Identifying Canal Drainage Structures: The Aqueduct

The question describes a specific type of hydraulic structure used in civil engineering, particularly in irrigation and water management. It asks to identify the structure where a canal passes over a natural drainage path (like a stream or drain), and crucially, the drainage water's Highest Flood Level (HFL) is positioned significantly lower than the bottom level of the canal structure. This positioning allows the drainage water to flow naturally using gravity.

Detailed Explanation of Options

Let's analyze the different types of cross-drainage structures:

1. Aqueduct: An aqueduct is a structure where the canal (waterway) is carried *over* the natural drainage path (roadway or drain). The condition specified in the question—where the HFL of the drain is below the bottom of the canal, enabling gravity flow for the drain—perfectly matches the definition and functional requirement of an aqueduct. The canal water flows in a trough or channel supported by piers or arches above the drain.

2. Super-passage: This is the inverse of an aqueduct. Here, the natural drainage path or drain is carried *over* the canal. The canal water flows below the drain.

3. Level Crossing: In a level crossing, the canal and the drainage path intersect each other at the same natural ground level. This requires careful management of water flow in both the canal and the drain, often involving regulators, and does not fit the description of free gravity flow for the drain below the canal structure.

4. None of the above: Since the description accurately defines an aqueduct, this option is incorrect.

Why the Canal Structure is an Aqueduct

The key factors from the question that point to an aqueduct are:

• The canal passes *over* the drain (implied by the relative levels).
• The Highest Flood Level (HFL) of the drain is *below* the bottom of the canal. The notation for Highest Flood Level is often represented as HFL.
• The drainage water flows *freely under gravity*.

These conditions are characteristic of an aqueduct, ensuring the canal operates without interference from the drain's floodwaters and the drain maintains its natural flow regime.