Flow-improving elements in circular drop manholes

ABSTRACT Drop manholes are widely employed in urban drainage systems. They experience operating conditions in which insufficient energy dissipation leads to hydraulic problems that are enhanced by particular configurations of the manhole bottom. Therefore, specific jet-breaker devices were considered to improve the flow conditions. Two different types of jet-breakers have been tested in the present work: plane jet-breaker (PJB) and wedge jet-breaker (WJB). Both have proved to be effective in inducing adequate energy loss, if properly sized, and to improve the overall drop manhole performance. The selection of a jet-breaker element should account for additional features of the drop manhole hydraulics, including the pool depth, the air entrainment phenomena, as well as further practical aspects including cost-effectiveness, simplicity of realization and clogging risk. This study provides the hydraulic basis for the design of the jet-breakers, particularly of the PJB, which has an overall optimum performance, as laboratory experiments have shown.