Reserch On The Local Scour At Bridge Piers In The Tidal Reach Of The Changjiang River

The destruction of bridge is dominantly caused by local scour around the bridge pier. The research on local scour of bridge pier has attracted worldwide attention. Since the beginning of the last century, constructions of some large-scale hydraulic engineering projects are springing up in the Changjiang River, and they have great influence on scoring around the bridge pier, which will cause riverbed erosion. More of crossing river bridge engineering projects have been constructed in the tidal reach of Changjiang River. They have a wide range of bridge pier structures, which lead to ineffective usage of empirical estimated formula of local scour depth from flume experiments and numerical analysis. Therefore, it is necessary to make in-situ observations, which is important for identification of scouring intensity and effective estimation of maximum scour of piers.The multi-beam bathymetric system is utilized to observe the geometry of scour pit and bed form around pier of seven bridges which in tidal reach of the Changjiang River (Tongling Changjiang Bridge, Wuhu Changjiang River Bridge, Dashengguan Changjiang River Bridge, Nanjing Changjiang River Bridge, Second Nanjing Changing River Bridge, Fourth Nanjing Changing River Bridge, Shanghai Changjiang River Bridge). Meantime, the data of runoff current velocity and tidal current velocity during flood and dry season is measured by ADCP (Acoustic Doppler Current Profiler) and historical data of water depth are analyzed using ArcGis 10.1 from the nautical Charts in 1998 and 2013 for the deposition and erosion of the riverbed. Some commonly used local scour formulas are adopted to calculate maximum scour depth and the outcomes are compared to the maximum depth measured, which can provides reference for calculating maximum scour depth under tidal current. Main conclusions are as follows:1. With the influence of runoff, the scour pit of large circular cofferdams is show in "C" shape. With the fluence of tidal current, the scour pit of group piles foundation is show in "double-kidneys" shape. Apart from the local scour pit, there is riverbed scour pit outer of the rectangular bridge pier.2. Flow condition and pier structure have impact on the maximum scour depth. The maximum scour depth of 4# pier of Tongling Changjiang River Bridge, that of 11# pier of Wuhu Changjiang River Bridge, that of 6# pier of Dashengguan Changjiang River Bridge, that of 8# pier Nanjing Changjiang River Bridge, that of south pier of the Second Nanjing Changjiang River Bridge, that of south pier of the Fourth Nanjing Changjiang River Bridge, that of south main pier of shanghai Changjiang River Bridge, that of north main pier of shanghai Changjiang River Bridge is 14.6 m,12.2 m,10.6 m, 7.6 m,14.8 m,7.3 m,2.7 m,4.2mrespectively. As there are routes between piles group, its maximum socur depth is much less than that of large dofferdam piers. The maximum scour depth of group piles foundation affected by tidal current is less than that affected by tidal current. The maximum riverbed scour depth is 47 to 155 percent of the maximum local scour on bridge of rectangular pier.3. The correction formulae of local scour depth of pier affected by tidal current in the dunes covered areas is improved. Based on the correction formula of local scour depth of pier affected by tidal current in the dunes covered areas (the correction formule of dunes type), the correction formule of dunes type is modified by using the bedload incipient velocity rather than dunes’incipient velocity. The bedload incipient velocity is obtained from the correlation between bedload transport velocity and water velocity which measured by ADCP. The results gained from new formule, formula 65-2 type, the correction formula of 65-1 type and the correction formula of dunes type are compared and evaluated. The outcome of new formula is most approaching to the measurement results, which porivides a reference for bridge design and construction.4. Large-scale hydraulic projects intensity local scour on piers. Sediment discharge has decreased significantly since the implementation of Three Gorges Project, which lead to fluvial processes in the tidal reach. The value of mean erosion depth of 4#,5# piers of Tongling Changjiang River Bridge, that of 4#～12# piers of Wuhu Changjiang River Bridge, that of 7#-9# piers of the Dashengguan Changjiang River Bridge, that of 2#～5# piers of the Nanjing Changjiang River Bridge, that of the Second Nanjing Changjiang River Bridge, that of south main pier of the Shanghai Changjiang River Bridge, that of north main pier of the Shanghai Changjiang River Bridge is 1.7 m,3.4 m,8 m,2.2 m,2.6 m,3 m,4 m respectively.