Flow Characteristics in Partly Vegetated Channels: An Experimental Investigation
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsI think that the particular work is of high significance, but I am not particularly sure as to how accurately represents the reality.
Turbulence is one of the unsolved problems in Mathematics and Physics and occasionally the word turbulence is used in a non-destructive manner.
The experimental set-up, it is claimed in the manuscript, that:
"The instantaneous three flow velocity components were accurately meas-14 ured using a 3D-Acoustic Doppler Velocimeter (ADV)-Vectrino system at high frequency. Flow be-15 haviors through the vegetated area, at the interface, and in the unobstructed area were analyzed via 16 time-averaged velocities, turbulence intensity, correlation properties, and spectral analysis."
Interfering with the natural flow field, in order to take measurements, usually leads to inconclusive results. Therefore I am not entirely convinced that the three characteristic flow zones: "
i) a vegetated zone, characterized by lower U- 505 velocity, due to high drag from the cylinder arrays, ii) a shear layer zone, along which the 506U-velocity increases hyperbolically up to a maximum value, and iii) a free-stream zone of 507 maximum and homogeneous velocity" are well defined as it I claimed in the manuscript. The authors do discuss their findings, but the discussion is centred on their results exclusively without reference to external physical factors. The latter will be difficult to accommodate due to the controlled experiments performed and reported in the manuscript, so perhaps a 'disclaimer' could be inserted to indicate that the results were not influenced by other 'physical' nature parameters, such wind direction, temperature of the soil, radiation from the sun (or time of day).
There is also no evidence of the spectral analysis that is mentioned within the manuscript. The latter would have shown in some detail the parameters that have been taken into account and how the analysis was applied. It would be nice for example to explain the usage of the empirical equations 1-4, that come rather late and occupy an unnecessary large amount of space.
I think that the paper can proceed to publication with a little more effort. Particularly in identifying the numerical aspect of the experimental work and how the experiment has affect the hardware analysis.
Comments on the Quality of English LanguagePerhaps a spell check will be beneficial.
Author Response
See attached file.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript " Flow characteristics in partly vegetated channels: an experimental investigation " by Ben Meftah et al is a comprehensive experimental study of turbulence properties in an open, partly vegetated channel . The manuscript is well written and the subject of the study is of practical interest. The authors present statistics of turbulence, including mean velocities, turbulence intensities, higher-order moments - skewness and kurtosis, as well as turbulence spectra. The measurements were performed with the 3D Acoustic Doppler Velocimeter.
A certain doubt which I have concerns the novelty aspects of the current study. I would ask the authors to specify what are the new contributions with respect to their previous studies.
Another issue concerns the results, mostly those of the higher-order statistics. The authors present them without any convergence study or error analyses. E.g. calculating the third and fourth order statistics requires significantly larger ensembles. To draw conclusions on the structure of turbulence, the errors should first be estimated and reported (e.g. by including the error bars).
An interesting aspect of the present study is the presence of the free surface which should be more deformed in the vegetated area. Previous works address the interactions between turbulence and the free surface:
Brocchini, M., Peregrine, D.H., 2001a. The dynamics of strong turbulence at free
surface. Part 1. Description. J. Fluid Mech. 449, 225–254.
Wacławczyk, M., Wacławczyk, T. 2015. A priori study for the modelling of velocity–interface correlations in the stratified air–water flows, Int. J. Heat Fluid Flow, 52, 40-49.
In particular, underformed surface damps the surface-normal velocity fluctuations stronger than the deformed one. The blocking effect of the surface could contribute to the difference in the intensities of different velocity components in Figs. 10 and 12. Can the authors comment on it?
Minor remarks:
Line 120: "knowing - should start with a capital letter
L. 187: " main velocity" or rather 'mean velocity'?
Fig. 11 description: 'y/b = o' is either written with smaller font or with "o" instead of zero.
Author Response
See attached file.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsSummary of the Work
The main purpose of this study is to provide a reference for numerical modeling and management of partially vegetated channels. To this end, the authors investigated experimentally the flow turbulence structure, within and outside the vegetated area, in a large channel at the Coastal Engineering Laboratory of the Department of Civil (Polytechnic University of Bari, Italy).
Main Results Obtained
Three characteristic flow zones were identified:
i) a vegetated zone, characterized by lower U-velocity, due to high drag from the cylinder arrays;
ii) a shear layer zone, along which the U-velocity increases hyperbolically up to a maximum value;
iii) a free-stream zone of maximum and homogeneous velocity.
General Considerations
- The manuscript does not report the statistical analysis of the results obtained.
- The physical interpretation of the results obtained is missing.
- Gaining a more nuanced understanding of the complex interactions between flow and vegetation in natural channels requires a holistic and integrated approach, combining field measurements, laboratory experiments, and advanced modeling techniques. Limitations and challenges related to the proposed approach have not been duly discussed.
- The objectives of the work are well explained. However, some points need to be clarified.
The following suggestions are intended to help fill in some gaps.
Suggestions
Experimentally investigating the structure of flow turbulence in a partially vegetated channel requires adequate and specific statistical analyses.
1) For completeness, please insert Table 2. reporting the root mean square (RMS) of the turbulent velocity fluctuations to the mean flow velocity. As we know, calculating the turbulence intensity, which is the RMS, measures the relative strength of turbulence in the flow.
2) Generally, it is significant to compute the turbulent kinetic energy (TKE) as it represents the energy associated with turbulent motion in the flow. Generally, this quantity is calculated as half of the sum of the squared turbulent velocity fluctuations. What are the values of the turbulent kinetic energy (TKE) in the three characteristic flow zones?
3) To understand the flow turbulence structure in a partly vegetated channel it is also crucial to assess the spatial correlation of turbulent quantities. This involves calculating autocorrelation functions for turbulent velocity components.
3a) Please provide the values of these coefficients found in the identified three flow areas.
3b) By the way, it is also very important to estimate the integral length scale, (ILS) which represents the average size of turbulent structures in the flow. This can be calculated using autocorrelation functions or spectral analysis. If possible, it would be useful to know the ILS values.
4) Significant improvements for calibrating numerical simulations of partly vegetated channels can be obtained by examining the Probability Density Functions (PDFs) of turbulent velocity fluctuations to understand the probability distribution of turbulence intensities. Deviations from a Gaussian distribution may indicate non-uniform or skewed turbulence characteristics. What can the authors say about the PDFs found experimentally? Additionally, have the authors computed higher-order statistical moments, such as skewness and kurtosis, to assess the shape of the probability distribution of turbulent fluctuations?
5) Did the author analyze the vortical structures in the flow? More specifically, have the authors employed vortex identification methods, such as the Q-criterion or λ2-criterion, to characterize vertical flow turbulence structure?
6) To estimate the smallest turbulent eddies, the Taylor microscale calculation is usually performed, which provides information on the scale at which velocity fluctuations are decorrelated. Did the authors perform Taylor microscale calculations?
7) As we know, the presence of vegetation introduces complexity to the channel's structure. The types, densities, and spatial distribution of vegetation can vary widely, making it challenging to characterize and model the flow patterns accurately. To investigate these aspects, it is crucial to examine the correlations between vegetation characteristics (density, height, etc.) and turbulence parameters to understand the influence of vegetation on flow turbulence. Have the authors performed these calculations? Please, discuss.
8) Vegetation in channels is dynamic, with changes in growth and decay over time. This dynamic nature can make it difficult to capture the full range of vegetation effects on flow characteristics in a static study, requiring long-term monitoring and analysis. Furthermore, the influence of biological factors (such as plant species, health, and age) and environmental factors (temperature, sediment transport, etc.) can introduce additional variability to flow characteristics. These factors can be challenging to control or account for in experimental studies. These limitations have not been mentioned in this study. The authors are asked to fill this gap.
9) We come now to a crucial point. Flow behavior in partly vegetated channels can exhibit scale-dependent characteristics. Studying flow at different scales is necessary, and experiments may need to consider both laboratory and field scales to understand the interactions between flow and vegetation accurately. It can be objected that in their study the authors did not take these particular drawbacks into account. The authors are invited to dispel this possible objection.
Conclusions
The topic is current and deserves consideration. However, the present work shows many vulnerable aspects, some of which have been mentioned above (such as the lack of adequate statistical analysis of the experimental data and in-depth discussion of the limitations of the present work). I invite the authors to take into account the above suggestions.
Comments on the Quality of English Language
A few typos were found. However, it is advisable to double-check the English language.
Author Response
See attached file.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript was corrected according to my previous comments and suggestions. In particular, information on uncertainties of measurements were added in Section 2, which is appreciated. Additionally, a new subsection 3.4 on the vortex identification was added.
Minor issue:
References on line 377 should be [29--32]
I think this study is interesting and can be published in the journal Water.
Author Response
Please see the attached file.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors have satisfactorily answered all the questions raised in my previous report. In my opinion, this revised version deserves to be published.
Author Response
Please see the attached file.
Author Response File: Author Response.pdf
