the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Marsh induced backwater: the influence of non-fluvial sedimentation on a delta's channel morphology and kinematics
John Shaw
Samuel Zapp
José Silvestre
Ripul Dutt
Kyle Straub
Abstract. We investigate the interaction of fluvial and non-fluvial sedimentation on the channel morphology and kinematics of an experimental river delta. We compare two deltas: one that evolved with a proxy for non-fluvial sedimentation (treatment experiment) and one that evolved without the proxy (control). We show that the addition of the non-fluvial sediment proxy alters the delta's channel morphology and kinematics. Notably, the flow outside the channels is significantly reduced in the treatment experiment and the channels are deeper (as a function of radial distance) and longer. We also find that the treatment channels have the same width from the entrance to the shoreline, while the control channels get narrower as they approach the shore. Interestingly, the channel beds in the treatment experiment often exist below sea level in the terrestrial portion of the delta top creating a ~0.7 m reach of steady, nonuniform backwater flow. However, in the control experiment, the channel beds generally exist at or above relative sea level, creating channel movement resembling morphodynamic backwater kinematics and topographic flow expansions. Differences between channel and far-field aggradation produce a longer channel in-filling timescale for the treatment as compared to the control, suggesting that the channel avulsions triggered by a peak in channel sedimentation occur less frequently in the treatment experiment. Despite this difference, the basin-wide timescale of lateral channel mobility remains similar. Ultimately, non-fluvial sedimentation on the delta top plays a key role in the channel morphology and kinematics of an experimental river delta, producing channels which are more analogous to channels in global river deltas, and which cannot be produced solely by increasing cohesion in an experimental river delta.
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Kelly Sanks et al.
Status: closed
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RC1: 'General comment on egusphere-2023-545', Anonymous Referee #1, 08 Jun 2023
In this paper, the authors have compared two experiments to evaluate how marsh sediment accretion may influence deltas. They show the influence of marsh sediment accretion on delta channel morphology, hydrodynamics and morphodynamics, using a set of well-chosen figures and simple but appropriate analyses.
I think this is a great paper. The questions are well defined; the experiments are suitably designed to address those questions; the findings are clearly presented and generally well explained, and the paper is very well-written (to the point that I think I will share the published version with my grad students as an example of good academic writing). I have recommended minor revisions as there are a few points I feel could receive more attention: a few experimental details need inclusion; a few figure details need tweaking; some results’ physical explanation could be better elaborated, and I feel the experiments’ temporal dynamics warrant some further comment. Other than that, my comments are mostly minor and editorial (these are included in attached pdf comments).
General comments:
I know these experiments are covered in another paper, but some very basic detail including the flow rate, sediment supply rate, fluvial sediment size and distribution thereof, and scaling method (or justification, if none used) should be added. I think this will help other experimentalists to contextualise your work.
For all your figures, I recommend checking the consistency of colours for the points marking shoreline positions, and check that text is not too cramped in the legend.
Third, I’ve commented a few places where I think you could give a bit more detail on the physical mechanisms that explain your observations.
Finally, I think the paper could benefit from some consideration of the temporal dynamics of the experiments – was there any temporal variability in behaviour? Either trend or periodicity? If so, did it differ between the experiments? If there was no real temporal pattern to the experiments’ behaviour then I think this is worth just mentioning too. For instance, I was initially expecting to see some discussion of progradation and changing behaviour as the delta prograded. In hindsight, it makes sense that your fixed SLR rate dampened progradation, but others might have the same question so I think it would be helpful to either discuss any interesting temporal dynamics or comment on why you have not.
Line comments: in attached pdf. Hope they’re helpful!
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AC1: 'Reply on RC1', Kelly Sanks, 23 Aug 2023
In this paper, the authors have compared two experiments to evaluate how marsh sediment accretion may influence deltas. They show the influence of marsh sediment accretion on delta channel morphology, hydrodynamics and morphodynamics, using a set of well-chosen figures and simple but appropriate analyses.
I think this is a great paper. The questions are well defined; the experiments are suitably designed to address those questions; the findings are clearly presented and generally well explained, and the paper is very well-written (to the point that I think I will share the published version with my grad students as an example of good academic writing). I have recommended minor revisions as there are a few points I feel could receive more attention: a few experimental details need inclusion; a few figure details need tweaking; some results’ physical explanation could be better elaborated, and I feel the experiments’ temporal dynamics warrant some further comment. Other than that, my comments are mostly minor and editorial (these are included in attached pdf comments).
Thank you very much for the helpful comments and kind words about the quality of writing. We appreciate your thorough review and believe the new version adequately addresses all the concerns raised. Specifically, we have added details about the experimental setup, tweaked the figures as needed, and expanded on the discussion of our results to better illustrate the physical mechanisms that are at play in both the control and treatment experiments. However, due to the length of this manuscript and the nature of temporal dynamics in the experiments, we did not expand much on this herein. Because the experiments were run in equilibrium conditions, the temporal dynamics observed are a function of autogenic dynamics (see response to last comment). While interesting and warranting further study, another manuscript that analyzes the temporal dynamics of shorelines and how that controls wetland sediment accumulation is in preparation (see lines 110-111). Please find our responses to your comments below in italicized text, as well as in the attached pdf where we have responded directly to your comments. All changes can be found in bold text in the revised manuscript.
General comments:
I know these experiments are covered in another paper, but some very basic detail including the flow rate, sediment supply rate, fluvial sediment size and distribution thereof, and scaling method (or justification, if none used) should be added. I think this will help other experimentalists to contextualise your work.
We agree that more context will help the reader better understand the experimental set-up. As such, we added Table 1 to outline the boundary conditions of the two experiments. Further, we have elaborated on the scaling justification for the marsh proxy (lines 119-122) and added information about the grain size distribution of the river sediment (lines 122-125).
For all your figures, I recommend checking the consistency of colours for the points marking shoreline positions, and check that text is not too cramped in the legend.
Thank you for pointing out the inconsistency with the shoreline diamonds colors. The legend was incorrect for all figures and has been fixed. Further, we moved the shoreline diamonds onto the line per the suggestion of R2, and changed them to open circles so they are easier to see. Other figure tweaks were also made, including increasing the spacing and size of the legends and removing black lines surrounding the 1 sigma standard deviation bands, as suggested in the pdf comments from you and by R2. We note here that we did not change the aerial images in Fig. 3c and d or Fig. 8 as suggested. In Fig. 3 c and d, the timesteps were chosen to represent average flow conditions and in Fig. 8, the timesteps were chosen to illustrate the difference in channel morphology when one main channel is active. Because the experiments are run in an equilibrium state, the different time steps are not in a different delta growth stage and since we are not comparing absolute elevations, time step is not a factor here. Please refer to the pdf for an in depth response to your comments related to these figures.
Third, I’ve commented a few places where I think you could give a bit more detail on the physical mechanisms that explain your observations.
We have responded directly to all comments you made in the pdf (please refer there for specifics) and expanded on the physical mechanisms where asked (e.g., additional text in lines 277-282, 304-307, 311-312, 337-349, 393-397, and lines 402-412).
Finally, I think the paper could benefit from some consideration of the temporal dynamics of the experiments – was there any temporal variability in behaviour? Either trend or periodicity? If so, did it differ between the experiments? If there was no real temporal pattern to the experiments’ behaviour then I think this is worth just mentioning too. For instance, I was initially expecting to see some discussion of progradation and changing behaviour as the delta prograded. In hindsight, it makes sense that your fixed SLR rate dampened progradation, but others might have the same question so I think it would be helpful to either discuss any interesting temporal dynamics or comment on why you have not.
Thank you for this comment. We did not do a great job explaining the experimental set-up in the initial manuscript, but we run the experiments at an equilibrium state. Therefore, all temporal dynamics arise from stochasticity and autogenic dynamics and not from progradation, as this stage of delta formation occurs prior to data collection. We have added some detail on this in the Methods section (see lines 110-111). However, R2 raised a similar question related to shoreline variability over time. The complexity of these experiments and the amount of data collected allows us to address a range of questions, and we have a manuscript in preparation related to stratigraphic preservation of the various units and how preservation is related to the shoreline position (using spectral analysis). For this reason and in an effort to keep the length of the manuscript in check, we stand firm in our decision to keep the scope of the current manuscript to channel properties and kinematics without much discussion of the shoreline dynamics and temporal variability. However, we did add text in the Results section (lines 248-251) to illustrate the range of shoreline positions for context.
Line comments: in attached pdf. Hope they’re helpful!
Thank you! Your comments were extremely helpful! We have responded to and addressed all relevant line comments directly. Please see attached pdf.
-
AC1: 'Reply on RC1', Kelly Sanks, 23 Aug 2023
-
RC2: 'Comment on egusphere-2023-545', Anonymous Referee #2, 11 Jul 2023
- AC2: 'Reply on RC2', Kelly Sanks, 23 Aug 2023
- AC3: 'Cover Letter', Kelly Sanks, 23 Aug 2023
Status: closed
-
RC1: 'General comment on egusphere-2023-545', Anonymous Referee #1, 08 Jun 2023
In this paper, the authors have compared two experiments to evaluate how marsh sediment accretion may influence deltas. They show the influence of marsh sediment accretion on delta channel morphology, hydrodynamics and morphodynamics, using a set of well-chosen figures and simple but appropriate analyses.
I think this is a great paper. The questions are well defined; the experiments are suitably designed to address those questions; the findings are clearly presented and generally well explained, and the paper is very well-written (to the point that I think I will share the published version with my grad students as an example of good academic writing). I have recommended minor revisions as there are a few points I feel could receive more attention: a few experimental details need inclusion; a few figure details need tweaking; some results’ physical explanation could be better elaborated, and I feel the experiments’ temporal dynamics warrant some further comment. Other than that, my comments are mostly minor and editorial (these are included in attached pdf comments).
General comments:
I know these experiments are covered in another paper, but some very basic detail including the flow rate, sediment supply rate, fluvial sediment size and distribution thereof, and scaling method (or justification, if none used) should be added. I think this will help other experimentalists to contextualise your work.
For all your figures, I recommend checking the consistency of colours for the points marking shoreline positions, and check that text is not too cramped in the legend.
Third, I’ve commented a few places where I think you could give a bit more detail on the physical mechanisms that explain your observations.
Finally, I think the paper could benefit from some consideration of the temporal dynamics of the experiments – was there any temporal variability in behaviour? Either trend or periodicity? If so, did it differ between the experiments? If there was no real temporal pattern to the experiments’ behaviour then I think this is worth just mentioning too. For instance, I was initially expecting to see some discussion of progradation and changing behaviour as the delta prograded. In hindsight, it makes sense that your fixed SLR rate dampened progradation, but others might have the same question so I think it would be helpful to either discuss any interesting temporal dynamics or comment on why you have not.
Line comments: in attached pdf. Hope they’re helpful!
-
AC1: 'Reply on RC1', Kelly Sanks, 23 Aug 2023
In this paper, the authors have compared two experiments to evaluate how marsh sediment accretion may influence deltas. They show the influence of marsh sediment accretion on delta channel morphology, hydrodynamics and morphodynamics, using a set of well-chosen figures and simple but appropriate analyses.
I think this is a great paper. The questions are well defined; the experiments are suitably designed to address those questions; the findings are clearly presented and generally well explained, and the paper is very well-written (to the point that I think I will share the published version with my grad students as an example of good academic writing). I have recommended minor revisions as there are a few points I feel could receive more attention: a few experimental details need inclusion; a few figure details need tweaking; some results’ physical explanation could be better elaborated, and I feel the experiments’ temporal dynamics warrant some further comment. Other than that, my comments are mostly minor and editorial (these are included in attached pdf comments).
Thank you very much for the helpful comments and kind words about the quality of writing. We appreciate your thorough review and believe the new version adequately addresses all the concerns raised. Specifically, we have added details about the experimental setup, tweaked the figures as needed, and expanded on the discussion of our results to better illustrate the physical mechanisms that are at play in both the control and treatment experiments. However, due to the length of this manuscript and the nature of temporal dynamics in the experiments, we did not expand much on this herein. Because the experiments were run in equilibrium conditions, the temporal dynamics observed are a function of autogenic dynamics (see response to last comment). While interesting and warranting further study, another manuscript that analyzes the temporal dynamics of shorelines and how that controls wetland sediment accumulation is in preparation (see lines 110-111). Please find our responses to your comments below in italicized text, as well as in the attached pdf where we have responded directly to your comments. All changes can be found in bold text in the revised manuscript.
General comments:
I know these experiments are covered in another paper, but some very basic detail including the flow rate, sediment supply rate, fluvial sediment size and distribution thereof, and scaling method (or justification, if none used) should be added. I think this will help other experimentalists to contextualise your work.
We agree that more context will help the reader better understand the experimental set-up. As such, we added Table 1 to outline the boundary conditions of the two experiments. Further, we have elaborated on the scaling justification for the marsh proxy (lines 119-122) and added information about the grain size distribution of the river sediment (lines 122-125).
For all your figures, I recommend checking the consistency of colours for the points marking shoreline positions, and check that text is not too cramped in the legend.
Thank you for pointing out the inconsistency with the shoreline diamonds colors. The legend was incorrect for all figures and has been fixed. Further, we moved the shoreline diamonds onto the line per the suggestion of R2, and changed them to open circles so they are easier to see. Other figure tweaks were also made, including increasing the spacing and size of the legends and removing black lines surrounding the 1 sigma standard deviation bands, as suggested in the pdf comments from you and by R2. We note here that we did not change the aerial images in Fig. 3c and d or Fig. 8 as suggested. In Fig. 3 c and d, the timesteps were chosen to represent average flow conditions and in Fig. 8, the timesteps were chosen to illustrate the difference in channel morphology when one main channel is active. Because the experiments are run in an equilibrium state, the different time steps are not in a different delta growth stage and since we are not comparing absolute elevations, time step is not a factor here. Please refer to the pdf for an in depth response to your comments related to these figures.
Third, I’ve commented a few places where I think you could give a bit more detail on the physical mechanisms that explain your observations.
We have responded directly to all comments you made in the pdf (please refer there for specifics) and expanded on the physical mechanisms where asked (e.g., additional text in lines 277-282, 304-307, 311-312, 337-349, 393-397, and lines 402-412).
Finally, I think the paper could benefit from some consideration of the temporal dynamics of the experiments – was there any temporal variability in behaviour? Either trend or periodicity? If so, did it differ between the experiments? If there was no real temporal pattern to the experiments’ behaviour then I think this is worth just mentioning too. For instance, I was initially expecting to see some discussion of progradation and changing behaviour as the delta prograded. In hindsight, it makes sense that your fixed SLR rate dampened progradation, but others might have the same question so I think it would be helpful to either discuss any interesting temporal dynamics or comment on why you have not.
Thank you for this comment. We did not do a great job explaining the experimental set-up in the initial manuscript, but we run the experiments at an equilibrium state. Therefore, all temporal dynamics arise from stochasticity and autogenic dynamics and not from progradation, as this stage of delta formation occurs prior to data collection. We have added some detail on this in the Methods section (see lines 110-111). However, R2 raised a similar question related to shoreline variability over time. The complexity of these experiments and the amount of data collected allows us to address a range of questions, and we have a manuscript in preparation related to stratigraphic preservation of the various units and how preservation is related to the shoreline position (using spectral analysis). For this reason and in an effort to keep the length of the manuscript in check, we stand firm in our decision to keep the scope of the current manuscript to channel properties and kinematics without much discussion of the shoreline dynamics and temporal variability. However, we did add text in the Results section (lines 248-251) to illustrate the range of shoreline positions for context.
Line comments: in attached pdf. Hope they’re helpful!
Thank you! Your comments were extremely helpful! We have responded to and addressed all relevant line comments directly. Please see attached pdf.
-
AC1: 'Reply on RC1', Kelly Sanks, 23 Aug 2023
-
RC2: 'Comment on egusphere-2023-545', Anonymous Referee #2, 11 Jul 2023
- AC2: 'Reply on RC2', Kelly Sanks, 23 Aug 2023
- AC3: 'Cover Letter', Kelly Sanks, 23 Aug 2023
Kelly Sanks et al.
Kelly Sanks et al.
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