Case studies of hydroplant intake design to avoid free-surface vortices.

Free surface vortices at hydroplant intakes can cause excessive vibration, efficiency loss, structural damage, and flow reduction in hydroelectric facilities. 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 surface 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. The parameters influencing intake vortices are reviewed. It is shown that a properly scaled Froude model will adequately represent the phenomena. The use of anti-vortex devices to eliminate vortices is reviewed and illustrated in two case studies. In order to assist in preliminary intake design, a plot of dimensionless submergence versus intake Froud number is presented for a number of vertical and horizontal intakes from both field and laboratory observations. The plot is divided into two regions: (1) a region where intake vortices are unlikely and a model study is not required except with poor approach conditions, and (2) a region with a good possibility of intake vortices, and a model study is recommended. (A)