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A Mechanistic Model for Mud Flocculation in Freshwater Rivers

Nghiem, Justin A. and Fischer, Woodward W. and Li, Gen K. and Lamb, Michael P. (2022) A Mechanistic Model for Mud Flocculation in Freshwater Rivers. Journal of Geophysical Research. Earth Surface, 127 (5). Art. No. e2021JF006392. ISSN 2169-9003. doi:10.1029/2021jf006392.

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The transport and deposition of mud in rivers are key processes in fluvial geomorphology and biogeochemical cycles. Recent work indicates that flocculation might regulate fluvial mud transport by increasing mud settling velocities, but we lack a calibrated mechanistic model for flocculation in freshwater rivers. Here, we developed and calibrated a semi-empirical model for floc diameter and settling velocity in rivers. We compiled a global data set of river suspended sediment concentration-depth profiles and inverted them for in situ settling velocity using the Rouse-Vanoni equation. On average, clay and silt (diameters <39 μm) are flocculated with settling velocity of 1.8 mm s⁻¹ and floc diameter of 130 μm. Among model variables, Kolmogorov microscale has the strongest positive correlation with floc diameter, supporting the idea that turbulent shear limits floc size. Sediment Al/Si (a mineralogy proxy) has the strongest negative correlation with floc diameter and settling velocity, indicating the importance of clay abundance and composition for flocculation. Floc settling velocity increases with greater mud and organic matter concentrations, consistent with flocculation driven by particle collisions and binding by organic matter which is often concentrated in mud. Relative charge density (a salinity proxy) correlates with smaller floc settling velocities, a finding that might reflect the primary particle size distribution and physical hosting of organic matter. The calibrated model explains river floc settling velocity data within a factor of about two. Results highlight that flocculation can impact the fate of mud and particulate organic carbon, holding implications for global biogeochemical cycles.

Item Type:Article
Related URLs:
URLURL TypeDescription Information
Nghiem, Justin A.0000-0003-2772-9945
Fischer, Woodward W.0000-0002-8836-3054
Li, Gen K.0000-0002-6300-3570
Lamb, Michael P.0000-0002-5701-0504
Additional Information:© 2022. American Geophysical Union. Issue Online: 17 May 2022; Version of Record online: 17 May 2022; Accepted manuscript online: 06 May 2022; Manuscript accepted: 02 April 2022; Manuscript revised: 11 March 2022; Manuscript received: 15 August 2021. JN acknowledges NASA FINESST Grant 80NSSC20K1645. WWF and MPL acknowledge funding by the Discovery Fund and the Resnick Sustainability Institute at Caltech. GKL acknowledges the Caltech Geology Option Postdoc Fellowship. MPL acknowledges funding from the NASA Delta-X project by the Science Mission Directorate's Earth Science Division through the Earth Venture Suborbital-3 Program NNH17ZDA001N-EVS3 and the National Science Foundation Geomorphology and Land-use Dynamics Grant No. 2136991. The authors thank reviewer Scott Wright and four anonymous reviewers for their feedback, which improved the paper. The authors also thank Editor Ton Hoitink and Associate Editor Florent Grasso. Data Availability Statement: The suspended sediment concentration-depth profile, grain size distribution, and geochemical data used in this paper are freely available in the respective original publications. The authors are grateful to J. Bouchez, E. Dingle, and J. Shelley for sharing data used in this paper. Derived data are available online at
Group:Resnick Sustainability Institute
Funding AgencyGrant Number
Caltech Discovery FundUNSPECIFIED
Resnick Sustainability InstituteUNSPECIFIED
Caltech Division of Geological and Planetary SciencesUNSPECIFIED
Subject Keywords:mud; flocculation; suspended sediment; cohesive sediment; organic carbon
Issue or Number:5
Record Number:CaltechAUTHORS:20220610-583154400
Persistent URL:
Official Citation:Nghiem, J. A., Fischer, W. W., Li, G. K., & Lamb, M. P. (2022). A mechanistic model for mud flocculation in freshwater rivers. Journal of Geophysical Research: Earth Surface, 127, e2021JF006392.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:115123
Deposited By: Tony Diaz
Deposited On:13 Jun 2022 17:28
Last Modified:13 Jun 2022 17:28

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