The paper investigates the variability of scour depths around monopile foundations in offshore wind farms (OWFs) and identifies the key drivers influencing these scour processes. The study utilizes high-resolution bathymetry data from 460 monopiles across nine British OWFs to analyze spatial and environmental factors affecting scour. It should be acknowledged that the measurement data from the present study are of great engineering value to predict the scour before the planning of OWF. The methodology of interpreting the data is reasonable, and the general conclusion from the interpretation is plausible. The primary problem is that the overall manuscript lacks physical analysis and correlation of the scour depth with the hydrodynamic quantities. This is understandable due to the lack of critical Shield parameters, which are directly linked to the scour process. Furthermore, in shallow water regions, it seems that the wave characteristics associated with the KC number play an important role in determining the scour depth. As pointed out by the authors, this parameter is not analyzed due to the limited measurement. Despite this lack, the physical interpretation of this manuscript can still be enhanced by estimating the values of the Shields and KC based on some assumptions. For example, combined with Re and the roughness associated with the grain size, the shear stress can be estimated based on a log law assumption. Combined with the dispersion relationship, knowing the water depth, the wave-induced velocity can be estimated so that the KC value can be estimated. Some other suggestions to improve the manuscript are listed as follows:

1. On Page 6, it is stated that the Froude number influences the scour depth. However, Fr is more related to the free-surface waves. How can it be related to the scour depth at the seabed? The analysis of the mechanism, which is stated to be related to the pressure gradients at the pile, is not clear. How can the free-surface waves affect the pressure gradients around the pile? How are the pressure gradients associated with the scour process? The authors should describe these problems in more detail, at least by providing some reference studies.
2. As an important component of the analysis, the process of performing the PCA should be elaborated in more detail. For example, how to arrange the current measurement data into matrices and how to compute the correlation angle should be introduced.
3. On Page 17, the discussion on the influence of the water depth on the scour depth is not sufficient, especially regarding the unexpected decrease in the scour depth with the increasing water depth. The authors stated that it is expected that a large water depth led to a large boundary layer thickness. However, the boundary layer thickness is not necessarily related to the water depth and more associated with Re and seabed roughness. Furthermore, the discussion on the pressure fields is rather superficial. Why does the increasing water depth result in more uniform pressure fields? Why does thinner boundary layer lead to a large shear stress?
4. It is difficult to understand the influence of Fr on the scour depth. Especially, as shown in Figure 6 (e), it seems that there are only two exceptional points, and the correlation is not strong.
5. On Page 30 from Line 592 to 597, it is stated that the wave dynamics play an important role in determining the scour depth. The authors should provide a more detailed discussion on this subject by estimating the KC number based on the wave height and water depth.

Dear Prof. Bachynski-Polić,

It has been a pleasure to review this manuscript. The authors present a comprehensive analysis of scour around OWFs. From a large data set of scour and environmental parameters at OWFs in British waters, they identify the main drivers of scour phenomena around the structures. They do so at three levels: (i) considering the overall dataset, (ii) chopping it up into median grain size clusters, (iii) zooming in on specific OWFs to see whether the overall correlations hold locally. The work is novel, interesting (a bit lengthy I must say) and in my opinion highly suitable for WES-audience. Also, the figures are helpful.

My specific concerns are on (i) the choice of a mix of dimensionless and dimensional parameters as the set of explaining parameters, (ii) the use of wave height instead of near-bed wave-induced orbital velocities, (iii) the background and limitations of the flow parameter (Uc,99) and (iv) the implication of the word “understanding” in the title. Please see my explanation below.

Please also see my other points further below. I think the manuscript benefit from a more precise and consistent presentation of the content, perhaps some restructuring, and even shortening. Particularly, using the structure of the conclusions (which I think is clear) to present the goals in the Introduction would be helpful.

Overall, based on my review, I recommend “minor revision”. Depending on your editorial decision, I would be happy to receive a revised version of the manuscript.

Anonymous, 21 May 2025

SPECIFIC CONCERNS

Choice of parameters: It strikes me that many but not all (five out of eight) variables are dimensionless. There seems to be some arbitrariness involved, which makes me wonder about the implications of that for the overall results and  conclusions. Why did you decide not to scale the other three: wave height, current velocity and sediment size? From physics-based scaling arguments, I would always expect that dimensionless parameters provide the most meaningful way of explaining dependencies. Please clarify.

To me, it would seem logical to convert wave height (along with wave period, which is now overlooked) into near-bed wave-induced orbital velocities (using linear wave theory). This quantity is more directly related to scour. Please comment.

What does the current speed Uc really represent? Is it depth-averaged, or near bed? And would this affect the results? And, furthermore, how is the tidal contribution extracted, by filtering out the wave contributions? Also, I do not see the need to add a subscript "c" for currents, as I do not see another type of velocity-related quantity such as wave current (and it also looks a lot like “cr” used for critical velocity).

Thirdly, I wonder whether “Understanding” (used in the title) is really the right term to use here, since the PCA-method does not involve a process-oriented analysis or process-based modelling. In my opinion, when adopting PCA as method, this is more about “Identifying” than “Understanding”. Please reconsider.

OTHER POINTS (PAPER STRUCTURE, GRAMMAR, SPELLING, MATH, FIGURES)

GENERAL Regarding writing structure, I find it contusing to see different types of paragraph breaks: (i) with vertical white-spacing. (which seems regular); (ii) with a hard return (which I think are unintended), (iii) with or without a horizontal indent (e.g., lines 120, 125). I expect this to be resolved in the final formatting.

GENERAL At many instances, the authors refer to a parameter (water depth, scour depth) when they actually mean the scaled version of that parameter (h/D and S/D). Please avoid such loose phrasing, since regarding dependencies this cannot be interchanged.

L19 sure you want to put symbols in abstract? This leaves me guessing. e.g., that D is pile diameter, which I think is undesirable. Please reconsider.

L39 Here you introduce OWES, but later you use the term turbine. Are these intended as synonyms? Please clarify/reconsider terminology.

L52 “but”: should be “and” (don't see the contradiction here)

L58 “the superposition of”: I would rather say "the combined effects of" (superposition suggests adding the separate influences)

L75 Curious what you mean with sediment mobility. What about the interaction between structures and seabed patterns such as tidal sand waves?

L78 remove one full stop dot

L96 (and further) “99^th quantile”: I think what you mean is what I know as the 99th percentile. To my knowledge, quantile is a more general way of chopping up distributions.

L97 um should be μm and I find the grain size values a bit odd (why so many digits, which suggests an unrealistic precision). And to my knowledge, the large value (19872 μm) is coarse gravel.

L99 Please add a sentence to prepare for and justify the OWF-site specific analyses in Section 3.5.

L104 “PCA (Principal Component Analysis)”: No need to explain this abbreviation twice (already done on line 99).

L113 Really necessary to repeat OWFs here? I think it is not needed and also inconsistent.

L114 (and further) current velocity magnitudes is also known as current speeds

L117 (and further) EMODET should be EMODNET

L122 omit “the quantile of”

L127 “Teeside” should be “Teesside”

P5, Figure 1: For consistency and clarity, why not add symbol h to both caption and figure denote water depth also here? (similar to how Hs and Uc are presented)

L175 Use of bot Ucr and Ucrit: please maintain notational consistency

L181 Upon introducing Fr and Re, please either immediately define them in the text below or immediately refer to the expressions where they are defined.

P7, Table 1: Why μm in boldface? Spelling: Gunfleet Sands

P8, Table 2, Eq.(1): I do not understand why there is 99 on the left-hand side and no 99 around the quantity on the right-hand side. In my opinion, this should be U_c,99 = [ sqrt(u0^2+v0^2) ]_99

P8, Table 2, Eq.(2): Uc should be Uc,99. [In Re-expresision this is indeed done but not here in Fr-expression]

P8, Table 2, Eq.(4): Substituting this relationship straight into Eq.(7) saves you one unnecessary intermediate quantity (s), equation (Eq.4) and text (line 200) to explain it.

L197-199: Please use equations instead of sentences to present parameter values, for example: \rho_s = 2650 kg/m^3.

L198: 1.3E-6 m/s^2: please correct units (should be m^2/s) and avoid computer notation

L200: Please also introduce D50

L212 (and further): Keulegan-Carpenter? wave velocities? Please give explanation, not just symbols.

L217: There seems to be a change of terminology: Turbine or OWES? It seems that in this piece of text you are using a different name than before. Or am I missing something?

L224: “analyzed in more detail”: This is a bit puzzling to me. Do you mean: "were included in our analysis"? What did you with the other 220: analyse them "in less detail" or simply discard them?

L227: “the next chapter” should be section 2.4? (please be specific, papers do not really have chapters)

P10, Figure 2: This figure is very clear and helpful. Typo in figure: should be "Acquisition"

L234: Here I welcome the use of “percentile” (rather than quantile). Please check everywhere.

L239: “foodprint” should be “footprint”

L246: In the spirit of PCA, I think it is most clear to speak of “linear combinations”.

L253: I am confused by the number of 692, as I thought that you earlier had only 460 OWES left (see line 224). Or am I missing something here?

L273 (as pointed out earlier): “scour depths” should be “relative scour depths”

L274: This is an incorrect way of present S/D-values: “0.29 S/D” should be “S/D = 0.29” or an “S/D-value of 0.29”.

L277: “influence” should be “influenced”

P13. Figure 3: I see letters A-I rather than numbered markers 1-9. Please adjust/clarify. Also, I think “colormap” should be “colorbar”.

P14, Figure 4: Please state that these angles are presented in degrees and I do not see the added value of presenting so many digits. Suggest rounding off to degrees. I don’t think this really is a percentage (number between 0 and 100) but a number between 0 and 1. To my knowledge the value in the right column is the absolute value of the cosine of the angle in the middle column. And why some printed in boldface and others in regular typeface?

L318 space missing

L321 “flow-related”

L323 “in-depth”

L326: why not more directly phrase as “sediment classes” instead of “soil classes”?

L336: This adds up to a total of 727 data points. How does this relate to the numbers mentioned earlier?

L350 another example of “relative scour depths”?

L359 “depth” (should be singular)

P19, Figure 6: Please be consistent in terminology: replace “normalised scour depth” with “relative scour depth”

L413: change “relatively minor” into “small”?

L416: change “trends” into “correlations” (the word trend suggests something evolving over time)

L424-425: can be shortened into “disagree”

L426 “depth” (should be singular)

L457-460: you mention the influence of water depth on the way wave height impacts the system. What about the correlation between water depth and current speeds?

L482, start of Section 3.5: This comes a bit as a surprise since it is not really prepared for in the Introduction. How does Section 3.5 contribute to the goal set out in Section 1? Please also see my earlier suggestion to add a sentence to the Introduction (L99).

L552-554: again confusing presentation of S/D-values. Please see my remark on L274.

L606, Section 4 in its entirety: I am a bit puzzled as to the role/meaning of this section. Is it a discussion, part of it, or rather a summary of all results so far. And how does it then relate to sections 5 and 6 that are still to come? Please reconsider merging the content of sections 4 and 5 - and consider the header "Discussion" as a title? This would avoid unnecessary doubling with the conclusion, which would certainly help me as a reader.

L629 Why emphasis on nonlinear?

L648, again please make sure that Keulegan-Carpenter is properly introduced/explained in the manuscript.

L665: I welcome the structure of the conclusion. Please see how you can incorporate this also in the aim set out in the Introduction, so as to help the reader in what to expect.