ANS:B - circular

In a separate sewerage system, where wastewater and stormwater are conveyed through separate networks, the choice of sewer cross-section aims to optimize flow efficiency, self-cleaning ability, and hydraulic performance. Among the options provided, the most efficient cross-section for sewers in a separate sewerage system is typically considered to be circular. Here's why circular sewers are often preferred:

1. Hydraulic Efficiency: Circular sewers offer excellent hydraulic efficiency due to their symmetrical shape, which minimizes energy losses and provides uniform flow characteristics. The circular profile distributes flow evenly around the perimeter, reducing the risk of stagnant zones and sediment deposition.
2. Self-Cleaning Ability: Circular sewers promote self-cleaning action by maintaining higher flow velocities across the entire cross-section, which helps prevent the accumulation of solids and debris. The smooth, continuous curvature of the circular shape reduces frictional resistance and enhances scouring effects, aiding in the removal of sediment and obstructions.
3. Optimal Flow Capacity: Circular sewers maximize flow capacity relative to their cross-sectional area compared to other shapes like rectangular or parabolic. This allows circular sewers to accommodate higher flow rates and peak stormwater events more effectively, reducing the risk of surcharging and overflow during heavy rainfall.
4. Structural Integrity: Circular sewers exhibit superior structural integrity and resistance to external loads, such as soil pressure, traffic loads, and groundwater fluctuations. The curved shape of the circular profile distributes external forces more evenly, minimizing stress concentrations and enhancing overall durability.

While other sewer cross-sections like rectangular or parabolic shapes may be suitable for specific applications or constraints, such as space limitations or aesthetic considerations, circular sewers are generally preferred in separate sewerage systems for their hydraulic efficiency, self-cleaning ability, and structural resilience. Therefore, the circular cross-section is often regarded as the most efficient choice for sewers in separate sewerage systems.