AbstractDrop manholes commonly are used in steep-slope urban areas to decrease pipeline slopes because flow velocity should be limited to a specific range in sewer and drainage systems. Surface runoff usually carries particles larger than a few millimeters to underground facilities, such as drop manholes. This sort of debris could accumulate inside drop manhole pools and clog the flow. As the residence time of particles in a drop manhole pool increases, the outlet blockage risk rises. This research experimentally examined the effects of flow discharge and inlet pipe filling ratio on the residence time of large buoyant particles in a circular drop manhole. Results showed that dimensionless residence time (TR) increases with flow rate to the peak value under Regime R2c, in which the falling jet partially collides with the manhole outlet. It decreases in Regime R3, in which the inlet jet impacts the manhole opposite wall. Thus, Regime R2 should be avoided in these structures. Moreover, statistical hypotheses testing revealed that different inlet pipe filling ratios did not cause significant differences in dimensionless residence time. It also was founded that under low inlet pipe filling ratios and flow rates, TR reveals some information about the dissipated energy.

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