Local scouring is a leading cause of bridge collapses. To protect bridges against local scouring, different countermeasures have been proposed and tested in the literature. In this study, the performance of collars was evaluated for scour reduction at two tandem piers aligned with four skew angles (h) of 0 ; 30 ; 60 ; 90 with respect to flow direction. For this purpose, long duration testswere conducted with uniform sediments under clear-water conditions with flow intensities close to the threshold of sediment motion. The results showed that increasing the skew angle increased the scour depth, while the performance of collars for scour reduction decreased. The maximum and minimum scour depths occurred at skew angles of h ¼ 60 and h ¼ 0 ; respectively, around both the piers with and without collars. Analysis of the results indicated that the maximum scour depth at the upstream pier shifted to the downstream pier from h 20 and h 28 for the tandem piers with and without collars, respectively. In addition, as the skew angle increased, the equilibrium time of scour increased. Moreover, the performance of collars for scour reduction around the piers decreased over time.

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