Drop manhole and alternative option

Drop manhole

A drop manhole is a manhole in which a branch sewer entering at a high level is connected to the lower main sewer by a vertical drop pipe, usually outside the manhole shaft, so that the sewage does not fall directly into the chamber.

   Ground
  ___________
  |  |    |  |
  |  |<---|--+--- branch sewer (high)
  |  |    |
  |  |  drop pipe (concrete encased)
  |  |    |
  |__|____v____ main sewer ---->

Importance

• Prevents splashing, noise, erosion of the manhole base and release of foul gases when sewage falls from more than about 0.6 m.
• Keeps workers safe and makes entry dry.
• Avoids deep excavation of the branch sewer on steep ground and so reduces cost.
• Reduces velocity and protects the downstream pipe from scour.

Alternative structural options

1. Steep sewer or cascade (stepped) gradient: the sewer itself is laid on a steep slope with cascades/step drops in the manhole, with the energy dissipated by a stilling basin. Applicable where large flows come on very steep ground, or for large trunk sewers where a drop pipe would not be big enough.
2. Ramp (inclined) drop: a sloping channel is built in the manhole to bring the sewage down smoothly; suitable for large sewers with a drop of about 1-3 m.
3. Backdrop or vortex drop structures: the flow spirals down a vertical shaft, dissipating its energy against the walls. Applicable in major trunk sewers where large flows must drop several metres.
4. Deeper excavation of the branch (laying at lower level) when the difference is small (<0.6 m), where an ordinary manhole with a steep channel can be used.

A drop manhole is chosen for small to medium branch sewers; cascades or ramps are used for large sewers.