Investigating the bottom free surface nappe (ogee profile) across a sharp-crested weir caused by the flow in an asymmetrical approach channel

Abstract

The shape of an ogee spillway is based on the shape of the lower nappe of water flowing over an aerated sharp-crested weir. At the design discharge, this shape minimises the possibility of sub-atmospheric pressure occurring on the spillway and maximises the discharge across the spillway. The formulae that are currently in use to approximate the ogee profile consider only two-dimensional flow parameters, being the depth of flow over the spillway crest, the inclination of the upstream wall face, and the pool depth upstream of the spillway. The current formulae for the ogee shape, does not consider the influence of three-dimensional flow. The most significant three-dimensional flow parameters that could affect the shape of the lower nappe are the flow velocity distribution upstream of the spillway, the orientation or angle of the water approaching the spillway, the asymmetrical cross-section of the approach channel, and the curvature of the dam wall. This paper reflects the influence of asymmetrical flow in the approach channel. The investigation was based on a physical model constructed at the Department of Water and Sanitation (DWS). The inclination of the model’s sidewalls of the upstream approach channel was varied to cause a change in the symmetricity, while the lower nappe profile was routinely measured. It was found that the flow in the asymmetrical approach channel caused a variation from the theoretical estimated ogee profile. A comparison between the measured nappe profile and the currently used formulae was investigated. It can be concluded that the symmetricity of the approach channel influences the shape of the bottom nappe, which differs from the shape as proposed by the current ogee formulae. It is recommended that three-dimensional flow should be examined when designing an ogee spillway.