Gravity conduits in water conveyance: In practice, gravity conduits that carry water under free-surface or mild pressure conditions commonly take which forms?

Introduction / Context:
Long-distance water transfer schemes frequently rely on gravity conveyance to minimize energy cost. Depending on terrain and geotechnical constraints, engineers use open channels (canals), elevated or supported conduits (flumes and aqueducts), and bored passages (tunnels) to maintain hydraulic grade and deliver required flows.

Given Data / Assumptions:

• System is designed for predominantly gravity flow.
• Route crosses varied terrain including valleys and ridges.
• Hydraulic grade line must remain above the conduit invert.

Concept / Approach:
Choice of conduit type balances hydraulic efficiency, structural feasibility, right-of-way, and capital cost. Canals are economical on gentle terrain; flumes and aqueducts carry water across depressions; tunnels maintain grade through hills. All are standard parts of gravity schemes, often combined along a single alignment.

Step-by-Step Solution:
Identify terrain constraints along the alignment.Select the appropriate structure: canal, flume/aqueduct, or tunnel.Recognize that comprehensive schemes use multiple forms.

Verification / Alternative check:
Famous projects (e.g., aqueduct systems supplying cities) showcase combined use of canals, tunnels, and elevated aqueducts to keep the hydraulic grade consistently descending.

Why Other Options Are Wrong:
Each single structure alone is not universally applicable; the correct holistic answer is that all listed forms are used as gravity conduits where appropriate.

Common Pitfalls:

• Assuming pressure pipelines are mandatory; gravity works efficiently if topography permits.
• Ignoring headlosses from transitions; smooth geometry and appurtenances are key.

Final Answer:
all the above