Drop manholes occur at the intersection points of pipes with large changes in elevation. More specifically, the downstream invert elevation of incoming pipes into a drop manhole is higher than the invert elevation of the manhole. This results in a drop in water surface elevation from upstream to downstream. By default, the start and stop invert elevation of connecting pipes are set to be equal to the connecting node. The main advantage in designing a sanitary system with drop manholes is to reduce the slopes in order to minimize the scouring effect of potentially high velocities and to reduce turbulence within the manhole itself. However, due to potential blockage problems drop manholes should be used sparingly.
The calculation of headloss within a drop manhole is challenging for methods dependent on an iterative solution process. AASHTO (Headloss - AASHTO Method) and HEC-22 (Headloss-HEC-22 Energy Method) methods calculate headloss when the incoming and outgoing velocities balance resulting in a headloss that is the delta between the hydraulic grade upstream and downstream of the manhole. A drop manhole adds additional complexity in that as upstream hydraulic grade increases, incoming pipes with varying invert elevations contribute additional incoming velocities to consider during the iteration. Also note that the resulting calculated headloss may result in invalid downstream control depths for incoming pipes. In these cases, gradually varied flow analysis continues upstream by assuming minimal applicable control depths.
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