Aqueduct hydraulics (grade aqueduct): A grade aqueduct conveying canal flow across a depression at (approximately) the canal grade is generally not permitted to run in which condition?

Introduction / Context:
A grade aqueduct carries an open-channel flow across an obstacle while maintaining free-surface behavior. Allowing it to run full converts it to a pressure conduit, which introduces uplift, structural, and control issues contrary to its intended function.

Given Data / Assumptions:

• Open-channel canal hydraulics intended (free-surface conditions).
• Grade aqueduct with vents or open top arrangements.
• Structural design not intended for internal pressure surges.

Concept / Approach:

Open-channel works must preserve a free surface to avoid converting to pressure flow. Running full can cause water hammer, increase uplift on the deck/lining, and prevent proper level control. Standard practice therefore avoids full-pipe pressurization in grade aqueducts.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals specify air vents and adequate freeboard for aqueduct barrels to prevent siphoning and pressurization, confirming the restriction on full running.

Why Other Options Are Wrong:

• (b)–(d) maintain free surface and can be designed safely.
• (e) is incorrect because one option (a) is definitively not permitted.

Common Pitfalls:

• Treating grade aqueducts like pressure mains.
• Ignoring venting and freeboard requirements.

Final Answer:

Under pressure (running full).