# Effects of Bridge Piers on Flood Hazards: A Case Study on the Jialing River in China
## 3 Results and Discussion
### Effects on Flow Velocity Fields  
Flow velocity fields during floods are also important because high velocity flow will cause the erosion of river beds and the destruction of banks and bridges. The velocity distribution and flow regime were analyzed under 10-year and 100-year flood conditions. Figure 9 shows the velocity distributions with and without piers for 10-year flood conditions. Except in the vicinity of the Fengxian Bridge, the bridges have a minor impact on the maximum flow velocity distribution of the river. The maximum flow velocity for 10-year flood conditions was higher than 6 m/s. Upstream of the Fengxian Bridge, the maximum flow velocity during the flood was significantly reduced owing to the presence of the bridge piers. Quantitative analysis indicated that the maximum flow velocities near the Jialing River Bridge were 5.81 and 5.93 m/s without and with piers, respectively, which is an increase of 2.07%. For the other three bridges, the maximum flow velocities were all reduced because of the presence of the bridges. For the E Ramp Bridge, the velocities were 6.91 and 6.86 m/s without and with bridges, respectively. For the Fengxian Bridge, the velocities were 6.45 and 4.99 m/s, respectively. For the Xizhuang Bridge, the velocities were 5.02 and 4.80 m/s, respectively. The ratios of the maximum velocity decrease were 0.81% for the E Ramp Bridge, 22.66% for the Fengxian Bridge, and 4.5% for the Xizhuang Bridge. From the layout of the Jialing River Bridge (Figure 9b), it is evident that the bridge is not across the river but on the outside of a river curve, which decreases the width of the cross-section. The flood flow was  
Figure 8: Differences in water levels with and without bridge piers for 100-year flood conditions. directed to the inside of the curvature, resulting in a slight increase in the maximum flow velocity. The other three bridges span the entire river channel, resulting in an increase in the water level of the river and a decrease in the maximum flow velocity. Generally, the impact of the E Ramp Bridge piers on flood velocity was the weakest and that of the Fengxian Bridge piers was the strongest. Near the Fengxian Bridge, the maximum flow velocity was reduced by 22.66%, which likely leads to sediment deposition in this section. For 100-year flood conditions, a comparison of the flood velocity distributions without and with bridge piers is found in Figure 10. The maximum velocity distribution of the river was similar to that in 10-year flood conditions, but the flood velocity of the river increased with the increase in flood discharge, as expected. With bridge piers, the local velocity of the river was significantly  
Figure 9: Distributions of the computed velocity magnitude for 10-year flood conditions. (**a**) Results without bridge piers, and (**b**) results with bridge piers. reduced, especially upstream of the Fengxian Bridge. This was similar to the results from the 10-year flood conditions. Quantitative analysis showed that the maximum flow velocities near the Jialing River Bridge still increased, and also that the decreases in the maximum velocities were induced near the other three bridges, similar to what occurred under the 10-year flood conditions. Near the Jialing River Bridge the maximum velocities were 7.77 and 8.39 m/s without and with piers, respectively, which represents an increase of 7.98%. The maximum velocities for the E Ramp Bridge were 8.40 and 8.07 m/s without  
Figure 10: Distributions of the computed velocity magnitude for 100-year flood conditions. (**a**) Results without bridge piers, and (**b**) results with bridge piers. and with piers, respectively. The maximum velocities for the Fengxian Bridge were 8.41 and 7.67 m/s, respectively. The maximum velocities for the Xizhuang Bridge were 8.77 and 8.83 m/s, respectively. The corresponding ratios of increase were \(-\)3.93% (E Ramp Bridge), \(-\)8.80% (Fengxian Bridge), and 0.68% (Xizhuang Bridge).