ABSTRACT
Spillway chutes are appurtenant dam outlet structures with the purpose to safely convey large discharges during extreme flood events. During such events, hydraulics plays a major role in the safety of the structure. Along a spillway chute, water is accelerated by gravity and may reach flow velocities in the order of 10 to 50 m/s, implying a considerable cavitation risk. On the spillway invert, turbulence is generated by shear stresses and surface roughness, which results in self-aeration of the flow once the turbulent boundary layer interacts with the free surface. For reliable design guidelines of spillways, knowledge of air concentrations along the spillway chute is essential, as entrained air concentrations can mitigate the risk of cavitation at the expense of risking overtopping of the chute walls due to flow bulking, or further accelerating the flow due to drag reduction. While it is well known that the invert roughness is the controlling parameter for boundary layer development and the self-aeration process (for a given slope and discharge), the quantitative understanding of roughness effects on air-water flow properties is still limited by the availability of data sets that target this variable. In this research, the effects of invert roughness on smooth spillway chute flow are investigated in a large-scale physical model. The investigated flow properties include the clear water and air-water mixture flow depths, depth-averaged flow velocities, air concentrations, and friction factors. Based on the experimental data, we demonstrate that the streamwise development of depth-averaged air concentration is significantly affected by invert roughness, which in turn also affects the bottom air concentration downstream of the inception point. Further, we found that friction factors are significantly affected by the relative boundary layer thickness in the developing non-aerated flow region, but also by bottom air concentrations in the aerated flow region. Good agreement between experimentally determined friction factors and established theoretical relations was found. Overall, our findings contribute to a qualitative description of invert roughness effects on air-water flow properties for a robust design of spillways, thus contributing to safer dam infrastructure.
