3.2.1. Geometry of Scour Hole

The experimental measurements carried out in the flume showed that scour starts upstream of the pier, and that the scour hole increases in size over time (Figure 1 shows a snapshot of the scour hole after an experimental run of 24 h, with various dimensions of scour hole). The slope of the scour hole is slightly milder downstream than upstream of the pier. In Figure 6, the length of the scour hole in a longitudinal direction along the centerline is shown, for different flow rates and increasing in seepage percentage. While the scour depth is decreasing, the scour hole length is increasing, and, moreover, the scour hole length is slightly shifting downstream.

**Figure 6.** Longitudinal bed profile along the centerline.

Upstream of the pier, in the seepage runs, the reversal flow hindered by the lateral flow through the channel boundaries results in a reduction of the scour depth and the scour length, while downstream the pier, the increased momentum transfer results in the erosion of the bed material and an increase in the scour length. Similarly, from Figure 7, it is possible to observe that the scour hole is wider in the seepage runs than in no seepage run.

**Figure 7.** Lateral bed profile upstream from the piers.

This phenomenon depends on the turbulent characteristics of the flow—increasing the turbulence near the bed, laterally to the pier, in the seepage runs, enhances the bed erosion, and, therefore, increases the scour hole width.
