Author(s): De Marinis Giovanni; Gargano Rudy; Granata Francesco; Willi H. Hager
Keywords: Circular dropshaft; Drop manhole; Energy loss; Falling jet; Sewer system
Abstract: Drop manholes have found a wide application in sewer systems, especially in steep urban catchment basins. However, these typical structures of sewer systems have not received much attention in the technical literature. This is attributed to the complexity of flow phenomena of drop manholes that demands for an experimental approach. The following introduces a novel research project on drop manholes and describes the experimental apparatus. Its flexibility allows to change the relevant geometric and hydraulic parameters in wide ranges. Preliminary observations show that the hydraulic performance of a dropshaft depends significantly on the impact zone of the inflow jet and on the jet behaviour beyond its impact against the opposite manhole wall. Six different working conditions were retained, according as the jet falls directly onto the manhole bottom, the jet hits the outlet manhole, or the jet impacts against the opposite manhole wall. The overall energy dissipation between the upstream and downstream pipes was evaluated. Higher energy losses were obtained when the jet impinges on the manhole bottom, while a smaller dissipation relates to the case in which the jet hits the outlet of the manhole. The study further focused on the manhole pool water depth, for which different behaviours in relation to the working condition were observed. The pool water depth was found independent from the inflow filling ratio only in the case of direct jet impact onto the manhole bottom. The dimensionless factor including the product of approach flow pipe filling times the approach flow Froude number represents an effective parameter to adequately describe both the energy loss and pool water depth.