# Local Scour Reduction around Cylindrical Piers Using Permeable Collars in Clear Water

^{1}

^{2}

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## Abstract

**:**

## 1. Introduction

## 2. Methodology

#### 2.1. Experimental System

^{−3}, the median particle size d

_{50}was 0.89 mm, the uniformity coefficient was 7.16, and the curvature coefficient was 0.95. The sediment had a continuous gradation and met the test requirements; the gradation curve of the sediment is shown in Figure 3. The outlet section was located downstream of the flume with a length of 6.0 m, including a sedimentation tank, baffle, and tailgate. The flow depth was 0.20 m, and the starting velocity of sediment was 0.349 m/s which was calculated using Zhang Ruijin’s formula [32]:

_{0}is the starting velocity of the sediment; h is the flow depth; d is the average particle size of the sediment. The average flow velocity is 0.325 m/s in this study, which is less than that of the starting velocity of the sediment. Therefore, the experiments carried out in this study were in clear water.

#### 2.2. Factor Analysis

_{w}), and the shape, shown in Figure 4. For the diameter of the collar, Fang [19] considered that the collar size should be greater than or equal to 3.0 times the diameter of the bridge pier. Jahangirzadeh [26] found that the scour reduction changed by only 1.7% when the collar diameter exceeded 3.5 times the diameter of the bridge pier. Therefore, Jahangirzadeh concluded that W = 3~3.5 D is a more cost-effective range. For the installation height of the collar, the current research results all agree that the reduction efficiency is the best when the collar is installed near the riverbed (h/h

_{w}= 0) [24]. For the shape of the collar, the reduction effect of the rectangular collar is better than that of the circular collar in most cases. However, considering the problem of water incidence angle, the rectangular collar does not guarantee the same effect under any angle of the incoming flow. In contrast, the circular collar does not have that problem, although its reduction efficiency is slightly lower than that of the rectangular collar. Therefore, the circular collar was used in this paper.

#### 2.3. Experimental Setup

_{w}= 0), and the collar size (W = 3.0 D) at first. Then, the effects of the installation height, collar size, and thickness on the reduction efficiency were studied based on the optimal porosity. The experimental conditions are shown in Table 1.

_{w}). There are three experiments numbered C1–C3 in order. There are three levels of installation height: 0.05, 0.10, and 0.15. Group D studied the local scour characteristics around the bridge pier by using permeable collars with different diameters (W/D). There are five experiments numbered D1–D5 in order. There are five levels of diameters: 2.0, 2.5, 3.5, 4.0, and 4.5. Group E studied the local scour characteristics around the bridge pier using permeable collars with different thicknesses (T/D). There are four experiments numbered E1–E3 in order. There are three levels of thickness: 0.30, 0.45, and 0.60.

#### 2.4. Reduction Efficiency

_{P}, was defined as follows:

_{se}is the average scour depth of the region without preventative measures; d

_{sec}is the average scour depth of the region in different conditions using the permeable collar.

## 3. Results and Discussion

#### 3.1. Local Scour around a Single Pier without a Permeable Collar (Group A)

#### 3.2. Influence of Porosity on the Reduction Efficiency of the Permeable Collar (Group B)

_{P}can be used.

_{P}is greater than 70%, especially when it is 50%, and the E

_{P}even reaches up to 78.1%. Therefore, the optimal range of porosity of the permeable collars recommended is from 37.5% to 62.5%, and the porosity of the permeable collar is 50% in the subsequent tests in this study.

#### 3.3. Influence of Installation Height on the Reduction Efficiency of the Permeable Collar (Group C)

_{w}is 0, which means the permeable collar is installed on the surface of the sediment, the local scour mainly occurred near the edges of the collar, no scour happened to the front or rear of the pier. Furthermore, the maximum local scour depth is the smallest. (2) With the increase in the installation height of the collar, the local scour depth around the pier increases gradually, and the location of the maximum local scour depth is moved from the sides of the collar to the front of the pier. The sedimentation near the rear of the pier increased rapidly and moved down from the pier. (3) When h/h

_{w}increases to 0.15, the maximum local scour depth increases to 5.3 cm, which is very close to that in the condition without the permeable collar, and it indicates that the local scour reduction effect of the permeable collar is lost.

_{w}is 0, the closer the location to the pier, the smaller the local scour is. When h/h

_{w}is 0.05, the average local scour depth of each arc is close to each other and is concentrated around −1.0 cm. When h/h

_{w}is larger than 0.1, the average local scour depth of each arc is exactly opposite to that when h/h

_{w}is 0, and the larger the h/h

_{w}is, the greater the local scour depth of each arc is.

_{w}, the reduction efficiency of the permeable collar decreases rapidly. Therefore, whether it is a permeable collar or a solid collar, to achieve the best reduction efficiency, the collar must be installed close to the riverbed. In the subsequent tests in this study, the installation height of the permeable collar was set as h/h

_{w}= 0.

#### 3.4. Influence of Diameter on the Reduction Efficiency of the Permeable Collar (Group D)

#### 3.5. Influence of Thickness on the Reduction Efficiency of the Permeable Collar (Group E)

## 4. Conclusions

- (1)
- The local scour depth around bridge piers can be reduced by using a permeable collar with different porosities. Compared with the solid collar, the reduction effect of the permeable collar is close to that of the solid collar when the porosity of the collar is less than 75%. The reduction efficiency can reach up to 78.1% when the porosity is 50%.
- (2)
- The installation height of the permeable collar has a greater impact on its reduction effect. The reduction efficiency decreases with the increase in the installation height, and it is best when the collar is installed on the surface of the riverbed.
- (3)
- With the increase of the collar diameter, the reduction efficiency of the permeable collar increases rapidly and then stabilizes. When the diameter is greater than 4.0, the reduction efficiency is close to 100%. The collar diameter should be determined based on comprehensive consideration of the economic parameters and the reduction efficiency in actual projects.
- (4)
- The reduction efficiency of the permeable collar is negatively correlated with its thickness, and Pearson’s correlation coefficient is −0.973.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Appendix A

No. | arc-0 | arc-0.25 | arc-0.5 | arc-0.75 | arc-1 | d_{sec} | E_{P} (%) |
---|---|---|---|---|---|---|---|

A1 | −5.30 | −4.69 | −3.95 | −3.09 | −2.19 | −3.84 | - |

B1 | −0.78 | −1.04 | −1.43 | −1.99 | −1.70 | −1.39 | 63.8 |

B2 | −0.73 | −0.95 | −1.43 | −2.06 | −1.86 | −1.41 | 63.3 |

B3 | −0.38 | −0.94 | −1.45 | −1.99 | −1.68 | −1.29 | 66.4 |

B4 | −0.16 | −0.60 | −1.23 | −1.75 | −1.68 | −1.08 | 71.9 |

B5 | −0.18 | −0.52 | −0.85 | −1.16 | −1.47 | −0.84 | 78.1 |

B6 | −1.38 | −1.43 | −1.03 | −0.70 | −1.20 | −1.15 | 70.0 |

B7 | −2.18 | −2.05 | −1.43 | −0.64 | −0.75 | −1.41 | 63.3 |

B8 | −3.75 | −3.45 | −2.50 | −1.83 | −1.08 | −2.52 | 34.4 |

C1 | −1.45 | −1.28 | −1.45 | −1.62 | −1.38 | −1.44 | 62.5 |

C2 | −3.30 | −2.83 | −2.40 | −2.00 | −1.49 | −2.40 | 37.5 |

C3 | −4.63 | −4.15 | −3.26 | −2.58 | −1.75 | −3.27 | 14.8 |

D1 | −3.35 | −3.18 | −2.85 | −2.05 | −0.85 | −2.46 | 35.9 |

D2 | −1.88 | −2.32 | −2.50 | −2.25 | −1.37 | −2.06 | 46.4 |

D3 | −0.05 | −0.13 | −0.18 | −0.53 | −1.13 | −0.40 | 89.6 |

D4 | −0.05 | −0.10 | −0.09 | −0.10 | −0.41 | −0.15 | 96.1 |

D5 | −0.05 | 0 | 0 | −0.10 | −0.15 | −0.06 | 98.4 |

E1 | −0.25 | −0.79 | −1.54 | −2.36 | −2.84 | −1.56 | 59.4 |

E2 | −2.23 | −2.59 | −3.13 | −3.40 | −3.54 | −2.98 | 22.4 |

E3 | −2.50 | −2.85 | −3.47 | −3.73 | −3.83 | −3.28 | 14.6 |

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**Figure 9.**Local scour pit morphology of test A. (

**a**) The experimental model of test A. (

**b**) Local scour of test A.

**Figure 11.**(

**a**) Variation of average local scour depth with porosity. (

**b**) Variation of reduction efficiency of the permeable collar with porosity.

**Figure 12.**Local scour around the pier using the permeable collar with different installation heights.

**Figure 13.**(

**a**) Variation of average scour depth and installation height. (

**b**) Variation of reduction efficiency of the collar and installation height.

**Figure 15.**(

**a**) Variation of average scour depth with collar diameter. (

**b**) Variation of reduction efficiency with collar diameter.

**Figure 17.**(

**a**) Variation of average scour depth with the thickness of the collar. (

**b**) Variation of reduction efficiency of the collar with the thickness of the collar.

No. | γ (%) | h/h_{w} | W/D | T/D | |
---|---|---|---|---|---|

A | Single pier | ||||

B1 | 0 | 0 | 3.0 | 0.15 | effect of porosity on reduction efficiency |

B2 | 12.5 | 0 | 3.0 | 0.15 | |

B3 | 25.0 | 0 | 3.0 | 0.15 | |

B4 | 37.5 | 0 | 3.0 | 0.15 | |

B5 | 50.0 | 0 | 3.0 | 0.15 | |

B6 | 62.5 | 0 | 3.0 | 0.15 | |

B7 | 75.0 | 0 | 3.0 | 0.15 | |

B8 | 87.5 | 0 | 3.0 | 0.15 | |

C1 | Optimum porosity | 0.05 | 3.0 | 0.15 | effect of installation height on reduction efficiency |

C2 | 0.10 | 3.0 | 0.15 | ||

C3 | 0.15 | 3.0 | 0.15 | ||

D1 | Optimum porosity | Optimum installation height | 2.0 | 0.15 | effect of diameter on reduction efficiency |

D2 | 2.5 | 0.15 | |||

D3 | 3.5 | 0.15 | |||

D4 | 4.0 | 0.15 | |||

D5 | 4.5 | 0.15 | |||

E1 | Optimum porosity | Optimum installation height | Suitable diameter | 0.30 | effect of thickness on reduction efficiency |

E2 | 0.45 | ||||

E3 | 0.60 |

**Table 2.**Maximum local scour depth around the pier using a permeable collar with different porosities.

No. | γ (%) | h/h_{w} | W/D | T/D | Z_{max}/cm |
---|---|---|---|---|---|

B1 | 0 | 0 | 3.0 | 0.15 | −2.6 |

B2 | 12.5 | −2.5 | |||

B3 | 25.0 | −2.4 | |||

B4 | 37.5 | −2.4 | |||

B5 | 50.0 | −2.5 | |||

B6 | 62.5 | −2.4 | |||

B7 | 75.0 | −2.7 | |||

B8 | 87.5 | −4.0 | |||

A | 100 | Single pier | −5.5 |

**Table 3.**Maximum scour depth around the pier using the permeable collar with different installation heights.

No. | γ (%) | h/h_{w} | W/D | T/D | Z_{max}/cm |
---|---|---|---|---|---|

B5 | 50 | 0 | 3 | 0.15 | −2.5 |

C1 | 0.05 | −2.9 | |||

C2 | 0.10 | −3.8 | |||

C3 | 0.15 | −5.3 |

No. | γ (%) | h/h_{w} | W/D | T/D | Z_{max}/cm |
---|---|---|---|---|---|

D1 | 50 | 0 | 2.0 | 0.15 | −3.5 |

D2 | 2.5 | −2.9 | |||

B5 | 3.0 | −2.5 | |||

D3 | 3.5 | −2.1 | |||

D4 | 4.0 | −1.8 | |||

D5 | 4.5 | −2.0 |

**Table 5.**Maximum local scour depth around the pier using a permeable collar with different thicknesses.

No. | γ (%) | h/h_{w} | W/D | T/D | Z_{max}/cm |
---|---|---|---|---|---|

D3 | 50 | 0 | 3.5 | 0.15 | −2.1 |

E1 | 0.30 | −3.5 | |||

E2 | 0.45 | −3.9 | |||

E3 | 0.60 | −4.3 |

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## Share and Cite

**MDPI and ACS Style**

Qi, H.; Yuan, T.; Zhao, F.; Chen, G.; Tian, W.; Li, J. Local Scour Reduction around Cylindrical Piers Using Permeable Collars in Clear Water. *Water* **2023**, *15*, 897.
https://doi.org/10.3390/w15050897

**AMA Style**

Qi H, Yuan T, Zhao F, Chen G, Tian W, Li J. Local Scour Reduction around Cylindrical Piers Using Permeable Collars in Clear Water. *Water*. 2023; 15(5):897.
https://doi.org/10.3390/w15050897

**Chicago/Turabian Style**

Qi, Hongliang, Tiangang Yuan, Fei Zhao, Guishan Chen, Weiping Tian, and Jiachun Li. 2023. "Local Scour Reduction around Cylindrical Piers Using Permeable Collars in Clear Water" *Water* 15, no. 5: 897.
https://doi.org/10.3390/w15050897