Understanding Cross Drainage Works in Civil Engineering

A cross drainage work is a structure constructed where an irrigation canal and a natural drainage (like a river or stream) intersect. These structures are essential in civil engineering to allow the smooth flow of water in both the canal and the drainage without them interfering with each other. Different types of cross drainage works are built depending on the relative levels of the canal bed, canal full supply level (FSL), drainage bed, and drainage high flood level (HFL), as well as the quantity of water carried by each.

Let's examine the specific type of cross drainage work described in the question: a structure where a canal is provided over the natural drainage.

Analyzing the Options for Cross Drainage Works

We need to identify which of the given options matches the description of a canal constructed over a natural drainage.

• Super-passage: In a super-passage, the natural drainage is carried over the irrigation canal. This is the opposite configuration to what is asked in the question. Typically, this type is chosen when the canal bed level is lower than the drainage bed level, and the canal's FSL is below the drainage's HFL.
• Canal Syphon: A canal syphon is a type of structure where the irrigation canal is taken below the natural drainage. The canal section is depressed, and it works on syphon action under the drainage. This arrangement is used when the drainage bed level is much higher than the canal bed level and the drainage HFL is considerably above the canal FSL.
• Level Crossing: A level crossing is constructed when the bed levels of the canal and the drainage are practically the same, and their full supply levels are also close to each other. At a level crossing, the canal and the drainage intersect at the same level, requiring regulators on both to manage flows. This does not involve one being built over the other in a continuous structure.
• Aqueduct: An aqueduct is a cross drainage work in which the irrigation canal is carried over the natural drainage. This perfectly matches the description given in the question. Aqueducts are constructed when the canal bed level is higher than the drainage bed level, and the canal's FSL is well above the drainage's HFL. The canal flows in a trough supported by piers over the drainage channel.

Identifying the Correct Cross Drainage Structure

Based on the definitions:

• A super-passage has the drainage over the canal.
• A canal syphon has the canal under the drainage.
• A level crossing has the canal and drainage at the same level.
• An aqueduct has the canal over the drainage.

The question asks for a cross drainage work where a canal is provided over the natural drainage. This configuration is specifically known as an aqueduct.

Therefore, the cross drainage work where a canal is provided over the natural drainage is called an aqueduct.

Revision Table: Cross Drainage Works

Additional Information on Cross Drainage Works Selection

The selection of the appropriate type of cross drainage work depends on several factors, including:

• Relative bed levels and water levels (FSL of canal, HFL of drainage).
• Discharges of the canal and the drainage.
• Foundation conditions and soil type.
• Economic considerations and cost of construction.
• Operational requirements and maintenance.

Aqueducts are common when the canal needs to maintain its level across a lower-lying drainage channel. Super-passages are used in the opposite scenario. Syphons or canal syphons are employed when the canal has to cross beneath a drainage, often to avoid large embankments or cuttings.