the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Sep 2023
The dynamics of marsh-channel slump blocks: an observational study using repeated drone imagery
Zhicheng Yang
Clark Alexander
Merryl Alber
Abstract. Slump blocks are widely distributed features along marsh shorelines that can play an important role in marsh dynamics. However, little is known about their spatial distribution patterns, nor their longevity and movement. We employed an Unmanned Aerial Vehicle (UAV) to track slump blocks in 11 monthly images (March 2020–March 2021) of Dean Creek, a tidal creek surrounded by salt marsh located on Sapelo Island (GA, USA). Slump blocks were observed along both convex and concave banks of the creek in all images, with sizes between 0.03 and 72.51 m2. Although the majority of blocks were categorized as persistent, there were also new blocks in each image. Most blocks were lost through submergence, and both decreased in area and moved towards the center of the channel over time. However, some blocks reconnected to the marsh platform, which has not been previously observed. These blocks were initially larger and located closer to the marsh edge than those that submerged, and increased in area over time. Only 13 out of a cohort of 61 newly created blocks observed in May 2020 remained after 5 months, suggesting that most blocks persist for only a short time. When taken together, the total area of new slump blocks was 886.13 m2 and that of reconnected blocks was 652.45 m2. This resulted in a net expansion of the channel by 233.68 m2 over the study period, accounting for about 66 % of the overall increase in the channel area of Dean Creek, and suggests that slump block processes play an important role in tidal creek channel widening. This study illustrates the power of repeated UAV surveys to monitor short-term geomorphological processes, such as slump block formation and loss, to provide new insights into marsh eco-geomorphological processes.
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Zhicheng Yang et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2023-180', Anonymous Referee #1, 06 Nov 2023
Summary
The manuscript “The dynamics of marsh-channel slump blocks: an observational study using repeated drone imagery” presents the evolution of slump blocks developed along Dean creek in Sapelo Island (Georgia, US) through UAV orthophoto analyses from March 2020 to March 2021. The paper is simple and concise, and although it does not “dig” into the process of slump block’s formation (since no other data besides the orthophotos are included to integrate the discussion), it still gives interesting information regarding a process that, personally, I did not find frequently discussed.
Herein I summarize the main questions and discussion. Then I present a list of specific comments (which are on the pdf as well).
The paper gives some context to the process, which must be introduced since it is not too obvious. Maybe more information regarding the process of slump formation taken from other studies could be useful, although I understand that there are no many studies. The analyses seem to be thorough; however, I have a few questions:
- I see in the additional information that you recorded the tidal level for each survey, that is indeed important otherwise it would be difficult to define which block can be considered as submerged and which was not. It is normal that you could not make photos perfectly at the same mean sea level, however I guess you considered the slumps submerged if they were underwater in the photo, in spite of what tidal level you had in that moment. Can you add a statement regarding this? Just to clarify what you considered underwater.
- I did not see any information regarding the flights. How many GCPs did you use? Where are they located? Is that drone using an RTK system? Did you measure some random points to validate the products? Can you show the GCPs on Fig. 1? I appreciate the validation considering the error derived by subjective identification of the slumps, but it could have been improved combining it with the horizontal and vertical Root Mean Square Error (since you have calculated several areas and rates). Anyway, due to the high resolution I would not expect large changes.
- For what concerns sediment deposition, you say in L 389 that sedimentation in the gap between block and edge is 1.5 – 2 times higher than the adjacent marsh, but you do not mention either the average rate of deposition of the study area or the rates of deposition in the gaps. In fact, how much was sedimentation inside the gaps? Then, what are the rates in the marsh? If you have measured it, it would be interesting to add them to the manuscript.
One important thing that I would recommend is to improve the discussion regarding the connection between channel widening and slump formation (e.g. L 402). You say that the slump formation processes is the main mechanism of channel widening, and indeed it seems important, but (based on what you said) it looks like the block formation is a process that is separated from the channel widening itself, like it affects the channel but at the same time is something separated from it. “The fact that bank slumping accounted for 66% of the increase in channel area in this study demonstrates that slumping can be an important mechanism for channel widening, …” What I suppose you are suggesting is that the erosion of the channel is occurring as slump detachment from the banks (i.e. discontinue) rather than following a continue and homogeneous lateral erosion. However, it looks to me that slump formation is one type of erosional process of the channel itself, hence I would suggest changing the storytelling of this part of the discussion highlighting how the slump formation is the main erosional process of channel widening, rather than saying that it plays an important role on the channel widening. This observation would be different if instead you meant to say that the block themselves change the flow patterns and alter bank erosion but does not look like you went in this direction to me.
Overall, the paper is good and easy to understand. It just needs some corrections and to clarify some statements.
Comments
L 29: The marsh is vegetated by definition, I don't think that the term "vegetated" makes sense here.
L 36/Fig. 1: I believe that it would be better to always use the letters for references (for example this one would be the Fig. 1b).
L 44: Why here you use cm/month but in the discussion you use m/year? Plus, why is this value (16 cm/year) not discussed in 4.3? You say that the value you calculated later are similar to other environments, but compared to this value the average seems higher. So the slumps you investigated creeped faster then the others, although the difference is low.
L 51: How? Maybe some examples could be usefull (e.g. ...).
L 104: First time should be always written full (Mean Sea Level).
L 120/Fig. 1: The Fig. 1 shows the segments. But did you think about adding the limits between the reaches in the orthophoto? It may be useful, for example using just a line to separatet each section. Unless you believe that it might become messy.
L 123: missing point (al.).
L 167: the Formation.
L 171: Delete “,”
L 230-232: Interesting, but should go in the discussion.
L 239/Fig. 5: I suppose you used this representation because you wanted to point out that the disappearance of the slumps was mainly driven by reconnection rather than submersion, but I think it confuses the reader because here I would expect the values of the submerged blocks.
L 254: I may have lost this, but I did not see the explanation of what p is in the text.
L 295: This part should go in the discussion. Despite this, 233 is 66% of the increase in the channel area; you should add the total increase as well, which I guess is around 350.
Fig. 9: It is not clear to me which are the points referred to the slumps changes and the channel changes. Can you please improve the graph and make it clearer?
L 325: Were the oyster reefs on both sides of the channel?
L 383: This statement seems a bit unprecise, because it depends on what kind of hydroperiod the plants need to survive, hence not necessarily the mean sea level. However, I guess that this is what you observed in this environment. Is this true?
L 389: Indeed, do you know the vertical changes of this study area? How much was the rate of sedimentation that you observed?
L 407: I don't clearly understand how vegetation loss can be described by a linear rate (m/year instead of m2/year).
L 636: This should go before the Zhao et al., 2022.
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RC2: 'Comment on bg-2023-180', Anonymous Referee #2, 26 Nov 2023
The paper investigates slump blocks along marsh shorelines and their impact on marsh dynamics, particularly within Dean Creek, a tidal creek on Sapelo Island, GA, USA. Using Unmanned Aerial Vehicle (UAV) imagery spanning 11 months (March 2020–March 2021), the study tracks the spatial distribution, movement, and longevity of slump blocks. Overall, the paper demonstrates the efficacy of repeated UAV surveys in monitoring short-term geomorphological processes. Here are my comments:
- An aspect deserving more detailed discussion is the influence of vegetation on slump block loss. Does vegetation have any effect on the loss?
- The discussion needs to be strengthened regarding the effect of the short observation period and whether the conclusion can be extrapolated to a larger area. Findings of this study are specific to Dean Creek, and what is the implication for other marshes or tidal creeks?
Specific comments:
Why is the slump block important? It is unclear in the abstract.
Citation: https://doi.org/10.5194/bg-2023-180-RC2
Zhicheng Yang et al.
Zhicheng Yang et al.
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