Canal offtake level setting — the Full Supply Level (FSL) of a canal at its head relative to the parent channel is generally kept:

Introduction / Context:
Proper setting of the canal Full Supply Level (FSL) at the head regulator ensures smooth entry and control of discharge. A modest positive head is desirable to avoid draw-down issues and to operate control gates within a workable range even as river stages fluctuate.

Given Data / Assumptions:

• Canal offtake from a parent (main) channel or river with a head regulator.
• Standard practice seeks a small positive head at offtake.
• Typical rule-of-thumb values are used in preliminary design.

Concept / Approach:

A small elevation difference (on the order of a few decimetres) between the canal FSL and the parent channel water level at the offtake permits stable and controllable inflow. Common practice adopts about 0.15 m (15 cm) higher FSL for the canal at its head to facilitate regulation and avoid sediment draw-in due to excessive suction.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Hydraulic model studies often confirm that a modest positive head improves control; too large a difference risks sediment ingress and approach disturbances.

Why Other Options Are Wrong:

“same level” or “lower” reduce controllability; “30 cm higher/lower” are outside the commonly adopted preliminary setting.

Common Pitfalls (misconceptions, mistakes):

Confusing canal bed level with canal FSL; assuming larger head is always better for entry.

Final Answer:

15 cm higher