Canal control works: In a main canal, what is the primary functional purpose of a cross regulator?

Introduction / Context:
Cross regulators are gated control structures provided in the body of a main canal. They play a crucial role in maintaining pond levels that allow distributary head regulators to receive their intended discharge, particularly when the parent canal is running with reduced supplies.

Given Data / Assumptions:

• Main canal with offtaking distributaries.
• Variable supply conditions across seasons.
• No special measuring flume specified at the cross regulator site.

Concept / Approach:
By partially closing gates, a cross regulator increases the upstream water level (creates a pond), ensuring sufficient head at the distributary head regulators. This helps maintain design discharge into distributaries and stabilizes flow division during low-supply periods.

Step-by-Step Solution:
1) Identify the control need: maintain sufficient head for offtakes.2) Provide a gated cross regulator in the main canal.3) Operate gates to raise upstream level during low flows, enabling accurate offtake regulation.

Verification / Alternative check:
Operation manuals commonly pair cross regulators (to set pond level) with distributary head regulators (to set offtake discharge). Escapes and measuring flumes are separate structures for different functions.

Why Other Options Are Wrong:

• Measuring discharge: dedicated flumes or measuring structures are used instead.
• Passing excessive flows: that is the job of canal escapes or surplus weirs.
• Permanently closing or desilting by vortex: not the cross regulator’s primary function.

Common Pitfalls:

• Confusing cross regulators with head regulators and escapes.
• Assuming regulators are primarily for measurement rather than level control.

Final Answer:
To raise and control the upstream water level so that distributaries can draw their design discharge during low supplies.