Concurrent Session 02B: Experimental Evaluation of Stilling Basins Downstream of Embankment-Sloped Stepped Spillways

Abstract Description: The design of hydraulic jump stilling basins typically follows standardized procedures from Federal organizations (Reclamation, NRCS, USACE) who developed said procedures using smooth chute criteria. The increasing number of spillway designs with stepped spillways has highlighted the need to more broadly understand the interaction of stilling basins with upstream stepped chutes. Therefore, this study employed a systematic experimental campaign at the Utah Water Research Laboratory to expand the use of Reclamation Type II and St. Anthony Falls basins to moderate-sloped stepped spillways common to embankment dams. The basins were each designed for an incoming Froude number of 4 and were tested for a range of unit discharges below, at, and above the design discharge. Sensitivity of the hydraulic jump surface and pressures within the basin to the step height and tailwater were assessed by testing two steps heights and three ratios of conjugate depth (10% below, 10% above, and equal to conjugate depth). A qualitative parametric study to evaluate possible improvements to the St. Anthony Falls basin was also performed by modifying the placement and size of the baffle blocks. It was found that streamwise basin floor pressure profiles do not follow hydrostatic conditions with maximum values occurring at the basin entrance and near basin elements. Fluctuations of flow depths and pressures were greatest at these locations. Between the St. Anthony Falls basin baffle blocks, some pressures were below atmospheric pressure. Increasing the step height by a factor of two reduced the mean flow depths and basin floor pressures as much as 20%. Tailwaters less than conjugate depth often resulted in sweep out or the hydraulic jump toe occurring within the basin while tailwaters above conjugate depth improved jump stability and placed the hydraulic jump toe upstream of the basin. The St. Anthony Falls basin was more robust to tailwater sensitivity than the Type II basin. To contextualize the results, comparisons of hydraulic jump profiles were made to the more common Type III basin, among others. The study concluded that standardized design procedures for Type II and St. Anthony Falls basins are applicable to moderate-sloped stepped spillways and Froude numbers less than 4 while modifications to the St. Anthony Falls basin baffle blocks may improve basin performance.