ANS:C - 1 in 120

A gradient of 1 in 120 for sewers is less common in general engineering practice compared to other typical gradients like 1 in 100 or 1 in 150. However, it can still be used in specific situations, depending on factors such as the diameter of the sewer, the flow rate, and the characteristics of the sewage. Here's an explanation of why a gradient of 1 in 120 might be chosen for sewers:

1. Hydraulic Considerations: The gradient or slope of a sewer line is crucial for ensuring proper flow and preventing the accumulation of solids and debris. A steeper gradient allows for faster flow velocities, which can help in self-cleansing of the sewer by preventing the deposition of solids. A gradient of 1 in 120 provides a moderate slope that facilitates adequate flow velocities without being too steep or too shallow.
2. Terrain and Site Conditions: The choice of gradient may also be influenced by terrain and site conditions. In some cases, where there are constraints such as limited available space or the need to navigate difficult terrain, a gradient of 1 in 120 may be selected to accommodate these site-specific challenges while still maintaining adequate flow characteristics.
3. Flow Rate and Capacity: The design of sewer systems considers the anticipated flow rates and capacities to ensure efficient conveyance of wastewater. A gradient of 1 in 120 may be suitable for sewer lines with moderate flow rates and capacities, where the goal is to balance flow velocity with hydraulic capacity.
4. Regulatory Requirements: Local regulations and standards may also influence the choice of sewer gradients. In some jurisdictions, specific guidelines or standards may recommend or require certain gradients for sewer design to ensure compliance with regulatory requirements and industry best practices.