*2.1. Experimental Facility*

Experimental measurements have been carried out in a recirculating plexi-glassed tilting flume, with dimensions of 20 m long, 1 m wide, and 0.72 m deep, supplied by an overhead tank, through a regulating valve (Figure 1). The slope of the channel was 0.5%. Detailed information on the experimental flume can be found in Chavan et al. [9]. Starting 2 m from the inlet, the bed of the flume was made porous, placing a fine mesh on a pressure chamber at a length 15.2 m, width of 1 m, and depth of 0.22 m. Two electromagnetic flow meters measured the seepage discharge. The test section was 5 m, that is, it is 5 m to 10 m from the tailgate of the flume.

For the fine mesh, two river sands of median diameters (*d*50), 0.395 mm and 0.5 mm, and standard deviation (σg), 1.85 and 1.65, respectively, were selected. Both sands had standard deviation values higher than 1.4, so they were non-uniform [27]. The sand bed thickness was 17 cm. Two perspex circular piers with a diameter of (*d*) 75 mm and 90 mm, respectively, which were both 150 mm high, were alternatively set at the center of the test section, 7.5 m from the tail end of the flume (i.e., 7.5, 0.5, and 0). Both diameters of the piers were less than 10% of the channel width, so as to avoid a sidewall effect on the scour depth [14].

**Figure 1.** Schematic diagram of the experimental set-up.
