The Superimposed Backwater Effect of Dense Bridges in Nanjing Section of the Yangtze River Under River Erosion Conditions

Numerous bridges have been constructed along the Nanjing section of the lower Yangtze River, with some in close proximity to one another forming a dense bridge group. The previous research on the water-blocking effect of dense bridge groups on large rivers based on realistic engineering data has been insufficient. This study focused on the Nanjing section as the research subject and created a 2D numerical hydrodynamic flow model to investigate the actual bridge backwater effect with different bridge densities, hydrological conditions, and river topography. The results demonstrated that for a single bridge with a separation greater than 10 km, the backwater effect caused by the bridge was insufficient to be superimposed, and the single bridge water-blocking rate came into play. The relationship between the maximum backwater height and the single bridge water-blocking rate within the Nanjing section was obtained by data fitting. For the dense bridge group with a separation of less than 10 km, it was found that the superimposition of the bridge group enhanced the water-blocking effect of a single bridge. The relationship between the backwater height and the water-blocking rate of the dense bridge group was also obtained by data fitting. By utilizing the actual calculation results for the bridge backwater, when the water-blocking rate for a single bridge and for the bridge group in the Nanjing section was 5%, the backwater heights were found to be approximately 2 and 5 cm, respectively. The quadratic function correlation between the bridge group backwater height and the bridge pier water-blocking ratio with respect to variations in hydrological conditions was determined. The backwater effect of the bridge group within the Nanjing section was more remarkable as the bridge group water-blocking effect increased during the high-water period in comparison with the low-water period. This study demonstrated that during different topography conditions before and after river erosion, the bridge group backwater height was effectively unchanged and there was no significant connection between the backwater height and topography variations.