Performance of Circular Drop Manholes in Ventilation Absence

A suitable ventilation is required for urban sewer systems to ensure that aerobic conditions are preserved within the system, and to prevent sulphate reduction and the risk of explosive gas development. However, the passive ventilation of urban drainage systems does generally not assure adequate air supply. This shortage of ventilation may also affect the hydraulic performance of sewers, particularly if drop manholes are implemented. This paper discusses the effects of ventilation absence on the hydraulics of circular drop manholes, with a particular reference to pool depth, energy dissipation and the choking risk in the downstream sewer. The results were obtained from an experimental research conducted at the University of Cassino on three manhole models of different sizes. The effects of the main hydraulic and geometric parameters were considered to present improved design information for sewer systems.