1. Head work:

• Consist of all the works to store, divert and control river water and regulate supplies into canal.

2. Canal network:

a. Main canal:

• Large capacity canal which supplies water to branch canals and the major distributaries.

b. Branch canal:

• Supply water to major and minor distributaries.

c. Major distributaries:

• Use for direct irrigation.
• Supply water to minor distributaries.

d. Minor distributaries:

• Discharge is less than 0.25 cumec.
• Supply water through outlet to water course for irrigation.

e. Water course:

• Small channel managed by farmers to take water from minor to the field.

3. Structure in canal:

a. Cross drainage structure:

• Structure constructed at the crossing of canal and natural drainage.
• Use to disposal of drainage water without interrupting the continuous canal supply.

b. Canal fall:

• Use to carry canal water below stream or drainage.

c. Cross regulator:

• Constructed across a canal to regulate the water level in canal.

d. Canal escape:

• Constructed to escape extra water from the canal into natural drain.

1.2 Classification of canal:

A. Based on their function:

1. Irrigation canal:

• Canal used to fulfill irrigation water requirement.

2. Navigation canal:

• Canal used for transportation through water.

3. Hydropower canal / power canal:

• Canal use to supply water to power house to generate electricity.

4. Feeder canal:

• Canal that supplies water to two or more canals.

5. Link canal:

• Canal that link one river with the other.

B. Based on carrying capacity:

• Main canal
• Branch canal
• Major distributary
• Minor distributary
• Water course

C. Based on alignment:

• Watershed or ridge canal
• Contour canal
• Side slope canal

D. Based on financial output:

1. Protective canal:

• To protect particular area from famine.

2. Productive canal:

• To generate revenue to the nation.

E. Based on canal surface:

1. Lined canal:

• Canal with their surface lined by impervious material like concrete, brick etc.

2. Unlined canal:

• Canal with their surface in natural condition.

F. Based on nature of source:

1. Permanent canal:

• Canal that have continuous supply from the source like lake or perennial river or  ice.

2. Inundation canal:

• Canal in which flows occurs only during the high stage of river.

1.3 Components of canal cross- section:

1. Side slope:

• Used  for stable canal.
• Depends upon type of soil.
• Steeper slope is provided in cutting.

2. Berm:

• Used for deposition of silt which acts as a lining.
• Reduce seepage.
• Protect bank from erosion.

3. Free- board:

• Margin between full supply level (FSL) and bank level.
• Depends upon size of canal:

4. Canal banks:

• Primary purpose is to retain water.
• Used for inspection and as means of communication.

5. Service road:

• Roads provided on canal for inspection purpose.
• Provided 0.4 m to 1 m above FSL (Full supply level) depending upon canal size.

6. Spoil bank:

• Constructed when earth work in excavation exceed earthwork in filling.
• Disposal of such soil may be uneconomical by mechanical means.

1.4 Alignment canal:

1. Watershed or ridge canal:

• Line separating the catchment of two streams is ridge line.
• No drainage can intersect watershed.

2. Contour canal:

• Economical than ridge canal.
• Suitable for contour farming.
• Irrigation only one side so serves small area.

3. Side slope canal:

• Canal aligned perpendicular to contour line.
• Doesn’t require cross drainage structure.

Consideration during alignment of canal:

• Alignment should not pass through valuable land, religious site, village etc.
• Alignment should be short as possible.
• Alignment should be straight as possible.
• Alignment should not involve heavy cutting or filling.
• Alignment should done such way minimum number of cross drainage are required.
• Alignment should cross natural drainage.
• Ridge or watershed is preferred so both side can be irrigated.

4.5 Canal seepage and evaporation and other losses:

1. Evaporation:

• Evaporation losses are about 2% to 3 % of total losses.
• Depeds upon meteorological factors like temperature, wind velocity and humidity.

2. Seepage:

a. Percolation:

• In percolation there exits a continuous zone of saturation from the canal to the water table.
• A direct flow is established between canal and ground water reservoir.

b. Absorption:

• In absorption a small saturated zone exits round the canal section.
• A small zone above water table is also saturated by capillary action.
• An unsaturated zone exits between these two saturated zone.

Factors on which seepage loss depends:

• Types of seepage (Percolation loss is more)
• Soil permeability (More permeable soil more seepage loss)
• Seepage through silted canal is more than from a new canal.
• More amount of silt less loss.
• More velocity less loss.
• Cross-section and wetted perimeter.

4.6 Tractive force approach in canal:

• The movement of sediment at bed is caused by a force exerted on its grains by the flowing water. The force is known as tractive force.
• Consider a grain of weight ‘w’  represented by dotted circle on the side slope and hollow circle on a horizontal bed of channel as shown in figure below.

Critical shear stress at horizontal loads = τ_L = τ_C = wtanΦ

Critical shear stress at slopping surface = τ_S

Angle of repose of soil = Φ

We know,

      (τ_S)² + (wsinθ)² = (wcosθ*tan Φ)²

or, (τ_S)² + ((τ_L * sinθ)/ tan Φ)² = ((τ_L* cosθ*tan Φ)/ tan Φ)²

or, (τ_S)² + ((τ_L * sinθ)/ tan Φ)² = (τ_L* cosθ)²

or, (τ_S / τ_L)² = cos²θ – sin²θ / tan² Φ

or, (τ_S / τ_L)² = (1- sin² θ – sin² θ / tan² Φ)

or, (τ_S / τ_L)² = (1- sin² θ (1+1/tan² Φ)

or, (τ_S / τ_L)² = (1- sin² θ (1+cot² Φ)

or, (τ_S / τ_L)² = (1- sin² θ *cosec² Φ)

or, (τ_S / τ_L)² = (1- sin² θ/sin² Φ)

∴ τ_S / τ_L = (1- sin² θ/sin² Φ)^1/2

Above equation is the required expression.

4.7 Silt theories:

Kennedy’s silt theory:

• According to Kennedy’s “regime channel” are those channels in which neither silting nor scouring takes place.
• This condition is not possible in natural channel.
• Expression of critical velocity.

V_o = 0.55y^0.64

Lacey’s regime theory:

• According to lacey a canal showing no silting and no scouring may not be in regime.
• He defined three regime condition:

1. True regime:

• Discharge is constant.
• Flow is uniform.
• Silt discharge is constant.
• Silt grade i.e. type and size of silt is constant.
• Practically not possible.

2. Initial regime:

• Bed slope of a channel varies.
• Cross- section or wetted perimeter remains unaffected.

3. Final regime:

• All variables such as perimeter, depth, slope etc. are equally free to vary and achieve permanent stability called final regime.

In such a channel:

• The coarse the silt, the flatter is the semi-ellipse.
• The finer the silt the more the section attains a semi circle.

Design of canal according to Lacey’s theory:

• Based on final regime.
• The empirical equation is:

V = (2FR/5)^1/2 ————-(i)

AF² = 140V⁵ —————-(ii)

V = 10.8 R^2/3 S^1/3————(iii)

Where,

V = Mean velocity of flow in m/s

F = Silt factor = 1.76(D_mm)^1/2

D_mm = Average size of particle in mm

A = Cross section area in m²

S = Bed slope

Relation derived from Lacey’s formula:

1. Perimeter discharge relation:

P = 4.75 (Q)^1/2

Proof:

From equation (i)

   V = (2FR/5)^1/2

or, V⁴ = 4F²R² / 25

From equation (ii)

AF² = 140V⁵

Eliminating F² from the equation and substituting R = A/P

   V⁴ = (4*140V⁵ *R²)/25A

or, V⁴ = (560V⁵ *A²)/(25*A*P²)   [∴R = A/P]

or, P² = 22.4Q

∴ P = 4.75 (Q)^1/2

2. Regime slope equation:

S = F^5/3/(3340*Q^1/6)

Proof:

From equation (iii)

V = 10.8 R^2/3 S^1/3

Cubing on both side

V³ = 1259.712 R² S

We have,

   R = 5 V² / 2F

or, R² = 25 V⁴ /4F²

Now,

      V³ = 1259.712S [(25 V⁴)/4F²]

or, S = F² / 78732V ———(iv)

Again,

V = (QF²/140)^1/6

Now, equation (iv) becomes

S = F²/[7873.2*(QF²/140)^1/6]

On simplifying we get

∴ S = F^5/3/(3340*Q^1/6)

3. Hydraulic Radius:

R = 5V²/2F

Proof:

From equation (i)

V = (2FR/5)^1/2

Squaring on both side

V² = 2FR/#

∴ R = 5V²/2F

Difference between Lacey’s theory and Kennedy’s theory:

4.8 Advantage and economics of canal lining:

Advantage of canal lining:

• Seepage control. (Treated with less permeable material)
• Prevention of water logging.
• Increase channel capacity.(Less resistance to flow of water)
• Reduction in maintenance cost.
• Elimination of flood risk.

Types of lining:

1. Hard surface lining:

• Cement concrete lining
• Brick lining
• Boulder lining

2. Earth type lining:

• Soil cement lining
• Compacted earth lining

References:

• WECS (1998), Design Guidelines for Surface Irrigation in Terai and Hills of Nepal, (Vol. I and II)
• Michael, A.M.(2011). Irrigation theory and practice
• FAO(1977). Guidelines for Predicting Crop Water Requirements. FAO Irrigation and Drainage Paper No. 24.

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