When are drop manholes provided in a sewerage system?
- AWhen there is change from gravity system to pressure system.
- BWhen there is change in the elevation of the ground level.Correct
- CWhen there is change in the diameter of the sewer.
- DWhen there is change in the direction of the sewer line.
Solution
Concept
Drop Manhole Construction and Purpose
Formula
N/A
Step-by-step solution
1. Review the purpose of drop manholes in a sewerage network. 2. Recall that drop manholes are installed when there is a significant change in ground or sewer elevation. 3. Select Option B as the correct choice.
Explanation
A drop manhole is installed when a branch sewer enters at a significantly higher elevation than the main sewer line. Instead of letting sewage fall freely and splash inside the chamber, a vertical drop pipe safely carries the wastewater down to the lower level, preventing splashing, bad odors, and structural damage.
Shortcut
To remember when Drop Manholes are used, use the mnemonic "DROP = Drop in Elevation": - Drop in Level: Used when a branch sewer enters at a higher elevation than the main sewer (significant difference in ground or invert level). - Prevents Splashing: Eliminates sewage dropping/falling freely from a height inside the chamber to prevent splash, odor, and damage.
Common mistake
A common mistake is selecting option D or C by confusing a drop manhole with a standard manhole, which is typically provided at changes in direction or pipe diameter. Students often overlook the word "drop," which specifically indicates a significant vertical elevation difference between the branch sewer and main sewer line.
Correct answer: B) When there is change in the elevation of the ground level.