Field study of scour around Bridge piers in the transitional section of the Qiantang River

With the rapid development of bridge construction, the safety of bridges under complex hydrological conditions has become an increasingly critical issue. This study investigates the local scour characteristics of bridge piers in the transition section of the Qiantang River, utilizing multi-beam echo sounding (MBES, vertical accuracy:±0.02 m) and high-resolution underwater 3D imaging sonar for data acquisition. The results reveal that the maximum local scour depth reaches 9.3 m, with the scour hole exhibiting an elliptical shape. When pier dimensions are similar, the scour hole width shows a strong correlation with the maximum scour depth, approximately four times the width of the pier cap. In contrast, the scour hole extension along the flow direction exhibits no significant correlation with scour depth but correlates with the pier cap length, approximately twice its dimension. Based on the measured data, the safety of bridge piers in the transition section was evaluated. It was found that the piers PM5-PM6 and PM8-PM10 of Bridge II, as well as the main piers of Bridge III, experienced significant scour depths, posing potential structural risks. The study recommends implementing protective measures for severely scoured piers and establishing long-term monitoring to ensure structural stability. The findings provide valuable insights for scour assessment and protection design of bridges under complex hydrological conditions.