From soil aggregates to riverine flocs: a laboratory experiment assessing the respective effects of soil type and flow shear stress on particles characteristics

Abstract

Abstract:Particles eroded from hillslopes and exported to rivers are recognized to be composite particles of high internal complexity. Their architecture and composition are known to influence their transport behaviour within the water column relative to discrete particles. To-date, hillslope erosion studies consider aggregates to be stable once they are detached from the soil matrix. However, lowland rivers and estuaries studies often suggest that particle structure and dynamics are controlled by flocculation within the water column. In order to improve the understanding of particle dynamics along the continuum from hillslopes to the lowland river environment, soil particle behaviour was tested under controlled laboratory conditions. Seven flume erosion and deposition experiments, designed to simulate a natural erosive event, and five shear cell experiments were performed using three contrasting materials: two of them were poorly developed and as such can not be considered as soils, whilst the third one was a calcareous brown soil. These experiments revealed that soil aggregates were prone to disaggregation within the water column and that flocculation may affect their size distribution during transport. Large differences in effective particle size were found between soil types during the rising limb of the bed shear stress sequence. Indeed, at the maximum applied bed shear stress, the aggregated particles median diameter was found to be three times larger for the well-developed soil than for the two others. Differences were smaller in the falling limb, suggesting that soil aggregates underwent structural changes. However, characterization of particles strength parameters showed that these changes did not fully turn soil aggregates into flocs, but rather into hybrid soil aggregate-floc particles.

Suspended Sediment Mineralogy and the Nature of Suspended Sediment Particles in Stormflow of the Southern Piedmont of the USA

“…SEM analysis of upland sediment from our watershed shows signs of composite particles existing as soil aggregates and kaolinite books (supporting information Figures S19 to S21), but we can find little evidence of composite suspended sediment particles in stormflow. The relative lack of composite particles in stormflow could be due to the forces exerted upon aggregate sediment particles upon erosion and entering turbulent stormflow, in agreement with Grangeon et al (2013) who showed that soil aggregates undergo disaggregation once introduced into river flow with high shear forces. In fact, commercial kaolin operations purposefully delaminate kaolinite by percussive/wet milling in water with small beads, in order to break apart kaolinite booklets along their basal planes (Gunn & Morris, 1965).…”

Microplastic deposition velocity in streams follows patterns for naturally occurring allochthonous particles

“…For example, this study was conducted at a single discharge and with only one type of sediment, and in artificial streams with little to no hyporheic exchange or suspended sediment. Previous research has used dynamic measurements of erosion, suspended sediment, and deposition to quantify the role of sediment flocculation, water velocity, and particle characteristics on sediment redistribution 44,47,48 . All of these factors will also affect microplastic movement and represent important factors to study for a greater understanding of their role in stream microplastic dynamics.…”

Assessment of erosion and settling properties of fine sediments stored in cobble bed rivers: the Arc and Isère alpine rivers before and after reservoir flushing

“…This procedure was developed and validated by Grangeon et al . () and Wendling et al . () to minimize sample disturbance.…”

“…500 and 1250 rpm respectively). This procedure was developed and validated by Grangeon et al (2014) and Wendling et al (2016) to minimize sample disturbance. Then sonication was activated and stirring and pumping were increased at their maximum levels.…”