## 5 Conclusions  
In this study, the impact of constructing concurrent bridges on river flow dynamics was examined using physical modeling. The subsequent conclusions were drawn by analyzing alterations in water levels and flow rates before and after the construction of these bridges under varying frequencies of flooding conditions:  
1. The hydraulic physical model constructed in this paper is basically consistent with the measured data in simulating the water level difference between the upstream and downstream of the bridge group river section, the average cross-sectional flow velocity, and the flow pattern of the river section. The relative error in sediment deposition simulations is within 20%, confirming the model's suitability for such studies. 2. Characteristics of river regime changes near the bridge location after parallel construction of bridges: The river course is basically stable, with the overall flow dynamics constrained by existing bank protection projects. The new bridge has a negligible impact on the reach-scale river regime. The main river line of the river course, which is typically meandering, generally follows the evolutionary pattern of \"straightening with high water and curving with low water.\"
3. Characteristics of water level rises in front of the bridge after parallel construction of the bridge: As the flow increases, the flood discharge section gradually expands. When the flow exceeds 5000 m\({}^{3}\)/s, the flood begins to overflow onto the banks and is affected by the backward effect of the newly constructed bridge. Under the 10-year design flood, the water level difference between the upstream and downstream observation sections adjacent to the bridge pier changed from 5 cm before the construction of the parallel bridge to 9 cm after construction, representing an 80% increase in the backwater effect. Under the 20-year design flood, the water level difference between the two measurement points changed from 7 cm before the construction of the parallel bridge to 12 cm after construction, indicating a 71% enhancement in the backwater effect. 4. Characteristics of the waterback length of water levels in front of the bridge after merging: As the flow increases, the backwater distance extends upstream. At the flow rate of a 10-year design flood, the backwater effect can reach about 3000 m upstream of the bridge site; when it is at the flow rate of a 20-year design flood, the affected area extends to about 3500 m.
5. Characteristics of flow velocity changes near the bridge location after a parallel construction of bridges: The flow velocity in the cross-section before and after the bridge decreased to a certain extent compared to before the bridge, and the distribution pattern of flow velocity across the sections is consistent. In the main channel area, the flow velocity generally increases, with the maximum change in flow velocity being 0.25 m/s; in the floodplain area, the flow velocity fluctuates significantly, and under characteristic flood conditions, the change in flow velocity in areas with dense bridge piers is greater than in areas with sparse piers, with the maximum difference in flow velocity in the floodplain area reaching 0.3 m/s. 6. Impact characteristics of the flood processes on changes in the riverbank: Significant bank erosion has been observed in the left bank shoal area of the bridge layout, causing the main channel to widen. It is necessary to further strengthen the monitoring of the bank area to protect the resources of the shoal area. After the flooding process, the river channel tends to evolve into a U-shaped channel, and measures should be taken to strengthen the protection of the piers at the junction of the floodplain and the channel. Based on the test results, it is advisable to install diversion facilities at the most affected piers (Piers 116 and P5 of the new bridge) to modify local flow dynamics, and thus, safeguard pier safety and mitigate localized backwater effects. Writing--original draft, B.C. ; Writing--review and editing, Y.Z.