Air-water flows in circular drop manholes

Drop manholes, a typical element of urban drainage networks in steep catchments and in reaches of supercritical flow, enhance air entrainment and entrapment. The air flow across drop shafts can be remarkably high. This paper discusses the air transport phenomena and the effects of ventilation absence in drop manholes. Based on an extensive experimental study, air entrainment mechanisms have been accurately described and air demand has been evaluated in different flow regimes. In addition, the effects of ventilation absence on the hydraulics of circular drop manholes, with emphasis on sub-atmospheric pressure onset and pool depth raising, have been investigated. The effects of a possible air flow recirculation have also been evaluated. Issues regarding any scale effects have been discussed. The influence of the main hydraulic and geometric parameters on drop manhole performance was contemplated to provide improved design concepts for sewer systems.