Effects of chemical amendments on aquatic floc structure, settling and strength

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.

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“…For detailed description of the annular flume and shear cell, refer to Droppo () and Droppo et al . (), respectively.…”

Section: Methodsmentioning

confidence: 95%

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

Grangeon

Droppo

Legoût

et al. 2013

Hydrological Processes

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“…The size distribution can be an important factor in addition to the median grain size. The aggregation of particles in water by cohesion is termed coagulation and that by adhesion is termed flocculation [ Droppo et al ., ]. Moreover, the factors that influence the strength of the aggregation bonds can also affect the rheological parameters owing to the direct relationship between rheology and the structure of the cohesive sediments, and then lead to variations in the critical condition.…”

Section: Discussionmentioning

confidence: 99%

Critical conditions of incipient motion of cohesive sediments

Zhang

2017

Water Resources Research

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The critical condition of incipient motion of cohesive sediments was investigated in a laboratory study. One‐hundred experimental runs were performed with sediment samples by varying the yield stress to determine the relationship between the critical condition of incipient motion and the rheological properties of the cohesive sediments. The results indicate that yield stress is a factor that has a major influence on the incipient motion of cohesive sediments. In addition, the critical Shields parameter is found to be exponentially proportional to the yield stress and inversely proportional to the median grain size. The effect of yield stress on the critical Shields parameter is significant for the cohesive sediments and becomes progressively weaker with increasing median grain size. Furthermore, an empirical formula for calculating the critical Shields parameter of cohesive sediments that includes a rheological term and a gravity term is proposed by introducing the yield stress. According to this formula, a modified Shields diagram is obtained in which the values of the critical Shields parameter for cohesive sediments vary within a band that contains countless curves (instead of on a single line) to reflect the influence of the yield stress. This modification of the traditional Shields curve is effective for fine sediments, but the effects tend to vanish for coarse sediments as the behavior of sediments changes from cohesive to noncohesive. Finally, potential further investigations are discussed.

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“…Moreover, the surface with a diffuse layer constitute a double layer. However, the surface plus a Stern layer plus a diffuse layer constitute a triple layer model.The point just outside the Stern layer is usually known as the shear plane since movement of the colloid through the suspension shears the diffuse layer away [22,23,[27][28][29]. In fact, in some cases, the shear plane can be quite displaced beyond the Stern plane.…”

Section: The Double Layer Modelmentioning

confidence: 99%

Controlling Coagulation Process: From Zeta Potential to Streaming Potential

Ghernaout¹

2015

AJEP

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This review concerns the using of Zeta and streaming potentials in the coagulation process control. Coagulation process is usually explained by charge neutralization mechanism. The negative charge may be quantified by Zeta potential or streaming potential measures. Prior to the advent of streaming current monitors, Zeta meters were the primary instruments for measuring electrokinetic properties as related to coagulant dose. Both instruments measure the potential and indirectly the particle surface charge, but use very different methods. Even if the on-line streaming current monitor can provide coagulation process optimization when properly installed, maintained, and interpreted, jar tests experiments and Zeta meters remain indispensable.

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Effects of chemical amendments on aquatic floc structure, settling and strength

Cited by 30 publications

References 30 publications

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

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

Critical conditions of incipient motion of cohesive sediments

Controlling Coagulation Process: From Zeta Potential to Streaming Potential

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