Sewer hydraulics — velocities for efficient service: For efficient working of a sewer, which condition(s) should be ensured regarding velocities at minimum and maximum flows?

Introduction / Context:
Sewer performance depends on maintaining velocities that avoid deposition at low flows and prevent abrasion or structural issues at high flows. Practical velocity bands are often specified in local codes or standard practice documents.

Given Data / Assumptions:

• Minimum velocity target is stated as about 0.45 m/s at low flow.
• Maximum velocity cap is stated as about 0.90 m/s at high flow.
• Question asks which condition(s) should be ensured.

Concept / Approach:
Self-cleansing requires attaining a lower-bound velocity to keep solids in motion; conversely, an upper bound prevents scouring and damage. The problem statement embeds these target figures and asks whether both apply.

Step-by-Step Solution:
Accept the given minimum and maximum velocity criteria as design targets.Ensure the sewer gradient/diameter meets or exceeds the minimum and does not exceed the maximum.Therefore, both conditions together are required for efficient service.

Verification / Alternative check:
While many handbooks cite ~0.6–0.75 m/s minimum and up to ~3 m/s maximum depending on material, within the scope of this problem the stated limits define the acceptability band.

Why Other Options Are Wrong:
Only (a) or only (b) satisfies half the requirement and can lead to either silting or abrasion.“Neither” contradicts basic hydraulic serviceability checks.

Common Pitfalls:

• Assuming one universal pair of values; actual limits depend on pipe material and sediment.
• Ignoring partial-flow velocity behavior in circular conduits.

Final Answer:
Both (a) and (b)