Bottom-Pressure Development Due to an Abrupt Slope Reduction at Stepped Spillways

Abstract

Fluctuating bottom-pressures on stepped chutes are relevant for the spillway design. An abrupt slope reduction causes a local alteration of the bottom-pressure development. Little information is available regarding the air–water flow properties near an abrupt slope reduction on stepped chutes, particularly on the local pressure evolution. Nevertheless, the option of providing a chute slope reduction may be of interest in spillway layout. The experiments presented herein include pressure distributions on both vertical and horizontal step faces, subsequent to an abrupt slope reduction on stepped chutes. A relatively large-scale physical model including abrupt slope reductions from 50° to 18.6° and from 50° to 30° was used, operated with skimming flow. The data indicate a substantial influence of the tested slope reductions on the bottom-pressure development. In the vicinity of the slope reduction, the mean pressure head near the edge of the horizontal step face reached 0.4 to 0.6 times the velocity head upstream of the slope reduction, for critical flow depths normalized by the step height ranging between 2.6 and 4.6.

Advances in Spillway Hydraulics: From Theory to Practice

“…In turn, Stilmant et al [14] derive a purely theoretical model, validated against experimental data for a broad range of upstream heads, to compute the discharge coefficient with an ogee crest, which does not depend on empirical coefficients. Regarding the second topic, spillway transport structures, a diversity of subjects are analyzed, such as examining the physical effects of beveled face steps on various hydraulic design parameters, of relevance to contemporary design and construction practices for stepped chutes (Hunt et al [20]); computing the flow parameters of engineering interest for smooth and stepped spillway chutes using the SpillwayPro program (Wahl et al [17]); evaluating the performance of 3D SPH simulations for nonaerated flow over smooth and stepped spillways with converging walls (Nóbrega et al [18]); estimating bottom-pressure development on stepped chutes caused by an abrupt slope reduction (Ostad Mirza Tehrani et al [22]); CFD modeling of a spillway offset aerator, using the two-phase volume of fluid versus the complete two-phase Euler models included in the OpenFOAM ® toolbox (Mendes et al [26]); and investigating the performance of a deflector aerator at the upstream reach of stepped chutes to prevent cavitation damage, and, therefore, allow high specific discharges (Terrier et al [27]). For the third topic, spillway outlet structures, the contributions include reasonably large-scale physical model studies of the air-water flow properties in rectangular free-falling jets (Carrillo et al [28]), stilling basin performance below 30 • and 50 • inclined smooth and stepped chutes (Stojnic et al [29]), and scour process occurring downstream of a PK weir in the presence of downstream apron (Lantz et al [30]).…”

“…According to the prevailing topography, these stepped spillways are designed with variable step heights and slope changes along the channel. Ostad Mirza Tehrani et al [22] present systematic laboratory tests studying the influence of abrupt slope changes on the flow characteristics of stepped spillways. A relatively large-scale physical model, including abrupt slope reductions from 50 • to 18.6 • and from 50 • to 30 • was used, operated with skimming flow.…”