Hydraulic investigation of flows at high-head overflow spillway with multiple aerators: a physical and numerical study of Mohmand Dam, Pakistan

A spillway is the essential part of the dam body, which releases surplus flows. At higher floods, the spillway operates at high heads, which results in high flow velocities along the chute and may cause negative pressures and cavitation. Therefore, to minimize such issues, aerators are provided along the spillway's chutes. This study aims to analyze the performance of the high-head overflow spillway of Mohmand Dam, Pakistan, having a steep chute of 32° with multiple aerators. Based on Froude's law of similitude, the physical model study was carried out at Irrigation Research Institute, Nandipur, on a scale of 1:60, while FLOW-3D numerical models were used to compare different hydraulic parameters, i.e., flow depth, velocity and pressure. The numerical models were validated with the results of a physical model, which were found in an acceptable range (i.e., 4.93%), and the hydraulic performance of two aerators was evaluated at different discharges. The models indicated negative pressures inside the aerator cavity, which allowed the suction of air to the lower nappe. The maximum air entrainment at the first aerator was about 8.5%. The results also showed that air entrainment to the lower nappe decreased when discharge was increased, whereas the maximum air detrainment reached 11.3% downstream of the second aerator.