Dear authors,

Thanks for writing this great manuscript. You did a great job in exploring various experimental conditions for sand-clay ripples by current/wave dominated conditions. I think you did a very thorough job with a lot of literature research. It is generally well written (although the amount of numbers in the text can be distracting), and the figures are nice and clear. It won’t be much work to get this manuscript ready for publication! It will be a very valuable contribution to the scientific community.

However, I should provide some critical notes.

First of all, on first sight I find it hard to differentiate this paper from Wu’s earlier publication in 2022 on deep cleaning. It might be worth it to state this difference a bit more, already in the abstract. In the introduction the difference becomes clear, but readers might be confused and mix up the two papers after only browsing through the abstract.

Following up on that, it is good that the conclusions in the 2022 paper are largely confirmed. However, this causes repetition of many of the findings. For example, the decrease in height/length etc with clay content already known from 2022 paper, and so is the notion of deep cleaning, and the longer time till onset/equilibrium of the ripples. The new finding of this publication is the phase diagram. It would be good to focus more on this, rather than a data analysis of the raw results. Maybe it is possible to compile it in more comprehensive graphs instead of rough visualization of data, such as done in the 2022 publication.

Regarding readability, it’s generally very easy to read. However, in the results a lot of numbers are given, including many abbreviations. Additionally, many comparisons are made. It causes me to get lost in your data, and I miss the main message. Maybe it would help to start giving the main take away, and support this with numbers, rather than giving a lot of numbers from which the reader has to derive the conclusion themselves, making it hard to read and hard to concentrate.  

All smaller comments can be found in the attached pdf document. Please contact me if you have any questions regarding my feedback!

Cheers, Sjoukje de Lange

The submitted manuscript provides rigorous analysis on wave-current driven ripple development in cohesive clay bed. The analysis is based on a series of laboratory experiments. The discussed topic is interesting and important to advance our understanding of coastal morphodynamics. However, I find it difficult to read this paper. The main reasons of the difficulties in reading this paper are mainly associated with too many presented numeric and abbreviation when things are explained in the result section. In addition, the authors refer earlier studies regarding experiment set-up and results from Wu et al. (2022). I often get lost what the main messages are in the texts or figures. Thus, some clarifications and rework in writing are needed before publication. I suggest the author focus on telling the main messages from their experiments and each figure in the revised manuscript. I also have some concerns about configurations of their experiment.

Major comments

I am sure that the authors meticulously design and run the experiments, but I have some concerns about their experiment configurations, which can affect experimental condition characterization and results. Please see below for the details.

• The experiments were conducted in a recirculating flume tank, where two different types of sediment are placed in the streamwise direction (mixed sand clay in upstream and clean sand downstream). In such configuration, only clean sand is likely to be recirculated during the experiments, which can change clay – sand mixture ratio in the test section. Although the authors argue the decreases in clay content after the experiments in Figure 5 are mainly associated with clay winnowing, the decreases in clay content can be also affected by the recirculated sand. Was the recirculated sand transport rate measured and negligibly small? More explanations are needed in this matter.
• I presume the flume is separated with the brick walls to maximize the number of experiments in one run. The height of the brick walls (0.2 m) is a half of the water depth (0.4 m). Why is it not entirely separated (the height of the brick walls > 0.4 m)? I can imagine that some three-dimensional flow structures are generated at the interface between top (> 0.2 m) and bottom (< 0.2 m) layers, which would not exist in normal flow conditions (i.e. without flume separations). Did authors check vertical velocity profiles before flume separation are comparable with the ones obtained after flume separation? If there are substantial differences between the velocity profiles, quite different bed shear stress can be obtained even with the same experiment conditions. This can affect your 3D phase diagram.
• Also, regarding the flume separation, there seems to be a preferential bed slope that can affect ripple development. For example, more sediment aggregation (larger ripples) is observed in the upper part of the panel c,f (channel 3) in Figure 4. Similarly, more sediment aggregation (larger ripples) is observed in the lower part of the panel b,d (channel 1) in Figure 4. This can influence ripple morphology characterization (particularly height and steepness). Some explanations are needed in this matter.
• Lastly, the test section is located fairly close to the current inlet (only ~ 2 times of the channel width excluding the gravel section). I wonder whether the space is enough for the flow to fully develop. If the flow is still in transition (not fully developed), this can explain why slightly inhomogeneous ripple fields are generated in different channels that are described above. Please address this matter if the authors have checked the flow is fully developed.

Specific comments

• In the abstract, please avoid speculative words, such as “likely associated with hydraulic conductivity” or “which may be significant for paleoenvironmental reconstruction”. I further wonder whether readers can understand the reason of your findings at this stage. It could be worth skipping such explanations and leaving them for the discussion section.
• In the introduction, please define combined wave-current flows before using it.
• L74-77: Importance of point (1) and (2) is not clearly stated in the introduction. I also think point (3) is the main scientific contribution of this paper. Please consider putting point (3) first.
• L83: temporal resolution of the velocity measurement looks low. From my understanding, Vectrino profiler can collect velocity samples up to 200 Hz. Is there any reason of low temporal resolution? If there is, please explain it in the revised manuscript.
• L83: please also provide the velocity measurement duration.
• Please consider change notation of wave velocity magnitude U0 into Uw for the sake of readability.
• L103: please elaborate how C0 is measured? Is it based on mass or volume? If it is based on the volume, how the bed porosity is considered?
• L115-116: please explain how you define “sufficiently not well mixed channel” Is it based on visual inspection? Or is it based on inhomogeneous ripple development?
• Please consider rearrange the order of experiments in Table 1. Consistent grouping based on experimental parameters seems to me more important than chronological order. To be specific, it would be better to present results of WC3 in an order of increasing C0.
• Please consider putting experimental parameters of the WC1 experiments in Table 1. Please clearly state that the WC1 results are from Wu et al. (2022).
• Figure 2: If I am not mistaken, WC1 results are from Wu et al. (2022). Please clearly state this. Also, last tick of 0.25 in the x-axis is cut out in Figure 2.
• L260: Please discuss how the threshold of C0 ~ 10 % will change under saline water (more realistic flow condition in coastal regions), not freshwater.