The Effect of Non-Homogeneous Sediment Bed on Local Scour Around Cylindrical Pier

Studies of local scour around bridge commonly employed homogenous non-cohesive sediment as their simplified sediment parameters. However, sediment in real water stream is non-homogeneous with varying grain size distribution. This study investigates the local scour behaviour for two non-homogeneous, non-cohesive sediment with gradation parameter 1.5 and 2.25. Laboratory experiments were conducted using cylindrical piers with varying sizes of 2.5, 3.5, 5 and 6 cm. Measurement of maximum scour depth, transverse and longitudinal scour lengths were measured using Vernier scale after each test. The scour depth and lengths were consistently increased as the pier size increased for both types of sediment. Increment of the longitudinal and transverse scour lengths were found to be exponential when the pier size was increased from 3.5 cm to 5 cm. The effect of non-homogeneity of sediment characteristics were clearly affected the local scour behaviour where the scour depth and lengths for sediment with higher gradation parameter were much lower. Futhermore, the obtained scour characteristics were significantly underestimated when compared with values predicted with the empirical equation. This indicate that the armouring effect has taken place, where the coarser sediment protects the finer sediment from being entrained. Consequently, the level of scouring is much less than what is anticipated and posed a possibility of bridge overdesign.