Definition - What does Drop Manhole mean?
A drop manhole is a manhole in which a vertical pipe is provided to allow for flow between the branch sewer and the main sewer. It is utilized in areas where the slope is steep or when an inlet pipe’s invert elevation is much higher than the outlet pipe’s invert level.
In a typical manhole arrangement, the invert elevation at the stop end of the inlet pipe is the same as the manhole invert. In a drop manhole, the invert at the stop end is at a significantly higher level than that of the manhole invert level.
Gradient differences between two sewer pipes can sometimes be more than 0.5m, which is too big of an elevation difference for unimpeded sewage flow between the branch sewer and main sewer. Drop manholes are used in this scenario as well as when larger sloping gradients are impractical or uneconomical.
Flow to the main sewer can either be routed outside of the manhole known or through the manhole channel’s interior. The pipe that passes through the manhole barrel is called the inside drop. The one that is on the exterior is the outside drop. Outside drops lack accessibility making routine inspections, maintenance, and cleaning difficult.
Figure 1: Typical Arrangement of Outside Drop Manhole in a Sewer Network (Source: WPS Group)
Figure 2: Typical Arrangement of Inside Drop Manhole in a Sewer Network (Source: Shaw Precast Solutions)
Trenchlesspedia explains Drop Manhole
Drop manholes form a part of steep, sloped, urban sewer systems as they can reduce flow velocities by lessening the slopes in the network’s pipes and dissipating energy. High speeds can result in damage to lines and overall poor hydraulic performance. To avoid damage to the manhole in continuous flow situations, drop manholes are sometimes constructed with protective lining and corrosion proofing.
The design of drop manholes ensures that there is no back up of flow in the collection system. During maintenance of the drop manhole, provisions may be implemented to store or divert flow in the upstream collection network so that there is no interruption of flow. It is essential that drop manholes also be designed to lessen the amount of entrained air from the flow transported to the main sewer.
The drop manhole is the interface between the branch sewers at the surface and the tunnel system. Therefore it serves as a preferred location to analyze the efficiency of the collection system. Odor control systems, flow monitoring, and flow control apparatus are installed at the drop manhole location for monitoring purposes.
Types of Drop Manhole Structures
The most common types of drop structures fall under vortex type and plunge type for wastewater sewer networks.
Vortex Type Drop Manholes
Flow spirals down and clings to the vertical pipe walls in vortex type drop manholes. Vortex drop structures are commonly used and are hydraulically efficient. It is recommended that a de-aeration chamber be installed with this system to reduce air entrainment. One disadvantage of the vortex type is the limited space within the manhole, restricting access for both people and equipment.
Plunge Type Drop Manholes
Flow enters the system radially and is plunged directly down the vertical pipe in the plunge type arrangement. This flow can occupy the entire cross-section of the line through which it is dropped. Plunge drop manholes are suitable for small volume flows and shallow drops. Its construction is not complicated, and it is not costly to install. Disadvantages of the plunge type system include lack of energy dissipation, unstable flow, air entrainment, and odor release.