Armoring Effect on Local Scouring around the Bridge Pier under Unsteady Flow Condition (Part 2)

Document Type : Research Article


Associate Professor, Civil Engineering Department, Isfahan University of Technology, Isfahan, Iran


In this study armoring effect on local scouring under unsteady flow condition was investigated based on model experimentation. To obtain unsteady flow condition, ten different triangular hydrographs were considered. Experiments were performed in an 8.5 m long and 0.405 m wide laboratory-scaled flume. The bed materials were overlain by a thin armor layer of coarser sediment. Different combinations of uniform bed-armor layers were employed. Three circular piers of 22, 33 and 42 mm diameters were used. For each experiment the armor-layer thickness was maintained at 3da (da, is the armor-layer material mean size). 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, and u*ca is the critical shear velocity of the armor-layer particles estimated based on the Shields’ diagram). Results indicated that the equilibrium scour depth is independent of the hydrograph shape. In addition, increasing the hydrograph base time does not affect the scouring hole formed downstream of the pier. Observations showed that the effect of n hydrographs with the same base time of T is almost identical to that of a continuous hydrograph with the base time of nT.


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