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.
