Understanding Canal Types and Irrigation

The question asks about a specific type of irrigation canal that is designed to irrigate land on only one side. This characteristic is determined by how the canal is aligned with respect to the natural topography of the land, particularly the contour lines.

Let's examine the different canal types mentioned in the options:

Analyzing Canal Types for Irrigation

• Ridge Canal (or Watershed Canal): This type of canal runs along the watershed line or the highest point of the area. Because it follows the ridge, the land slopes away on both sides. Therefore, ridge canals are typically capable of irrigating land on both sides of the canal.
• Contour Canal: A contour canal is aligned roughly parallel to the contour lines of the land. Contour lines connect points of equal elevation. When a canal follows a contour, the land on the downhill side slopes away from the canal, making it suitable for irrigation. However, the land on the uphill side (the other side) is higher in elevation than the canal water level. It is not possible to irrigate land that is higher than the water source by gravity flow. Hence, contour canals typically irrigate only on the side where the land slopes downwards, i.e., one side.
• Watershed Canal: As mentioned, this is synonymous with a Ridge canal and runs along the highest elevation line, allowing irrigation on both sides.
• Side Slope Canal: This canal runs along the natural slope of the land, often roughly perpendicular to the contour lines. These canals are usually constructed across the contours. Their primary purpose might be conveyance rather than direct irrigation along their length, or they may supply water to channels that irrigate specific areas. They don't inherently irrigate only on one side due to higher ground on the other in the same way a contour canal does.

Identifying the Canal Irrigating on One Side

Based on the analysis of how each canal type interacts with the land's elevation and slope, the canal that irrigates only on one side because the area on the other side is higher is the Contour canal. The uphill side of the contour canal is at a higher elevation, preventing irrigation on that side by gravity.

Here is a summary comparison:

Therefore, the canal type that fits the description of irrigating only on one side due to the higher elevation on the other side is the Contour canal.

Revision Table: Irrigation Canal Types

Additional Information on Canal Alignment and Irrigation

The choice of canal alignment is a critical aspect of irrigation engineering. It directly impacts the command area (the area that can be irrigated) and the cost of the canal system.

• Gravity Flow: Irrigation using canals typically relies on gravity. Water flows from a higher elevation to a lower elevation. This is why the relative elevation between the canal water level and the land surface is crucial for determining which areas can be irrigated.
• Topography: The topography of the land (its shape and elevation changes) is the primary factor influencing canal alignment. Ridge lines, valley lines, and contour lines are key topographic features considered during canal design.
• Command Area: The area commanded by a canal is the total area that can potentially receive water from the canal. The extent and shape of the command area are largely determined by the canal's alignment and the local topography.
• Cross-Drainage Works: When canals cross natural drainage lines (like streams or rivers), structures like aqueducts, syphon aqueducts, or super passages are required. Aligning canals along ridges or contours can help minimize the number and cost of such cross-drainage works. Contour canals, however, often intersect drainages, requiring many cross-drainage structures.

Understanding these principles helps in appreciating why different canal types are used in different topographic conditions and how they affect irrigation efficiency and coverage.