Why circular-section gravity aqueducts are generally not preferred:\r Identify the main practical reason a circular gravity aqueduct is avoided for open-channel conveyance on ground supports.

Introduction / Context:
Section shape heavily influences both hydraulic performance and constructability of aqueducts and channels. While a circular section has attractive hydraulic properties for closed conduits under pressure, it is not the go-to shape for gravity aqueducts laid on ground or supported spans.

Given Data / Assumptions:

• “Circular gravity aqueduct” here denotes a conduit intended to flow partly full by gravity, supported on ground or piers.
• Structural and construction considerations are as important as hydraulics.
• Alternatives include rectangular, trapezoidal, horseshoe, and elliptical sections.

Concept / Approach:
A circular cross-section lacks a flat bearing surface, complicating stable support and anchorage as an open channel on ground or trestles. For open-channel conditions, shapes with flat or gently curved inverts (e.g., horseshoe or rectangular) are easier to seat, line, and maintain. Although circles have favorable area-to-wetted-perimeter ratios when flowing full, they are not optimal when partially full and when structural seating dominates design.

Step-by-Step Solution:
Compare hydraulic versus structural needs for gravity (part-full) operation.Note circular invert provides poor bearing; cradle supports become complex and costly.Hence, difficulty of providing proper support is the practical reason for avoidance.

Verification / Alternative check:
Historic and modern aqueducts commonly adopt horseshoe or rectangular sections; where circular pipes are used, they usually operate pressurized or are encased with cradles and saddles, reflecting the support concern.

Why Other Options Are Wrong:

• Hydraulic mean depth and wetted perimeter claims (A and B) favor circles when flowing full, not against them.
• Cost (C) depends on materials and supports; not inherently “always most expensive.”
• Statement (E) is too absolute; circular conduits can carry sediment-laden flows with proper velocities.

Common Pitfalls:

• Assuming the best pressure-pipe shape is also best for open-channel conveyance.
• Ignoring maintenance access and seating when selecting section geometry.

Final Answer:
Difficulty in providing proper support on the ground (unstable bearing geometry)