Experimental investigation of transient bed shear stress and scour formation at seawalls under tsunami bore impact using particle image velocimetry (PIV)

Abstract Tsunami bores generate short-duration, highly turbulent flows that impose intense bed shear stress on coastal structures, often leading to toe scour and structural instability. This study experimentally examines the transient evolution of bed shear stress and scour formation at four seawall geometries (box, curve, slope, and step types) using a dam-break wave testing facility and high-resolution Particle Image Velocimetry (PIV). Controlled bores with initial reservoir depths ( H = 13, 15, 17 cm) propagated over a movable bed of 1 mm glass beads for scour experiments. PIV measurements on an identical fixed-bed setup from which near-bed shear rate and bed shear stress were derived. Results show scour initiated within the first second after impact and corresponded to brief shear intensifications rather than sustained bed shear stress levels. The box-type seawall produced the deepest scour due to a concentrated downward jet at the toe, even though its peak bed shear stress was lower than that of the other geometries. The curve, slope, and step-type seawalls generated higher instantaneous bed shear stress values but negligible scour because their geometries redirected momentum and shortened the duration of near-bed shear. These results indicate that the bed shear stress magnitude alone is not enough to determine sediment response, but that the duration and direction of transient shear events determine the initiation of motion under tsunami-like bores. Profiles with curved, sloped and stepped form were especially effective in reducing toe scour by dispersing bore momentum and limiting the bed exposure to high shear. This study provides integrated laboratory dataset linking PIV-derived transient bed shear stress with measured scour initiation under wet-bed tsunami bore conditions.