Characteristics of Free Surface Vortices at Low-Head Hydropower Intakes

Tools for engineers who assess and optimize hydropower intakes are provided to help them measure and quantify the characteristics of free surface vortices (characteristic radius, bulk circulation, tip depth, nominal depression slope) that form at the intakes. Accessible methods are proposed for measuring and modelling vortex characteristics and the processes that affect their generation and strength. Common mechanisms that produce and strengthen the vortices (flow separation, shear, asymmetric approach flow) are discussed. An analytical model, based on Burgers's vortex model and laboratory measurements, is described that incorporates the effect of the approach flow and intake geometry on vortex characteristics. Simple measurement techniques (acoustic Doppler velocimetry and surface particle tracking velocimetry) are presented by which the flow and vortex characteristics can be documented, allowing the model to be adjusted to the particularities of the specific intake under consideration. The analytical model is then used to help understand how the different processes affect the scaling of vortex characteristics.