On stratification control of the velocity fluctuations in sedimentation

International audienceWe have tested whether stratification can govern local velocity fluctuations in suspensions of sedimenting spheres. Comparison of the proposed scaling for local control of fluctuations by stratification to experimental data demonstrates that this mechanism cannot account for the reduction of the observed velocity fluctuations

Fluctuations and stratification in sedimentation of dilute suspensions of spheres

International audienceWe have tested in experiments and simulations whether stratification can control velocity fluctuations in suspensions of sedimenting spheres. The initial value and early decay of the velocity fluctuations are not affected by stratification. On the other hand, in the descending front where the stratification is strong and well defined, the velocity fluctuations are inhibited according to a previously proposed scaling. In between, after the initial decay and before the arrival of the front, the local value of the stratification does not always play a role

Spreading fronts in sedimentation of dilute suspension of spheres

International audienceThe thickness of the diffuse front between a sedimenting dilute suspension and the clear fluid above grows linearly in time due to polydispersity in the size of the particles and due to a hydrodynamic effect in which randomly heavy clusters fall out of the front leaving it depleted. Experiments and simplified point-particle numerical simulations agree that these two effects are not simply linearly additive

Adapting Landscape Mosaics of medIteranean Rainfed Agrosystems for a sustainable management of crop production, water and soil resources: the ALMIRA project

International audienceWater erosion of cultivated soils is a threat to the sustainability of agriculture, especially in Mediterranean areas. For a long time, Mediterranean farmers have thus adopted some soil conservation practices. Actual ditch networks, which are generally associated with terraces, result from historical successive farmer settlements and are one of these soil conservation practices. By intercepting surface run-off, ditches decrease slope length and prevent soil erosion on downstream plots. However, since water erosion hazard and ditch network geometries are highly variable in vineyards landscape and since ditch building and maintaining are costly, the objective of this study was to identify and map the resulting efficiency of ditch networks in preventing soil erosion. For a given area, a ditch network efficiency is defined here as the balance between the network density, i.e. network cumulated length for a given area unit, and the erosion sensitivity over an area which measures the performance of the ditch network in limiting soil erosion. The erosion efficiency of ditch networks was thus identified using both i) computer generated ditch networks with various spatial configurations and ii) the stream power index as an erosion sensitivity indicator, computed from a DTM in which each ditch network was burned. The stream power index of the actual networks were compared with a set of generated networks whose density and topology were selected to maximize the performance in preventing soil erosion thanks to the use of a self-developed optimized stochastic network generator. For four 1 km2 hillslopes, we showed that the performances of actual networks to prevent soil erosion was among the best that were obtained by simulated networks with even greater densities. Furthermore, we showed that the stream power index values that accounted for the actual ditch networks to prevent soil erosion hazard was both minimal and weakly variable in the whole study area (30 km2 ) at hillslope scale, whatever the other erosion factors, namely topography and landuse. This suggested that densities and topologies of actual ditch networks in the catchment have been optimized by farmers through individual acts along the last centuries in order to limit the soil erosion hazard. This also confirms there is very little room to propose new ditch network spatial configurations that better prevent vineyards soil erosion