Laboratory-Scale Investigation of the Pressurization of T-Junctions in Hydraulic Systems

Round 1

Reviewer 1 Report

After reading the manuscript, I a have a major concern on the experimental configuration used to perform the analysis.

The experimental device is not realistic. There is lack of connection from the model to realistic geometric and hydraulic conditions.

In a T-junction, an addition of inflows is occurring, so usually, there is a need of a higher diameter downstream. Here, maybe the hydraulic capacity needed downstream is enough, because the D-branch has a higher slope, but this is not a usual configuration.

Moreover, the system is completely sealed, and this is also not representative of reality. At least in the T-junction, there might be a manhole, that should have been considered in the model because it supposes an entrance/exit for entrapped air. In addition, it is not usual to have not a single manhole along the branches. But, especially, best practices for designing high slope sewers (like the D-branch) recommend to set up device to ensure air entrance and exit from/to the sewer, precisely to avoid pressurization problems, pulsating flows...

Another important missing justification within the experimental setups are related to the geometric and hydraulic scales of the physical model. Why 10 cm diameters? What about the influence of roughness in the model against real behaviours in reality? What similitudes have been adopted for the model: Reynolds, Froude, another similitude…? Explain the normalisation factor (line 120). What is the geometric scale of the model? It is important to show the real dimensions of the analysed prototype as well as the range of real flows.

The downstream conditions used to pressurise downstream are not, again, realistic. Please, give examples of real situations that would yield to such pressurisation condition in a stormwater sewer system, where a Q=0 condition is being adopted.

If all these preliminary issues on the methodological framework are not well explained and justified, the work lacks of realistic foundations.

Author Response

After reading the manuscript, I a have a major concern on the experimental configuration used to perform the analysis.

• The authors would like to thank the reviewer for his/her considerations. We acknowledge that the goal of the manuscript wasn’t clear enough in its original version. As described below, the manuscript presented an experimental investigation – not a physical model study – of possible mechanisms of junction pressurization that could take place in stormwater systems. Conditions in the experiment related to geometries, inflows, and experimental methodology were meant to lead to pressurization, thus the different mechanisms of pressurization could be observed, described, and characterized.
• Due to the lack of experimental works in related fields, we believe that such results will contribute to better understand the unknowledge gap describing how the pressurization process can be developed in T-junctions.

The experimental device is not realistic. There is a lack of connection from the model to realistic geometric and hydraulic conditions.

• As stated above, while the apparatus has some geometric similarities to existing stormwater systems, it was not meant to be a physical model, but rather a simple geometry for an experimental investigation. As a result, the geometry is unique to the apparatus as it was designed to facilitate the investigation of the pressurization mechanisms in stormwater junctions.

In a T-junction, an addition of inflows is occurring, so usually, there is a need of a higher diameter downstream. Here, maybe the hydraulic capacity needed downstream is enough, because the D-branch has a higher slope, but this is not a usual configuration.

• Again, a good point from the reviewer who is evaluating this from the standpoint of a physical model. While typically the outflow reach diameter could increase in actual designs, the use of the same diameter was meant to facilitate the observation of different junction pressurization mechanisms. We have considered this point in the conclusion, by stating our plans of performing related experiments considering other geometries, including reaches and junction configurations in future experimental work

Moreover, the system is completely sealed, and this is also not representative of reality. At least in the T-junction, there might be a manhole, that should have been considered in the model because it supposes an entrance/exit for entrapped air.

• Indeed, for most stormwater junctions, there is contact with the atmosphere at junctions. In preliminary experiments, it was observed that the propagation of pressurization conditions upstream was delayed by storage in the junction due to the manhole, for instance. Given that our goal was to understand how a pressurization front arriving in a junction would spread to reaches at the upstream end, storage devices like manholes was not used to facilitate the propagation of the pressurization fronts. To address the reviewer’s comment, this clarification is now added in the manuscript as the experimental apparatus is described.

 

In addition, it is not usual to have not a single manhole along the branches. But, especially, best practices for designing high slope sewers (like the D-branch) recommend to set up device to ensure air entrance and exit from/to the sewer, precisely to avoid pressurization problems, pulsating flows...

• We would like to refer to our previous response on what motivated these experiments and how this experiment differs from physical models and actual design geometries. Again, our intent was to create conditions where junction pressurization will occur more easily enabling the study of the process. Moreover, we point out that while some design approaches can be successful in reducing air-water interactions, sometimes the lack of maintenance or design/construction errors could lead to such pressurization events. The authors have presented some of these cases in this reference, now added to the manuscript

Allasia, D. G. ; Pachaly, R. L. ; Tassi, R. ; Vasconcelos, J. G. ; Hodges, B. R. ; Dickinson, R. E. (2020) “Challenges of modeling stormwater transients in developing countries”. Hydrolink, Madrid, p. 61 – 63.

Another important missing justification within the experimental setups are related to the geometric and hydraulic scales of the physical model. Why 10 cm diameters?

• One important issue to be avoided when developing such experimental studies is the surface tension effects. According to Zukoski (1966, https://doi.org/10.1017/S0022112066000442), 10-cm pipes will avoid such issues between air and water. Moreover, the range of flows that were aimed for the presented experiments (normalized by the factor ) would be achievable with 10-cm pipes. Again, following the reviewer's comments, this point was added to the manuscript for clarification.

What about the influence of roughness in the model against real behaviours in reality?

• The authors acknowledge that the PVC/plexiglass pipes used in the experiment have much smoother surfaces compared with typical pipe materials in stormwater systems (i.e., concrete), and the resulting energy gradient should be shallower. However, given the very short length in which the physical processes take place (in the range of 1 meter around the junction), the impact of steeper energy gradients in the development of pressurization interfaces would be barely noticeable.

What similitudes have been adopted for the model: Reynolds, Froude, another similitude…?

• As pointed above, the apparatus was not a physical model of an existing (or proposed) system. Thus, a Froude similitude, which would have been appropriate, does not exist.

Explain the normalisation factor (line 120).

• This followed a related investigation by Zhou et al. (2002, https://doi.org/10.1061/(ASCE)0733-9429(2002)128:6(625)), which focused on air-water compression in a straight pipe geometry. In other investigations (e.g. Vasconcelos and Wright, 2017 https://doi.org/10.1061/(ASCE)HY.1943-7900.0001250), this normalization was adopted as a means to quantify the intensity of inflow rates. We added more information in the manuscript clarifying the use of this normalization.

What is the geometric scale of the model? It is important to show the real dimensions of the analysed prototype as well as the range of real flows.

• As pointed above, the apparatus was not a physical model of an existing (or proposed) system, hence, there is no geometric scale associated with the apparatus.

The downstream conditions used to pressurise downstream are not, again, realistic. Please, give examples of real situations that would yield to such pressurisation condition in a stormwater sewer system, where a Q=0 condition is being adopted.

• We agree with the reviewer that the strategy used to trigger pressurization, which was the valve closure, was designed to facilitate the spread of pressurized conditions in the apparatus. However, we would like to point out that near-complete flow blockage situations and thus rapid pressurization can happen in real stormwater systems. Mechanisms leading up to this include the failure of stormwater pumps, blockage created by debris/sediment accumulation, among others. Past work that made use of rapid and complete valve closure in related cases include the experimental works by Trajkovic et al. (1999 https://doi.org/10.2166/wst.1999.0453) and Vasconcelos and Leite (2012, https://doi.org/10.1061/(ASCE)HY.1943-7900.0000616). This is now clarified in the manuscript.

If all these preliminary issues on the methodological framework are not well explained and justified, the work lacks of realistic foundations.

• We hope that the comments above have addressed the points raised by the reviewer and that the manuscript after all the clarifications are added, would be acceptable.

Reviewer 2 Report

The article deals with important issues. It may be the first step for further research.

In my opinion, it need some improvements:

 The title should be corrected – the research was conducted in the laboratory not  in stormwater systems. Maybe just add in the end of the title: “Laboratory tests”

line 120-124. It should be explained how standardization has been carried out. In my opinion, this fragment should be supplemented with the following data: specify what values of flows (inflows to branches U and L) were achieved (m³/s)

Why the tests were carried out with the sudden closure of the valve? In such a situation, we may encounter a hydraulic jump and water hammer (when completely filled). Is it justifiable? Referring to the typical conditions in a stormwater system, it would be better to leave the valve open permanently and check the filling.

Author Response

The article deals with important issues. It may be the first step for further research.

In my opinion, it need some improvements:

The title should be corrected – the research was conducted in the laboratory not in stormwater systems. Maybe just add in the end of the title: “Laboratory tests”

• We are in agreement with the reviewer, the title was re-written to ”Laboratory-scale investigation of the pressurization of T-junctions in stormwater systems”

line 120-124. It should be explained how standardization has been carried out. In my opinion, this fragment should be supplemented with the following data: specify what values of flows (inflows to branches U and L) were achieved (m3/s)

• We agree with the suggestion, and we have added a new Table 1 that contains the dimensional values for the inflows in L/s. Moreover, the rationale of why the range of flows was set in the values used in the research is also explained in this portion of the manuscript.

Why the tests were carried out with the sudden closure of the valve? In such a situation, we may encounter a hydraulic jump and water hammer (when completely filled). Is it justifiable? Referring to the typical conditions in a stormwater system, it would be better to leave the valve open permanently and check the filling.

Thanks for raising the point and enabling us to clarify this in the manuscript. The strategy used to trigger pressurization, which was the valve closure, was designed to facilitate the spread of pressurized conditions in the apparatus. Past work that made use of rapid and complete valve closure in related cases include the experimental works by Trajkovic et al. (1999 https://doi.org/10.2166/wst.1999.0453) and Vasconcelos and Leite (2012, https://doi.org/10.1061/(ASCE)HY.1943-7900.0000616). Also, related to the complete valve closure, we would like to point out that near-complete flow blockage situations and thus rapid pressurization can happen in actual stormwater systems. Mechanisms leading up to this include the failure of stormwater pumps, blockage created 

Reviewer 3 Report

The paper is very interesting and its structure is correct, but some aspects of the paper should be revised and improved:

• The authors should clarify the significant novelty that justifies its publication as a research paper in a prestigious journal. If there are novelty elements, they have to be clearly identified in the abstract and introduction. It would also be interesting for the authors to explain clearly the usefulness of the work carried out.
• The bibliographic review carried out in the introduction should be more exhaustive. There are several research groups in the world, which have worked on topics related to the problem of entrapped air in pipes.
• The paper provides a good description of the experiments carried out by the authors. However, a better justification of the phenomena observed in the experiments is lacking.
• The authors must complete bibliographic references.Many bibliographic references are very old. There are 35 references, but there are only 9 references from the last 6 years.

To conclude, in my opinion, the paper needs some modifications to be published in a prestigious scientific journal, mainly related to completing the bibliographic review and more clearly explaining some issues.

 

Author Response

The paper is very interesting and its structure is correct, but some aspects of the paper should be revised and improved:

The authors should clarify the significant novelty that justifies its publication as a research paper in a prestigious journal. If there are novelty elements, they have to be clearly identified in the abstract and introduction. It would also be interesting for the authors to explain clearly the usefulness of the work carried out.

• The authors thank the reviewer for the suggestion, and the point is taken. We have made edits in the manuscript to better outline introduce the abstract and the contributions from this research. Also, the applications of the manuscript were better articulated in the manuscript conclusion.

The bibliographic review carried out in the introduction should be more exhaustive. There are several research groups in the world, which have worked on topics related to the problem of entrapped air in pipes.

The authors must complete bibliographic references. Many bibliographic references are very old. There are 35 references, but there are only 9 references from the last 6 years.

• The edited version of the manuscript has included many more references from research in the area of air-water flows in pipes. One point to be made, which in part motivates this research, is the relative lack of research contributions in the area focused in the manuscript. While some other areas of air-water flow in water systems (e.g., air entrainment in hydraulic structures, water pipelines priming/emptying, geysering created by uncontrolled air release, etc.) have had more attention in past years, pressurization processes in stormwater reaches have not been much investigated. And, considering the existing body of knowledge in the field, this is the first study that systematically looked into the pressurization of stormwater junctions. This is reflected in the number of citations, particularly new citations, that are presented in the manuscript.

The paper provides a good description of the experiments carried out by the authors. However, a better justification of the phenomena observed in the experiments is lacking.

• Through the editing of the Results section, the authors have attempted to improve why the flow features that were reported did occur, and with that, we hope that we addressed the reviewer’s comment.

To conclude, in my opinion, the paper needs some modifications to be published in a prestigious scientific journal, mainly related to completing the bibliographic review and more clearly explaining some issues.

• We thank you for the comments and hope that the new version will have addressed the comments raised by the reviewer.

Reviewer 4 Report

The topic of this manuscript is interesting and fall within the scope of the journal. This manuscript presented a simple experiment on the pressurization of junctions in stormwater systems that are subject to rapid filling. The main contributions of this manuscript are to describe the pressurization processes in a junction undergoing rapid filling by the experiment, but without significant new findings. The experimental program considered a lot of unique combinations, with variables including different slopes of the upstream and lateral pipes, as well as different inflow rates in each one of these conduits. The layout of the paper is logical and readable. With appropriate improvements, I agree to recommend it for publication in the journal.

 

Shortcomings and Suggestions

• The practical significance of the study is not outstanding, so it is suggested to add some practical cases.
• It seems that the title didn't accurately describe the content of this article.
• The experiment is a qualitative study, which is not comprehensive and specific in data collection and regular analysis.

Author Response

The topic of this manuscript is interesting and fall within the scope of the journal. This manuscript presented a simple experiment on the pressurization of junctions in stormwater systems that are subject to rapid filling. The main contributions of this manuscript are to describe the pressurization processes in a junction undergoing rapid filling by the experiment, but without significant new findings. The experimental program considered a lot of unique combinations, with variables including different slopes of the upstream and lateral pipes, as well as different inflow rates in each one of these conduits. The layout of the paper is logical and readable. With appropriate improvements, I agree to recommend it for publication in the journal.

 

Shortcomings and Suggestions

The practical significance of the study is not outstanding, so it is suggested to add some practical cases.

• The authors thank the reviewer for the suggestion, and the point is taken. We have made edits in the manuscript to better contribute to this research in the abstract and introduction. Also, the applications of the manuscript were better articulated in the manuscript conclusion.

It seems that the title didn't accurately describe the content of this article.

• We are in agreement with the reviewer, the title was re-written to ”Laboratory-scale investigation of the pressurization of T-junctions in stormwater systems”

The experiment is a qualitative study, which is not comprehensive and specific in data collection and regular analysis.

• We agree that the data collection is not comprehensive in that scale effects were not considered and those other geometries could have an impact on the conclusions. Nevertheless, this is an initial investigation on this topic, which we hope to expand in future experimental works.

Reviewer 5 Report

Manuscript under the title Investigation on the pressurization of T-junctions in stormwater systems, presents research on the periodic operation of gravity drainage systems under pressure.

The title corresponds to the research presented in the manuscript.

The abstract describes the purpose of the research in a synthetic way, explains the necessity to conduct this research and explains the general conclusions - the abstract is complete and well introduces the reader to the topic of the article.

Introduction: The introduction was carefully done and well-thought-out. Describes the scientific background and explains the importance of research and its purpose. In the introduction, the relevant literature review describes the current approach to modeling drainage systems and the previously negligible effect of hydraulic disturbances in connection nodes.

Material and methods. In this section, the authors focused on the subject of research and the description of the case study. They described in detail the test stand, its parameters and the work performed. In this section, I would suggest the authors to refer to the scale effect. In the case they described, DN100 pipes were taken into account. Last year, I conducted similar laboratory scale research (not yet published), although mine concerned pipe retention, the specificity of the research itself was similar. My research was carried out on the diameters of the cables dn 50 dn 70 dn 100 and dn 200. In my case, the adopted diameter of the pipes influenced the observed phenomena to a large extent and the larger it was, the less "spectacular" my theses were.I would suggest the authors take this into account in future research.

Result. The results were presented clearly. I would suggest completing the units, e.g. in table 1. The discussions were extensive, and the conclusions were accurately drawn. Taking into account the laboratory conditions, I would suggest the authors to refer to the scale effect. Perhaps it should be strongly emphasized in the manuscript that laboratory conditions will be extended to different diameters in the future. Considering the subject, there is an obvious problem with mapping the real conditions in a controlled environment. However, in the future, the article requires an EXTENDER with a larger diameter range.

Summary. The conclusions describe the research carried out well and point to the specific results obtained. Further directions of research are also described here, which in my opinion I would extend.

Author Response

Manuscript under the title Investigation on the pressurization of T-junctions in stormwater systems, presents research on the periodic operation of gravity drainage systems under pressure.

The title corresponds to the research presented in the manuscript. The abstract describes the purpose of the research in a synthetic way, explains the necessity to conduct this research and explains the general conclusions - the abstract is complete and well introduces the reader to the topic of the article.

• The authors thank the reviewer for his/her comments.

Introduction: The introduction was carefully done and well-thought-out. Describes the scientific background and explains the importance of research and its purpose. In the introduction, the relevant literature review describes the current approach to modeling drainage systems and the previously negligible effect of hydraulic disturbances in connection nodes.

• The authors thank the reviewer for his/her comments.

Material and methods. In this section, the authors focused on the subject of research and the description of the case study. They described in detail the test stand, its parameters and the work performed. In this section, I would suggest the authors to refer to the scale effect. In the case they described, DN100 pipes were taken into account. Last year, I conducted similar laboratory-scale research (not yet published), although mine concerned pipe retention, the specificity of the research itself was similar. My research was carried out on the diameters of the cables dn 50 dn 70 dn 100 and dn 200. In my case, the adopted diameter of the pipes influenced the observed phenomena to a large extent and the larger it was, the less "spectacular" my theses were. I would suggest the authors take this into account in future research.

• This is a very good point, indeed overlooked in our initial version, but addressed in the present version. Scale effects are very important in these applications (Falvey, 1980; Mortensen et al. 2013; Schulz et al. 2020) and we now discussed the potential effects of performing these experiments with large diameters. This point is now raised in the manuscript conclusion, and future investigation should be looking into this effect.

Result. The results were presented clearly. I would suggest completing the units, e.g. in table 1. 

• A table with experimental variables and the range of variability (in dimensional form) was added to the manuscript as a new Table 1. We hope this addresses this point.

The discussions were extensive, and the conclusions were accurately drawn. Taking into account the laboratory conditions, I would suggest the authors to refer to the scale effect. Perhaps it should be strongly emphasized in the manuscript that laboratory conditions will be extended to different diameters in the future. Considering the subject, there is an obvious problem with mapping the real conditions in a controlled environment. However, in the future, the article requires an EXTENDER with a larger diameter range.

• Again, a great suggestion that is now included in the manuscript, and indeed reflects the plans for our research team to continue this work.

Summary. The conclusions describe the research carried out well and point to the specific results obtained. Further directions of research are also described here, which in my opinion I would extend.

• The authors thank the reviewer for his/her comments. We hope that the new manuscript version is improved and would be acceptable.

Round 2

Reviewer 1 Report

Given the responses to my previous comments, it is obvious that the author's aim is not to represent the reality of a stormwater system, but just raise conclusions from a given laboratory-scale experiment, without justifying any possible scale effects. The geometric similarities are not enough. Hydraulic similiraties should have been taken into account to justify, for instance, the tested flow range. Given that, the reference to "stormwater systems" should be removed from the manuscript title. Otherwise, the title is misleading. In addition, the limitations of the study derived from the disconnection between experiment and reality should be clearly stated in the abstract, introduction and conclusions. If this issues are taken into account, I will accept the manuscript to be considered for publication.

Author Response

Thanks for your review and comments. The request of changing the manuscript title and presenting the study limitations in the abstract, introduction, and conclusions have been implemented. More specifically, we go over the reviewer comments in the attached PDF file.

Author Response File: Author Response.pdf

Reviewer 3 Report

After this last review, the authors have made changes and the paper has been improved significantly. The paper can be published. However, I think that a better justification for the phenomenon observed in the experiments is lacking.

 

Author Response

Thanks for your review and comments. We go over the reviewer comments in the attached PDF file.

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

The new additions to the paper showing clearly the limitations of the study improve its overall quality. From my side, the paper is now OK for publication.