# Characteristics and Mechanism of Local Scour Reduction around Pier Using Permeable Sacrificial Pile in Clear Water

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## Abstract

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## 1. Introduction

## 2. Experiment Setup and Procedures

#### 2.1. Experiment Equipments

_{50}= 0.89 mm, the non-uniformity coefficient Cu = 7.16, and the curvature coefficient Cc = 0.95. The sediment is well-graded and continuous and is qualified for the test. The grading characteristics of the sediment are shown in Figure 2.

#### 2.2. Design of the Experiments

_{1}/D), and the submergence rate (H/h), on the local scour reduction characteristics of the permeable sacrificial pile were studied in the condition of uniform flow. There were 6 groups of experiments in this study, named A, B, C, D, E, and F, shown in Table 1. Group A was designed for local scour around the pier using an unsubmerged solid sacrificial pile. The diameter of the pile was the same as that of the pier, and L was 4.0 D. Group B studied the influence of gravel particle size of the permeable pile on local scour reduction. There were five levels: 8–12 mm (0.1–0.15 D), 12–16 mm (0.15–0.2 D), 16–20 mm (0.2–0.25 D), 20–24 mm (0.25–0.3 D), and 24–28 mm (0.3–0.35 D). The ratio of the maximum length to the minimum thickness of the filler gravel was less than 2, and the porosity for each gravel level was 0.445 mm, 0.457 mm, 0.491 mm, 0.493 mm, and 0.497 mm, respectively. Group C focused on the influence of L, and six levels were taken: 2.0 D, 3.0 D, 4.0 D, 5.0 D, 6.0 D, and 7.0 D. Group D studied the influence of the diameter ratio (D1/D) on the local scour reduction, and five levels were selected: 0.6, 0.8, 1.0, 1.2, and 1.5. Group E studied the influence of submergence rate (H/h) on local scour reduction at five levels: 0.2, 0.4, 0.6, 0.8, and 1.0. The layout of the experimental model is shown in Figure 3. The permeable sacrificial pile was a cylinder-shaped steel grid and filled with gravel.

## 3. Experiment Results and Analysis

#### 3.1. Local Scour around a Single Pier

#### 3.2. Influence of Gravel Size on the Protective Effect of Permeable Sacrificial Pile

#### 3.3. Influence of Layout Distance on the Protect Effect of Permeable Sacrificial Pile

#### 3.4. Influence of Diameter Ratio on the Protect Effect of Permeable Sacrificial Pile

_{1}/D). From D1–D5, the diameter ratio was 0.6, 0.8, 1.0, 1.2, and 1.5, respectively. The gravel size was 0.2–0.25 D and the layout distance was 3.0 D. It can be seen from Figure 10 that with the increase of diameter ratio, the area of sediment siltation behind the permeable pile becomes larger and larger, and the local scour depth and range of piers decrease gradually. However, the scour depth and range of permeable sacrificial pile increase gradually.

#### 3.5. Influence of Submergence Rate on Protect Effect of Permeable Sacrificial Pile

_{1}/D) was 1.0. The submergence rate of E1-E5 was 0.2, 0.4, 0.6, 0.8, and 1.0, respectively. As the submergence rate increases, the scour depth and scope around the permeable sacrificial pile increase, however the scour depth and scope around the pier decrease gradually. When the submergence rate reaches to 0.6, the change is no longer significant.

## 4. Numerical Simulation

#### 4.1. Simulation Verification

#### 4.2. Model Setup

#### 4.3. Boundary Conditions and Input Parameters

_{50}(d

_{50}is the median particle size of the sediment in the experiment). The sides of the domain were set as the wall boundary. The top surface of the model was set as pressure boundaries. The permeable sacrificial pile was set to porous. The viscous resistance coefficient and inertial resistance coefficient of porous material can be calculated by the Ergun formula:

_{p}is the equivalent diameter of porous material.

_{2}is the inertial resistance coefficient.

_{2}was 0.491, and the equivalent diameter was 0.018 m, so the corresponding viscous resistance coefficient was 1,013,298.35 and the inertial resistance coefficient was 836.12 in the simulation.

#### 4.4. Simulation Results and Analysis

#### 4.4.1. Velocity of the Flow Field

#### 4.4.2. Distribution of Shear Stress on the Riverbed

## 5. Conclusions

- (1)
- In the same conditions, the reduction effect of the permeable sacrificial pile on the local scour of the pier is close to that using the solid sacrificial pile. However, the local scour depth around the permeable sacrificial pile is smaller than that of the solid sacrificial pile. That is to say, the permeable sacrificial pile has an excellent effect on reducing local scour on itself while providing close protection to the pier
- (2)
- As the size of the filling gravel of the permeable sacrificial pile increases, the local scour depth around the pier shows a trend of decreasing and then increasing. The local scour depth is the minimum when the size of the gravel is 0.2–0.25 D. The trend of the local scour depth in changing with the distance between the pier and the pile is the same as the trend with gravel size. The local scour depth is the smallest when the distance is 3.0 D. The local scour reduction effect of permeable piles increases with the increase of diameter ratio. The amount of local scour depths around the sacrificial pile itself and the pier are the smallest when the diameter ratio is 1.0, and do not change much when the diameter ratio is greater than 1.0. The local scour reduction effect increases with the increase in the submergence rate and reaches its best when the submergence rate is 0.8.
- (3)
- Compared with the test case using a solid sacrificial pile, the permeability of the sacrificial pile weakens the downflow in front of the pile and the velocity of the flow around the pile effectively. Thus, the local scour around itself can be reduced. The pressure difference inside and outside the permeable pile leads the water to flow along the direction which is perpendicular to the isobars. This impacts the velocity of the flow, the vortex system, and the shear stress on the riverbed around the pile and pier significantly. The lower-velocity area around the pile expands a lot compared to that around the solid sacrificial pile, which expands the lower-velocity area around the pier indirectly.
- (4)
- Compared with the shear stress on the riverbed in experiment A, the maximum shear stress on both sides of the permeable sacrificial pile is about half of those of the solid sacrificial pile, and the maximum shear stress on both sides of the pier in experiment C2 is about a quarter of that on both sides of the pier in experiment A. Meanwhile, there is a larger lower-stress area behind the permeable sacrificial pile in experiment C2.
- (5)
- Limitations of this study include: the conclusions of the numerical simulation are qualitative and only used to reveal the mechanism of local scour reduction of permeable sacrificial pile. This study is not based on prototypes and cannot be applied to real cases to a certain scale.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**Layout of the test system. (

**a**) Top view of the test system. (

**b**) Side view of the test section.

**Figure 6.**Local scour around the pier protected by permeable sacrificial pile with different particle sizes.

**Figure 7.**Local scour depth around the pier and permeable sacrificial pile with different gravel sizes.

**Figure 8.**Topographic maps of scour around pier protected by permeable sacrificial pile with different distances.

**Figure 9.**Local scour depths around the pier and permeable sacrificial pile with different distances.

**Figure 10.**Scour around pier protected by permeable sacrificial pile with different diameter ratios.

**Figure 11.**Local scour depth around the pier and permeable sacrificial pile with different diameter ratios.

**Figure 12.**Scour around pier protected by permeable sacrificial pile with different submergence rates.

**Figure 13.**Local scour depths around the pier and permeable sacrificial pile with different submergence rates.

**Figure 14.**Comparison of flow field characteristics near the piers. (

**a**) Flow field around the pier. (

**b**) The flow field in front of the pier.

**Figure 16.**Flow velocity vector and streamline at z = 0.05 m above the riverbed. (

**a**) Experiment A. (

**b**) Experiment C2.

**Figure 18.**Velocity vector and streamline at the central longitudinal section of the model. (

**a**) Experiment A. (

**b**) Experiment C2.

Group | Experimental Factors | Levels | Filling Particle Size | Layout Distance | Diameter Ratio | Submergence Rate |
---|---|---|---|---|---|---|

A | - | Solid sacrificial pile | - | 4.0 D | 1.0 | 1.0 |

B | Particle size (d) | 0.1–0.15 D, 0.15–0.2 D, 0.2–0.25 D, 0.25–0.3 D, 0.3–0.35 D | - | 4.0 D | 1.0 | 1.0 |

C | Distance (L) | 2.0 D, 3.0 D, 4.0 D, 5.0 D, 6.0 D, 7.0 D | Optimum particle size | - | 1.0 | 1.0 |

D | Diameter ratio (D_{1}/D) | 0.6, 0.8, 1.0, 1.2, 1.5 | Optimum distance | - | 1.0 | |

E | Submergence rate (H/h) | 0.2, 0.4, 0.6, 0.8, 1.0 | 1.0 | - |

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**MDPI and ACS Style**

Qi, H.; Chen, G.; Zou, W.; Yuan, T.; Tian, W.; Li, J.
Characteristics and Mechanism of Local Scour Reduction around Pier Using Permeable Sacrificial Pile in Clear Water. *Water* **2022**, *14*, 4051.
https://doi.org/10.3390/w14244051

**AMA Style**

Qi H, Chen G, Zou W, Yuan T, Tian W, Li J.
Characteristics and Mechanism of Local Scour Reduction around Pier Using Permeable Sacrificial Pile in Clear Water. *Water*. 2022; 14(24):4051.
https://doi.org/10.3390/w14244051

**Chicago/Turabian Style**

Qi, Hongliang, Guishan Chen, Wen Zou, Tiangang Yuan, Weiping Tian, and Jiachun Li.
2022. "Characteristics and Mechanism of Local Scour Reduction around Pier Using Permeable Sacrificial Pile in Clear Water" *Water* 14, no. 24: 4051.
https://doi.org/10.3390/w14244051