Reorganization of the horseshoe vortex system by developing scour-hole geometry at a cylindrical pier

The horseshoe vortex (HSV) system is the principal agent of local scour at bridge piers, yet its response to the geometry it excavates is difficult to measure: during active scour the boundary evolves while the flow is sampled, and the mobile bed obstructs optical access. Here, scour holes generated in mobile-bed tests were laser-scanned from within a transparent pier, digitally reconstructed, scaled, and printed as three-dimensional models, molded and cast in clear resin. The casts, representing bed states after 0–20 h of scour development, enabled repeatable time-resolved particle image velocimetry at a circular pier, with vortex cores tracked in every velocity field. The results show that the developing geometry does not merely amplify the HSV: it relocates and confines it. The primary core descends by 0.125D±0.004D (D, pier diameter), spends 95% of the time below the initial bed level in the 20 h geometry, and maintains a standoff of only 0.06D–0.11D above the local bed as it deepens. The area enclosed by the core trajectory falls by about two-thirds, core circulation strengthens by 28%, mean downflow deepens by 62%, and organized bed-parallel Reynolds shear stress over the slope rises to 1.7–2.9 times its flat-bed value; these percentages are condition-specific, whereas the geometry-relative behavior, with descent tracking the deepening bed at a standoff of order 0.1D and progressive confinement, is robust. The measurements show the scour hole acting as a progressive hydrodynamic organizer of the vortex-system, continually re-seating erosive forcing at the deepening bed; the resulting sediment response is inferred rather than measured.