*2.2. Description of Anti-Scour Collar*

The commonly used highway bridges in the valley river areas always adopt cylindrical piers with a diameter of 2 m, as illustrated in Figure 3. A cylindrical pile with a diameter of 2 m was also used for scour analysis in some literature [36]. In this study, a length scale of 1:20 is adopted to design the model and flow conditions. The pier model was made of cylindrical acrylic tube with an outer diameter of 10 cm and a thickness of 1 cm.

**Figure 3.** Schematic diagram of anti-scour collar: (**a**) the pier with 2 m diameter in highway bridges; (**b**) the installation of 360◦ collar in pier; and (**c**) indication of collar with a protection angle a (◦).

The downward flow and horseshoe vortex are two main hydraulic effects that affect the local scour. The anti-scour collar installed at the bottom of the pier can protect the sediment particles far from erosion by reducing the downward flow particle velocity and the horseshoe vortex.

In this study, the anti-scour collar used in the physical model tests is an acrylic circle plate with a thickness of 0.5 cm. As shown in Figure 3, three key design parameters of the collar need to be addressed, including collar installation height *h* (a distance from the collar to the riverbed), collar external diameter *W* and collar protection range α. D is the outer diameter of pier, H is the water depth.
