TIME EVOLUTION OF SCOUR AROUND BRIDGE ABUTMENTS

Local phenomena around bridge piers and abutments are generally considered to be similar, nevertheless the presence of the incoming boundary layer on the side wall in the abutment case generates extra pressure gradients and consequently a more complex vortex pattern. In the literature, experimental data for bridge abutments are relatively scarce; in particular almost no data are available for the time evolution of the scour. In this work we present the results of several long duration (3 days longrightarrow5weeks) clear water scour laboratory tests around bridge abutments; the time evolution of the erosion process is analysed with respect to local and global characteristic values (maxima, volume, hole shape). In particular we analyse the effect of the constriction ratio b/B between the transversal obstacle dimension and the flume width: in many practical situations abutments (or piers) obstruct a significant portion of the channel, so that the average acceleration due to constriction is expected to increase the scour effects of the local acceleration around the obstacle. Measured values for maximum scour are poorly predicted by literature formulas. Scour depths are positively correlated with the constriction ratio, but increases are smaller than expected from literature indications. Experimental results show that models for bridge piers cannot be directly applied to abutments; in particular, time scales for the latter are significantly larger than for piers.