Canal Alignment in Irrigation Projects – As per IS 5968
Introduction
Effective planning of canal alignment is critical in irrigation project design. It ensures not only the economic viability of the project but also the operational efficiency and sustainability of the system. The Indian Standard IS 5968 provides detailed guidelines on how to determine the alignment of canals, covering aspects such as topographic considerations, watershed dynamics, construction feasibility, and safety.
1. Survey and Mapping
Before alignment planning, survey maps should be prepared at a preferred scale of 1:15,000, displaying contours, spot levels, and important land features across the full area of development. These maps are foundational for the logical positioning of canals and associated structures.
2. Preliminary Canal Alignment
2.1 Tentative Layout
- Main canals, branch canals, and distributaries should be tentatively marked on the survey maps.
- The main canal is typically aligned as a contour canal, following elevation lines to serve the maximum area.
- Branches and distributaries are generally aligned along watershed lines or side slopes.
3. Main Canal Alignment Principles
3.1 Initial Contour Alignment
The main canal should initially follow a contour alignment to maximize command and reduce structural complexity, continuing until:
- It commands the entire designated irrigation area, or
- It reaches the top of a watershed.
3.2 Transition to Watershed
Once the watershed top is attained, the canal alignment should shift down along the main watershed line, transitioning from a contour canal to a watershed canal.
4. Branch and Distributary Alignment
- Branch canals and distributaries should originate near the points where the main canal intersects the watershed.
- They should follow secondary watershed ridges to efficiently irrigate adjacent areas.
This ensures efficient gravity flow, full coverage, and minimal excavation.
5. Economic Considerations in Alignment
5.1 Comparative Cost Evaluation
In upper reaches, multiple alternative alignments should be studied and compared based on:
- Cost of excavation and earthworks
- Land acquisition and usage
- Risk of seepage and failures
- Structural complexity and operational risk
5.2 Avoidance of Deep Cuttings or High Embankments
Avoid alignments requiring excessive earthwork. Where unavoidable, consider detouring after a cost-benefit analysis. High embankments also pose seepage risks and structural hazards.
6. Drainage Crossings
Canal alignments should be chosen to cross the least number of natural drainage lines. Where crossings are necessary, special structures like aqueducts or siphons must be considered.
7. Geometric Design
7.1 Canal Curves
Canal alignments should consist of straight sections connected with circular curves. The radius of these curves should typically be 3 to 7 times the water surface width, based on IS 5968 specifications.
7.2 Spacing of Distributaries
Distributary canals should be spaced based on the topography and water demand of the region to optimize coverage and delivery.
Conclusion
Canal alignment is both a technical and strategic task, requiring topographical analysis, economic planning, and engineering judgment. Following IS 5968 ensures:
- Efficient irrigation command
- Low construction and maintenance costs
- Long-term system resilience and safety
Proper planning, combined with field surveys and economic evaluation, leads to a reliable and effective irrigation network.
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