In a gravity canal, the Full Supply Level (F.S.L.) relative to the natural ground is generally kept at what elevation to ensure economy and safety in earthwork?

Introduction / Context:
Setting the Full Supply Level (F.S.L.) correctly is crucial in canal design. Too high an F.S.L. causes excessive embankment, instability, and cost, while too low an F.S.L. increases excavation and may hinder offtake command. The standard practice in gravity canals is to keep F.S.L. slightly above the natural ground to maintain command while minimizing earthwork.

Given Data / Assumptions:

• Earth canal in reasonably uniform terrain.
• Desire for economical balance between cutting and filling.
• Need for command at outlets.

Concept / Approach:

Economical cross sections are achieved when the canal water surface lies just above natural ground, providing adequate hydraulic head for outlets without demanding tall embankments. This also mitigates overtopping risk and erosion.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals and economic analyses of canal sections consistently recommend setting F.S.L. close to ground for stable and cost-effective alignments.

Why Other Options Are Wrong:

At or below ground level: poor command and potential siltation in cut sections.
4–5 m above ground: impractical embankments and high risk.
“Several metres below” is neither economic nor hydraulic efficient for gravity supply.

Common Pitfalls:

Ignoring local topography; not accounting for freeboard and settlement in embankment design.

Final Answer:

Only a few centimetres above the ground level