# Effects of Bridge Piers on Flood Hazards: A Case Study on the Jialing River in China
## 3 Results and Discussion
### Backwater Effects of Bridge Piers  
The differences in water levels with and without bridge piers under 10-year flood conditions were obtained by subtracting the water levels when there are no piers from the water levels when there are piers (Figure 6). Figure 6 shows that there were clear backwater effects due to the presence of bridge piers upstream of the Fengxian Bridge, and the maximum water level rise was about 1 m. Upstream of the three other bridges, slight differences (less than 0.5 m) were found. The backwater lengths of the four bridges were 160 m (Xizhuang Bridge), 270 m (Fengxian Bridge), 180 m (E Ramp Bridge), and 270 m (Jialing River Bridge) for a 10-year flood. The impact of the Fengxian Bridge on flooding was more serious than for the three other bridges. The results of statistical analysis showed that, under the conditions of a 10-year flood, 48 piers of the Fengxian Bridge were in the inundation area of the flood. The numbers of piers of the other three bridges in the flood inundation area were 28 (Jialing River Bridge), ten (E Ramp Bridge), and ten (Xizhuang Bridge). Figure 7 presents the distribution of computed water depth with piers near the Fengxian Bridge and the locations of the piers. This is the reason why the Fengxian Bridge had the greatest impact on flooding.  
The results of the flood water level rise and the backwater length upstream of bridge piers are of great interest in the design of levees. The planned levees along the banks of this river reach have been designed for 10-year floods (Figure 6). Elevations at the top of the levees are determined by the designed flood levels, plus a 1 m freeboard. There must be proper accounting of the impact of bridge piers on the designed levees. Table 2 shows the comparisons between planned and computed levee elevations at cross-sections just upstream of the four bridges, where there is a maximum water level rise due to the presence of piers. It was observed that, near the four bridges, the top elevations of the planned levees are all less than the computed ones with piers. This means that the elevations of the planned levees have to be raised.  
600 m (Jiangling River Bridge) for the four bridges. Compared with the 10-year flood conditions, the increase in backwater length by bridge piers was limited, which is probably due to the meandering of this river. Similarly, statistical analysis showed that, under the conditions of a 100-year flood, the numbers of piers of the four bridges in the flood inundation area were 37 (Jialing River Bridge), 13 (E Ramp Bridge), 67 (Fengxian Bridge), and ten (Xizhuang Bridge). This explains why the impacts of water level rise are more serious near the Fengxian and Jialing River bridges. Furthermore, the impact on the transverse variations of the water level was computed at cross-sections just upstream of the Fengxian Bridge, which is 1090 m from the upstream boundary. The transverse water level slopes were 0.136% without bridges and 0.115% with bridges. This is negligible because the width of the cross-section is small (about 110 m).  
In general, for both flood conditions, water level rise and backwater lengths due to the bridges were obvious, especially near the Fengxian and Jialing River bridges. These effects must be carefully taken into account when designing planned levees.