Canal Design: Components of surface gravity irrigation system & Classification of canal

1.1 Components of surface gravity irrigation system:

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:

Discharge in cumec	Extent of FBL
1 to 5	0.5
5 to 10	0.6
10 to 30	0.75
30 to 50	0.9

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:

Lacey’s theory	Kennedy’s theory
i. Relation between V and R.	i. Relation between V and y.
ii. Silt factor is taken.	ii. Critical velocity is taken.
iii. Equation for bed slope is taken.	iii. Equation for bed slope is not taken.
iv. Does not involve trial and error.	iv. Trial and error method is taken.

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.