Author(s): F. Granata; R. Gargano; G. De Marinis; W. H. Hager
Linked Author(s): Willi H. Hager
Keywords: No Keywords
Abstract: Drop manholes are implemented in steep urban sewer systems to reduce flow velocities. The dominant hydraulic features of drop manholes depend on the operating conditions, affected by the manhole geometry and the approach flow features, characterized in terms of the dimensionless impact parameter. A water pool develops onto the manhole bottom from which the outflow is discharged into the downstream sewer. A high pool level results in a large discharge but a minimum drop height is required to avoid negative backwater effects to the approach flow sewer. Moreover, the presence of a drop manhole may promote pipe choking in the downstream, corresponding to the sudden and abrupt transition from free surface to pressurized sewer flow. An experimental research has been conducted on two circular manhole models of different size. It was observed that t he pool water level inside a drop manhole in supercritical flow depends mainly on the flow regime. Besides, the pool height is affected by various dimensionless parameters as changes in the flow regime. A novel parameter to characterize choking inception in the downstream pipe of the manhole was introduced to account for the relative drop height and the approach flow conditions. Empirical equations to estimate the pool depth and to predict choking inception are proposed in this paper.