the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Feb 2024
Validating floc settling velocity models in rivers and freshwater wetlands
Abstract. Flocculation controls mud sedimentation and organic carbon burial rates by increasing mud settling velocity. Floc settling velocity can be predicted using a semi-empirical model that depends on turbulence, sediment concentration, and geochemical variables or an explicit Stokes law-type model that depends on floc diameter, permeability, and fractal properties. However, validation of the semi-empirical and explicit models with direct field measurements is lacking. We employed a camera, in situ particle sizing, and analysis of grain size-specific suspended sediment concentration profiles to measure flocs in the freshwater channels and wetlands of Wax Lake Delta, Louisiana. Sediment finer than ~20 to 50 μm flocculates with median floc diameter of 30 to 90 μm, bulk solid fraction of 0.05 to 0.3, and floc settling velocity of ~0.1 to 1 mm s-1, with little variation along depth. These values are consistent with the semi-empirical model, which indicates that turbulence limits variation in floc settling velocity on flood-to-seasonal time scales. In the explicit model, the effective primary particle diameter, commonly assumed to be the median primary particle diameter, differs by a factor of ~2 to 6 smaller than the median and can be better described using a simple fractal theory. Flow through the floc increases settling velocity by a factor of ~2 and can be explained by parameterizing flocs as effectively permeable clusters of primary particles. Our results provide the first full field validation of effective primary particle diameter and floc permeability theories, which improve floc settling velocity predictions of the explicit model.
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RC1: 'Comment on egusphere-2024-524', Anonymous Referee #1, 06 Apr 2024
This is a very interesting and meticulously written paper that explores in detail the settling velocity of flocs. The knowledge gap is clearly identified, the methods are extensively explained, and the findings of this study are useful and clearly presented. I only have a few comments that may add some clarity or extend the discussion a little bit. My most important question is how representative a depth-averaged Kolmogorov microscale value is to infer the effect of turbulence on flocculation and floc settling velocity. In lines 245-249, the authors show that they estimate profiles of Kolmogorov microscales across the depth (based on turbulence dissipation rate profiles), but later on, the influence of the Kolmogorov microscale is evaluated as a depth-averaged quantity (as mentioned in Table 2 and e.g. line 533). It is not clear to me if the authors coupled the vertical variation of the Kolmogorov microscale (to introduce an expression for turbulence intensity) with the vertical variation of the suspended sediment concentration or they used the depth-averaged value of the Kolmogorov microscale across the whole depth for the variations across the sediment concentration profile. I would appreciate it if you further elaborate on this. A followup question is about the role of bed-generated turbulence on the sediment concentration in general. The authors mention that there is limited vertical variation in sediment diameter and concentration (e.g., lines 481, 484, 522), but the turbulence intensity varies a lot with depth, especially near the bed. How does this fact relate to the discussion in Section 6.3? Is this relevant, or do the authors prefer to focus on depth-averaged quantities? Finally, in lines 451-452, does the sediment stratification affect the velocity (and turbulence) profile, or is it not that strong?
Another point for clarification comes from line 245 and the rest of the paragraph: How representative was the law of the wall for the measured velocity profiles? What were the coefficients of determination? In other words, did the measured velocity profiles exhibit a fully developed turbulent boundary layer to allow for the proper calculation of the shear velocity at the bed? Were there wind waves at the shallower sites affecting the velocity profiles (and sediment concentrations)? All sites except one have flow depths less than 4 m (Table 2), thus the 20% of the depth that was used for the law of the wall fit, was less than 0.8 m. How many measurement cells did you use to fit the law of the wall to measurement data from such shallow waters? How was the ADCP deployed (looking downwards or upwards), and what was the sampling resolution (cell size, sampling frequency, averaging duration)? How close to the bed did you measure? This is particularly relevant for the sites M1 Spring and M2 Spring which have depths of less than one meter. Finally, were there any vegetation effects in sites M1 and M2 Springs?
Some other minor comments in order of appearance:
Lines 9-12: When I first read this sentence in the abstract, I was expecting the development of a new model but the analysis relies a lot on models from the literature (Lines 48 and 57). Can this be clarified in the abstract?
Equation (1): Please explain each variable when it is firstly mentioned (Df is explained in line 94 instead of here).
Line 81: What does b express? Is the value of 20 a result of calibration or observation or something else from the cited study? Since you explain in detail everything else, maybe it's a good idea to also explain this in one line.
Line 121: How does Eq. (2) support that "Flocs tend to be less dense at their edges"?
Lines 285-286: Can you please elaborate a bit on the "empirically-determined gradient cutoff"?
Line 300: Correct to "in a given time-scale"
Figure 15a: Equation 2a is mentioned but there is no such equation. Maybe Equation 6a is meant?
Citation: https://doi.org/10.5194/egusphere-2024-524-RC1 -
RC2: 'Comment on egusphere-2024-524', Anonymous Referee #2, 08 Apr 2024
This is a very well planned, well thought-out and very well-written paper. In particular the finding that the effective primary particle diameter is not related to a particular length scale was revealing to me. I only have a couple of very basic comments:
1) not being familiar with the area at all, is the Wax Lake Delta truly a freshwater delta? It seems odd to me that Mike Island, only 6-8 km from the Gulf of Mexico would be an all-freshwater environment? Do the authors have some CTD measurements or similar they could add to the results to show this?
2) I am personally not a big fan of satellite images as the ones in 1A and 1C - a proper map that shows the system is less cluttered as far as I'm concerned. I would also be curious to know how the shorelines of Mike Island changes from spring to summer during the high and low discharges. This would provide a better system understanding by showing how much of it is submerged at various flood stages.
Citation: https://doi.org/10.5194/egusphere-2024-524-RC2 -
RC3: 'Comment on egusphere-2024-524', Anonymous Referee #3, 13 Apr 2024
MY REVIEW COMMENTS:
I see these finding to be quite interesting. Improving our understanding of the flocculation within coastal and estuarine regions is one of current challenges. It is of great importance to understand how sediment transport for any sediment type that is even partly cohesive, including mud:sand/silt mixtures, can influence the depositional characteristics of microplastics. This will also mean that the flocculation is a vital component of the sedimentary settling process. The manuscript is generally well structured, some relevant illustrations, and a reasonable range of relevant literature cited and referenced. The manuscript is of an appropriate length for egusphere.
Before publication can be considered, I would like to see the following comments and modifications accounted for within the manuscript.
Good to include a few key quantitative findings within the Abstract.
Although there are numerous references cited, I would suggest that a number of references reporting key advances in flocculation and near-bed processes are included in this manuscript, being cited initially in the Introduction (1) and also in relevant parts of the manuscript such as Results and Discussion sections.
* Soulsby, R.L., Manning, A.J., Spearman, J. and Whitehouse, R.J.S. (2013). Settling velocity and mass settling flux of flocculated estuarine sediments. Marine Geology, doi.org/10.1016/j.margeo.2013.04.006.
* Wolanski, E. and Elliott, M. (2015). Estuarine Ecohydrology. An Introduction. Elsevier, Amsterdam. 322p.
* Craig, M.J., Baas, J.H., Amos, K.J., Strachan, L.J., Manning, A.J., Paterson, D.M., Hope, J.A., Nodder, S.D., and Baker, M.L. (2019). Biomediation of submarine sediment gravity flow dynamics. Geology, vol. 48, no. 1, pp. 72-76. https://doi.org/10.1130/G46837.1
* Lawrence, T.J., Carr, S.J., Wheatland, J.A.T., Manning, A.J., and Spencer, K.L. (2022). Quantifying the 3D structure and function of porosity and pore space in natural sediment flocs. Journal of Sediments and Soils 22, 3176-3188, https://doi.org/10.1007/s11368-022-03304-x.
* Winterwerp, J. C., van Kesteren, W. G. M. (2004). Introduction to the physics of cohesive sediment in the marine environment. In: van Loon, T. (ed.), Developments in Sedimentology, 56. Amsterdam: Elsevier.
* Whitehouse, R. J. S., Soulsby, R.L., Robert, W. and Mitchener, H.J. (2000). Dynamics of Estuarine Muds: A manual for practical applications. Thomas Telford, London, ISBN 0-7277-2864-4.
* Mehta, A.J. (2022). An Introduction to Hydraulics of Fine Sediment Transport. 2nd Edition. Advanced Series on Ocean Engineering, Vol. 38. Hackensack, NJ: World Scientific Publishing Co.
* Mietta, F., Chassagne, C., Manning, A.J. and Winterwerp, J.C. (2009). Influence of shear rate, organic matter content, pH and salinity on mud flocculation. Ocean Dynamics, 59, 751-763, doi: 10.1007/s10236-009-0231-4.
In the Methodology sections I would like to see a slightly clearer scientific statement indicating the rationale for the experimental set-up and protocols. This will assist future scientist researching within this particular Sedimentary-MP field.
I would like to see just a few more comments on the effects of sedimentary organic cohesion levels in the Discussion and earlier in the Introduction. I see some of the reference listed in the manuscript mention organic material effects. Some other more recent research on the role of organic cohesion relating to purely cohesive and naturally mixed sediment bedform are listed below. I would suggest that key points could be included from some of these publications (listed below) in both the literature review and also within the discussion and interpretation:
* Eisma, D., (1986). Flocculation and de-flocculation of suspended matter in estuaries. Neth. Journal of Sea Res., 20 (2/3): 183-199.
* Deng, Z., He, Q., Manning, A.J. and Chassagne, C. (2023). A laboratory study on the behavior of estuarine sediment flocculation as function of salinity, EPS and living algae. Marine Geology 459:107029-107029, doi.org/10.1016/j.margeo.2023.107029
* Malarkey, J., Baas, J.H., Hope, J.A., Aspden, R.J., Parsons, D.R., Peakall, J., Paterson, D.M., Schindler, R.J., Ye, L., Lichtman, I.D., Bass, S.J., Davies, A.G., Manning, A.J., Thorne, P.D. (2015). The pervasive role of biological cohesion in bedform development. Nature Communications, DOI: 10.1038/ncomms7257.
* Gregory, J. and Barany, S. (2011). Adsorption and flocculation by polymers and polymer mixtures. Advances in Colloid and Interface Science, 169(1), 1–12.
* Parsons, D.R., Schindler, R.J., Hope, J.A., Malarkey, J., Baas, J.H., Peakall, J., Manning, A.J., Ye, L., Simmons, S., Paterson, D.M., Aspden, R.J., Bass, S.J., Davies, A.G., Lichtman, I.D. and Thorne, P.D. (2016). The role of biophysical cohesion on subaqueous bed form size. Geophysical Research Letters, 43, doi:10.1002/2016GL067667.
* Paterson, D.M., Crawford, R.M. and Little, C. (1990). Subaerial exposure and changes in the stability of intertidal estuarine sediments. Estuarine Coastal and Shelf Science, 30, 541-556.
* Paterson, D.M. and Hagerthey, S.E. (2001). Microphytobenthos in contrasting coastal ecosystems: Biology and dynamics. In: Ecological comparisons of sedimentary shores (K. Reise, Ed.), Ecological studies, pp. 105-125.
* Gregory, J. (2005). Particles in Water: Properties and Processes. CRC Press, zeroth edition.
* Schindler, R.J., Parsons, D.R., Ye, L., Hope, J.A., Baas, J.H., Peakall, J., Manning, A.J., Aspden, R.J., Malarkey, J., Simmons, S., Paterson, D.M., Lichtman, I.D., Davies, A.G., Thorne, P.D. and Bass, S.J. (2015). Sticky stuff: Redefining bedform prediction in modern and ancient environments. Geology, doi: 10.1130/G36262.1.
* Wolanski, E. and Elliott, M. (2015). Estuarine Ecohydrology. An Introduction. Elsevier, Amsterdam. 322p.
* Tolhurst, T.J., Gust. G. and Paterson, D.M. (2002). The influence on an extra-cellular polymeric substance (EPS) on cohesive sediment stability. In: J.C. Winterwerp and C. Kranenburg (Eds), Fine Sediment Dynamics in the Marine Environment - Proc. In Marine Science 5, Amsterdam: Elsevier, pp. 409-425, ISBN: 0-444-51136-9.
For completeness, I think it is worth mentioning Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, as this is often used to quantify particle-to-particle interactions. This requires relevant referencing, including:
* Chassagne, C. (2019). Introduction to Colloid Science. Delft Academic Press.
* Kruyt, H. R. (1949). Colloid Science. Technical report, Elsevier Pub. Co.
* van Leussen, W., 1994. Estuarine macroflocs and their role in fine-grained sediment transport. Ph.D. Thesis, University of Utrecht, The Netherlands, 488pp.
Furthermore, in estuarine areas, the suspended sediment particle will, according to DLVO theory, be destabilized and flocculate because of the increase in salinity between the river fresh water and the sea. However, in the presence of organic matter however, DLVO theory cannot be applied, as the flocculation mechanisms will be driven by the presence of polyelectrolytes and microorganisms which are not accounted for in the DLVO theory (see Deng et al., 2023). I would like to see a comment on this point within the Discussion and implications for cohesive sedimentary settling.
* Deng, Z., He, Q., Manning, A.J. and Chassagne, C. (2023). A laboratory study on the behavior of estuarine sediment flocculation as function of salinity, EPS and living algae. Marine Geology 459:107029-107029, https://doi.org/10.1016/j.margeo .2023.107029
I would not agree with the statement on line 153 that sand grains are not incorporated within mud flocs. As much of the study focuses on the roles of both cohesive and non-cohesive sedimentary dynamics - sandy / silts / clays, recent key publications on the flocculation processes of cohesive and mixed fine-grained sediment suspension (experimental results, applied modeling; floc properties; floc structure settling and deposition) need to be considered (and cited) within both the Introduction and Discussion / Interpretation, as these outline key processes relating to these suspended sediment types - including:
* van Ledden, M. (2002). A process-based sand-mud model. In: J.C. Winterwerp and C. Kranenburg (Eds.), Fine Sediment Dynamics in the Marine Environment - Proc. in
Mar. Science 5, Amsterdam: Elsevier, pp.577-594, ISBN: 0-444-51136-9.
* Manning, A.J., Baugh, J.V., Spearman, J.R., Pidduck, E.L. and Whitehouse, R.J.S. (2011). The settling dynamics of flocculating mud:sand mixtures: Part 1 – Empirical algorithm development. Ocean Dynamics, INTERCOH 2009 special issue, doi: 10.1007/s10236-011-0394-7.
* van Ledden, M., 2003. Sand-mud segregation in estuaries and tidal basins. Ph.D. Thesis, Delft University of Technology, The Netherlands, Report No. 03–2, ISSN 0169-6548, 217pp.
* Manning, A.J., Baugh, J.V., Spearman, J. and Whitehouse, R.J.S. (2010). Flocculation Settling Characteristics of Mud:Sand Mixtures. Ocean Dynamics, doi: 10.1007/s10236-009-0251-0.
* Dankers, P.J.T., Sills, G.C. and Winterwerp, J.C. (2007). On the hindered settling of highly concentrated mud-sand mixtures. In: T. Kudusa, H. Yamanishi, J. Spearman and J.Z. Gailani (Eds), Sediment and Ecohydraulics - Proc. in Marine Science, INTERCOH 2005, Amsterdam: Elsevier, pp. 255-274.
* Waeles, B., Le Hir, P. and Lesueur, P. (2008). A 3D morphodynamic process-based modelling of a mixed sand/mud coastal environment : the Seine Estuary, France. In: T. Kudusa, H. Yamanishi, J. Spearman and J.Z. Galiani, (eds.), Sediment and Ecohydraulics - Proc. in Marine Science 9, Amsterdam: Elsevier, pp. 477-498, ISBN: 978-0-444-53184-1.
* Spearman, J.R., Manning, A.J. and Whitehouse, R.J.S. (2011). The settling dynamics of flocculating mud:sand mixtures: Part 2 – Numerical modelling. Ocean Dynamics, doi: 10.1007/s10236-011-0385-8.
* van Wijngaarden, M., Venema, L.B., De Meijer, R.J., Zwolsman, J.J.G., Van Os, B. and Gieske, J.M.J. (2002a). Radiometric sand-mud characterisation in the Rhine-Meuse estuary, Part A: Fingerprinting. Geomorphology, 43, 87-101.
* Spencer, K.L., Manning, A.J., Droppo, I.G., Leppard, G.G. and Benson, T. (2010). Dynamic interactions between cohesive sediment tracers and natural mud. Journal of Soils and Sediments, Volume 10 (7), doi:10.1007/s11368-010-0291-6
Much of the modelling is based on flocs being represented by a fractal structure. Although this can be ustilised, I would like to see the authors including comments on the non-ftractal structure of natural flocs. See:
* Spencer, K.L., Wheatland, J.A.T., Bushby, A.J., Carr, S.J., Droppo, I.G. and Manning, A.J. (2021). A structure–function based approach to floc hierarchy and evidence for the non‑fractal nature of natural sediment flocs. Nature - Scientific Reports, 11:14012, doi.org/10.1038/s41598-021-93302-9.
In the latter part of the Introduction (page 6), I would like to see a slightly clear list of key Research Questions and the ones that are being addressed in this manuscript in terms of clear aims and objectives. This would greatly assist future researcher that follow this work.
I see that a LISST-200X was used to measure floc population distributions. I would like the authors to provide some comments on the limitations of this instrumentation (e.g. SSC low turbidity range, Mie Theory application, sphere particle shape assumption. This is important to consider, as the LISST is the primary instrument being used to measure the floc/particle size distributions. References on floc measurements relating to the LISST that I suggest need to be mentioned in both the literature review and included in the discussion are:
* Gratiot, N. and Manning, A.J. (2004). An experimental investigation of floc characteristics in a diffusive turbulent flow. Journal of Coastal Research, SI 41, 105-113.
* Agrawal, Y. C., Whitmire, A., Mikkelsen, O. A., & Pottsmith, H. C. (2008). Light scattering by random shaped particles and consequences on measuring suspended sediments by laser diffraction. Journal of Geophysical Research, 113 (C4), C04023. doi: 10.1029/2007JC004403
* Manning, A.J. and Dyer, K.R. (2002). The use of optics for the in-situ determination of flocculated mud characteristics. J. Optics A: Pure and Applied Optics, Institute of Physics Publishing, 4, S71-S81.
* Fall, K. A., Friedrichs, C. T., Massey, G. M., Bowers, D. G., & Smith, S. J. (2021). The Importance of Organic Content to Fractal Floc Properties in Estuarine Surface Waters: Insights From Video, LISST, and Pump Sampling. Journal of Geophysical Research: Oceans, 126 (1). doi:672 10.1029/2020JC016787
* Manning, A.J., Friend, P.L., Prowse, N. and Amos, C.L. (2007). Preliminary Findings from a Study of Medway Estuary (UK) Natural Mud Floc Properties Using a Laboratory Mini-flume and the LabSFLOC system. Continental Shelf Research, doi:10.1016/j.csr.2006.04.011.
* Fugate, D. C., & Friedrichs, C. T. (2002). Determining concentration and fall velocity of estuarine particle populations using ADV, OBS and LISST. Continental Shelf Research, 22 (11), 1867–1886.
* Manning, A.J. and Schoellhamer, D.H. (2013). Factors controlling floc settling velocity along a longitudinal estuarine transect. Marine Geology, San Francisco Bay special issue, doi.org/10.1016/j.margeo.2013.04.006.
* Livsey, D. N., Downing-Kunz, M. A., Schoellhamer, D. H., & Manning, A. (2020). Suspended Sediment Flux in the San Francisco Estuary: Part I—Changes in the Vertical Distribution of Suspended Sediment and Bias in Estuarine Sediment Flux Measurements. Estuaries and Coasts. doi: 10.1007/s12237-020-00734-z
* Mietta, F., Chassagne, C., Manning, A.J. and Winterwerp, J.C. (2009). Influence of shear rate, organic matter content, pH and salinity on mud flocculation. Ocean Dynamics, 59, 751-763, doi: 10.1007/s10236-009-0231-4.
Also, as only particle / floc sizes are estimated by the LISST-200X Type C, I would like to see some comments on how wide ranges in floc density within populations are accounted for in the interpretations, or what limits this may place on the data interpretation and subsequent application. This also includes any floc settling and depositional rate behaviour interpretation.
I would like to see a little more key quantitative results placed in and commented on in the Discussion.
I hope these comments are of help and look forward to reading the final draft.
RECOMMENDATION: Minor Revisions
Citation: https://doi.org/10.5194/egusphere-2024-524-RC3 - AC1: 'Response to referee comments', Justin A Nghiem, 03 Jun 2024
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-524', Anonymous Referee #1, 06 Apr 2024
This is a very interesting and meticulously written paper that explores in detail the settling velocity of flocs. The knowledge gap is clearly identified, the methods are extensively explained, and the findings of this study are useful and clearly presented. I only have a few comments that may add some clarity or extend the discussion a little bit. My most important question is how representative a depth-averaged Kolmogorov microscale value is to infer the effect of turbulence on flocculation and floc settling velocity. In lines 245-249, the authors show that they estimate profiles of Kolmogorov microscales across the depth (based on turbulence dissipation rate profiles), but later on, the influence of the Kolmogorov microscale is evaluated as a depth-averaged quantity (as mentioned in Table 2 and e.g. line 533). It is not clear to me if the authors coupled the vertical variation of the Kolmogorov microscale (to introduce an expression for turbulence intensity) with the vertical variation of the suspended sediment concentration or they used the depth-averaged value of the Kolmogorov microscale across the whole depth for the variations across the sediment concentration profile. I would appreciate it if you further elaborate on this. A followup question is about the role of bed-generated turbulence on the sediment concentration in general. The authors mention that there is limited vertical variation in sediment diameter and concentration (e.g., lines 481, 484, 522), but the turbulence intensity varies a lot with depth, especially near the bed. How does this fact relate to the discussion in Section 6.3? Is this relevant, or do the authors prefer to focus on depth-averaged quantities? Finally, in lines 451-452, does the sediment stratification affect the velocity (and turbulence) profile, or is it not that strong?
Another point for clarification comes from line 245 and the rest of the paragraph: How representative was the law of the wall for the measured velocity profiles? What were the coefficients of determination? In other words, did the measured velocity profiles exhibit a fully developed turbulent boundary layer to allow for the proper calculation of the shear velocity at the bed? Were there wind waves at the shallower sites affecting the velocity profiles (and sediment concentrations)? All sites except one have flow depths less than 4 m (Table 2), thus the 20% of the depth that was used for the law of the wall fit, was less than 0.8 m. How many measurement cells did you use to fit the law of the wall to measurement data from such shallow waters? How was the ADCP deployed (looking downwards or upwards), and what was the sampling resolution (cell size, sampling frequency, averaging duration)? How close to the bed did you measure? This is particularly relevant for the sites M1 Spring and M2 Spring which have depths of less than one meter. Finally, were there any vegetation effects in sites M1 and M2 Springs?
Some other minor comments in order of appearance:
Lines 9-12: When I first read this sentence in the abstract, I was expecting the development of a new model but the analysis relies a lot on models from the literature (Lines 48 and 57). Can this be clarified in the abstract?
Equation (1): Please explain each variable when it is firstly mentioned (Df is explained in line 94 instead of here).
Line 81: What does b express? Is the value of 20 a result of calibration or observation or something else from the cited study? Since you explain in detail everything else, maybe it's a good idea to also explain this in one line.
Line 121: How does Eq. (2) support that "Flocs tend to be less dense at their edges"?
Lines 285-286: Can you please elaborate a bit on the "empirically-determined gradient cutoff"?
Line 300: Correct to "in a given time-scale"
Figure 15a: Equation 2a is mentioned but there is no such equation. Maybe Equation 6a is meant?
Citation: https://doi.org/10.5194/egusphere-2024-524-RC1 -
RC2: 'Comment on egusphere-2024-524', Anonymous Referee #2, 08 Apr 2024
This is a very well planned, well thought-out and very well-written paper. In particular the finding that the effective primary particle diameter is not related to a particular length scale was revealing to me. I only have a couple of very basic comments:
1) not being familiar with the area at all, is the Wax Lake Delta truly a freshwater delta? It seems odd to me that Mike Island, only 6-8 km from the Gulf of Mexico would be an all-freshwater environment? Do the authors have some CTD measurements or similar they could add to the results to show this?
2) I am personally not a big fan of satellite images as the ones in 1A and 1C - a proper map that shows the system is less cluttered as far as I'm concerned. I would also be curious to know how the shorelines of Mike Island changes from spring to summer during the high and low discharges. This would provide a better system understanding by showing how much of it is submerged at various flood stages.
Citation: https://doi.org/10.5194/egusphere-2024-524-RC2 -
RC3: 'Comment on egusphere-2024-524', Anonymous Referee #3, 13 Apr 2024
MY REVIEW COMMENTS:
I see these finding to be quite interesting. Improving our understanding of the flocculation within coastal and estuarine regions is one of current challenges. It is of great importance to understand how sediment transport for any sediment type that is even partly cohesive, including mud:sand/silt mixtures, can influence the depositional characteristics of microplastics. This will also mean that the flocculation is a vital component of the sedimentary settling process. The manuscript is generally well structured, some relevant illustrations, and a reasonable range of relevant literature cited and referenced. The manuscript is of an appropriate length for egusphere.
Before publication can be considered, I would like to see the following comments and modifications accounted for within the manuscript.
Good to include a few key quantitative findings within the Abstract.
Although there are numerous references cited, I would suggest that a number of references reporting key advances in flocculation and near-bed processes are included in this manuscript, being cited initially in the Introduction (1) and also in relevant parts of the manuscript such as Results and Discussion sections.
* Soulsby, R.L., Manning, A.J., Spearman, J. and Whitehouse, R.J.S. (2013). Settling velocity and mass settling flux of flocculated estuarine sediments. Marine Geology, doi.org/10.1016/j.margeo.2013.04.006.
* Wolanski, E. and Elliott, M. (2015). Estuarine Ecohydrology. An Introduction. Elsevier, Amsterdam. 322p.
* Craig, M.J., Baas, J.H., Amos, K.J., Strachan, L.J., Manning, A.J., Paterson, D.M., Hope, J.A., Nodder, S.D., and Baker, M.L. (2019). Biomediation of submarine sediment gravity flow dynamics. Geology, vol. 48, no. 1, pp. 72-76. https://doi.org/10.1130/G46837.1
* Lawrence, T.J., Carr, S.J., Wheatland, J.A.T., Manning, A.J., and Spencer, K.L. (2022). Quantifying the 3D structure and function of porosity and pore space in natural sediment flocs. Journal of Sediments and Soils 22, 3176-3188, https://doi.org/10.1007/s11368-022-03304-x.
* Winterwerp, J. C., van Kesteren, W. G. M. (2004). Introduction to the physics of cohesive sediment in the marine environment. In: van Loon, T. (ed.), Developments in Sedimentology, 56. Amsterdam: Elsevier.
* Whitehouse, R. J. S., Soulsby, R.L., Robert, W. and Mitchener, H.J. (2000). Dynamics of Estuarine Muds: A manual for practical applications. Thomas Telford, London, ISBN 0-7277-2864-4.
* Mehta, A.J. (2022). An Introduction to Hydraulics of Fine Sediment Transport. 2nd Edition. Advanced Series on Ocean Engineering, Vol. 38. Hackensack, NJ: World Scientific Publishing Co.
* Mietta, F., Chassagne, C., Manning, A.J. and Winterwerp, J.C. (2009). Influence of shear rate, organic matter content, pH and salinity on mud flocculation. Ocean Dynamics, 59, 751-763, doi: 10.1007/s10236-009-0231-4.
In the Methodology sections I would like to see a slightly clearer scientific statement indicating the rationale for the experimental set-up and protocols. This will assist future scientist researching within this particular Sedimentary-MP field.
I would like to see just a few more comments on the effects of sedimentary organic cohesion levels in the Discussion and earlier in the Introduction. I see some of the reference listed in the manuscript mention organic material effects. Some other more recent research on the role of organic cohesion relating to purely cohesive and naturally mixed sediment bedform are listed below. I would suggest that key points could be included from some of these publications (listed below) in both the literature review and also within the discussion and interpretation:
* Eisma, D., (1986). Flocculation and de-flocculation of suspended matter in estuaries. Neth. Journal of Sea Res., 20 (2/3): 183-199.
* Deng, Z., He, Q., Manning, A.J. and Chassagne, C. (2023). A laboratory study on the behavior of estuarine sediment flocculation as function of salinity, EPS and living algae. Marine Geology 459:107029-107029, doi.org/10.1016/j.margeo.2023.107029
* Malarkey, J., Baas, J.H., Hope, J.A., Aspden, R.J., Parsons, D.R., Peakall, J., Paterson, D.M., Schindler, R.J., Ye, L., Lichtman, I.D., Bass, S.J., Davies, A.G., Manning, A.J., Thorne, P.D. (2015). The pervasive role of biological cohesion in bedform development. Nature Communications, DOI: 10.1038/ncomms7257.
* Gregory, J. and Barany, S. (2011). Adsorption and flocculation by polymers and polymer mixtures. Advances in Colloid and Interface Science, 169(1), 1–12.
* Parsons, D.R., Schindler, R.J., Hope, J.A., Malarkey, J., Baas, J.H., Peakall, J., Manning, A.J., Ye, L., Simmons, S., Paterson, D.M., Aspden, R.J., Bass, S.J., Davies, A.G., Lichtman, I.D. and Thorne, P.D. (2016). The role of biophysical cohesion on subaqueous bed form size. Geophysical Research Letters, 43, doi:10.1002/2016GL067667.
* Paterson, D.M., Crawford, R.M. and Little, C. (1990). Subaerial exposure and changes in the stability of intertidal estuarine sediments. Estuarine Coastal and Shelf Science, 30, 541-556.
* Paterson, D.M. and Hagerthey, S.E. (2001). Microphytobenthos in contrasting coastal ecosystems: Biology and dynamics. In: Ecological comparisons of sedimentary shores (K. Reise, Ed.), Ecological studies, pp. 105-125.
* Gregory, J. (2005). Particles in Water: Properties and Processes. CRC Press, zeroth edition.
* Schindler, R.J., Parsons, D.R., Ye, L., Hope, J.A., Baas, J.H., Peakall, J., Manning, A.J., Aspden, R.J., Malarkey, J., Simmons, S., Paterson, D.M., Lichtman, I.D., Davies, A.G., Thorne, P.D. and Bass, S.J. (2015). Sticky stuff: Redefining bedform prediction in modern and ancient environments. Geology, doi: 10.1130/G36262.1.
* Wolanski, E. and Elliott, M. (2015). Estuarine Ecohydrology. An Introduction. Elsevier, Amsterdam. 322p.
* Tolhurst, T.J., Gust. G. and Paterson, D.M. (2002). The influence on an extra-cellular polymeric substance (EPS) on cohesive sediment stability. In: J.C. Winterwerp and C. Kranenburg (Eds), Fine Sediment Dynamics in the Marine Environment - Proc. In Marine Science 5, Amsterdam: Elsevier, pp. 409-425, ISBN: 0-444-51136-9.
For completeness, I think it is worth mentioning Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, as this is often used to quantify particle-to-particle interactions. This requires relevant referencing, including:
* Chassagne, C. (2019). Introduction to Colloid Science. Delft Academic Press.
* Kruyt, H. R. (1949). Colloid Science. Technical report, Elsevier Pub. Co.
* van Leussen, W., 1994. Estuarine macroflocs and their role in fine-grained sediment transport. Ph.D. Thesis, University of Utrecht, The Netherlands, 488pp.
Furthermore, in estuarine areas, the suspended sediment particle will, according to DLVO theory, be destabilized and flocculate because of the increase in salinity between the river fresh water and the sea. However, in the presence of organic matter however, DLVO theory cannot be applied, as the flocculation mechanisms will be driven by the presence of polyelectrolytes and microorganisms which are not accounted for in the DLVO theory (see Deng et al., 2023). I would like to see a comment on this point within the Discussion and implications for cohesive sedimentary settling.
* Deng, Z., He, Q., Manning, A.J. and Chassagne, C. (2023). A laboratory study on the behavior of estuarine sediment flocculation as function of salinity, EPS and living algae. Marine Geology 459:107029-107029, https://doi.org/10.1016/j.margeo .2023.107029
I would not agree with the statement on line 153 that sand grains are not incorporated within mud flocs. As much of the study focuses on the roles of both cohesive and non-cohesive sedimentary dynamics - sandy / silts / clays, recent key publications on the flocculation processes of cohesive and mixed fine-grained sediment suspension (experimental results, applied modeling; floc properties; floc structure settling and deposition) need to be considered (and cited) within both the Introduction and Discussion / Interpretation, as these outline key processes relating to these suspended sediment types - including:
* van Ledden, M. (2002). A process-based sand-mud model. In: J.C. Winterwerp and C. Kranenburg (Eds.), Fine Sediment Dynamics in the Marine Environment - Proc. in
Mar. Science 5, Amsterdam: Elsevier, pp.577-594, ISBN: 0-444-51136-9.
* Manning, A.J., Baugh, J.V., Spearman, J.R., Pidduck, E.L. and Whitehouse, R.J.S. (2011). The settling dynamics of flocculating mud:sand mixtures: Part 1 – Empirical algorithm development. Ocean Dynamics, INTERCOH 2009 special issue, doi: 10.1007/s10236-011-0394-7.
* van Ledden, M., 2003. Sand-mud segregation in estuaries and tidal basins. Ph.D. Thesis, Delft University of Technology, The Netherlands, Report No. 03–2, ISSN 0169-6548, 217pp.
* Manning, A.J., Baugh, J.V., Spearman, J. and Whitehouse, R.J.S. (2010). Flocculation Settling Characteristics of Mud:Sand Mixtures. Ocean Dynamics, doi: 10.1007/s10236-009-0251-0.
* Dankers, P.J.T., Sills, G.C. and Winterwerp, J.C. (2007). On the hindered settling of highly concentrated mud-sand mixtures. In: T. Kudusa, H. Yamanishi, J. Spearman and J.Z. Gailani (Eds), Sediment and Ecohydraulics - Proc. in Marine Science, INTERCOH 2005, Amsterdam: Elsevier, pp. 255-274.
* Waeles, B., Le Hir, P. and Lesueur, P. (2008). A 3D morphodynamic process-based modelling of a mixed sand/mud coastal environment : the Seine Estuary, France. In: T. Kudusa, H. Yamanishi, J. Spearman and J.Z. Galiani, (eds.), Sediment and Ecohydraulics - Proc. in Marine Science 9, Amsterdam: Elsevier, pp. 477-498, ISBN: 978-0-444-53184-1.
* Spearman, J.R., Manning, A.J. and Whitehouse, R.J.S. (2011). The settling dynamics of flocculating mud:sand mixtures: Part 2 – Numerical modelling. Ocean Dynamics, doi: 10.1007/s10236-011-0385-8.
* van Wijngaarden, M., Venema, L.B., De Meijer, R.J., Zwolsman, J.J.G., Van Os, B. and Gieske, J.M.J. (2002a). Radiometric sand-mud characterisation in the Rhine-Meuse estuary, Part A: Fingerprinting. Geomorphology, 43, 87-101.
* Spencer, K.L., Manning, A.J., Droppo, I.G., Leppard, G.G. and Benson, T. (2010). Dynamic interactions between cohesive sediment tracers and natural mud. Journal of Soils and Sediments, Volume 10 (7), doi:10.1007/s11368-010-0291-6
Much of the modelling is based on flocs being represented by a fractal structure. Although this can be ustilised, I would like to see the authors including comments on the non-ftractal structure of natural flocs. See:
* Spencer, K.L., Wheatland, J.A.T., Bushby, A.J., Carr, S.J., Droppo, I.G. and Manning, A.J. (2021). A structure–function based approach to floc hierarchy and evidence for the non‑fractal nature of natural sediment flocs. Nature - Scientific Reports, 11:14012, doi.org/10.1038/s41598-021-93302-9.
In the latter part of the Introduction (page 6), I would like to see a slightly clear list of key Research Questions and the ones that are being addressed in this manuscript in terms of clear aims and objectives. This would greatly assist future researcher that follow this work.
I see that a LISST-200X was used to measure floc population distributions. I would like the authors to provide some comments on the limitations of this instrumentation (e.g. SSC low turbidity range, Mie Theory application, sphere particle shape assumption. This is important to consider, as the LISST is the primary instrument being used to measure the floc/particle size distributions. References on floc measurements relating to the LISST that I suggest need to be mentioned in both the literature review and included in the discussion are:
* Gratiot, N. and Manning, A.J. (2004). An experimental investigation of floc characteristics in a diffusive turbulent flow. Journal of Coastal Research, SI 41, 105-113.
* Agrawal, Y. C., Whitmire, A., Mikkelsen, O. A., & Pottsmith, H. C. (2008). Light scattering by random shaped particles and consequences on measuring suspended sediments by laser diffraction. Journal of Geophysical Research, 113 (C4), C04023. doi: 10.1029/2007JC004403
* Manning, A.J. and Dyer, K.R. (2002). The use of optics for the in-situ determination of flocculated mud characteristics. J. Optics A: Pure and Applied Optics, Institute of Physics Publishing, 4, S71-S81.
* Fall, K. A., Friedrichs, C. T., Massey, G. M., Bowers, D. G., & Smith, S. J. (2021). The Importance of Organic Content to Fractal Floc Properties in Estuarine Surface Waters: Insights From Video, LISST, and Pump Sampling. Journal of Geophysical Research: Oceans, 126 (1). doi:672 10.1029/2020JC016787
* Manning, A.J., Friend, P.L., Prowse, N. and Amos, C.L. (2007). Preliminary Findings from a Study of Medway Estuary (UK) Natural Mud Floc Properties Using a Laboratory Mini-flume and the LabSFLOC system. Continental Shelf Research, doi:10.1016/j.csr.2006.04.011.
* Fugate, D. C., & Friedrichs, C. T. (2002). Determining concentration and fall velocity of estuarine particle populations using ADV, OBS and LISST. Continental Shelf Research, 22 (11), 1867–1886.
* Manning, A.J. and Schoellhamer, D.H. (2013). Factors controlling floc settling velocity along a longitudinal estuarine transect. Marine Geology, San Francisco Bay special issue, doi.org/10.1016/j.margeo.2013.04.006.
* Livsey, D. N., Downing-Kunz, M. A., Schoellhamer, D. H., & Manning, A. (2020). Suspended Sediment Flux in the San Francisco Estuary: Part I—Changes in the Vertical Distribution of Suspended Sediment and Bias in Estuarine Sediment Flux Measurements. Estuaries and Coasts. doi: 10.1007/s12237-020-00734-z
* Mietta, F., Chassagne, C., Manning, A.J. and Winterwerp, J.C. (2009). Influence of shear rate, organic matter content, pH and salinity on mud flocculation. Ocean Dynamics, 59, 751-763, doi: 10.1007/s10236-009-0231-4.
Also, as only particle / floc sizes are estimated by the LISST-200X Type C, I would like to see some comments on how wide ranges in floc density within populations are accounted for in the interpretations, or what limits this may place on the data interpretation and subsequent application. This also includes any floc settling and depositional rate behaviour interpretation.
I would like to see a little more key quantitative results placed in and commented on in the Discussion.
I hope these comments are of help and look forward to reading the final draft.
RECOMMENDATION: Minor Revisions
Citation: https://doi.org/10.5194/egusphere-2024-524-RC3 - AC1: 'Response to referee comments', Justin A Nghiem, 03 Jun 2024
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