the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
On the relative role of abiotic and biotic controls on channel network development: insights from scaled tidal flume experiments
Sarah Hautekiet
Jan-Eike Rossius
Olivier Gourgue
Maarten G. Kleinhans
Stijn Temmerman
Abstract. Tidal marshes provide highly valued ecosystem services, which depend on variations in the geometric properties of the tidal channel networks dissecting marsh landscapes. The development and evolution of channel network properties are controlled by abiotic (dynamic flow-landform feedback) and biotic processes (e.g., vegetation-flow-landform feedback). However, the relative role of biotic and abiotic processes, and under which condition one or the other is more dominant, remains poorly understood. In this study, we investigated the impact of spatio-temporal plant colonization patterns on tidal channel network development through flume experiments. Four scaled experiments mimicking tidal landscape development were conducted in a tidal flume facility: two control experiments without vegetation, a third experiment with hydrochorous vegetation colonization (i.e., seed dispersal via the tidal flow), and a fourth with patchy colonization (i.e., by direct seeding on the sediment bed). Our results show that more dense and efficient channel networks are found in the vegetation experiments, especially in the hydrochorous seeding experiment with slower vegetation colonization. Further, an interdependency between abiotic and biotic controls on channel development can be deduced. Whether biotic factors affect channel network development seems to depend on the force of the hydrodynamic energy and the stage of the system development. Vegetation-flow-landform feedbacks are only dominant in contributing to channel development in places where intermediate hydrodynamic energy levels occur and mainly have an impact during the transition phase from a bare to a vegetated landscape state. Overall, our results suggest a zonal domination of abiotic processes at the seaward side of intertidal basins, while biotic processes dominate system development more towards the landward side.
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Sarah Hautekiet et al.
Status: open (until 24 Oct 2023)
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RC1: 'Comment on egusphere-2023-1515', Anonymous Referee #1, 18 Sep 2023
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Review of "On the relative role of abiotic and biotic controls on channel network development: insights from scaled tidal flume experiments" by Sarah Hautekiet et al.
Dear Editor and Associate Editor,
the paper describes a large-scale experiment on vegetation effects on the evolution of a tidal network. The experiments were carried out in the Metronome at the Utrecht University. Vegetation seeds were sown by dispersing them or manually creating some patches during the evolution of the system. The experiments were then compared to control experiments where no vegetation was used. Pictures of the tidal system were taken at different stages of the experiments to monitor vegetation growth and the channel geometric properties (with and without vegetation). In the experiments, vegetation favoured the development of a denser and more efficient network.
The issue is timely and of broad interest, particularly in view of the current great concern about the fate of coastal environments threatened as they are by climate change. The paper is well-written and well-conceived, but it would benefit from some minor clarifications and corrections. Thus, I recommend publication with minor revisions as the manuscript meets the kind of broad interest commanded by the readership of your Journal.
Please read below my comments (italics refer to the text of the manuscript).
Questions
- What was the duration of each experiment (days) and what would it correspond to in a real tidal system (years)?
- How did you ensure that the morphodynamic equilibrium was reached? Did you use a local or global criterion? The average difference between two successive DSM was computed and, if the difference was smaller than a certain threshold, the morphodynamic equilibrium was reached? Did the vegetation encroachment increase the speed at which the system reached the morphodynamic equilibrium? Could this trend be in line with previous field investigations in tidal environments?
- What are the limitations of using this vegetation species for large-scale experiments on the morphodynamic evolution of a tidal network? Vegetation has many functions in a tidal environment, for example it increases friction and favours deposition by sediment trapping and organic production. Are all these functions potentially reproduced (or reproducible) in the experiments? Or is there a function that is more dominant than the others in the experiments?
- Does the vegetation grow in a specific elevation range related to the tidal range? Does the vegetation biomass peak at a specific elevation?
Figures
Figures 4-11 and their labels look a bit blurred on my pdf file. This could be due to the pdf file compression but, anyway, please make sure that the overall resolution of each picture is high enough (usually 300 dpi).
Figure 1. On the top left, the panel labelling seems starting with “b)” and it gets a bit confusing when I look at the inset on the top right. I would replace label “b)” on the top left with “a)” and delete the labels in the inset on the top right leaving only the black square.
Figure 2. Could you add photographs where you see the channel network and the vegetation on the floodplains? I am thinking of photographs like those in Figure 5 in Weisscher et al., 2022. It helps to better visualise the experimental setup.
Figures 3, 4, Sup_f01, Sup_f02, Sup_f03, Sup_f04, Sup_f05. Tick values on some vertical axes are missing.
Figure Sup_f05. In the panel referring to hydrochorus vegetation at 3071 cycles, the sea basin is represented with a darker blue. I think it is only a plotting issue, but it could be worth checking it out.
Figure 5. This is a very nice picture. It seems that you have a figure composed of only a single column. Is it done on purpose? If you stretch the horizontal axis as in Figure 8, does it get too distorted? Moreover, tick values on some vertical axes are missing. If you use panel labelling, you can refer to specific panels when describing the figure in Lines 256-269.
References
Lines 606, 625. “/a -n/a” appears in the references Temmerman et al. (2005) and Vandenbruwaene et al. (2011). Is that right?
Lined comments
- Line 27-28. “Tidal channel networks dissect the landscape, which play a key role in the ecosystem functioning since the channels act as major flow paths for water, nutrients, sediments, and biota”. The sentence is grammatically correct, but by reading it quickly, “which” seems referred to “landscape” rather than “tidal networks”. I would suggest replacing “which” with “and” or using a passive version in the main clause such as “Tidal environments are dissected by channel networks that play …”
- Line 121. “…allowing connection of the intertidal basin with the “sea basin”…”. Please use “connecting the intertidal basin with the “sea basin””
- Line 146. “Based on Lokhorst et al. (2019), experience in the Weisscher et al. (2022) experiments and pilot experiments not reported here”. Is there a reference (paper or conference abstract) for the "pilot experiments not reported here" (are the pilot experiments mentioned in line 173?)? I would simplify the sentence as "Based on previous experiments (Lokhorst et al., 2019; Wesscher et al., 2022;, ...), a single plant species was ..."
- Line 147. Please use “a single plant species” instead of “one plant species”
- Line 149. “alfalfa”. Plant species were written in italics in the manuscript. Please write alfalfa in italics.
- Line 150. “it does not establish in unsuitable locations”. Do you mean “grow”?
- Line 196. “… with a resolution of 4000 of 6000 pixels …”. Is it 4K or 6K?
- Lines 198-199. “The images were calibrated for internal and external parameters (i.e., lens correction, geometric rectification) before they were stitched.”. Which calibration method did you use for correcting lens distortion? Checkerboard method by Zhang (1998) or another one? Could you add the reference for the method?
Zhang, Z. (1998). A flexible new technique for camera calibration. Technical Report MSRTR-98-71 Microsoft.
Was the geometric rectification carried out using python built-in functions in OpenCV? If so, please add a reference of the python package (e.g., Bradsky, 2000).
Bradski, G. (2000). The OpenCV Library. Dr. Dobb’s Journal of Software Tools, 120; 122-125.
- Line 164-165. “… (to avoid subsurface drainage) and avoid plant growth in channels …”. Please delete brackets and the repetition of "avoid" as "...to avoid subsurface drainage and plant growth channels..."
- Line 206. “Basic corrections were applied.”. Please give a brief description of the basic corrections applied.
- Lines 215-16. “…along the length direction of the flume and over the complete width of the flume (3 m).” Could it be “along the full length of the tidal basin (… m) and across the complete width of the flume (3 m)”?
- Lines 252-253. “the time scale […] was shifted by up to about 1000-2000 cycles”. "was shifted by" or "was shifted up to" ? Please choose the one that best describe the procedure.
- Line 259. By introducing panel labelling, you could refer to specific panels in figure 5 when describing evolution phases.
- Lines 283-284. “Even though the vegetation cover remained lower than expected, it still affected the channel network characteristics (see section 3.3).”. Since this point is addressed in the following subsection, I would suggest deleting this line here.
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RC2: 'Comment on egusphere-2023-1515', Luca Carniello, 22 Sep 2023
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I can definitely say I enjoyed reading the paper that is well written, well organized, concise but exhaustive in describing the methodology and in presenting and discussing the results.
Based on scaled flume experiments the study investigates the relative roles of abiotic and biotic processes in the development of tidal landscapes. Experiments are indeed novel as, for the first time (to my knowledge) they combine the analysis of the role of vegetation on the channel network formation and development in an intertidal basin context.
The experiments highlighted the role of vegetation in driving the development and evolution of channel networks investigating the differences associated with two different mechanisms of plant colonization (patchy and hydrochorous). Based on my personal experience I know the effort necessary to design, set up, and carry out physical experiments which made me further appreciate the study. I particularly appreciated the interesting way of concluding the introduction saying what the Authors expect (hypothesize) from their experiments.
Globally speaking I can say I recommend the manuscript for publication in Earth Surface Dynamics after minor revision. I provide the following few suggestions that are mostly issues of clarification and comments aimed, I hope, at further improving the readability and the quality of an already very good paper. No need to stay anonymous I am Luca Carniello.
SPECIFIC POINTS
Line 45: “Such channels are ebb-dominant in the unvegetated state, …” I suggest supporting this statement in some way. Being in the introduction the best option is to add some references.
Line 60: “history traits, such as plant recruitment strategies, can influence” I suggest adding two commas.
Line 160: “The first 10 tidal cycles were necessary to wet the tidal basin and re-establish a normal flow pattern.” Why is this necessary? Did you stop the experiment before each sowing event? I suppose not as at the beginning you state "During the experiment, Lotus seeds were dropped..." so, why do you need to re-establish the flow pattern? please clarify.
Line 165: “Around 160 000 seeds (i.e., 200 g) were supplied per sowing event to obtain a vegetation cover equaling about half the tidal basin at the end of the experiment.” Please explain why 160000 seeds are necessary to cover half of the basin.
Line 170: “while tilting of the flume was halted.” How long was the tilting halted? For the Hydrochloric sowing experiment, you say the experiment was stopped for 4 days. What happened when adopting the patchy sowing procedure? This important piece of information is actually missing.
Line 190: Can you explain the rationale that suggested you to run longer only one of the experiments with vegetation to check if the morphodynamic equilibrium was indeed reached? Why did you choose the patchy one? This is just a curiosity.
Line 201: “First, the raw laser line scanner data underwent a calibration and correction process for the laser-camera system.” Can you specify a little bit more in detail what the calibration and correction process consists of?
Line 201: “If the difference between the window median and the local pixel elevation was below a certain threshold (respectively, 0.0015, 0.005 and 0.0055 m) for at least one window size, the local pixel was identified as a channel.” I do not understand why this occurrence can ensure the selected pixel is in a channel. I suppose that the difference between the window median and the local pixel elevation can be below a certain threshold also for pixels pertaining to the adjacent flat areas. Can you clarify, please?
Line 286: “we observed that the left channel bend (as seen from vertical top-view)” I guess this is to explain to the reader what you mean by "left" channel bend but it is not clear to me what vertical means. I suggest defining left and right for example assuming an observer looking the the experiment from the inlet landward.
Line 337: I suggest remembering here that “DL” is the local drainage densities. It has been defined quite far above and I personally forgot.
Line 380: “In the hydrochorous seeding experiment, the vegetation cover increased slowly over time and remained lower than in the patchy seeding experiment.” It would be very interesting the investigate the effect of increasing the amount of seeds supplied per sowing event performing other hydrochorous seeding experiments. This is of course not a request of integration for this contribution but a suggestion for a further paper.
Citation: https://doi.org/10.5194/egusphere-2023-1515-RC2
Sarah Hautekiet et al.
Sarah Hautekiet et al.
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