The inception point of flow aeration on a rough stepped spillway

Stepped spillways are used in hydraulic engineering projects such as small and large dams. The inception point of aeration on such hydraulic structures is essential in determining the zones of single and two-phase flow. In this study, the effect of the surface roughness of steps on the location of IFPA on a stepped spillway is investigated. This paper investigated the effect of the surface roughness of steps on the location of the inception point of flow aeration (IPFA). For this purpose, a laboratory model of Ogee-stepped spillway was designed based on the maximum energy dissipation guidelines and its steps were roughened with gravel (with a specific grain size). The experiments were conducted in a channel with a longitudinal slope of 0.001, length of 12m, width of 0.5m, and depth of 0.8m on a stepped spillway with a height of 0.6m that has 9 steps. The flow discharge ranged between 6 (l/s) and 16(l/s). It was found that aeration starts from about 12 times the critical depth and by doubling the critical depth, its distance from the crest increased up to 50 percent. Notably, roughing the step surface reduces the length of the non-aeration area by about 15%.