Final answer:
The equivalent diameter of a circular sewer flowing half-full is the same as its actual diameter, which is option B.
Step-by-step explanation:
The equivalent diameter of a circular sewer when flowing half-full is referred to as the hydraulic diameter. This diameter is crucial in determining the hydraulic radius, which is used in calculations for things like flow rate and velocity in partially full pipes. When a circular pipe is flowing half-full, its cross-sectional area of flow is half of the full pipe's area. However, because the perimeter that contributes to flow is not halved (since the semi-circular perimeter is still involved in the flow), the equivalent diameter of a circular sewer flowing half-full remains the same as its actual diameter.
Therefore, the correct answer is:
B. its diameter