ANS:D - all of these.

Sewer manholes are essential components of sewer systems and are strategically placed at various points along the network to facilitate maintenance, inspection, and access to the sewer lines. Let's explore the reasons why sewer manholes are generally provided at the following locations:

1. At the Change of Gradient: Sewer manholes are often installed at points where there is a significant change in the gradient or slope of the sewer line. Changes in gradient can affect the flow velocity and direction of wastewater within the sewer pipe. By placing manholes at these points, engineers can access the sewer line to inspect and maintain the integrity of the pipe, ensure proper flow conditions, and address any blockages or buildup of debris that may occur due to changes in slope.
2. At the Change of Direction: Manholes are also commonly installed at locations where there is a change in the direction of the sewer line, such as bends, curves, or intersections. Changes in direction can impact the flow dynamics and hydraulic performance of the sewer system. Manholes provide access points for inspection and maintenance activities, allowing engineers to monitor the condition of the sewer lines, remove obstructions, and perform repairs if necessary. Additionally, manholes at change-of-direction points facilitate the flushing of sediment and debris that may accumulate in the pipes, helping to maintain the overall efficiency of the sewer network.
3. At the Junctions of Sewers: Junctions, where multiple sewer lines converge or intersect, are critical points in the sewer network. Manholes are typically installed at these junctions to facilitate the connection and branching of sewer lines. These manholes serve as access points for inspecting and maintaining the junction structure, ensuring proper alignment and sealing of the pipes, and preventing infiltration or exfiltration of groundwater or surface water. Additionally, manholes at sewer junctions allow for the installation of flow control devices, such as valves or gates, to regulate the flow of wastewater within the network and prevent overloading or surcharging of the system during heavy rainfall events.

In summary, sewer manholes are provided at the change of gradient, the change of direction, and the junctions of sewers to facilitate inspection, maintenance, and access to the sewer network. By strategically placing manholes at these locations, engineers can effectively manage the operation and performance of the sewer system, ensuring reliable and efficient conveyance of wastewater while minimizing the risk of blockages, overflows, and other operational issues.