ANS:B - half full

Designing sewer pipes to run at "half full" means that the flow depth within the pipe is equal to half of the pipe's diameter. In other words, the water level within the pipe is at half of its maximum capacity. When a sewer pipe is running at half full, it means that there is still space available within the pipe to accommodate additional flow if needed. This approach provides some degree of safety margin against surges in flow rates, preventing the risk of overflowing the pipe during peak flow conditions. Designing sewer pipes to run at half full offers several advantages:

1. Hydraulic Efficiency: Running the pipe at half full helps to optimize hydraulic efficiency. At this flow depth, the velocity of the flow is higher compared to shallower depths, reducing the likelihood of sedimentation and maintaining self-cleaning velocities.
2. Minimization of Sedimentation: Keeping the flow depth at half full helps to minimize the settling of solids within the pipe. Adequate flow velocities prevent the accumulation of sediment and debris, reducing the risk of blockages and maintaining the conveyance capacity of the sewer system.
3. Safety Margin: Designing the sewer pipe to run at half full provides a safety margin against peak flow conditions. It ensures that the sewer system can accommodate sudden increases in flow rates without risking overflow or flooding.
4. Cost-Effectiveness: Operating the sewer pipe at half full allows for the efficient conveyance of wastewater while minimizing the size and cost of the infrastructure. It strikes a balance between capacity and construction costs, ensuring that the sewer system meets its operational requirements economically.