Comparison of downstream scour of single and combined free-fall jets in co-axial and non-axial modes

Abstract

Downstream plunge pools of dams have been exposed to erosion caused by falling jets of the hydraulic structure’s outlets. This damages the foundation and body of the dam and its constituent structures. Therefore, investigating the scouring phenomenon of plunge pools is one of the important issues in hydraulic engineering. The present research investigates the characteristics of scour holes by single jets and combined jets in different operational modes of dam spillway and outlet. Using experimental modeling, 36 experiments were conducted with various discharges and two tail water depths in single and combined jet modes. The results show that combined jet has lower scour depth and hill height than single jets in all conditions. Comparing the depth and length of the hole and hill height caused by scouring reveals that combined jet is approximately between 5 and 10% greater for a co-axial than for non-axial combined jets in all tail water conditions. But the width of the scour hole of the non-axial combined jets is more than the co-axial one. With the aid of dimensional analysis, an equation is proposed for calculating the scouring characteristics of the combined jets, which is a good determination coefficient between calculated and observational data. Also, the dimensionless results show that the length of the scour hole and the length of mound are about one and a half times the position where the maximum scour depth is formed. Thus, a unit non-dimensional curve can be obtained for the longitudinal profile of scour. The results of the study indicate that, given the case of combining jets as non-axial, the depth and length of the scour holes are lower than the single mode, and that downstream scour can be controlled and reduced relative to the co-axial state by combining the non-axial spillway outlet jet with the orifice jet.