# Turbulence Characteristics before and after Scour Upstream of a Scaled-Down Bridge Pier Model

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

_{b}) represented by a pier width as a characteristics length. However, Muzzammil and Gangadhariah [17] found that the relative vortex size, which is the ratio of vortex diameter and pier width, is weakly influenced by the pier Reynolds number for higher values on a rigid flat bed. This means that vortex size is only dependent on the pier width for higher values of the pier Reynolds number (Re

_{b}>10

^{4}). Based on analytical models relating scour depth to horseshoe vortex properties, Muzzammil and Gangadhariah [17] proposed that equilibrium scour depth is a function of the horseshoe vortex size, tangential velocity, and vortex strength in the scour hole. They found that the mean size of the horseshoe vortex is ~20% of the cylindrical pier diameter, and the vortex tangential velocity is ~50% of the mean velocity of approach flow for ${10}^{4}\le R{e}_{b}\le 1.4\times {10}^{5}$ at fixed flat-bed conditions. The size of the vortex is assumed to be independent of the sediment mobility.

## 2. Methodology

#### 2.1. Experimental Setup

#### 2.2. Experimental Procedure

^{3}/s) was established with the flow depth adjusted well above the desired value. Then, the flow depth was gradually decreased by changing the height of the tailgate until the target approach flow depth was obtained. During this time, the point gage (uncertainty of $\pm 1mm$) was used to monitor the flow depth. Once the target flowrate and the flow depth had been reached, scour continued for 2 to 3 days until equilibrium was achieved. The equilibrium was defined when the increment of scour depth is less than 5% of the bride pier diameter during 24 h. During the scouring process, instantaneous point velocities and turbulence quantities were measured by ADV in front of the pier. Furthermore, temporal change of bed elevations were measured periodically using ADV temporarily positioned for a moment above the point of scouring. At the end of scouring (equilibrium state), the velocity flow field was measured throughout the test section both in the near field next to the pier and in the far field at relative elevations for the comparison of turbulence and flow characteristics between after scour and before scour. During the velocity measurements, ADV sampling frequency was chosen to be 25 Hz with a duration of at least 2 min and perhaps as much as five minutes depending on the magnitude of turbulence at each measuring location. The correlation values in these measurements were greater than 80% and the Signal Noise Ratio (SNR) was greater than 15. The phase-space despiking algorithm was also employed to remove any spikes in the time record caused by aliasing of the Doppler signal which sometimes occurs near a boundary. More detailed filtering protocol can be found in Lee and Sturm [22] and Hong et al. [26]. After the completion of each experiment, the final bed elevations were measured using the ADV and the point gage.

^{3}/s. The kaolinite suspension was mixed to achieve a concentration of 1.0 mg/cm

^{3}in the tank. The flow rate of tracer was adjusted with the aid of rotameter to produce a released velocity that was the same as the open channel mean velocity. As the tracer was released at a constant rate, a high speed video camera (30 FPS) was used to capture the unsteady dynamics of the swirl of the horseshoe vortex as it amplified and partially collapsed in size.

## 3. Results and Discussion

_{1}is approach section water depth, V

_{1}is approach section velocity, T

_{eq}is time to the equilibrium scour, and d

_{s}is the equilibrium scour depth in front of the bridge pier.

#### 3.1. Velocity Field

#### 3.2. Temporal Variation of Flow and Turbulence Characteristics Upstream of the Bridge Pier

#### 3.3. Flow Characteristics Upstream of the Bridge Pier

## 4. Summary and Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Hydraulic laboratory model in previous laboratory studies and bridge pier model for this study: (

**a**,

**c**) 1:23 scaled model for Chattahoochee River bridge and (

**b**,

**d**) 1:33 scaled model for Flint River bridge.

**Figure 2.**Pier model in the flume: (

**a**) Chattahoochee River bridge model and (

**b**) Flint River bridge model.

**Figure 3.**Normalized mean velocity vectors: Measured at 40 percent of the approach depth for experimental Run 1 before and after scour in (

**a**,

**b**), respectively, and Run 3 before and after scour in (

**c**,

**d**), respectively.

**Figure 4.**Schematic diagram of the locations for measuring the temporal variation of flow characteristics.

**Figure 7.**Flow visualization of horseshoe vortex with tracer injection in experimental Run 4: sequential snap shots from (

**a**–

**d**).

**Figure 8.**Schematic of the plate for quadrant analysis [36].

**Figure 9.**Joint probability density function of u’ and w’ for (

**a**) before-scour case and (

**b**) after-scour case measured at x/b = −0.33 and z/b = 0.17 for Run 2.

Run | Model | Scale | Q (m^{3}/s) | b (m) | y_{1} (m) | V_{1} (m/s) | T_{eq} (h) | d_{s} (m) | Conditions |
---|---|---|---|---|---|---|---|---|---|

1 | CR | 1:23 | 0.051 | 0.046 | 0.191 | 0.257 | 30 | 0.093 | Fixed & Moveable-bed |

2 | 0.044 | 0.046 | 0.142 | 0.304 | 12 | 0.090 | |||

3 | FR | 1:33 | 0.054 | 0.055 | 0.241 | 0.215 | 48 | 0.046 | Fixed & Moveable-bed |

4 | 0.052 | 0.055 | 0.170 | 0.281 | 24 | 0.085 |

Before Scour, Sec | After Scour, Sec | |
---|---|---|

Streamwise direction | 1.8 | 1.3 |

Vertical direction | 4.3 | 0.7 |

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**MDPI and ACS Style**

Lee, S.O.; Hong, S.H. Turbulence Characteristics before and after Scour Upstream of a Scaled-Down Bridge Pier Model. *Water* **2019**, *11*, 1900.
https://doi.org/10.3390/w11091900

**AMA Style**

Lee SO, Hong SH. Turbulence Characteristics before and after Scour Upstream of a Scaled-Down Bridge Pier Model. *Water*. 2019; 11(9):1900.
https://doi.org/10.3390/w11091900

**Chicago/Turabian Style**

Lee, Seung Oh, and Seung Ho Hong. 2019. "Turbulence Characteristics before and after Scour Upstream of a Scaled-Down Bridge Pier Model" *Water* 11, no. 9: 1900.
https://doi.org/10.3390/w11091900