A hydraulic structure is constructed when a Full Supply Level (FSL) of a canal is much higher than the High Flood Level (HFL) of the stream which in turn, is lower than the bottom of the canal trough. Such a structure is called as:
Aqueduct
Aqueduct: Understanding Hydraulic Structures
When designing hydraulic structures, engineers must consider the relative levels of the canal and the natural stream or drain it needs to cross. The choice of structure depends heavily on these water levels and the ground conditions. The question describes a specific scenario where the canal's water level is significantly higher than the stream's high flood level, and crucially, the stream's highest water level is still below the bottom of the canal itself.
Hydraulic Structure Selection Criteria
The type of hydraulic structure chosen for a canal and stream crossing is determined by the relative levels of their beds and water surfaces. Key levels to consider are:
- Canal Full Supply Level (FSL): The maximum operating water level in the canal.
- Canal Bed Level: The bottom level of the canal trough.
- Stream High Flood Level (HFL): The highest water level observed in the stream during floods.
- Stream Bed Level: The bottom level of the stream.
Aqueduct: The Optimal Solution
The scenario described in the question states:
- The Full Supply Level (FSL) of the canal is much higher than the High Flood Level (HFL) of the stream.
- The High Flood Level (HFL) of the stream is lower than the bottom of the canal trough.
This means the canal needs to pass over the stream, and there is sufficient vertical clearance for the stream water to flow freely underneath the canal, even during high floods. This specific arrangement is characteristic of an Aqueduct.
In an aqueduct, the canal is carried in a trough (or series of pipes) constructed above the stream. The stream flows unimpeded below the canal. This design is preferred because it ensures gravity flow in the canal is maintained without interruption, and the stream's natural flow is also preserved.
Comparing with Other Hydraulic Structures
To further understand why an aqueduct is the correct choice, let's briefly look at the other options:
| Structure Type | Description and Conditions |
|---|---|
| Super Passage | This is the opposite of an aqueduct. In a super passage, the stream (or drain) is carried over the canal. This occurs when the High Flood Level (HFL) of the stream is much higher than the Full Supply Level (FSL) of the canal, and the stream's bed level is higher than the canal's Full Supply Level. |
| Inlets | Inlets are structures designed to admit water from a stream or natural drain directly into a canal. They are not used for crossing purposes but rather for supplementing the canal's water supply or diverting floodwaters. |
| Level Crossing | A level crossing is used when a canal and a stream meet at approximately the same level. At such a crossing, the water of the canal and the stream intermingle. This typically involves a structure that allows water to be diverted or regulated, such as a regulator or a mixture of flows, which is more complex than a simple over-under arrangement described by an aqueduct or super passage. It does not fit the condition where the canal is significantly above the stream. |
Based on the defined conditions, where the canal is well above the stream's highest flood level, an aqueduct is the most suitable hydraulic structure.
Important Questions from Cross Drainage Works - Teaching
The uplift pressure on the roof of a siphon aqueduct is maximum when
When an irrigation canal is taken over a drainage channel the crossing is called
