For cement concrete sewers, select the acceptable range of non-scouring (safe maximum) velocity so that the pipe surface is not eroded by the flowing sewage.

Introduction / Context:
Designing a gravity sewer requires keeping velocities within a safe band. The lower bound (self-cleansing velocity) prevents sediment deposition, and the upper bound (non-scouring or safe maximum velocity) prevents erosion of the pipe material. This problem asks about the permissible upper range for cement concrete sewers.

Given Data / Assumptions:

• Material: cement concrete (plain or reinforced cement concrete).
• Flowing fluid: municipal sewage containing grit.
• Objective: avoid abrasion and erosion of the concrete lining by keeping maximum velocity below the scouring threshold.

Concept / Approach:
Non-scouring velocity is the maximum velocity that a sewer can safely withstand without damaging its interior surface. For concrete, grit-laden flows can abrade the matrix at high velocities, so design manuals specify a safe range. Typical practice places concrete's upper safe velocity between about 3.5 m/sec and 4.5 m/sec, above which erosion risk rises sharply, especially during peak wet-weather flows.

Step-by-Step Solution:
1) Identify the material-specific scouring limit: concrete has a moderate resistance to abrasion compared to cast iron or ductile iron.2) Compare the provided velocity ranges with commonly accepted limits.3) The band 3.5 to 4.5 m/sec is widely cited as the safe upper range for cement concrete sewers.4) Select the option that matches this safe non-scouring range.

Verification / Alternative check:
As a cross-check, note that minimum self-cleansing velocities for sanitary sewers are typically around 0.6 to 0.9 m/sec, much lower than the scouring limit. The correct option represents the upper bound, not the lower bound, which aligns with standard design practice for concrete conduits.

Why Other Options Are Wrong:

• 2.5 to 3.0 m/sec: Too low for the non-scouring limit of concrete; usually still safe but unnecessarily restrictive as a maximum.
• 3.0 to 4.0 m/sec: Lower portion overlaps safe range but does not capture the commonly accepted upper bound of approximately 4.5 m/sec.
• 4.5 to 5.5 m/sec.: Velocities above about 4.5 m/sec risk abrasion and surface damage in concrete sewers.
• 2.0 to 2.5 m/sec: Far below typical upper limits; may be used for other materials or conservative designs, but not the standard concrete scouring threshold.

Common Pitfalls:

• Mixing up self-cleansing (minimum) velocity with non-scouring (maximum) velocity.
• Ignoring that grit concentration during storms increases abrasion risk, justifying a realistic but safe upper design limit.

Final Answer:
3.5 to 4.5 m/sec