## 3 Results
### The Influence of the Parallel Bridge Construction on the Flood Level of the River Channel  
#### 3.3.1 Analysis of Along-Channel Water Levels Under Different Flow Conditions  
During the 2021 flood season, a comprehensive study was conducted on the river section to observe the water level changes corresponding to five distinct flow rates (600 m\({}^{3}\)/s\(-\)5000 m\({}^{3}\)/s). The results of this study are depicted in Figure 10. As illustrated in Figure 8, under low flow conditions, the river section's water surface line near the bridge group exhibits greater stability and uniformity. However, fluctuations are observed in the vicinity of the bridge, both upstream and downstream, with the phenomenon of water backing up in front of the piers. The bridge reconstruction and expansion project features five piers within the river channel, with one pier located in the main channel and four piers situated on the banks. As the flow rate increases, the water level along the river also rises, resulting in a pronounced backscatter effect at the bridge abutment area. The ratio drop along the river fluctuates between 0.9 and 1.36 parts per million, aligning closely with the average ratio drop of approximately one part per million for the river section.  
Figure 10: Variations in water level along the course of 600–5000 m\({}^{3}\)/s flow class conditions.  
Figure 9: Mainstream line changes at different flow levels.  
As the flow increases, the water level rises. During the flood, as water flows through the river section with the bridge group, the flood cross-section expands from the main channel to the dike within the beach area. Consequently, the bridge abutments are increasingly submerged, with the water level rising from the main channel abutments to the beach abutments. Following the construction of the bridge and its adjacent river sections, water level records were taken during a one-in-ten-year flood (Q = 9600 m\({}^{3}\)/s) and a one-in-twenty-year flood (Q = 11,000 m\({}^{3}\)/s). With the rise in flow levels, the bridge abutment situated within the river channel encroaches upon a portion of the flood cross-section, thereby reducing the area available for a water passage and increasing the resistance to the flow. The water level behind the bridge exhibits backwater in front of the piers and a rapid drawdown behind them. Under the 10-year flood scenario (Q = 9600 m\({}^{3}\)/s), the water level difference between the two monitoring points increased from 0.05 m before bridge construction to 0.09 m after construction. Under the 20-year flood scenario (Q = 11,000 m\({}^{3}\)/s), this difference rose from 0.07 m pre-construction to 0.12 m post-construction. This leads to a rise in water levels upstream of the bridge within a certain range, and as the flow intensifies, the distance of waterback extends further upstream. During the design flood with a flow rate of Q = 9600 m\({}^{3}\)/s, the impact of waterback on the bridge is substantial, with the water level at the bridge serving not only as a good example but also as an excellent case study. At this design flood flow rate, the waterback affects an area extending approximately 3000 m upstream of the bridge. For the design flood with a flow rate of Q = 11,000 m\({}^{3}\)/s, the impact of waterback extends to about 3500 m upstream of the bridge.