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Modeling flocculation in a hypertidal estuary

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Abstract

When fine particles are involved, cohesive properties of sediment can result in flocculation and significantly complicate sediment process studies. We combine data from field observations and state-of-the-art modeling to investigate and predict flocculation processes within a hypertidal estuary. The study site is the Welsh Channel located at the entrance of the Dee Estuary in Liverpool Bay. Field data consist of measurements from a fixed site deployment during 12–22 February 2008. Grain size, suspended sediment volume concentration, and current velocity were obtained hourly from moored instruments at 1.5 m above bed. Near-bottom water samples taken every hour from a research vessel are used to convert volume concentrations to mass concentrations for the moored measurements. We use the hydrodynamic model Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) coupled with the turbulence model General Ocean Turbulence Model (GOTM) and a sediment module to obtain three-dimensional distributions of suspended particulate matter (SPM). Flocculation is identified by changes in grain size. Small flocs were found during flood and ebb periods—and correlate with strong currents—due to breakup, while coarse flocs were present during slack waters because of aggregation. A fractal number of 2.4 is found for the study site. Turbulent stresses and particle settling velocities are estimated and are found to be related via an exponential function. The result is a simple semiempirical formulation for the fall velocity of the particles solely depending on turbulent stresses. The formula is implemented in the full three-dimensional model to represent changes in particle size due to flocculation processes. Predictions from the model are in agreement with observations for both settling velocity and SPM. The SPM fortnight variability was reproduced by the model and the concentration peaks are almost in phase with those from field data.

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Acknowledgments

We want to acknowledge the organizations of Mexico and the UK, which provided the funding and opportunity for this investigation to be carried out: Consejo Nacional de Ciencia y Tecnología (CONACyT, through the PhD scholarship ID 212026), Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), National Environmental Research Council (NERC, via the iCOASST project, grant NE/J005444/1), and the National Oceanography Centre (NOC). We also want to thank the anonymous reviewers for their important comments to improve this manuscript.

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Authors and Affiliations

  1. National Oceanography Centre, Joseph Proudman Building, 6 Brownlow Street, Liverpool, L3 5DA, UK

    Rafael Ramírez-Mendoza, Alejandro J. Souza & Laurent O. Amoudry

Authors
  1. Rafael Ramírez-Mendoza
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  2. Alejandro J. Souza
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  3. Laurent O. Amoudry
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Correspondence to Rafael Ramírez-Mendoza.

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Responsible Editor: Bob Chant

This article is part of the Topical Collection on Physics of Estuaries and Coastal Seas 2012

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Ramírez-Mendoza, R., Souza, A.J. & Amoudry, L.O. Modeling flocculation in a hypertidal estuary. Ocean Dynamics 64, 301–313 (2014). https://doi.org/10.1007/s10236-013-0675-4

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  • DOI: https://doi.org/10.1007/s10236-013-0675-4

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