Glossary of terms

Drop structure

A drop structure is used in a manhole when the incoming sewer invert is much higher than the outgoing invert and a direct connection would create excessive wastewater velocity. Instead of a very steep pipe section, the level difference is handled vertically inside the manhole by a drop pipe or a shaped channel. On a longitudinal profile it appears as a jump in invert level at one node while the adjacent sections keep reasonable slopes. Check the drop height, inlet and outlet invert levels, and the slopes on both sides.