Diversion head works in canal engineering — identify the primary function of a river diversion headworks constructed at the canal offtake (choose the most appropriate main purpose while other roles may be secondary).

Introduction / Context:
In irrigation and water resources engineering, a diversion headworks is constructed at the offtake where a canal meets a river. Students are often asked what the main purpose of this structure is, because it performs several roles (sediment exclusion, flow regulation, etc.). Understanding the primary objective clarifies design choices such as crest level, undersluices, and regulator settings.

Given Data / Assumptions:

• The structure in question is a diversion headworks located on a river at the canal offtake.
• It may include a weir or barrage, scouring sluices (undersluices), a canal head regulator, fish ladder, and sediment excluders.
• The question asks for the principal function (the one most central to its purpose).

Concept / Approach:

The canal can only draw water reliably if the river water surface is raised sufficiently above the canal Full Supply Level (FSL). A weir/barrage within the headworks raises and stabilizes the upstream water level to provide a dependable head. While the system also helps in sediment management and flow control, those are enabling or secondary functions to the main goal of diverting water into the canal by raising the river level.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Cross-check with standard layout: the canal head regulator works effectively only when the upstream water surface is sufficiently high; undersluices fine-tune sediment and flow but do not replace the need for raised level.

Why Other Options Are Wrong:

“to remove silt” — sediment excluders/undersluices help, but this is not the main purpose.

“to control floods” — a diversion headworks can pass floods safely, but flood control is not its primary raison d’être.

“to store water” — storage is minimal (pondage), not a reservoir for regulation over long periods.

“all the above” — overstates the equally primary nature of secondary benefits; the most fundamental purpose is raising the water level.

Common Pitfalls (misconceptions, mistakes):

Confusing diversion headworks with storage dams; assuming silt control is the main goal; ignoring the central need for hydraulic head at the offtake.

Final Answer:

to raise water level