Gravity conduits in water supply engineering:\r Which statements correctly describe gravity conduits used to carry water by gravity from a source to a service area?

Introduction / Context:
Gravity conduits are the backbone of many cost-effective water transmission systems. By exploiting elevation head between source and demand nodes, engineers can move large flows with minimal operational energy. This question checks the key attributes and alignments used for gravity aqueducts and pipelines in rugged terrain.

Given Data / Assumptions:

• No pumps are intended; flow is driven by elevation head.
• Design must maintain a continuous downward hydraulic grade.
• Topographic obstacles (valleys, ridges) are present along the route.

Concept / Approach:
To sustain gravity flow, the hydraulic gradient line (HGL) must remain above the pipe centerline (or channel invert) after accounting for head losses. Alignments therefore adapt to terrain using trestles (aqueduct bridges) over valleys and tunnels through ridges to reduce excessive headlosses or negative pressures. The overall aim is to preserve the gravity head and avoid cavitation or air-binding.

Step-by-Step Solution:
Recognize gravity conduits function without pumping (Option A true).They are laid to match the HGL within practical limits (Option B true).Structural solutions like trestles cross depressions (Option C true).Tunnels are used to pass through high ground efficiently (Option D true).Therefore, “All the above” is correct.

Verification / Alternative check:
Classical aqueduct systems and modern raw water conduits widely use siphons, bridges, and tunnels to maintain gravity-driven conveyance, validating the statements collectively.

Why Other Options Are Wrong:
Each single statement is correct but partial; only Option E captures the complete, typical design toolkit for gravity conduits.

Common Pitfalls:

• Confusing HGL with pipe crown level; it is the energy grade that matters.
• Underestimating headlosses at appurtenances, which can drop the HGL below the conduit.

Final Answer:
All the above