# Effects of Bridge Piers on Flood Hazards: A Case Study on the Jialing River in China
## 4 Conclusions  
To investigate the impact of four bridges on the flooding of the Jialing River in China, a 2D hydrodynamic model based on an unstructured mesh was developed, in which the TVD finite volume method was used to solve the 2D shallow water equations. The impacts of four bridges on flooding were analyzed by comparing results without and with bridge piers under the conditions of 10-year and 100-year floods. Backwater effects and flow velocity fields due to the presence of piers were analyzed in detail. The main conclusions are drawn as follows.  
Sensitivity analysis showed that the maximum flood water level differences were 0.38 m for three different meshes and 0.36 m for the different Manning coefficients. The difference in the water level for validation was 0.45 m. This indicates that the numerical model developed in this study was acceptable because the differences were small and comparative.  
For the backwater effects caused by bridge piers, under 10-year flood conditions, obvious water level rises were found upstream of the Fengxian Bridge; the maximum water level rise was about 1 m owing to the presence of this bridge. The extent of the maximum water level rise was much less for the other three bridges. For the planned levees near the bridges, the top elevations of the levees should be increased by 0.15-0.36 m. Under 100-year flood conditions, the water level rises were more significant upstream of the Fengxian and Jialing River bridges. The maximum value was higher than 2 m. The backwater lengths were also increased.  
With respect to the effects on the flood flow field, it was found that the presence of the Jialing River Bridge caused the maximum flow velocity in its vicinity to increase, but the other three bridges could induce the maximum flow velocity in their vicinity to decrease. This was probably because the Jialing River Bridge is not across the river but on the outside of the river curvature. For both floods, significant effects of the Fengxian Bridge on the maximum flow velocity were obtained. These were increased by 22.66% and 8.80% for the 10-year and 100-year flood conditions, respectively. The effects caused by the Jialing River Bridge were less significant, with decreases of 2.07% and 7.98% for the 10-year and 100-year flood conditions, respectively. It is recommended that more emphasis should be placed on the protection measures near these two bridges.  
The flow field structure near piers and the resulting sediment erosion are critical to the safety of bridges and are an issue of interest to engineers, and thus this subject deserves further study.  
[PERSON] and [PERSON] collected topography and hydrological data; [PERSON] and [PERSON] developed the numerical model and performed the simulation; [PERSON], [PERSON], [PERSON], and [PERSON] analyzed the results and drafted the manuscript; all authors contributed in reviewing and editing the manuscript. This research was funded by the National Key R & D Program of China (2017 YFC0504704; 41731289); the Scientific Research Program Funded by the National Natural Science Foundation of China (Grant Nos. 51609198; 11605136); the Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2017 JQ5063); the Innovative Research Team of Institute of Water Resources and Hydro-electric Engineering, Xi'an University of Technology (2016 ZZKT-14/104-451217015/104-45115004). We thank [PERSON], BSc (Econ), from Liwen Bianji, Edanz Editing China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript. The authors declare no conflict of interest.