Canal Irrigation System

A canal irrigation system takes water from a river (or reservoir) and carries it by a network of canals to the fields.

 River --> HEADWORKS (weir/barrage + head regulator)
              |
         MAIN CANAL ---> escape (to river)
         /     |    \
   Branch   Branch   Branch      (cross regulator at each take-off)
      |
  Distributary ---- Distributary
      |
   Minor ---- Minor
      |
   OUTLET (module)
      |
   Watercourse (field channel) --> FIELD

Components

1. Headworks: diversion weir/barrage, under-sluices, divide wall, head regulator, fish ladder and silt excluder. They raise the water level and divert flow to the canal.
2. Main canal: the largest canal, carrying the whole supply, with a gentle slope; usually not used for direct irrigation.
3. Branch canals: take off from the main canal; carry 5-15 cumec or more.
4. Distributaries and minors: carry water to the field from the branch; minors are the smallest in the series.
5. Watercourses/field channels: carry water from the outlet to the fields (maintained by the farmers).
6. Regulating structures: head regulator, cross regulator, distributary head regulator, escape, outlets/modules.
7. Falls (drops) and cross-drainage works: reduce bed slope on steep ground, and carry the canal across streams (aqueduct, siphon, level crossing).
8. Other structures: bridges, escapes, measuring flumes, canal roads, and drains.

Main and branch canals are carrier canals; distributaries, minors and watercourses are distribution canals.

Canals are classified by alignment according to the ground they follow.

1. Watershed (ridge) canal

Aligned along the ridge line between two drainage basins. Water can flow by gravity to both sides, there is no cross-drainage work, and the whole area can be commanded.

        ridge
   fields /\ fields
    ____ /  \ ____
   command  canal  command

2. Contour canal

Runs along a contour line with a very small slope; used in hilly areas. It irrigates only the land on its lower side. It meets streams, so it needs cross-drainage works and is liable to landslides.

    hill (higher side)
   ===== canal along contour ======>
    irrigated area on one side only

3. Side-slope canal

Runs roughly at right angles to the contours, on the side slope of the ground, between a watershed and a drainage line. It meets no cross-drainage, but is liable to be damaged by runoff from the hill side, so it is protected by catch drains.

     ridge
       \  canal  (perpendicular to contours)
        \ -->
         \
      drainage line

Comparison

In plains the main canal is generally a watershed canal; in hills contour or side-slope canals are used.

A canal in partial cutting and partial filling has its bed below ground level and its banks built up above ground, using the excavated soil.

          b                       b
        |----|                  |----|
   Bank_______           _______Bank
   top /    \  FSL ~~~~~~~~~~~  /    \ outer
      /      \ ___ free board /      \ slope
 ----/--berm--\_______________/--berm--\---- G.L.
 outer         \  full supply  /
 slope          \   depth     /
                 \___________/ bed
                    bed width B

Labelled elements

1. Bed (bed width B): the bottom of the canal.
2. Full supply level (FSL) and full supply depth (FSD): the design water level and depth.
3. Side slopes: 1.5H:1V in cutting and 2H:1V in filling, typically.
4. Free board: vertical distance between FSL and the top of the bank (0.5-1.0 m).
5. Banks: embankments on both sides. Top width 1.5-3 m for ordinary banks; the service bank is 5 m or wider for inspection road.
6. Berm: horizontal strip between the toe of the bank and the edge of the canal, which provides stability and reduces seepage. In balancing it is given as 0.5-1 m wide.
7. Ground level (GL) and borrow pits/spoil banks beyond the berm.
8. Hydraulic gradient line (0.5-1.5 m below bank top) to keep it inside the bank.

When the cutting equals the filling, the section is economical (balancing depth).

Given: $B = 20$ m, cutting slope $n_1 = 1.5$ (H:V), banking slope $n_2 = 2$ (H:V), top width of bank $b = 3$ m, height of bank above ground $h = 4$ m. Both faces of each bank are taken at 2:1, and no berm is considered.

Condition: cutting area = filling area (two banks).

Filling area (both banks):

$$A_f = 2h\,(b + n_2 h) = 2\times4\times(3 + 2\times4) = 88\ \text{m}^2$$

Cutting area:

$$A_c = (B + n_1 d)\,d = (20 + 1.5d)\,d$$

Equate:

$$1.5d^2 + 20d - 88 = 0$$ $$d = \frac{-20 + \sqrt{20^2 + 4\times1.5\times88}}{2\times1.5} = \frac{-20+\sqrt{928}}{3} = 3.488\ \text{m}$$

Check: $A_c = (20+1.5\times3.488)3.488 = 88.00$ m$^2$, equal to $A_f = 88$ m$^2$.

Answer: balancing depth = 3.49 m.

Given: $B = 25$ m, $n_1 = 1.75$ (cutting), $n_2 = 2.5$ (banking), $b = 4$ m, $h = 5$ m. Both banks are built with slope 2.5:1 on both faces; no berm.

Filling area (two banks):

$$A_f = 2h\,(b + n_2 h) = 2\times5\times(4+2.5\times5) = 165\ \text{m}^2$$

Cutting area: $A_c = (25 + 1.75d)\,d$

Equate $A_c = A_f$:

$$1.75d^2 + 25d - 165 = 0$$ $$d = \frac{-25+\sqrt{25^2+4\times1.75\times165}}{2\times1.75} = \frac{-25+\sqrt{1780}}{3.5} = 4.911\ \text{m}$$

Check: $A_c = 165.00$ m$^2$ = $A_f = 165$ m$^2$.

Answer: balancing depth = 4.91 m.

Alluvial canals

Canals excavated in alluvial soil (deposited by rivers, e.g. the Gangetic and Terai plains), which is fine, loose and easily eroded. The canal carries silt, so the bed and banks can be scoured or silted. The channel must be designed for a stable regime section, using Kennedy's or Lacey's regime theory; the slope and shape adjust themselves until the canal neither silts nor scours.

Non-alluvial canals

Canals constructed in hard soil, rocky terrain or non-erodible strata, e.g. hilly and peninsular areas, where the bed and banks are stable and the canal does not carry silt that changes its shape. They are designed with Manning's or Chezy's formula using permissible velocity and rugosity, so no regime theory is needed. They may be lined to reduce seepage.

In the Terai the river carries water from the hills and the land has a gentle slope, so a gravity canal network is used. Water is diverted by a barrage or weir and carried in stages.

 River --[Barrage/Headworks]
             |
        MAIN CANAL
        /          \
   BRANCH CANAL    BRANCH CANAL
      |    \
 DISTRIBUTARY  DISTRIBUTARY
      |
    MINOR
      |
    OUTLET
      |
 WATERCOURSE / FIELD CHANNELS --> FIELDS

• Main canal: takes off at the head regulator; aligned along the ridge (watershed) as far as possible.
• Branch canals: diverted from the main canal through cross-regulators; irrigate large blocks.
• Distributaries and minors: supply 0.1-3 cumec; run roughly along the ridge between watercourses.
• Outlets: control the discharge to watercourses; modules or pipe outlets.
• Field channels: farmers' channels, with a design of rotational supply.
• Drains and cross-drainage works (aqueducts, culverts) keep the system safe from floods and waterlogging.

Canals run along ridge lines (watershed alignment) so that command is maximum.

Balancing depth is the depth of excavation (depth of the bed below ground level) at which the volume of earth cut from the canal equals the volume of earth needed to form the banks, so that no earth has to be borrowed or spoiled. It gives the most economical section.

Notation

$B$ = bed width, $d$ = depth of the bed below ground level (balancing depth), $n_1$ = side slope in cutting ($n_1$H:1V), $n_2$ = side slope of the banks, $b$ = top width of bank, $h$ = height of bank above ground level (for a bank-on-both-sides canal).

        b                      b
      |---|                  |---|
      ____                    ____
     /    \                  /    \    h
 ---/      \__    GL       __/      \---
              \            /   d
               \__________/
                    B

Derivation (per metre length of canal)

Cutting area (trapezoid of bottom width $B$, depth $d$, side slope $n_1$):

$$A_c = (B + n_1 d)\,d$$

Filling area of one bank (top width $b$, height $h$, slope $n_2$ on both faces):

$$A_f = h\,(b + n_2 h)$$

For two banks, $A_f = 2h\,(b + n_2 h)$.

For balancing, cutting = filling:

$$(B + n_1 d)\,d = 2h\,(b + n_2 h)$$ $$n_1 d^2 + B d - 2h(b+n_2 h) = 0$$ $$d = \frac{-B + \sqrt{B^2 + 8 n_1 h\,(b + n_2 h)}}{2 n_1}$$

When the bank height depends on d

For a canal of full supply depth $D$ and free board $F$, the top of the bank is at $D + F - d$ above ground level, so $h = D + F - d$. Substituting in the equation above gives a quadratic in $d$ whose smaller positive root is the balancing depth. If a service bank of top width $b_s$ is provided on one side, then fill $= h(b + n_2 h) + h(b_s + n_2 h)$.

If the actual depth is greater than $d$, earth is surplus (spoil banks are needed); if it is less, borrow pits are needed.

Components of a canal section

    spoil bank          bank          bank   borrow pit
        __              ____          ____     ___
   ____/  \____ GL ____/    \__ FSL __/    \___/   \____
                       berm  \______/ berm
                              bed

• Bed and side slopes: bottom and inclined sides of the channel.
• Full supply level (FSL) and free board: water level and the safety height above it.
• Banks: earth embankments on one or both sides; top width carries a road (service road).
• Berm: horizontal strip between the canal and the bank toe; gives stability, protects the bank from erosion and room for silting.
• Dowel (dowla): a small bank on the outer side of the road for safety and protection.
• Borrow pit: a pit from which earth is taken to build the bank when the excavation of the canal is not sufficient (canal in filling).
• Spoil bank: a heap of surplus excavated soil placed in layers along the canal when the excavated earth is more than that required for the banks (canal in deep cutting).

Requirement of borrow pit and spoil bank

• If the canal depth is less than the balancing depth, excavation is less than the filling requirement, and earth must be taken from borrow pits (placed 5-10 m or more from the toe, shallow, continuous strips outside the berm, not within the canal section, to avoid seepage and breaches).
• If the depth is greater than the balancing depth, there is surplus earth, which is dumped as a spoil bank away from the canal edge at a safe distance so that it does not slide back or block drainage.
• Both are located to avoid damage to land, with the minimum possible lead and cost.

The alignment should give maximum command at minimum cost and ensure safety. The factors are:

1. Command: keep the canal on the highest ground (watershed/ridge) so that gravity irrigation covers the maximum area, with full supply level above the field levels.
2. Topography and contours: avoid unnecessary deep cuttings and high fillings; follow the ground slope as far as possible; keep to balancing depth for economy of earthwork.
3. Cross-drainage works: minimise the number of drainage crossings (aqueducts, culverts), as each is costly.
4. Geology and soil: avoid rocky, sandy, porous, or landslide-prone areas; avoid saline or swampy ground.
5. Straight and smooth alignment: avoid sharp bends (use curves of radius at least 5 times the bed width) and avoid cutting through villages and fertile land.
6. Land acquisition and existing structures: avoid roads, houses and religious places; low compensation.
7. Head and drops: the length and bed slope should allow for the needed head and a minimum number of falls.
8. Command boundaries and field boundaries: align along property boundaries; avoid splitting farms.
9. Future extension and drainage: possibility of extension and avoidance of waterlogging.
10. Accessibility for construction and O&M and the proximity of construction material (borrow area).

Factors in fixing canal alignment

1. Maximum command: align on ridge lines so that the greatest area is irrigated by gravity.
2. Economy of earthwork: keep the section near balancing depth; avoid deep cutting and high banks.
3. Few cross-drainage works, as they are costly.
4. Good soil and geology; avoid rocks, sand, landslide zones.
5. Gentle curves, minimum length, avoiding private lands, villages and forests.
6. Needs of falls, bridges and roads; accessibility for O&M.

Types of canals and their suitability

In the Terai, perennial, unlined/lined alluvial watershed canals are common; in the hills, contour or side-slope non-alluvial lined canals are preferred.

Canal standards are the standard design values adopted for canal sections, so that canals are safe and economical: side slopes (1.5:1 in cutting, 2:1 in filling in the usual case), free board (0.5-1 m by discharge), berm width, bank top width (1.5-6 m, wider for service roads), bed width to depth ratio, permissible velocity and bed slope, curve radius, and the hydraulic gradient inside the bank.

Balancing depth is the depth of canal bed below ground at which the earth excavated equals the earth required for the banks. Then there is neither a borrow pit nor a spoil bank, so cost is least. It is found from $(B+n_1d)d =$ area of filling in banks.

Given: $B = 8$ m, full supply depth $D = 1.5$ m, cutting slope $n_1 = 1$ (1:1), banking slope $n_2 = 1.5$ (1.5:1), top width of ordinary bank $b_1 = 1.8$ m, top width of service bank $b_2 = 5.0$ m, free board $F = 0.6$ m.

Set-up. Let $d$ = depth of the bed below ground level (balancing depth). The water surface is $D - d$ above ground, and the top of the bank is at $D + F - d$ above ground:

$$h = D + F - d = 2.1 - d$$

Cutting area: $A_c = (8 + d)\,d$

Filling area (one ordinary bank and one service bank, slope 1.5:1 on both faces):

$$A_f = h(b_1 + 1.5h) + h(b_2 + 1.5h) = h(6.8 + 3h)$$

Equate with $h = 2.1 - d$:

$$\begin{aligned} d^2 + 8d &= (2.1-d)(6.8 + 6.3 - 3d)\\ d^2 + 8d &= 3d^2 - 19.4d + 27.51\\ 2d^2 - 27.4d + 27.51 &= 0 \end{aligned}$$ $$d = \frac{27.4 - \sqrt{27.4^2 - 4\times2\times27.51}}{4} = \frac{27.4-\sqrt{530.68}}{4} = 1.091\ \text{m}$$

(The larger root, $d>2.1$ m, is rejected because it gives a negative bank height.)

Check: $h = 1.009$ m; $A_c = 9.917$ m$^2$ and $A_f = 9.917$ m$^2$.

Answer: balancing depth (bed below ground level) = 1.09 m; bank height above ground = 1.01 m.

Given: $B = 8$ m, $n_1 = 1.5$ (cutting), $n_2 = 2$ (filling), crest width $b = 2$ m, bank height above ground $h = 3$ m. Banks are on both sides with 2:1 slopes on both faces; no berm.

Filling area:

$$A_f = 2h\,(b + n_2h) = 2\times3\times(2+2\times3) = 48\ \text{m}^2$$

Cutting area: $A_c = (8 + 1.5d)\,d$

Equate:

$$1.5d^2 + 8d - 48 = 0\;\Rightarrow\; d = \frac{-8+\sqrt{64+4\times1.5\times48}}{3} = \frac{-8+\sqrt{352}}{3} = 3.587\ \text{m}$$

Answer: balancing depth = 3.59 m.

Cross-section (level with the ground at 0.00):

      2.0 m                          2.0 m
   |---|                          |---|
 +3.00 ____                       ____ +3.00
      /    \                     /    \
 2:1 /      \  ground 0.00       /      \ 2:1
 ---/        \--             --/        \---
                \ 1.5:1  1.5:1/
                 \            /   depth 3.59 m
                  \__________/ -3.59
                     8.0 m bed

Bed level is 3.59 m below ground; crest of each bank is 3.0 m above ground; bed width 8 m; cutting slopes 1.5H:1V; bank slopes 2H:1V; crest width 2 m.

Irrigation canals in the hills of Nepal face steep, unstable and rocky terrain, high rainfall and small command areas, so they need special design.

1. Alignment: contour or side-slope alignment, following the ground with a gentle bed slope. Avoid landslide-prone, fractured and saturated areas; keep a safe distance from gullies; use short tunnels where needed.
2. Section and lining: small rectangular or trapezoidal, often masonry or concrete-lined, to avoid seepage and erosion. In unstable ground use pipes (HDPE/GI) or covered channels.
3. Bed slope and velocity: slopes are steeper (1 in 500 to 1 in 1000 or more), so velocity is high; check against erosion and use drops/chutes and energy dissipators to take up excess fall.
4. Cross-drainage works: many streams are crossed, so aqueducts, culverts, siphons or flumes are needed; provide catch drains above the canal for runoff.
5. Spillways and escapes: frequent escape structures to release excess flow in storms and protect the canal.
6. Sediment control: intake with a sediment trap/settling basin and flushing arrangements.
7. Retaining and breast walls on the hillside; bioengineering (grasses, shrubs) to stabilise the slope.
8. Small structures: simple, cheap, locally constructed, easy for WUA to operate and maintain.
9. Access for repair, and farmer participation; allow for lower efficiency and seasonal damage.
10. Safety: guard rails and covers at crossings.