Flocculation depositional properties of freshly eroded aggregates

In Europe, 260,000 km² of soils already suffer erosion by water. This worrying level of land degradation is expected to increase in the context of climate change, with situations particularly critical in mountainous environments. As any multi-use resources, there is also a growing human pressure on mountainous rivers. Data is required to obtain a better understanding of the cycles of deposition and erosion and of the respective contributions of natural and human-induced processes to fine, cohesive sediment fluxes. When placed in suspension in rivers, cohesive materials are principally transported as flocs. The quantity of suspended solids exported downstream depends mainly of their morphometric properties, including: size, shape and effective density. Once eroded on hillslopes, the conveyance of fine sediment particles on continental surfaces requires the maintenance of particles in suspension by nearbed turbulence and its advection downstream by the mean flow. Traditional Rouse profile relationships do not readily hold for highly concentrated riverine suspensions (~10 g/l), where processes such as flocculation, hindered settling and stratification interplay. This study aims at improving sediment transport parameterisation, by examining the kinetics of fine soil aggregates (size, settling velocity, density), once immersed in a turbulent flow. Particle properties of three Mediterranean materials (clay loam soil, black marl and molasse, all sampled in Badlands environments) were tested in the grid stirred experiment following the protocol previously used by Gratiot and Manning (2004). Hydrodynamic properties were monitored with ADV and turbidity sensors. For each soil, three sediment loads (1.5; 5; 10 g.l^-1) representative of flood conditions were tested. Aggregate properties were obtained after sampling at four depths above the grid, using the LabSFLOC – Laboratory Spectral Flocculation Characteristics – technique (Manning, 2006) and laser techniques. These acquisition heights are associated with the corresponding turbulence dissipation rates G of 1.5, 3, 7 and 19 s^-1. Flocculation rates and dynamics are reported and discussed.