Definition - What does Drop Manhole mean?
A manhole in which a lateral service line or the main line enters a manhole above the manhole's channel or above the flow line so that the flow drops to a lower level is called a drop manhole. If the flow's elevation changes before it reaches the manhole, this is an "outside drop" Manhole. If the flow's elevation drops inside the manhole's confines, this is an "inside drop" manhole.
Trenchlesspedia explains Drop Manhole
Outside drop manholes may have a "T" connection that enters the manhole shaft at the original elevation of the line. This "T" can be used for inspection or to divert flow into the channel at the bottom of the manhole. The opening may be covered or have a one-way check valve to prevent the unrestrained discharge of flow into the manhole, or it may have a connection to a vertical downflow pipe, called an internal drop shaft, to channel overflow into the manhole's channel.