The extent of partial transport in a wide gravel-bed river (Tagliamento river, Italy)

Sediment transport is the most important factor affecting the physical habitat of a riverine ecosystem and is essential to understanding the morphological changes occurring in rivers. Unfortunately, it is exceedingly difficult to measure it directly using sediment traps, especially in large and complex braided rivers. Here we present an attempt of assessing sediment mobility in a large gravel-bed river (Tagliamento River, north-eastern Italy) as a first step to the quantification of sediment transport rate using the Virtual velocity approach. The analyses were carried out on three cross-sections where a number of areas were painted and the size and travel distance of the mobilized particles were measured after flood events. The results show that partial transport occurs with dimensionless shear stress ranging between 0.015 and 0.039 while with stresses > 0.045 the bed experiences full mobility. The analysis of the particle displacement length revealed that under partial transport conditions the travel distance of a certain size particle increases with the shear stress, and that a certain threshold between equal-mobility and size-selectivity conditions can be identify.