Rivers may carry a considerable amount of debris during floods, and the accumulation of this debris around bridge piers can change the hydraulic behaviour of flow fields and influence scour depth development. In this study, the effect of debris jam on scour depth at cylindrical bridge piers was investigated and a comparison carried out for the main existing empirical formulas. A series of 27 tests of rectangular geometry in three different groups according to width, length, submerged depth, and blocked area were studied. The results showed that the accumulation of debris increased scour depth from 10.3% to 62.1%, 22.4% to 103.4%, and 20.7% to 139.7% as the percentage blocked area of debris increased from 2.07% to 6.2%, 5.52% to 16.55%, and 8.96% to 26.89% for groups (1), (2), and (3) respectively. The comparison of the experimental results with the most commonly used empirical formulas revealed general overestimation of scour depth with debris accumulation. The Richardson and Davis (1995) and Lagasse et al. (2009) formulas were the most acceptable methods for scour prediction under debris jams, though further study is recommended for bridge pier scour depth estimation in the presence of debris.
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