Experimental and Numerical Investigation of Floating Large Woody Debris Impact on a Masonry Arch Bridge

Round 1

Reviewer 1 Report

This manuscript deals with an important topic related to fluid-structure and structure-structure interactions. It examines numerically and experimentally its impact on overall damage of bridges and their surroudings. The reviewer believe that the experiments and theoritical background has well supports the conclusion drawn out and thus this paper is close to publication. So I recommend its publication in this journal.

Author Response

Thank you for your constructive feedback.

Reviewer 2 Report

The paper "Experimental and numerical investigation of floating large 2 woody debris impact on a masonry arch bridge" focuses on experimental and numerical analyses of fluid-structure interactions in the context of debris flow in the vicinity of masonry arch bridges. The experiments were conducted on a recirculating flume test section at the University of Manchester. The numerical methodology is Smoothed Particle Hydrodynamics, a meshless method capable of solving free-surface flows that are pertinent to the subject of work proposed in this paper.

Overall the paper is of good quality and adds value to other literature studies centered on this subject. The reviewer feels the paper can be slightly improved before being accepted for publications, and therefore suggests the following minor revisions without the need to revise the manuscript a second time:

 

- "Video recordings at 50 frames per second 247 (fps) with a resolution of 1920 x 1080 pixel were recorded within the camera in accordance 248 with the movements of the debris were used." This sentence needs to be fixed.

- Although a mention of uncertainties for measured water levels in Fig6 is made in the text, it would probably be a good idea to include these uncertainties in the plots, to make for a more thorough illustration.

- It's not completely clear how you set up the boundary conditions in your numerical simulations in part 3. You have a parabolic velocity profile in the vertical direction (depth). What did you use at the edges of your inlet, i.e. spanwise direction? Also, I understand your SPH code is based on weakly-compressible SPH, so you must have had to assign a density condition at the inlet, this is not clear in this section. Finally, can you elaborate a little on how the outlet velocity was assigned? Did you calculate the average velocity from mass conservation and apply it as is to the entire outlet depth? Wouldn't this cause unwanted shear at the bottom?

- It appears your cylindrical debris diameter is 5x the particle spacing, which seems a bit low with respect to the number of particles one would assume enough to use for spanning the characteristic length of an object in the flow (10 or 20 particles). Could you elaborate on this?

- Views in Figure 12 should be clearly explained. Also, the quality of this figure is not ideal, not sure if it's a PDF issue or a figure issue.

Author Response

• The sentence has been corrected as follows:

Line 263-265: Videos of each test run were recorded at 50 frames per second (fps) with a resolution of 1920 x 1080 pixels.

• As detailed in Line 379-385, a parabolic velocity profile with the free-surface velocity of 0.2 m/s and associated water depth were defined at the inlet, while water depth and constant velocity profile were imposed considering the water depth measured at location V9 in the experiments and conservation of mass between these two locations. Note that the velocity profile measured at the location V9 was at the centreline of the flume width which was not imposed at the outlet considering its highest velocity value along location V9 and mass conversation. Yes, the SPH code is based on weakly-compressible SPH and the density was extrapolated at the inlet and outlet. Regarding this, the following sentence has now been added to the text:

  Line 385-386: “Note that the density was extrapolated at both the inlet and outlet.”

• Yes, the number of particles is 5 through the debris diameter. Three particle sizes were examined considering the characteristic length of the bridge and the optimal particle size was chosen based on the convergence study as detailed in Table 4 and Figure 11 (Line 401-419). Also, note that the debris diameter is relatively small compared to flume and bridge dimensions and choosing the particle size based on the debris diameter would significantly increase the computational demand within this application.

• Figure 12 has been explained in the caption:

  Line 423-424: “Plan view (left) and side view (right) with the velocity distribution and water depth at the upstream and downstream of the bridge”

  To improve the quality of the figures, the figures have been reproduced and copied to the word file with 330 ppi.

Reviewer 3 Report

The problem studied in this paper is quite interesting. In a word, the whole paper is also well written. I recommend it to be accepted after addressing the following comments:

1.       There are too many Keywords which will distract the focus of the paper, so it is recommended to only keep 5 keywords

2.       In introduction part, the meshless particle method is introduced to study the movement of the structure. At the same time, it is also necessary to introduce some mesh arrangement strategy in the FVM or FEM mesh based applications such as dynamic mesh in “Study on the motion of a freely falling horizontal cylinder into water using OpenFOAM”(OE); overset mesh in “A flux correction approach for the pressure equation in incompressible flows on overset meshes in OpenFOAM”(CPC)

3.        In Figure 16, why are the pressure time curves for left and right probes very different? Please also explain why there are two peaks in the pressure time curve.

Author Response

Thank you for your comments. We have addressed them as follows:

1. We have removed one keyword.

2. We have updated the manuscript to include more information associated with this, please see Line 117-122.

3. These dual peaks were due to differences in the debris orientation at impact compared to the initial orientation whereby the long axis was parallel to the bridge span. This was also highlighted in the experiment for force values (Line 366-367).

Reviewer 4 Report

 

The paper describes a very relevant problem; interesting data are collected.

In the introduction a good description is given of the problem of debris impact on masonry bridges. Next part in the paper is the description of the physical model (one arch, one cylindrical log, approaching the arch perpendicularly). What I miss between these parts is a short discussion why this schematisation is selected, and what is by definition then left out of the model tests.

Personally, I think that logs will float towards a bridge not perpendicular to the flow, but parallel to the flow. So, the log will probably hit the abutment head-on, and then turn 90 degrees and end-up in a position as was tested in the model. Will this have effect on the final results? Is this planned to be investigated in a later stage? See for example https://www.youtube.com/watch?v=MQcE8UaIJmQ

 

Also, a nice cylindrical log is used; is this an appropriate schematisation? When a log has still some branches (or roots) the total force from the log on the structure will probably be more, but the impact might be dampened considerable.

It is clear that because of modelling you have to schematise the real world and to make choices. But it would be good to pay some attention to these matters in the paper. Or refer to another paper where this has been discussed. But also, in the earlier paper (nr. 6 in your reflist) this is not discussed.

I also could not find a research plan on this matter somewhere on the website of your university. I think such information would be helpful to assess the quality of this specific paper.

When water flows with high velocities through gaps there will be vortex shedding, causing a dynamic load on the structure. I can imagine that especially for a brittle structure like a masonry bridge this may cause considerable damage. This effect is not included in your physical model. What is the effect of this? This is also related to the scaling. As usual, you used Froude scaling for the hydraulics. But this implies that scaling of the stiffness of the structure is not correct. Especially for dynamic effect on the structure this may have repercussions. Of course, this cannot be solved in the present model, but some discussion on this issue may be needed.

I have no comments on your mathematical modelling; the model as such looks fine. It may be a good tool to  be used in your follow-up research.

Small remarks

In fig 6a and 13a   the vertical scale of the water depth is as such that the figure does not provide info. Maybe scale should go from 0.15 – 0.25 m.

It is a pity that you did not measure the velocities under the bridge

Author Response

Many thanks for your comments. We have addressed them as follows:

• Thank you very much for the video. Yes, the tree log can be perpendicular to the flow which was considered in our previous publication (Majtan et al. [6]). Considering only submerged abutment and no interaction with bridge and debris with 90-degree orientation, this is not discussed in the present manuscript as it is part of a separate future publication.

• Thank you for your comments and suggestions. This is the first study investigating this type of phenomenon. However, this can be considered in future studies and the following statement has been rewritten by adding this:

  Line 193-197: “To represent a tree log in the rivers (Figure 2), a cylindrical debris was designated based on the span length of the bridge and the ratio between the length and diameter of the debris with 0.059 based on several studies [50–52]. In reality, tree logs are likely to incorporate branches etc. which may have an effect on behaviour, however this is outside the scope of the present study.”

• You can find more information about our work and the research interest of the authors here:

  https://sites.manchester.ac.uk/sph/

   

  https://sites.manchester.ac.uk/structural-resilience/

• The stiffness of the structure does not represent the masonry arch bridges. However, the 12-mm thick Perspex sheets were assembled to obtain a rigid bridge in the experiment considering previous studies (Line 179-181).

• Unfortunately, the velocity under the bridge could not be captured due to using the ADV for the velocity measurements.