Experimental Study of Local Scour Around Circularcrossing and Square-crossing Piles in Waves and Current

Abstract

Experimental tests of local scour around a circular-crossing pile (CCP) and a square-crossing pile (SCP) were carried out to study the effects of superimposed waves upon current in the local scour process and the differences in characteristics in the two shaped piles. Non-uniform sediment with a gradation of 1.6 was used in the tests. Stokes waves of 0.11 m in height and 1.6 s in period were generated in a 50 cm water depth flume. The ratios of Shields number to critical Shields number in the current-only tests, the waves-only test, and the waves-current tests were individually 0.90, 1.48, and 1.83, respectively. The Keulegan-Carpenter number (KC) was kept at a constant value of 2.9, which was expected not to generate a horseshoe vortex or a wake vortex in the waves-only condition. The topography at various scour durations, temporal bed elevation profiles, and temporal scour depths were measured and discussed. The results showed that sediment backfilling into the scour hole was caused by moving sand ripples. The limit value under which the horseshoe vortex or wake vortex was absent in the waves-only condition was reduced in the waves-current condition. The temporal bed elevation profiles and scour depth development were very similar between the SCP and CCP, although the scour hole and sand dunes in the former were individually deeper and larger than the latter. The maximum scour depth in the SCP was nearly equal to that of the CCP in the waves-current condition, but it was much larger in the current-only condition.