Abstract
Circular drop manholes have been an important device for energy dissipation and reduction of flow velocities in urban drainage networks. The energy dissipation in a drop manhole depends on the manhole flow patterns, the outflow regimes in the exit pipe and the downstream operation conditions, and is closely related to the hydraulic and geometric parameters of the manhole. In the present work, the energy dissipation of a drop manhole with three drop heights was experimentally investigated under free outflow conditions and constrained outflow conditions. The results demonstrate that the local head loss coefficient is solely related to the dimensionless drop parameter for free surface outflow without a downstream backwater effect, whereas it depends on the dimensionless submerge parameter for constrained outflow. Moreover, it is concluded that the energy dissipation is largely promoted when outlet choking occurs.
Highlights
Drop manholes are hydraulic features that are widely implemented in urban drainage networks for steep catchments
The energy dissipation of a drop manhole is related to many factors, which can be grouped into four categories: (a) the approach flow conditions associated with the filling ratio of the upstream pipe and the approach flow Froude number; (b) the outlet flow conditions, such as free outflow conditions, including free surface flow and pressurized flow, and constrained outflow conditions, in which backwater effects downstream of the exit pipe are imposed on the outlet flow; (c) manhole configurations and dimensions, such as inlet or outlet entrance configurations, baffles in the manhole, drop height, and manhole diameter, etc.; and (d) air supply conditions
For a manhole under free outflow conditions, the energy dissipation is strongly related to the flow flow patterns describing the drop manhole flow and the outflow regimes in the exit pipe
Summary
Drop manholes are hydraulic features that are widely implemented in urban drainage networks for steep catchments. The energy dissipation of plunge flow in drop manholes is one of the major concerns for urban system drainage designers. As pointed out by Christodoulou [1] and Granata et al [2], adequate energy dissipation in drop manholes should be achieved in order to avoid excessive flow velocities—and, erosion—in the exit pipe. This cannot be always attained, because of the wide range of discharges experienced in sewer systems during a flood event [3]. The interest has been focused on the effects of ventilation absence on the sub-atmospheric pressure and pool depth, which can strongly influence the interaction between water and airflow and the dissipation energy in drop manholes [11]
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