Evolution of Water Conveyance Capacity through Hydraulic Transition Processes in Circular Drop Manholes

Circular drop manholes are widely implemented for steep catchments in urban drainage networks. Poor downstream hydraulic transition processes of a manhole system, i.e., the formation of hydraulic jump near the outlet entrance, and the sudden transition from free surface to pressurized flow with bursts of air in the outflow pipe can severely constraint the capacity of water conveyance. In this paper, we defined four basic hydraulic stages that indicate where hydraulic transition processes begin and end. The measurements of typical manhole models with different drop heights were conducted under different inflow and outflow conditions. Three types of transition processes covering all flow patterns have resulted into a graphical visualization by analyzing two pairs of dimensionless parameters. The flow inside a circular drop manhole was considered to reach its discharge capacity when the abrupt drop of manhole water level is visible in the fully aerated flow pattern. Four empirical equations revealing the water level filling ratio and discharge efficiency at different hydraulic stages were also presented for further predictions of choking risks.