Measuring the geometry of a developing scour hole in clear-water conditions using underwater sonar scanning

A novel scanning technique using a rotating-head sonar profiler attached to a slider mechanism is presented as a means to directly measure the complex erosion and deposition features of local scour holes developing in clear-water conditions around vertical cylinders mimicking bridge piers. Extensive validation shows that the method produces high-density elevation surfaces to within y1.5 ± 2 mm accuracy in a quasi-non-invasive manner. This equates to 0.5 ± 0.7% relative to the flow depth which sonar resolution is well known to scale with. Experimental data from three trials using different cylinder diameters indicate that monitoring of the entire scour hole over time (instead of only the maximum depth as is commonly done in laboratory experiments) can reveal important information about local scour evolution. In particular, results show that the scour-hole volume scales with the maximum scour depth cubed (VOL y3s ) through three linear regimes. The transition to the third linear regime was found to represent a step change in the scour evolution process. Following the recent theoretical framework proposed by Manes and Brocchini (2015), this change, termed the crossover point, was interpreted as the point where the production of turbulent kinetic energy plateaus which corresponds to a stabilization in the erosive power of the horseshoe vortex. Scour development beyond the crossover point is characterised by a significant reduction in the rate of volumetric scour, relative to the steadily-increasing maximum scour depth. This overall reduction in volume-development is attributed to a balance between erosion from in-front of the pier and deposition around the sides using topography analysis. It is speculated that the existence of the crossover point may help to identify the characteristic length and time scales describing the evolution of local scour, which may be used for modeling purposes.