rio Cordon exhibited accentuated threshold conditions with lower transport distance and virtual velocity, confirming that steep mountain streams are generally influenced by a reduced transport efficiency due to protruding bedforms and macro-roughness that cause a pronounced energy dissipation. Interestingly, such condition seems to have progressively reduced from under- to near-bankfull flows. The results were compared to the bedload mobility observed in the Rio Cordon during 1993–1998, enabling to quantitatively assess how the bedload dispersion varied between a setting of stable armouring layer with protruding bedforms (2012–2018) and a partial alteration of these (1993–1998). An evident difference was observed in terms of transport distance, while critical conditions did not significantly change. Lastly, the long-term bedload tracing investigation highlighted that a certain legacy on the transport efficiency was produced by the persistent high frequency flow conditions. Over the study period, a general decrease of transport distance was observed that could be explained by a progressive stabilization of streambed material. Previous studies suggested that such condition can be interrupted by high magnitude/low frequency floods.