Hydraulics of a physical model of supercritical vortex drop shaft

Vortex drop shafts are special drop structures typically installed in urban drainage systems or rarely as spillways for hydropower plants. The knowledge about the hydraulic behavior of vortex drop shafts was improved over the years but some gaps still have to be filled, with particular concern to the hydraulic aspects of the intake structure and of the vertical shaft. In this paper, an experimental campaign on a reduced-scaled physical model of vortex drop shaft serving an urban sewer system is described. Free-surface observations were used to characterize the hydraulics of the spiral inlet at the head of the shaft. A computational procedure based on the momentum approach is applied to derive velocity and flow rotation angle profiles over the vertical shaft. Such hydraulic features are fundamental into the design of a vortex drop shaft. If they are known with a sufficient reliability, then the energy dissipation efficiency of the shaft can be easily derived.