An experimental study of critical submergence to avoid free-surface vortices at vertical intakes

Free surface vortices at intakes can cause excessive vibration, efficiency loss, structural damage, and flow reduction in hydroturbines, pumps, culverts, etc. They can also be a safety hazard and a potential loss of life. One of the major problems encountered during intake design is the specification of submergence and other design parameters in order to avoid strong free surf ace vortex formation. A properly conceived model study will determine whether free surface vortices are likely to occur. Before that point, however, the engineer needs to develop a preliminary design and then decide if a model study is needed. In order to assist in preliminary intake design, a plot of dimensionless submergence versus intake Froude number is presented for a number of vertical and horizontal intakes from both field and laboratory observations. The pilot is divided into two regions: 1) a region where intake vortices are unlikely and a model study is not required except with extremely poor approach conditions, and 2) a region with a good possibility of intake vortices, and a model study is recommended. Region 2, where intake vortices are a good possibility, is very large; encompassing many intake facilities. This is because minimum intake submergence to avoid vortex formation is highly dependent upon approach conditions, which are site specific. In order to add some clarity to this limited design criteria, an experimental study was undertaken which focused upon typical intake approach conditions. Most hydropower intakes have a forebay to avoid high circulation near the intake, so the experimental study simulated approach conditions with a fore bay or approach channel of varying length and width. The experiments were limited to vertical be11mouth intakes. The tendency for vortex formation is enhanced by separation around the leading edge of the approach channel walls. A long, narrow forebay will reduce the tendency for vortex formation.