An experimental set-up for the spatio-temporal quantification of fine particle infiltration in porous beds

Suspended load is a major part of the solid fluxes transported by rivers, mostly made of fine particles. They eventually settle and, when reaching the bottom, may infiltrate the porous medium forming the riverbed, often made of larger sediments. This can clog the riverbed up to various points, hence disturbing the various processes occurring at this interface, the hyporheic zone, such as the exchange of water, nutrients, and other chemical species. Studying this infiltration process in the field is challenging because performing the measurements is difficult, but also because many processes are likely to affect this clogging mechanism, such as bioclogging or unsteady and complex flow conditions. Hence, many studies have used idealized analogue experimental setups to characterize this mechanism. Unfortunately, accessing the temporal dynamics is particularly challenging as the porous beds, usually made of glass beads, sand or gravel, are optically opaque and prevent as such from following the infiltration of fine particles [1]. Here, we present a flume experiment allowing for the spatio-temporal monitoring of the fine particle infiltration within the underlying porous medium. The key point consists in using hydrogel beads, which have a refractive index close to that of water, to build the riverbed. By using a camera, we can follow the intrusion of fine particles within the porous bed by light attenuation. In addition, we also use ultrasound backscattering measurements to characterize the overlying flow and suspension. In this set-up, we can vary the properties of the suspension (size, density), the flow (height, velocity profile) and the porous bed (porosity, heterogeneity) systematically and in a controlled way. Hence, in the future, this set-up will be able to map systematically the parameter space and relate clogging situations and their spatio-temporal dynamics to the corresponding external parameters.                                                                                Figure 1: Snapshot of an experiment. The dotted orange line indicates the separation between the suspension                                                                                       flow (above, from right to left), and the porous bed (below). References      [1] Romain Dubuis and Giovanni De Cesare. The clogging of riverbeds: A review of the physical processes. Earth-Science Reviews, 239:104374, apr 2023.