A Study on Interaction between Overfall Types and Scour at Bridge Piers with a Moving-Bed Experiment

Round 1

Reviewer 1 Report

Article resubmitted for review. I have posted my comments and remarks in the file.

Comments for author File: Comments.pdf

Author Response

We thank the Reviewer for the positive comments and constructive suggestions. We modified the manuscript according to the suggestions/comments. All the corrections are highlighted by red color in the revised MS, please see the attachment. Point-to-point answers to the reviewers’ comments are reported below.

Response to Referee 1’s Comments

Comment 1: For me, it is still unclear why the diameter of the debris equal to d = 0,46 mm was used for the analyses.

• Response: The experiment was designed that the moving-bed was composed of medium sand. We add several sentences for this issue in the second paragraph of the Section procedure of experiment work as below: -

“For the sediment using in the experiment, we assumed that the river bed was composed of medium sand. The median diameter (D₅₀) of sediments in this experiment was 0.46 mm, and the standard deviation of the sediments (σ_g) was 1.69. Randkivi and Ettema (1977) suggested that the σ_g should be smaller than 1.3 to avoid the armor layer in the development of local scour [29]. The flume was paved using homogenous sediments at a 2.5 cm depth in the zone out of the scour area to provide a similar roughness in the alluvial bed.”

Comment 2: Table 2 is incomprehensible to me. What the coefficients α₁ α₂ α₃ α₄ mean in the text. Do they refer to individual authors of studies.

• Response: We add several sentences for this issue in the fourth paragraph of the Section 3.1 as below:

From line 226 to line 228

“… These coefficients were represented by different values in individual studies, and our study lists some suggested values from Mason and Arumugam (1985) in Table 2 [21]. …”

Comment 3: There are many equations in the literature that empirically describe the depth of the maximum local blur please refer to them in the discussion or results.

• Response: We add several sentences for this issue in the fourth paragraph of the Section 3.1 as below:

From line 221 to line 223

“…Many researchers proposed different empirical formulas for the maximum equilibrium scour depth under varied conditions of structure, sediment material, and approach flow [3, 6, 10, 11, 17, 18, 21, 31]. For the condition of free overfall, Mason and Arumugam (1985) mentioned that the empirical formula for the maximum equilibrium scour depth has general form …”

Comment 4: The article lacks discussion of the results with the results obtained by other authors.

• Response: Now, we added the discussion of maximum scour depth and its location change due to the overfall type and pier’s location. We also gave a suggestion for other author’s empirical formula. New paragraph and Table 3. for this issue were added in the revised MS it reads as: -

“The maximum scour depth and its location change due to the interaction between overfall type and pier’s location can be investigated based on our experiments. The pier’s location (L_i), the maximum scour depth (d_s), and its location (L_scour) in each experiment were listed in Table 3. By comparing the conditions with and without pier, d_s/d_o-s and L_scour/L_o-scour, the effect of pier's location on the maximum scour depth and its location can be investigated. In the condition of free overfall, when L_i>L_o-scour, the maximum scour depth and its location due to drop structure were not affected by the pier. When L_i<L_o-scour, the location of maximum scour depth was changed according to L_i, and the maximum scour depth became smaller than in the case of w/o pier. In the condition of submerged overfall, when L_i>L_o-scour, the location of maximum scour depth was changed based on L_i, and the maximum scour depth was larger than in the case of w/o pier obviously. This result implied that the empirical formulas for the characteristic of local scour due to the drop structure, i.e. Mason and Arumugam (1985) [21], could be used when the overfall condition is free type and L_i>L_o-scour.

Table 3. Maximum scour depth and its location change due to the overfall type and pier’s location

”

Author Response File: Author Response.docx

Reviewer 2 Report

Dear authors,

thanks a lot for your interesting manuscript. As I have worked in a working group concerned with scouring processes which were also observed in a flume, I was particularly interested in your findings. I have a few remarks/questions, which you'll find below:

Generally spoken, you have to give some finishing touches to your grammar and syntax – there are numerous mistakes.

46-90: It's really good that you stress out all other studies in your research field, but it seems a bit wordy/exaggerated in the way you do it. Just a suggestion – maybe try to summarize this part a bit more.

l.194: Maybe I’ve missed it, but how do you measure or visualize your turbulent eddying? We’ve used a tracer that was rather long lasting and stable, which allowed a real visualization of turbulences: Euler et al. (2014): Influence of inclination and permeability of solitary woody riparian plants on local hydraulic and sedimentary processes. Hydrological Processes, 28, 1358-1371. (DOI: 10.1002/hyp.9655).

Apart from these remarks, your manuscript follows a clear and stringent structure, your figures are easily understandable and appropriate. Maybe try to find more references, our paper from 2014 might be helpful as we – as I said – hadalmost the same methodical approach.

Author Response

We thank the Reviewer for the positive comments and constructive suggestions. We modified the manuscript according to the suggestions/comments. All the corrections are highlighted by red color in the revised MS, please see the attachment. Point-to-point answers to the reviewers’ comments are reported below.

Response to Referee 2’s Comments

Comment 1: Generally spoken, you have to give some finishing touches to your grammar and syntax – there are numerous mistakes.

• Response: Done as suggested. We have sent our revised manuscript to MDPI author services for proofreading.

Comment 2: 46-90: It's really good that you stress out all other studies in your research field, but it seems a bit wordy/exaggerated in the way you do it. Just a suggestion – maybe try to summarize this part a bit more.

• Response: Done as suggested. The second paragraph of the Section introduction was separated into several parts and we added several sentences for this issue as below:-

“Many researchers devoted themselves to studying local scour below drop structures and at bridge piers (Dey 2014) [13]. Some researchers focused on the local scour below drop structures; for example, Schoklitsch (1932) has been the pioneering researcher and proposed an empirical relationship to estimate the equilibrium scour depth for flow-over structures [31]. Moore (1943), Rand (1955), Akram (1979), and Little and Murphey (1982) studied the energy change due to the drop [27,28,2,20]. Smith and Strang (1967) found that the profile change of a riverbed was strongly affected by the size of the river bed materials [33]. Mason and Arumugam (1985) reviewed the empirical formulas of equilibrium scour depth under a falling jet that started in 1932, and they proposed a modified formula that includes the effect of tailwater depth [21]. Hoffmans (1998) derived relations to predict the maximum scour depth in the equilibrium phase based on the Newton’s second law of motion [17].

Hoffmans (2009) introduced an index to represent the strength of loose material and extended previous relations to predict the sum of the maximum scour depth and the tailwater depth [18]. D’ Agostino and Ferro (2004) proposed an empirical formula to estimate the equilibrium scour depth of weir type drop structures based on the high crest of the weir and the flow depth over a weir [11]. Yager et al. (2012) extrapolated an approach to predict the scour depth and geometry of A-, U-, and W-shaped rock weirs from the case of two-dimensional flow [34]. Melville (2014) used a small-scale experiment to investigate the scour at a bridge foundation in the vicinity of a sluice gate and low wire [26].

With the aforementioned research, the effect of different types of drop structures, different conditions of approach flow, and different materials of sediment have been investigated, and varied empirical formulas for the characteristic of local scour due to the drop structure have been proposed. On the other hand, some researchers focused on the local scour at bridge piers; for example, Breusers et al. (1977) reviewed a series of literature regarding theory, model, and field data about the local scour around cylindrical piers and suggested a set of designs for protection against scour [7]. Ahmed and Rajaratnam (1998) reported that smooth, rough, and mobile beds impacted the flow features and pier scour [1]. Graf and Istiarto (2002) experimented with the equilibrium scour depth around a cylinder pier and investigated the vortex system around a cylinder pier based on measurement of the acoustic Doppler velocity profiler (ADVP) [16].

 Dey and Raikar (2007) experimented with the developing scour depth around a cylinder pier and investigated the features of the vortex system in the intermediate and equilibrium stages [12]. Ataie-Ashtiani and Aslani-Kordkandi (2013) experimented with the developing scour depth around a single pier and two piers in tandem on a roughly flat bed and investigated the difference of flow features in the implemented experiments [4]. Euler et al. (2014) investigated the local scour in the vicinity of pillar-like objects through experimental studies and compared the results with field data [15].

These studies contribute to the flow feature impact on pier scour. Other studies focused on scoring features; for example, Baker (1980) derived a formula to estimate the equilibrium scour depth in front of a cylindrical bridge pier and compared the results with the results of Baker (1979), Breusers et al. (1977), and Chabert and Engeldinger (1956) [6,5,7,8]. Chiew and Melville (1987) proposed an empirical relationship that was related to the equilibrium depth scour, particle size of sediment, and flow condition, and compared their results with the findings of Chee (1982) and Melville (1984) [10,9,22]. Elliott and Baker (1985) investigated the feature of scour depth under the effect of lateral spacing between bridge piers [14]. Melville and Chiew (1999) indicated that the development of the equilibrium scour depth can be related with the size of the pier, size of the sediment, and approach flow velocity [24].

 Sheppard et al. (2004) indicated that the wash load concentration impacts the scale of the equilibrium scour depth under clear-water conditions [32]. Ataie-Ashtiani and Beheshti (2006) derived an empirical relationship to estimate the maximum local scour depth for the pile group and compared their results with the reports of Melville and Coleman (2000) and Richardson and Davis (2001) [3,25,30]. Khosronejad et al. (2012) investigated the features of clear-water scour around the geometry of cylindrical, square, and diamond bridge piers through experiments and numerical simulation [19]. According to the aforementioned research, the mechanism of local scour at bridge piers has been investigated comprehensively by the theory, experiment, field data, and numerical model, and empirical formulas for the equilibrium depth scour at bridge piers have been proposed. However, there are few papers, to the authors’ knowledge, focusing on the interaction between overfall types and scour at bridge piers.”

Comment 3: l.194: Maybe I’ve missed it, but how do you measure or visualize your turbulent eddying? We’ve used a tracer that was rather long lasting and stable, which allowed a real visualization of turbulences: Euler et al. (2014): Influence of inclination and permeability of solitary woody riparian plants on local hydraulic and sedimentary processes. Hydrological Processes, 28, 1358-1371. (DOI: 10.1002/hyp.9655).

• Response: In the experiments, the turbulent eddying appearing during the development of scouring hole can be observed roughly when the water flow mixed with a few sediment particles and bubbles, but it was not a real visualization. As your mention, the mechanism of development of scouring hole was similar to the study of Euler et al. (2014). Hence, we add several sentences for this issue in the third paragraph of the Section 3.1 as below: -

“The mechanism of development of the scouring hole was that the two counter-rotating eddies brought up the sediment to the slope at the downstream side, and gradually a small dune was formed. Euler et al. (2014) investigated the mechanism using a tracer, which allowed a visualization of the turbulent eddying and was similar to the observations in our experiments [15]. While a deeper and wider scouring hole was dug by the overfall, the dune was moved further downstream. On the other hand, the sediment of the slope of the scouring hole upstream occasionally slid into the hole while the hole was being dug wider and deeper. The sliding sediment was brought away downstream randomly. The slope at the downstream of the scouring hole was steeper than the original at-rest angle of the sediment deposits because the eddies provided a floating force along the slope surface that supported the sediments to stay at the same location until slope instability due to the occurrence of toe erosion induced by scour.”

Comment 4: Apart from these remarks, your manuscript follows a clear and stringent structure, your figures are easily understandable and appropriate. Maybe try to find more references, our paper from 2014 might be helpful as we – as I said – hadalmost the same methodical approach.

• Response: Done as suggested. We add several sentences for your kind suggestion in the fifth paragraph of the Section introduction as below: -

“Dey and Raikar (2007) experimented with the developing scour depth around a cylinder pier and investigated the features of the vortex system in the intermediate and equilibrium stages [12]. Ataie-Ashtiani and Aslani-Kordkandi (2013) experimented with the developing scour depth around a single pier and two piers in tandem on a roughly flat bed and investigated the difference of flow features in the implemented experiments [4]. Euler et al. (2014) investigated the local scour in the vicinity of pillar-like objects through experimental studies and compared the results with field data [15].”

• Now the following Ref. is added.
  • Euler, T.; Zemke, J.; Rodrigues, S.; Herget, J. Influence of inclination and permeability of solitary woody riparian plants on local hydraulic and sedimentary processes. Hydrological Processes, 2014, 28(3), 1358-1371. https://0-doi-org.brum.beds.ac.uk/10.1002/hyp.9655

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Dear authors, 

I have read your draft about bridge scour and been inspired.

However, this paper should be improved with 1) a clear purpose of the research 2) scientific inspiration 3) engineering novelty .

briefly, I suggest several comments about your paper and its weak points as below. And I hope you to consider them.

a) Correlation/interaction between earthquake and bride scour

Although, this study was conducted due to the occurrence of a unpredictable step due to the earthquake and the resulting bridge scour, I believe that the introduction of the study does not include any comments of the earthquake. More general description of the bridge scour and its mechanism can be required for the readers. 
This is because, from an engineering basis, it is difficult to infer the correlation between seismic occurrence and bridge scour. The first paragraph is very likely to confuse the reader.

b) Experimental setup and its apparatus/measuring equipment

are the most interesting part of the physical modeling paper to the readers. Because hydraulic tests with a movable bed has been really difficult for the previous researchers including me. You need add some more detailed information about test and its setup/measuring.

c) Equilibrium state of the scour depth

A clear evidence is needed to determine that the bridge scour depth has reached equilibrium state. If it is not secured, engineering conclusions can be made when using the evidence of the existing researchers or when the author determines an engineering standard and uses it as the value of equilibrium state based on it.

Engineering conclusions of this paper should be added, and many weak points have been found.

Therefore, my decision about this submission is 'reject'.

Reviewer 2 Report

The article is very interesting. However, it requires some elements to be added. There is particularly a lack of discussion of results in the context of other studies. I present other comments in the attachment.

Comments for author File: Comments.pdf