Explanation:
When comparing non-circular and circular sewers, it can be said that they are hydraulically equal when the hydraulic mean depths are the same. This means that option 'B' is the correct answer.

Hydraulic Mean Depth:
Hydraulic mean depth (HMD) is a measure of the efficiency of flow in an open channel or a pipe. It is defined as the ratio of the cross-sectional area of flow to the wetted perimeter. HMD is an important parameter in determining the flow characteristics of a sewer.

Comparison of Non-Circular and Circular Sewers:
Non-circular sewers, such as rectangular or trapezoidal channels, have different shapes compared to circular sewers. The shape of the sewer affects its hydraulic characteristics.

Cross-Sectional Areas:
- The cross-sectional area of a sewer is the area of the flow section perpendicular to the flow direction.
- In non-circular sewers, the cross-sectional area can be different from that of circular sewers with the same discharge.
- Therefore, option 'A' is not correct as the cross-sectional areas of the two sewers can be different.

Discharge Velocities:
- Discharge velocity is the speed at which the flow is moving through the sewer.
- It is determined by the cross-sectional area and the discharge of the sewer.
- In non-circular sewers, the shape affects the flow pattern and velocity distribution.
- Therefore, option 'C' is not correct as the discharge velocities of the two sewers can be different.

Discharge:
- Discharge is the volume of water passing through a section of the sewer per unit time.
- In non-circular sewers, the shape affects the flow pattern and can result in different discharges.
- Therefore, option 'D' is not correct as the discharges of the two sewers can be different.

Hydraulic Mean Depths:
- Hydraulic mean depth is a measure of the average depth of flow in a sewer.
- It is calculated by dividing the cross-sectional area of flow by the wetted perimeter.
- The hydraulic mean depth takes into account both the cross-sectional area and the wetted perimeter of the sewer.
- When the hydraulic mean depths of non-circular and circular sewers are the same, it indicates that the flow efficiency is the same.
- Therefore, option 'B' is correct as the hydraulic mean depths being the same indicates hydraulic equality between the two sewers.

Conclusion:
In conclusion, non-circular and circular sewers are hydraulically equal when their hydraulic mean depths are the same. This is because the hydraulic mean depth takes into account both the cross-sectional area and the wetted perimeter of the sewer, providing a more accurate measure of flow efficiency.