It is a special type of manhole on a sewer line which is Constructed to provide a connection between a high-level branch sewer to a low-level main sewer. When, a branch sewer enters a manhole by, more than 0.5 to 0.6 m above the main sewer, the sewage is not allowed to fall directly into the manhole. instead, the sewage 0f the branch sewer is brought into the manhole of the main sewer either by inclined pipe (inclined at 45°) or through a downpipe (vertical pipe). In the former case, it is known a ramp while in the latter case it is known as a drop manhole.
Fig. shows details of a circular drog manhole. The branch sewer is joined to the manhole of the main sewer through a vertical pipe (or drop pipe). The sewage coming from the branch sewer flows down in the vertical pipe and emerges out through a horizontal pipe just above the benching. A plug is provided at the point where the branch sewer, if prolonged straight, intersects with the vertical wall of the manhole. The prolonged length of branch sewer, beyond the vertical pipe, serves as the inspection arm. After opening the plug, it can be used for inspecting or cleaning the branch sewer.