Proceedings of the 4 th World Congress on Civil, Structural, and Environmental Engineering (CSEE’19) Rome, Italy – April, 2019 Paper No. ICGRE 124 DOI: 10.11159/icgre19.124 ICGRE 124-1 Flow Structures Around a Circular Bridge Pier with a Submerged Prism at Upstream Mohsen Ranjbar-Zahedani 1 , Alireza Keshavarzi 2 , Hadi Khabbaz 1 , James Ball 1 1 School of Civil and Environmental Engineering, University of Technology Sydney (UTS) 15 Broadway, Ultimo, NSW 2007, Sydney, Australia Mohsen.Ranjbarzahedani@student.uts.edu.au; Hadi.Khabbaz@uts.edu.au; James.Ball@uts.edu.au 2 Centre for Infrastructure Engineering, Western Sydney University Second Ave, Kingswood, NSW 2747, Sydney, Australia A.Keshavarzy@westernsydney.edu.au Abstract - Previous investigations have indicated that local scour around bridge piers and abutments causes around 60% of waterway bridge failures. In order to decrease the potential of pier-scour failure, the authors previously proposed an upstream prism as a new countermeasure against local scour. The proposed prism was examined in a comprehensive experimental program to find the most efficient size, submergence ratio, and installation location of the prism. The experimental results showed that the submerged prism could reduce around 40% of the maximum scour depth, and 60% of the scour-hole volume. In this study, in order to find out how this submerged prism affects the flow structure around the pier and reduces the pier-scour, the flow structure analysis was conducted using particle image velocimetry (PIV). The velocity components were measured for two cases of a single circular pier with and without the submerged prism. Analysis of the results indicated that the proposed prism could change the flow structure at the upstream and downstream of the pier. In fact, this submerged prism formed a wake region behind itself, and the bridge pier was located at this wake region. The produced wake resisted the down-flow at the upstream side of the pier and also disturbed the formation of the horseshoe vortices around the pier. In addition, this submerged prism reduced the strength of wake vortices behind the pier. Consequently, the pier-scour was significantly reduced by the substantial changes in the flow structure. Keywords: Bridge pier, Local scour, Flow structures, Flow-altering devices. 1. Introduction Bridges on waterways are important structures for transportation. Every year many bridges collapse due to different reasons, including scour, flood, earthquake, fire, collision, wind, overloading, environmental degradation, faulty design, inappropriate construction, low-quality materials, lack of maintenance, etc. Previous investigations indicate that among all the causes, scour is a significant cause and more than 60% of waterway bridge failures are due to this problem. Pier-scour is a specific form of waterway erosion. As defined by Melville and Coleman [1], scour means the lowering of the level of the river bed by water erosion such that there is a tendency to expose the foundations of a bridge. Pier-scour is caused by the interference of the pier with the flow and characterised by the formation of the scour hole immediately around the pier. Numerous studies have been conducted to understand the mechanism of pier-scour (e.g., Melville and Raudkivi [2], Ahmed and Rajaratnam [3], Ettema et al. [4], Keshavarzi et al. [5-7]). According to Melville and Coleman [1], there are four principal features of the flow structure around a single bridge pier – down-flow, horseshoe vortices, surface rollers, and wake vortices. As shown in Figure 1, down-flow and surface rollers occur upstream of a pier, while horseshoe and wake vortices are formed at the base and the downstream of the pier, respectively. The down-flow impinges on the bed materials. The horseshoe vortex increases the velocity near the bed, resulting in an increase in the sediment transport capacity of the flow. The wake vortex system keeps the sediment suspended. It also acts as a ‘vacuum cleaner’ with the bed material carried to the downstream side by the eddies shaded from the pier. Therefore, an effective solution to control and reduce the pier-scour can be to change the complicated flow structure around the pier.