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Using an optical settling column to assess suspension characteristics within the free, flocculation, and hindered settling regimes

Journal of Soils and Sediments volume 15, pages 1991–2003 (2015)Cite this article

Abstract

Purpose

Instruments able to measure the settling velocity distribution (SVD) and investigate the flocculation behavior of suspensions for a wide range of concentrations and settling regimes are required to understand and model sediment transport in headwater catchments. Such knowledge will improve our water resource management capabilities.

Materials and methods

An optical settling column, equipped with a vertical array of optical sensors, was used to provide light transmission through a suspension during quiescent settling. A new method to determine the settling velocity and the propensity of suspensions to flocculate is proposed. Its reliability was evaluated based on settling tests for (1) noncohesive sediments, (2) cohesive sediments at medium (~1 g l−1) concentration in a natural and deflocculated state, and (3) a cohesive sediment at a very high concentration (~10 g l−1). This choice of sediments and concentrations allowed for the assessment of free, flocculated, and hindered settling regimes.

Results and discussion

The proposed data processing method provides measurements for a range of test conditions. The result showed that different populations of particles with different settling behaviors can be identified within the suspensions. In the case of noncohesive sediments, the proposed method provided SVD similar to those obtained with reference methods. The propensity to flocculate was zero as expected for inert material. The natural cohesive sediment at medium concentration exhibited a large range of SVD (10−2–10−5 m s−1) and high propensity to flocculate. These were both reduced with the addition of a deflocculant. Identified particle behaviors were consistent with independent measurement of size distribution, microscopic, and erosion properties. In the hindered regime, a narrow SVD corresponded to the hindered front settling velocity (~10−4 m s−1).

Conclusions

An optical settling column was able to provide reliable SVD and an evaluation of the propensity of particles to flocculate. The settling column was able to detect variations of the settling velocities with settled depth, thus highlighting that settling columns with a single measurement point may provide erroneous results by not accounting for the full spectrum of settling depth.

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Acknowledgments

This work has been funded by the French Agence Nationale de la Recherche (ANR-12-JS06-0006, and EC2CO BIOHEFECT research projects) and by the Explora’doc program of the region Rhône-Alpes. We are also grateful to the Franch Institut national de recherche en sciences et technologies pour l’environnement et l’agriculture (IRSTEA) and the Saint-Venant laboratory involved in the hydraulic flush operation, to all the technicians providing everyday technical help, and to the researchers involved the OSC development project.

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Authors and Affiliations

  1. University Grenoble Alpes, LTHE, BP 53, 38041, Grenoble Cedex 09, France

    Valentin Wendling & Cédric Legout

  2. IRD, LTHE, BP 53, 38041, Grenoble Cedex 09, France

    Nicolas Gratiot

  3. Environment Canada, 867 Lakeshore Rd., Burlington, ON, L7R 4A6, Canada

    Ian G. Droppo

  4. CNRS, LTHE, BP 53, 38041, Grenoble Cedex 09, France

    Catherine Coulaud & Bernard Mercier

Authors
  1. Valentin Wendling
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  2. Nicolas Gratiot
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  3. Cédric Legout
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  4. Ian G. Droppo
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  5. Catherine Coulaud
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  6. Bernard Mercier
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Correspondence to Valentin Wendling.

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Responsible editor: Ellen L. Petticrew

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Wendling, V., Gratiot, N., Legout, C. et al. Using an optical settling column to assess suspension characteristics within the free, flocculation, and hindered settling regimes. J Soils Sediments 15, 1991–2003 (2015). https://doi.org/10.1007/s11368-015-1135-1

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  • DOI: https://doi.org/10.1007/s11368-015-1135-1

Keywords

  • Cohesive sediments
  • Flocculation
  • Settling regimes
  • Optical measurement
  • Settling velocity
  • Suspended sediment transport