Hydraulic checks for sewers: During design, sewer pipes should be checked for which of the following hydraulic conditions?

Introduction / Context:
Sanitary and combined sewers must function reliably across a wide range of flows. Hydraulic checks at both ends of the operating spectrum ensure prevention of siltation at low flows and avoidance of excessive velocities or surcharge at high flows.

Given Data / Assumptions:

• Minimum flow occurs during off-peak hours or dry seasons.
• Maximum flow occurs during peak sanitary loads and/or storm events (combined systems).
• Pipe material and grade selected to meet both criteria.

Concept / Approach:
Two principal checks are required: (1) Minimum-velocity (self-cleansing) check to prevent deposition and odor problems; (2) Maximum-velocity/headroom check to prevent scouring, erosion, and structural or operational issues such as surcharge and manhole flooding.

Step-by-Step Reasoning:
At minimum flow: verify velocity ≥ self-cleansing threshold (e.g., about 0.6–0.75 m/s, depending on standards and sediments).At maximum flow: verify velocity ≤ allowable limit for the pipe material and that hydraulic grade line stays within design constraints (no unacceptable surcharge).Adjust slope/diameter as needed to satisfy both checks.

Verification / Alternative check:
Design manuals require both checks and provide recommended velocity ranges and depth-of-flow criteria for partial-full conditions in circular pipes.

Why Other Options Are Wrong:
Minimum-only or maximum-only ignores one side of the operational envelope.Structural-only check is insufficient; hydraulics govern serviceability.Average-only ignores extremes that cause most failures.

Common Pitfalls:

• Using one diameter for all reaches without confirming local grades and flows.
• Neglecting infiltration/inflow in maximum-flow checks.

Final Answer:
Both minimum and maximum flow conditions