ANS:E - All the above.

1. The circular section of sewers provides maximum hydraulic mean depth: This statement is correct. Among different sewer shapes (such as circular, rectangular, or egg-shaped), a circular section provides the maximum hydraulic mean depth for a given flow rate and cross-sectional area. Hydraulic mean depth is an important parameter in hydraulic calculations, as it influences the velocity of flow and the efficiency of sediment transport.
2. The circular sewers are provided for separate sewerage system: This statement is generally true. Circular sewers can be used in both combined and separate sewerage systems. In a separate sewerage system, separate pipes are used to convey wastewater (from households) and stormwater (from rainfall). Circular sewers are commonly used for both types of systems due to their hydraulic efficiency and structural stability.
3. The circular sewers work efficiently if the sections run at least half full: This statement is generally true. Circular sewers are most efficient when flowing at least half full. When the flow depth is less than half, the hydraulic efficiency decreases, resulting in higher friction losses and lower conveyance capacity. Therefore, designing circular sewers to run at least half full helps optimize their performance.
4. Two sewers of different shapes are said to be hydraulically equivalent if they discharge at the same rate while running full on the same grade: This statement is also correct. Hydraulically equivalent sewers are those that have different shapes but can carry the same flow rate under similar hydraulic conditions. This concept is important in sewer design and analysis, as it allows engineers to compare and select sewer shapes based on factors such as cost, construction feasibility, and hydraulic efficiency.