Velocity characteristics in the bottom boundary layer of an undular bore over a horizontal bed

The characteristics of velocity fields and profiles in the laminar boundary layer of an undular bore propagating on a horizontal bed are experimentally investigated, employing a high speed particle image velocimeter. A suddenly lifted gate installed in a flume was used to produce an undular bore (with a water depth ratio of 1.70). Based on these measured results, the temporal variation of the horizontal pressure gradient in the free stream is delineated to highlight the relationship between the spatiotemporal acceleration/deceleration in the laminar boundary layer and the pattern of the horizontal pressure gradient. In addition, various shapes of velocity profiles (including flow reversal and overshooting) obtained at different instants and locations are classified into eight kinds, along with distinct definitions of the boundary layer thickness as well as the representative length and velocity scales clearly identified. It is found that the boundary layer thickness keeps increasing with increasing time even when the bottom boundary layer is undergoing different patterns of the horizontal pressure gradient. Four similarity profiles with the dimensionless forms of horizontal velocity profiles in the bottom boundary layer are established temporally and spatially under distinct horizontal pressure gradients, together with the counterparts of solitary waves incorporated for comparison. This study provides a unique example that elucidates the related similarity profiles in the boundary layer of the unsteady undular bore with different types of horizontal pressure gradients, including favorable, zero, and adverse ones.