Scouring is a natural phenomena which is the result of the erosive action of flowing water, excavating and carrying away materials from the bed and banks of streams and from around the piers and abutments of the bridges. Different materials scour at different rates, e.g., loose granular soils are rapidly eroded, while cohesive or cemented soils are more scour-resistant. Failure of bridge due to scour at their foundations, including abutments and piers, is a common occurrence. Based on the mode of sediment traort by the approaching flow, local scour is In this study armoring effect on local scouring under unsteady flow condition was investigated based on model experimentation. Experiments were performed in a 8.5 m long, 0.405 m wide and 0.7 m high laboratory-scaled flume. Different combinations of uniform sediment of bed-armor layer were employed. Three circular piers were used and ten triangular hydrograph considered. For each experiment the armor-layer thickness was maintained at 3d a (d a , is the armor-layer material mean size). The bed materials were relatively fine non-cohesive sediment, overlain by a thin armor layer of coarser sediment. In this study the clear water scour condition is considered. The approaching flow velocity is restricted to the clear water scour condition with respect to the armor layer particles (u * /u *ca =0.8, where u * is the flow shear velocity, estimated based on the Shields’ diagram and u *ca is the critical shear velocity of the armor-layer particles). Results showed that, in armored beds, the effect of hydrograph shape on scour depth is negligible, and the base time is the most important parameter. Observations illustrated that the effect of hydrographs which composed of repetition of n triangular hydrographs with the same base time (T) is almost identical to that of a continues hydrograph with the base time of nT.
