The roles of geometry and viscosity in the mobilization of coarse sediment by finer sediment.

In rivers, the addition of finer sediment to a coarser riverbed is known to increase the mobility of the coarser fraction. Two mechanisms have been suggested for this: a geometric mechanism whereby smaller sizes smooth the bed, increasing near-bed velocity and thus mobility of the larger sizes, and a viscous mechanism whereby a transitionally smooth turbulent boundary layer forms, rendering the coarser grains more mobile. Here, we report on experiments using two sediment mixtures to better understand these proposed mechanisms. In Mixture 1, we used 0.5 and 5 mm grains, and in Mixture 2, we used 2 and 20 mm grains. If the entrainment of coarse gravel by finer sediment is a purely geometric effect, then the addition of finer material should produce the same effect on the mobility of the coarser material for both mixtures because they have the same size ratio. We show that addition of finer material has a different effect on the two mixtures. We observed an increase in the mobility of the coarse fraction for both mixtures, but the increase in coarse fraction mobility for Mixture 1 was almost twice that for Mixture 2. Our experiments show that in addition to the geometric effect, enhancement of coarse gravel transport by finer sediment is also driven by a viscous effect.

Sediment mobility and connectivity in a catchment: A new mapping approach.

In fluvial basin analysis, sediment connectivity is an important element for defining channel dynamics. Nevertheless, although several approaches to quantify this concept have been trialed, there is considerable discussion about ways to measure and assess sediment connectivity. The present study investigates sediment connectivity through the definition of a new index, aiming to integrate functional aspects within a structural component. Our objective is to produce a sediment flow connectivity index (SCI) map, directly applicable to monitoring and management activities. Our SCI is defined as the result of the gradient-based flow accumulation of a sediment mobility index, which is in turn a simple function of rainfall, geotechnical properties of soil and land use. This method is here applied to the Vernazza basin (eastern Liguria, Italy), producing a sediment connectivity map that shows good performance in predicting the positions and accumulation paths of mobilized deposits detected on the ground after the October 25th, 2011, flood event. A further evaluation of the proposed index is performed through a comparison of the maps derived using the SCI and connectivity index (IC) developed by Cavalli et al. (2013), which highlights comparable quantitative overall performances, together with a slightly better qualitative identification of subtle sediment flow paths by the SCI. In spite of current limitations due to, e.g., the local nature of the final index, the availability of input information through open global datasets promises the potential application of this method to larger-scale assessments, paying attention to properly addressing upscaling and standardization issues.