Modelling the Wave Overtopping Flow over the Crest and the Landward Slope of Grass-Covered Flood Defences
Round 1
Reviewer 1 Report
The paper regards the numerical simulation of the wave overtopping flow over the crest and the landward slope of grass-covered flood defences.
General comments
The numerical study is carried out by an existing non-original numerical model (OpenFoam). From a scientific point of view, it is difficult to find element of originality in the paper. The main contribution of the paper is a technical contribution, consisting in the application of the non-original existing numerical model to a specific technical problem, In this form, the paper is just a validation of the OpenFoam code to the specific problem concerning the wave overtopping of a grass-covered flood defence.
As stated by the authors, the openFoam model adopt a specific technique for the free-surface tracking that introduce some limitation in the simulation of the wave front and some limitation in the minimum thickness of the water. Others three dimensional numerical models adopt different approaches.
The authors should better clarify the numerical characteristics of the adopted 3D free-surface model and its limitations.
Minor comments
On line 107 the acronym WOS should be explained.
On line 238, why is the dynamic pressure computed by eq 7? An explanation is needed.
On line 543 “grid size of 2.3 mm” seems a misprint. In other parts of the paper the grid size is 2-3 cm.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper “Modelling the wave overtopping flow over the crest and the landward slope of grass-covered flood defences” developed a detailed hydrodynamic model to simulate the overtopping flow over a levee covered with grass. I think the paper is really interesting and it definitely deserves to be published after a minor revision. The objective is clearly defined, the methodology is well described, and the results are properly reported and discussed. Below I have reported a few minor comments and suggestions.
Overall, I found the paper quite long, with many information that sometimes can be difficult to follow and remember. I would suggest the author trying (if possible) to shorten the paper avoiding redundant information. Moreover, I would suggest reducing the number of figures (17 in the current manuscript) if not needed. For example, is figure 10 necessaries to explain the irregularities of figure 8b?
Have you checked that the roughness value derived from calibration represents the physical characteristics of the levee cover?
How can you justify the rapid changes of h(t) found in figure 6.d? Is that a numerical instability? The authors mentioned at line 373 that “These irregularities are also observed during overtopping tests on grass-covered dikes (Figure 9)”. However, it is not straightforward to observe such irregularities from figure 9. Moreover, how the authors can justify the instability that occurred after time 7?
Can you provide more details about the computation time of the simulations (see line 573 “Our numerical model is computationally fast”) and the limitation of the proposed method/model?
Maybe this can be out of the scope of the paper but did you investigated (or are you planning to) the effect of different levee covers on the erosion process and the consequent breach (which, at the end of the day, is the most important information during overtopping mechanism).
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
Modelling the wave overtopping flow over the crest and the landward slope of grass-covered flood defences.
van Bergeijk et al.,
General Comments
Authors present the results of an interesting study, which is based on a numerical model in the open-source software to simulate the overtopping flow on the grass-covered crest and slope of individual overtopping waves for a range of landward slope angles. Details and results given in the manuscript are comprehensive. However, the objective of the study is limited in scope and thus the methodology is too limited. Therefore, I suggest the following improvements to the manuscript so that it can be accepted for publications.
Authors have carried out model simulations only for discrete wave overtopping volumes. However, results would be more meaningful if there is an input variable with time series of wave heights where wave overtopping volumes too are related to the same time series. Even if a single wave-overtopping volume is selected for the particular wave height, that should be sustained over the dike crest for a sufficient period. For instance, authors may measure a single wave and then two waves together, three waves, so on. That is because, in reality, the wave overtopping causes over a certain period with several waves attacking the dike continuously in an extreme event. Measurement of several variables for a single wave with certain overtopping volume is rather too idealistic.
The results are presented for hydraulic variables such as the near-bed velocity, pressure, shear stress and normal stress as the forces that may lead to erosion of the cover (topsoil and subsoil) and maybe the core. Even if the erosion of the cover and core is the main issue of wave overtopping, there is no measurement of depths of erosion and its spatial variability over the dike slope and the crest in the field studies carried out for validating the model nor numerical simulation results of the same.
Thus, the objectives and result of the study are limited in scope. Authors can do more simulations and present results accordingly in a revised manuscript.
The approach can be used to derive relationships between the flow velocity with slope, pressure with a slope; shear stress with slope; normal stress with slope, for a given wave overtopping volume.
Specific Comments
Avoid symbols or abbreviations in the abstract without prior definitions.
Fig 1 can be improved by defining the physical variables of a dike.
Line 8-9: Were the grass-covered dikes used in real flood defences or were they built as a part of the physical model to validate the numerical model simulations?
Line 9-10: Authors say that the calibrated roughness height of 8 mm is representative for overtopping flow over grass-covered dikes. Are the physical characteristics of the vegetation same over both dikes?
Line 9-10: Despite the calibrated roughness height indicates the roughness induced by grass-cover over dikes, is there any significance to highlight the roughness height singularly in the abstract? Better to include only a brief explanation of the approach in the abstract rather than giving only one variable.
Line 15-17: Do both maximum pressure and maximum normal stress increase by 225% in magnitude when the slope change from 1:8 to 1:3? It would be more meaningful to derive relationships between the ratio of slope (ratio of the tangent value of slopes) and the corresponding maximum pressure and maximum normal pressure.
Line 74-77 “:… the slope is mostly below the water level.” This part is not clear.
Line: 95-100: Better to give a general figure with physical variables (symbols) marked on it.
Line 110-112: Better to give possible reasons for outliers.
Line 107-109 and Line 124: Is there any significance of volumes tested to any real situation of wave overtopping over dikes with vegetation cover?
Line 116-130. Say authors carried out a physical model test on one of the test sites for one overtopping volume, did authors repair the eroded areas of the dike before the next wave overtopping volume was tested?
Line 128-130: Reasons for not working instruments at locations M3, M5, M7 and M8.
Line 161-162 and Line 239-247: Does the roughness of grass-cover is same at both sites?
Line 174-176: define symbols such as k-w SST, k-e and k-w
Fig 12b: Horizontal axis is name is not visible.
Table 3: Is this table only related to the dike configurations in Vechtdijk? Give dimensions of dike configuration in the caption of the table.
Fig 15: Derive the functions of trend lines for each wave overtopping volume. Then synthesis those 4 functions in each graph to a single function to find a relationship between variables.
For instance,
Max U = f (slope, Wave overtopping volume).
Max Pressure = f (slope, Wave overtopping volume).
Max Sheer stress = f (slope, Wave overtopping volume).
Max Normal stress = f (slope, Wave overtopping volume).
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
In the first revision of this manuscript, I suggested to the authors to better clarify the numerical characteristics of the adopted 3D free-surface model and its limitations. The modifications of the manuscript introduced in this revised version to address my suggestion are really poor.
In this form, the paper is still lacking in scientific contribution to the matter.
As stated in my previous comments, the main contribution of the paper is a technical contribution, consisting in the application of an existing numerical model to a new specific problem. Also under this point of view, some explanation about the modifications that they introduced in the numerical model would improve the quality of the manuscript,
I suggest a minor revision in which the authors should explicitly describe some of these modifications, and not simply refer to the OpenFoam user guide.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
Authors have made sufficient improvement to the manuscript to address the concerns of the first review. The cover letter provides clarifications to certain issues identified in the original manuscript. Still, there is scope to improve the methodology of the numerical modelling approach while addressing the research gaps io incorporate many other variables of wave overtopping over vegetated dykes. However, the revised manuscript addresses certain aspects of research on wave overtopping over vegetated dykes. Therefore, I recommend accepting the revised manuscript for publication.
Author Response
We want to thank the reviewer for his/her time to revise our manuscript. We are happy to hear that the revisions are sufficient and the reviewer recommends accepting the revised manuscript for publication.
