# Effects of Bridge Piers on Flood Hazards: A Case Study on the Jialing River in China

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Study Area

^{2}, and the average bed slope of the river is 1.4%. This is a warm temperate mountain climate area with an annual average temperature of 11.4 °C, an average annual rainfall of 623.6 mm, a maximum rainfall of 939.3 mm (in 1958), and a minimum rainfall of 422.3 mm (in 1969). The precipitation from April to October (wet season) is on average 577.3 mm, which is 92% of the entire year’s precipitation. From November to March (dry season), the precipitation is on average 46.3 mm (8% of the entire year’s precipitation). The elevation on both sides of the Jialing River is in the range of 1400–1800 m, the relative height is 500–800 m with a steep slope of about 30–40 degree. Due to the narrow and steep river channel, the floods are of short durations and high peaks and most have a single peak. According to the statistics of the measured data, the floods in the upper reach of the Jialing River are mainly formed by high-intensity rain [30]. For example, the 1933 rainstorm, with a rainfall intensity of 73.6 mm/day, produced 500 m

^{3}/s peak flow at the upper reach and 970 m

^{3}/s peak flow in the lower reach, which is approximately a 10-year flood. Heavy rains generally occur from July to September.

^{2}, 2722 km

^{2}and 19,206 km

^{2}, respectively. The maximum peak flow rate measured at Fengzhou Station was 972 m

^{3}/s (in 1990). The maximum peak flow rate measured at Ciba Station was 4670 m

^{3}/s (in 1981). The Lueyang Station’s maximum flood peak flow in 1981 reached 8630 m

^{3}/s.

#### 2.2. Model Description

#### 2.3. Mesh Generation and Model Parameters

^{2}, a smallest allowable angle of 29°, and a maximum number of nodes of 100,000. By using an unstructured mesh, the circular bridge piers with diameters of 2 m are directly described (i.e., solid wall conditions) (Figure 2). Two sets of meshes with and without piers were used to investigate the impacts of bridge piers on the flow fields. Mesh independence is analyzed in Section 2.4. The input topography for the study area is presented in Figure 3.

^{−1/3}s. In this study, a constant Manning coefficient (n = 0.038 m

^{−1/3}s) was used in the entire study area for the sake of safety. A sensitivity analysis was also carried out to investigate the effects of the Manning coefficient on the computed results, as described in Section 2.4.

#### 2.4. Sensitivity Analysis

^{3}and a smallest allowable angle of 29° were used for the simulations both with and without piers.

^{−1/3}s, and the results were compared under the 100-year flood condition. The computed water levels at four cross-sections near the bridges are presented in Figure 5. The results show that a smaller Manning coefficient resulted in a lower water level, as expected. The maximum difference for this test was 0.36 m. The effects of the Manning coefficient on the computed results are thus limited in this study.

#### 2.5. Validation of the Numerical Model

^{3}/s measured at Fengzhou Station and 2750 m

^{3}/s measured at Ciba Station. The surveyed flood elevation at the cross-section, 1260 m from the upstream boundary, was 976.80 m. These data were used for validation of the numerical model. The corresponding flow discharge was 1440 m

^{3}/s, calculated by the interpolation of values from the Fengzhou Station and the Ciba Station. From our model, the computed water level at the cross-section under these flood conditions was 977.25 m, 0.45 m higher than the surveyed flood level. When comparing the effects of mesh resolution and the Manning coefficient, the difference between the computed and surveyed water levels is acceptable. Additionally, due to river bed evolution, the topography used in the numerical model may be different from the topography in 1990 and this may be the reason for the difference [32]. Generally, the numerical model developed was validated to be accurate enough for further study.

## 3. Results and Discussion

#### 3.1. Backwater Effects of Bridge Piers

#### 3.2. Effects on Flow Velocity Fields

## 4. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 7.**Distribution of water depth near the Fengxian Bridge for 10-year flood conditions (circles denote bridge piers).

**Figure 9.**Distributions of the computed velocity magnitude for 10-year flood conditions. (

**a**) Results without bridge piers, and (

**b**) results with bridge piers.

**Figure 10.**Distributions of the computed velocity magnitude for 100-year flood conditions. (

**a**) Results without bridge piers, and (

**b**) results with bridge piers.

Mesh | Number of Elements | Smallest Allowable Angle | Maximum Element Area (m^{3}) |
---|---|---|---|

1 | 14,412 | 29° | 200 |

2 | 33,865 | 29° | 80 |

3 | 41,524 | 29° | 50 |

Location | Distance to the Upstream Boundary (m) | Designed Elevations at the Top of Levees (m) | Computed (with Piers) Elevations at the Top of Levees (m) | Differences (m) |
---|---|---|---|---|

Xizhuang Bridge | 631 | 978.83 | 978.98 | −0.15 |

Fengxian Bridge | 1220 | 976.64 | 976.97 | −0.33 |

E Ramp Bridge | 2400 | 973.24 | 973.6 | −0.36 |

Jialing River Bridge | 2856 | 971.03 | 971.38 | −0.35 |

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**MDPI and ACS Style**

Wang, W.; Zhou, K.; Jing, H.; Zuo, J.; Li, P.; Li, Z.
Effects of Bridge Piers on Flood Hazards: A Case Study on the Jialing River in China. *Water* **2019**, *11*, 1181.
https://doi.org/10.3390/w11061181

**AMA Style**

Wang W, Zhou K, Jing H, Zuo J, Li P, Li Z.
Effects of Bridge Piers on Flood Hazards: A Case Study on the Jialing River in China. *Water*. 2019; 11(6):1181.
https://doi.org/10.3390/w11061181

**Chicago/Turabian Style**

Wang, Wen, Kaibo Zhou, Haixiao Jing, Juanli Zuo, Peng Li, and Zhanbin Li.
2019. "Effects of Bridge Piers on Flood Hazards: A Case Study on the Jialing River in China" *Water* 11, no. 6: 1181.
https://doi.org/10.3390/w11061181