What is the Purpose of Drop Manhole?

A drop manhole is a fall manhole that is provided in a sewer line when there is a difference in elevation between the incoming and outgoing sewers. Manholes are located in a sewer line system wherever there is a change in sewer geometry or where there is a change in flow parameters. 

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Inside Drop Manhole
Image Courtesy: Municipal Water & Sewer

The figure-1 below shows the plan and cross-section of a sewer line with different manholes installed. The manhole mentioned 'A' is a drop hole and is represented separately for better understanding.

Fig.1. Drop Manhole Cross-section and Details

Working of Drop Manhole

• When a branch sewer enters a manhole by an elevation more than 0.5 to 0.6m above the main sewer, the sewage is not allowed to fall directly into the manhole to avoid sewage splashing or throwing. Instead, the sewage from the branch sewer is brought into the manhole by using an 
• Inclined pipe or ramp
• Downpipe or drop manhole
• Sudden drop of wastewater can cause turbulence, resulting in splashing, the release of odorous gas, and damage to the manhole. Problems associated with air and pulsations are addressed. Hence, while designing the drop height, particular attention must be paid to the jet trajectory geometry. 
• Drop manhole avoids unnecessary steep gradient of branch sewer, reduces the above-mentioned issues, and also the amount of the earthwork.
• Drop manholes are considered special manhole structures that involve hydraulic problems at the intake in the shaft and the outlet. 
• Usually the difference between incoming and outflow sewers is greater than 0.5 m. Maximum drop height of 7 to 10 m.

Construction of a Drop Manhole

The figure below shows the construction of a circular drop manhole. The branch sewer is connected to the manhole of the main sewer through a vertical pipe called the drop pipe. The sewer from the branch sewer moves down the vertical pipe and through the horizontal pipe reaches the benching. 

Fig.2. Construction Details of Drop Manhole

A prolonged length of branch sewer with a plug is provided for inspecting or cleaning the branch sewer. This is a single type of drop manhole arrangement. The design of a manhole varies based on the drop height and the sewer motion.

Dimensions of Drop Manhole

One critical aspect of constructing drop manholes is ensuring the appropriate dimensions, which directly impact their effectiveness and safety. 

Diameter and Shape

The diameter of a drop manhole is a crucial consideration when designing these structures. It determines the accessibility of personnel, equipment, and materials during maintenance and repairs. The diameter typically varies based on the utility or pipeline size. 

As per standard codes such as the American Society of Civil Engineers (ASCE) standards, common sizes for drop manholes range from 900mm to 1800mm. Larger diameters may be required for larger pipes. The shape of the manhole can be cylindrical or rectangular, depending on the specific project requirements and available space.

The diameter of the drop pipe shall preferably be larger than, or of the same diameter as, the entering sewer. The diameter of the drop pipe shall not be smaller than the diameter of the entering sewer by more than two nominal diameters (e.g. for twelve-inch, fifteen-inch, and eighteen-inch entering sewer, the drip shall be eight inches, ten inches, and twelve inches, respectively).

Depth

The depth of a drop manhole is determined by the elevation differences between the incoming and outgoing pipelines. Standard codes, such as ASCE, provide guidelines for calculating the appropriate depth. The depth ensures efficient flow and prevents overflow or backflow issues. Designers must adhere to these guidelines to maintain a proper gradient and hydraulic performance within the system.

Access Opening

The access opening of a drop manhole is another critical dimension to consider. It allows personnel to enter the manhole for inspection, maintenance, and repairs. Standard codes specify minimum access opening dimensions to ensure safe and convenient entry. 

For example, the Occupational Safety and Health Administration (OSHA) in the United States recommends a minimum access opening size of 24 inches (61 cm) in diameter.

Drop Length

The drop length, also known as the vertical distance between the incoming and outgoing pipes within the manhole, is an essential dimension to consider. It affects the hydraulic performance and energy dissipation within the structure. Standard codes provide guidelines on drop lengths to ensure smooth flow transitions and prevent turbulence or pressure fluctuations.

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