4.3. Design of Prismatic Canals

1. Introduction to Canal Design

Canal design is a crucial aspect of irrigation engineering, as it ensures the efficient transport of water from the source (such as a river or reservoir) to agricultural fields. A well-designed canal system minimizes water losses, prevents erosion, and ensures uniform water distribution. Key aspects of canal design include the design of prismatic canals, determining canal alignments, and choosing appropriate canal lining.

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2. Design of Prismatic Canals

A. Definition:

A prismatic canal has a uniform cross-section along its length, meaning the shape and size of the canal do not change. This uniformity is crucial for maintaining a consistent flow of water and simplifying the design and construction process.

B. Key Design Parameters:

1. Canal Cross-Section:

   • Shape: Typically trapezoidal, rectangular, or triangular.
   • Trapezoidal Cross-Section: Most commonly used due to its stability and ease of construction. It allows for greater water-carrying capacity and stability.

2. Side Slopes:

   • Definition: The angle of the canal's side relative to the horizontal plane.
   • Factors Influencing Side Slopes: Soil type, canal depth, and potential erosion.
   • Typical Side Slope Ratios: Commonly used side slopes are 1.5:1 to 2:1 (horizontal: vertical).

3. Bottom Width (B):

   • Definition: The width of the canal's bottom, which directly influences the water-carrying capacity.
   • Determination: Depends on the discharge requirement, canal depth, and side slope.

4. Depth of Water (D):

   • Definition: The vertical distance between the canal's bottom and the water surface.
   • Considerations: Should be sufficient to carry the designed discharge while maintaining freeboard to prevent overflow.

5. Freeboard:

   • Definition: The vertical distance between the water surface and the top edge of the canal bank.
   • Purpose: Provides a safety margin to prevent overflow during high flow conditions.

6. Longitudinal Slope (S):

   • Definition: The slope along the length of the canal, which facilitates water flow by gravity.
   • Typical Slopes: Ranges from 0.05% to 0.3%, depending on the terrain and soil conditions.

C. Design Process:

1. Estimating Discharge (Q):

   • Method: Use empirical formulas or flow measurement data to estimate the amount of water to be conveyed.
   • Example: Use the Manning’s formula to estimate discharge: $$Q = \frac{1}{n} \times A \times R^{2/3} \times S^{1/2}$$ Where:
     • $Q$ = Discharge (m³/s)
     • $n$ = Manning’s roughness coefficient
     • $A$ = Cross-sectional area of flow (m²)
     • $R$ = Hydraulic radius (m)
     • $S$ = Longitudinal slope

2. Selecting the Cross-Section:

   • Choose a trapezoidal cross-section for stability and ease of construction.
   • Determine the bottom width, depth, and side slopes to meet discharge requirements.

3. Calculating Hydraulic Radius (R):

   • For a trapezoidal canal: $$R = \frac{A}{P}$$ Where:
     • $A$ = Area of flow = $B \times D + Z \times D^2$
     • $P$ = Wetted perimeter = $B + 2D \sqrt{1+Z^2}$
     • $Z$ = Side slope ratio (horizontal)

4. Checking for Velocity and Erosion:

   • Calculate the flow velocity using: $$V = \frac{Q}{A}$$
   • Ensure the velocity is within acceptable limits to prevent erosion (typically 0.3-1.5 m/s for earthen canals).

5. Determining Freeboard:

   • Freeboard is typically 0.3-0.5 m for small canals and 0.6-1.0 m for larger canals.

D. Example Calculation:

Design a prismatic trapezoidal canal with the following parameters:

• Discharge (Q): 10 m³/s
• Manning’s roughness coefficient (n): 0.025 (for earthen canals)
• Longitudinal slope (S): 0.0001
• Side slope (Z): 2:1 (horizontal
  )

Steps:

1. Assume a depth (D): Start with $D = 2$ m (initial guess).

2. Calculate the bottom width (B) and cross-sectional area (A):

   • $B=$ m
   • $A = B \times D + Z \times D^2 = 3 \times 2 + 2 \times 2^2 = 14$ m²

3. Calculate the hydraulic radius (R):

   • $P=B+2D\sqrt{1+Z^2}=3+2\times 2\times \sqrt{1+2^2}=11.6$ m
   • $R = \frac{A}{P} = \frac{14}{11.6} =$ m

4. Calculate the flow velocity (V):

   • Using Manning’s formula: $$Q = \frac{1}{n} \times A \times R^{2/3} \times S^{1/2} = \frac{1}{0.025} \times 14 \times 1.21^{2/3} \times 0.0001^{1/2} \approx 9.8 \, \text{m}^3/\text{s}$$
   • Adjust $D$, $B$, or $S$ if needed to meet exact $Q=10$ m³/s.

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3. Canal Alignments

A. Definition:

Canal alignment refers to the horizontal layout or path of a canal on the ground. Proper alignment is crucial for minimizing construction costs, ensuring efficient water flow, and reducing maintenance needs.

B. Types of Canal Alignments:

1. Contour Alignment:

   • Follows the natural contours of the land, maintaining a consistent elevation.
   • Suitable for undulating terrain to minimize excavation and embankment.
   • Helps in minimizing erosion and waterlogging.

2. Straight Alignment:

   • Canal is laid out in a straight line, typically used in flat terrain.
   • Easier construction and maintenance but may require more land acquisition.
   • May involve cut-and-fill to maintain a uniform slope.

3. Combined Alignment:

   • Uses a combination of straight and contour alignments.
   • Balances the benefits of both methods, suitable for varied terrain.
   • Optimizes water flow while minimizing construction and maintenance costs.

C. Factors Influencing Canal Alignment:

1. Topography:

   • The natural slope and contour of the land influence the alignment.
   • Aim to follow natural ridges and avoid low-lying areas prone to flooding.

2. Soil Conditions:

   • Soil type affects the stability of canal banks and the risk of erosion.
   • Avoid areas with highly permeable soils to reduce seepage losses.

3. Land Use:

   • Consider existing land use, such as agriculture, settlements, and forests.
   • Minimize displacement of communities and avoid environmentally sensitive areas.

4. Hydrology:

   • Ensure alignment provides access to reliable water sources and minimizes the risk of flooding.
   • Consider the impact of alignment on natural drainage patterns and ecosystems.

5. Economic Factors:

   • Construction and maintenance costs influence the choice of alignment.
   • Choose routes that minimize the need for expensive structures (bridges, aqueducts) and land acquisition.

D. Example of Canal Alignment:

Indira Gandhi Canal, India:

• Follows a contour alignment to efficiently irrigate arid regions of Rajasthan.
• Minimizes excavation and embankment costs by following natural terrain.
• Aligns through areas with stable soil conditions to prevent erosion and seepage.

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4. Canal Lining

A. Definition:

Canal lining involves covering the canal bed and sides with impermeable materials to reduce water losses due to seepage, prevent erosion, and improve the efficiency of water delivery.

B. Types of Canal Lining Materials:

1. Concrete Lining:

   • Material: Portland cement concrete.
   • Advantages: Durable, low seepage rates, reduces maintenance, and prevents weed growth.
   • Disadvantages: High initial cost, requires skilled labor and equipment, potential for cracking.
   • Example: Widely used in large irrigation canals, such as the Narmada Canal in India.

2. Brick Lining:

   • Material: Burnt clay bricks laid in cement mortar.
   • Advantages: Moderate cost, locally available materials, reduces seepage.
   • Disadvantages: Prone to damage and requires maintenance, less durable than concrete.
   • Example: Used in smaller canals or where local materials are available.

3. Stone Masonry Lining:

   • Material: Stones or boulders set in cement mortar.
   • Advantages: Durable, suitable for high-velocity flow, reduces erosion.
   • Disadvantages: Labor-intensive, higher cost than earthen linings.
   • Example: Used in regions with abundant stone availability, such as hilly areas.

4. Clay Lining:

   • Material: Compacted clay soil.
   • Advantages: Low cost, natural material, effective in reducing seepage.
   • Disadvantages: Requires proper compaction, susceptible to cracking and erosion.
   • Example: Suitable for areas with high clay content and low permeability.

5. Plastic or Geomembrane Lining:

   • Material: Polyethylene or PVC sheets.
   • Advantages: Low seepage, flexible, resistant to chemicals, quick installation.
   • Disadvantages: Vulnerable to damage from animals, UV degradation, and punctures.
   • Example: Used in temporary or small-scale irrigation projects.

6. Asphalt Lining:

   • Material: Asphaltic concrete or bitumen.
   • Advantages: Flexible, reduces seepage, resistant to cracking.
   • Disadvantages: Requires specialized equipment, higher cost.
   • Example: Suitable for regions with asphalt availability and need for flexibility.

C. Benefits of Canal Lining:

1. Reduction in Seepage Losses:

   • Lining reduces water loss due to seepage, ensuring more water reaches the fields.
   • Helps conserve water resources, especially in arid regions.

2. Prevention of Erosion:

   • Lining protects canal banks and beds from erosion caused by flowing water.
   • Reduces the need for frequent maintenance and repairs.

3. Improved Water Quality:

   • Prevents the infiltration of contaminants from surrounding soil into the canal.
   • Reduces weed growth and algae, improving water quality.

4. Enhanced Flow Efficiency:

   • Lined canals have smoother surfaces, reducing friction and improving flow rates.
   • Ensures uniform water distribution to all parts of the irrigation network.

D. Example of Canal Lining:

Grand Canal, China:

• One of the oldest and longest artificial waterways, extending over 1,700 km.
• Concrete and stone masonry lining used to prevent seepage and erosion.
• Lining has helped maintain the canal's efficiency over centuries, supporting navigation and irrigation.

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5. Conclusion

• The design of prismatic canals, alignment, and lining are critical components of an efficient irrigation system.
• Proper design and alignment minimize construction costs, ensure reliable water supply, and reduce maintenance needs.
• Choosing appropriate canal lining materials helps conserve water, prevent erosion, and maintain water quality.
• By understanding these principles, engineers can design sustainable and effective canal systems to support agriculture and livelihoods.

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These lecture notes provide a comprehensive overview of the key concepts related to the design of prismatic canals, canal alignments, and canal lining. The use of real-life examples and numerical calculations helps to illustrate the practical application of these concepts in irrigation engineering.