Hydraulics of sharp-crested weir culverts with downstream ramps in free-flow, partially, and fully submerged-flow conditions

Laboratory experiments were conducted to investigate the hydraulics and discharge characteristics of sharp-crested weir culverts with downstream ramps for hydraulically smooth wall boundary and for free, partially submerged, and fully submerged-flow conditions. Four weir-culvert models were tested with different weir heights, ramp lengths, and culvert heights. The partially submerged-flow conditions were created when a slight increase in tailwater caused the headwater to rise due to partial submergence of the culvert. In this flow regime, supercritical flow over the ramp interacted with culvert outflow and this flow regime was classified as Supercritical Jet Interaction Regime or SJIR. Once tailwater reached the weir crest, the weir flow regime became submerged indicating fully submerged-flow condition. Based on the variations of water surface profiles, submerged flows were further classified into Surface Jump Regime (SJR), Surface Wave Regime (SWR), and Deeply Submerged Regime (DSR) and the boundaries between each flow regime were defined by regime plots. To predict discharge of weir culverts with downstream ramps, variations of discharge coefficient with weir geometry and flow regimes were studied and empirical formulations were developed. In addition, free and partially submerged flow discharges were predicted by implementing the interaction factor. It was found that the interaction factor was independent of the Froude number in SJR, while it correlated with the Froude number in SJIR. Under submerged-flow conditions, evaluation of the discharge characteristics of the proposed weir-culvert model showed correlations with water surface flow regimes and a three-stage prediction formula was proposed for estimation of submerged flow. The energy losses over weir-culvert models were also calculated and it decreased with submergence.