ANS:D - > 1.5 m.

The spacing of manholes along a straight portion of a sewer is a crucial aspect of sewer system design, aimed at facilitating maintenance, inspection, and overall system functionality. Manholes serve as access points to the sewer system, allowing personnel to enter for maintenance activities, inspection of conditions, and removal of blockages or debris. The spacing between manholes is determined based on various factors, including hydraulic considerations, construction requirements, regulatory guidelines, and practical accessibility needs. Here's a more comprehensive breakdown of the factors influencing the spacing of manholes:

1. Hydraulic Considerations: The spacing of manholes is influenced by the hydraulic characteristics of the sewer system. Hydraulic design principles ensure that the flow velocity within the sewer remains within acceptable limits to prevent sediment deposition, scouring, or excessive turbulence. Manholes are strategically placed to maintain adequate flow velocities and minimize the risk of sediment buildup or flow restrictions.
2. Gradient and Slope: The gradient or slope of the sewer plays a crucial role in determining the spacing of manholes. Steeper slopes may require shorter distances between manholes to accommodate the increased flow velocity and prevent surcharging. Conversely, gentler slopes may allow for longer spacing between manholes while maintaining adequate flow velocities.
3. Flow Capacity and Design Flow: The diameter of the sewer, along with the anticipated flow rates and design flow, influences the spacing of manholes. Larger-diameter sewers typically have greater flow capacity and can accommodate higher flow rates, allowing for longer distances between manholes. Conversely, smaller-diameter sewers or sewers serving areas with high flow rates may require more frequent manhole spacing to ensure proper flow management.
4. Regulatory Requirements: Local regulations and design standards may specify minimum requirements for manhole spacing based on safety, accessibility, and operational considerations. Design codes and standards often provide guidelines for determining appropriate spacing based on factors such as pipe diameter, flow rates, and accessibility requirements.
5. Maintenance and Inspection Needs: Manholes serve as access points for maintenance crews to conduct routine inspections, cleaning, repairs, and emergency interventions. Adequate spacing between manholes ensures that maintenance activities can be carried out efficiently and safely, without the need for excessive entry points or long distances between access points.
6. Construction and Installation Constraints: Practical considerations such as construction methods, site conditions, and cost-effectiveness also influence the spacing of manholes. Construction crews need sufficient space and access to install, maintain, and repair the sewer system components, including manholes.