Drop Manholes in Supercritical Pipelines

In urban drainage networks on steep terrain, drop manholes are often employed for energy dissipation and reduction of flow velocities. The hydraulic behavior of such circular manholes located between supercritical pipe segments is studied by means of laboratory experiments. It is found that the local head-loss coefficient is governed solely by a dimensionless drop parameter, expressed in terms of the drop height and the inflow velocity. An analytical expression based on the experimental results is proposed for practical applications. The water level inside the manhole depends also on the angle between the inflow and outflow pipes, being higher when they are normal as compared to parallel. Optimum ranges of the drop parameter, in order to avoid impediment to the flow by formation of a hydraulic jump or surcharging of the pipe, may be determined and used in engineering practice.