*3.1. Subcritical Flows*

The researchers in [45] conducted an experimental study of clear water scouring around a circular cylinder, identifying the HSV as the main scour mechanism, due to the enhanced bed shear stress under the horseshoe vortices. The authors in [46] used numerical simulations of the flow field in the scour hole around a cylinder to show the importance of side slides in the scouring process. The study in [47] showed differences in the scour mechanism, depending on the flow intensity (*I*), i.e., the ratio between the flow velocity and the critical velocity for the incipient motion of sediment particles. From the minimum scour formation threshold at *I* ≈ 0.4 to 0.6, to the incipient motion condition at *I* = 1, clear-water conditions dominate, and the HSV and downflow produce sediment entrainment and landslides, which enlarge the scour hole over time. For intensities of 1.0 < *I* < ≈ 4.0, a bedload occurs, and fluctuations of the scour depth in time can be observed due to the bedforms entering the scour hole. For *I* > 4.0, entrainment into suspension occurs at the undisturbed bed, and refilling of the scour hole due to deposition is expected during the falling stage of floods. The study in [48] described the morphological evolution of dune-like bed forms downstream of bridge piers and abutments

that are generated by local scour. Recently, the authors in [49] conducted experiments that clearly distinguished two different scour modes: one, at the onset of erosion arising at the base of the cylinder and usually ascribed to the wrapping horseshoe vortex, which was determined and rationalized by a flow contraction effect, and another one, visible downstream of the cylinder, which consists of two side-by-side elongated holes. This pattern is observed for flow regimes close to the horseshoe scour onset, whose growth usually inhibits its spatio-temporal development.
