Control of hydraulic jump in the stilling basins with perforated sill

Stilling basins are the most common structures for energy dissipation downstream of spillways. A properly designed stilling basin can ensure 60–70% dissipation of energy in the basin. The purpose of this study is to evaluate the performance of perforated end sill and effect of the sill on characteristics of hydraulic jump. The perforated sills with different heights and four ratios of opening 12, 25, 50 and 75% were tested for three tail water depths. Results of experiments confirmed significant effect of the perforated sill on dissipation of energy, reduction of the basin length and reducing dependence of jump location on tail water depth. Based on experimental observation, in a constant Froude number, decreasing of the tail water depth causes the hydraulic jump to move forward toward the end of the basin. In this case, to control the jump and to avoid the hydraulic jump being swept out of the basin, sill distance is decreased. Also, observation showed the sill height needs to be increased as inflow Froude number increases and tail water decreases. In the other words, relative sill height increases up to 30% as tail water depth ratio decreases from 1 to 0.8. Furthermore, the results showed that sills with the 50% opening in comparison with other openings have most effective in energy dissipation and reduction of the hydraulic jump length. The comparison of stilling basin with perforated sill and free jump indicated that the sill with perforation ratio of 50% dissipated 60–90% of water energy in inflow Froude number range of 4.5-12. Moreover, once Froude number increases from 4.5 to 12, stilling basin length increases about 19% for this perforated sill in the tail water depth ratio of 1.