Energy Dissipation in Circular Drop Manholes

Energy dissipation is one of the main considerations in the design of drop manholes and is closely related to the flow regimes observed. Four flow regimes in a drop manhole are classified according to different flow conditions at the inlet and outlet: free overfall flow, orifice flow, pressurized outflow, and fully submerged manhole condition. Theoretical predictions of energy dissipation in drop manholes under different flow regimes compare well with previous experimental data. The drop height limitation for drop manholes is suggested to be 3 m in this study, as larger drop heights can cause the water flow to disintegrate into drops and result in excessive air entrainment. Designs of drop manholes based on different flow regimes are presented. A single drop manhole is also compared with a prior reported stacked drop manhole in terms of energy dissipation. It is found that a well-designed single drop manhole has a similar performance in energy dissipation compared with a stacked drop manhole. (C) 2017 American Society of Civil Engineers.