Energy loss in circular drop manholes

Drop manholes are often used as energy dissipators in sewer systems. A supercritical approach flow to a drop manhole has considerable hydraulic energy causing excessive air entrainment, tailwater waves, pipe abrasion and structural damage. Drop manholes may undergo various flow types. An experimental research has been conducted at the Laboratorio di Ingegneria delle Acque, University of Cassino, Italy on two circular manhole models of different sizes. It was observed that the energy dissipation of a drop manhole is mainly affected by operational conditions, i.e. the flow regime, depending on the manhole geometry and the approach flow features. The energy loss is also affected by the manhole diameter and by the drop height. It may become smaller than expected under certain circumstances, resulting in a high tailwater velocity. This undesirable condition must be avoided because drop manholes should significantly reduce peak velocities. The maximum velocity can be predicted by an empirical equation proposed herein.