Understanding Canal Head Structures and Flood Prevention

Canal headworks are important structures built at the starting point of a canal to divert water from a river or reservoir into the canal. These headworks serve multiple purposes, including regulating the water supply, controlling sediment entry, and preventing floodwater from damaging the canal system.

The question asks about the structure specifically designed to prevent river floods from entering the canal and to regulate the water supply. Let's look at the provided options:

• Artificial cut-offs: These are artificial channels created in rivers to shorten the flow path, often used for navigation or flood control along the river course itself, not primarily at the canal head for regulation or flood prevention into the canal.
• Divide wall: A divide wall is constructed parallel to the barrage axis to separate the under-sluice portion from the main weir portion. It helps in creating a relatively still pocket of water near the canal head regulator to promote sediment settling, but it's not the main structure for preventing flood entry into the canal.
• Head Sluices: Head Sluices are gates or shutters provided at the mouth of the canal head regulator. These gates control the amount of water entering the canal from the river. During periods of high flood, the head sluice gates are closed to prevent the excessive floodwater and sediment load from entering and damaging the canal. They can also be partially opened or closed to regulate the required supply of water into the canal under normal flow conditions. This function perfectly matches the description given in the question.
• Fish Ladder: A fish ladder is a structure designed to allow fish to bypass obstacles in a river, such as dams or weirs, enabling them to migrate upstream. It has no role in regulating water supply into a canal or preventing flood entry.

Based on the functions described, the structure responsible for preventing river floodwater from entering the canal and regulating the water supply is the Head Sluices.

Head sluices are crucial components of a canal head regulator system. They act as a control gate, ensuring that only the required amount of water enters the canal and providing a barrier against unwanted flood surges and debris.