Observations on sediment mobility in a large gravel-bed river

This study investigates sediment mobility in a large gravel-bed river (Tagliamento River, northeastern Italy). Field data were used to identify the morphological effectiveness of a range of flows (floods with recurrence interval < 1 to 3.5 year) and for a detailed analysis of the partial transport condition. The analyses were carried out on three cross sections where a number of areas representative of different morphological units (main and secondary channels, low and high bars, islands) were painted. Grain size of the painted area was measured using a photographic method. Monitoring of bed mobility during the study period supplied new photographs of the painted areas, measurements of size and travel distance of the mobilized particles, and estimate of flow depth after each flood event.Our analyses have shown that with dimensionless shear stress around 0.073 (17 N m− 2), the morphological effect over the painted areas was primarily partial transport; while with stresses > 0.1 (25 N m− 2), the bed experienced full mobility, i.e., all the painted particles were transported or totally buried. Full sediment mobility was observed on lower morphological units (i.e., main and secondary channels) under floods with very high frequency (RI < 1 y), whereas on the low bars full mobility occurred under floods with a recurrence interval slightly > 1 year. Instead, fine deposition and partial transport conditions were observed on the higher bars even after the largest of the monitored floods (RI = 3.5 y).The analysis of the partial transport condition relies on more than 3500 measurements of particle travel distance (travel distances range from a few centimetres up to 22 m). The transported particles were generally finer than the painted sediments, and the analysis of the sediment transport ratio revealed that partial transport occurred under equal-mobility conditions for particles finer than 32 mm and under size-selective mobility for coarser fractions. This appears to be corroborated (for D < 32 mm) by the insensitiveness of particle transport distance to particle sizes, even if the exponent (≈ 1) of the relationship between the dimensionless shear stress and the maximum transported diameter would suggest that all fractions are transported under equal-mobility conditions.