In this research work, besides analyzing the Scour hole at the downstream of the siphon spillway physical model, some equations have been presented to predict Scour hole developing. A physical model of siphon spillway along with three flip buckets of angle 30°, , 45°,and 60°,for three types of sedimentary materials of mean size 1. 8, 3. 5 and 1. 4 (mm) were used to study Scour hole formation process. Geometric characteristics of the Scour hole were collected for four flow discharges of 39. 2, 42. 12, 42. 12, 49. 76 (lit/s) and five tail-water depths of 15, 20, 25, 30 and 35 (cm) using a mesh grid size of 10 (cm) ×10 (cm). Three dimensionless parameters were extracted for analyzing experimental results using dimensional analysis. Of all Scour hole dimensions, Scour depth (ds), Scour length (Ls) and distance of downstream hill up to bucket lip (L6) were adjusted for numerical modeling. Results showed that increasing flow rate causes growth and developing of Scour hole geometric properties simultaneously. Increasing the particle size showed an inverse relationship with three dimensions of the Scour hole. Also, the reduction of the bucket's angle led to a reduction in ds, Ls and L6. The maximum correlation coefficient obtained between ds, Ls and L6 with particle size, flip bucket angle and discharge/tailwater, respectively. Finally, predictors were presented to describe above-mentioned dimensions of Scour hole dimensions.