Mock Test Mitra™
📖 Loading…
0%
0 sections · 0 topics · 0 min reading · 0 sessions

Irrigation Engineering – Complete Study Notes

GATE ESE / IES SSC JE State PSC State PWD

Comprehensive chapter-wise notes covering all 9 topics of Irrigation Engineering — necessity and types of irrigation, duty and delta, canal design (Kennedy's and Lacey's theories), distribution systems, hydraulic structures (weirs and barrages), gravity dams, waterlogging and drainage, and command area development. All key formulae, derivations, worked examples, and exam-focused tables included.

Ch 1 · Necessity & Types of Irrigation Ch 2 · Duty, Delta & Base Period Ch 3 · Canal Design — Kennedy's Theory Ch 4 · Canal Design — Lacey's Theory Ch 5 · Canal Distribution Systems Ch 6 · Weirs & Barrages Ch 7 · Gravity Dams Ch 8 · Waterlogging & Drainage Ch 9 · Command Area Development ★ Quick Revision
1Necessity & Types of Irrigation►

1.1 Necessity of Irrigation

Irrigation is the artificial application of water to land to assist the growing of agricultural crops. In India, about 60% of net sown area is rain-fed. Irrigation is necessary due to: (1) uneven spatial distribution of rainfall; (2) uneven temporal distribution — monsoon pattern; (3) deficit in soil moisture during crop growth periods.

1.2 Types of Irrigation

TypeMethodSuitability
Flow / Canal irrigationWater flows by gravity from river/reservoir through canal networkFlat terrain; large areas; perennial rivers
Lift irrigationWater pumped from river/well/tank to fieldWhere gravity flow not possible; wells, tanks
Tank / Reservoir irrigationSmall impoundments store runoff; gravity supplySouth India; hilly and undulating terrain
Drip (trickle) irrigationWater delivered directly to root zone through emittersHigh-value crops; water-scarce areas; 40–60% water saving
Sprinkler irrigationWater sprayed over crop canopy like rainfallUndulating land; sandy soils; 30–50% saving over surface
Sub-surface irrigationWater applied below soil surface through buried pipesLimited use; high-water-table areas

1.3 Soil–Water–Plant Relationships

Field Capacity (FC): maximum water held by soil after excess drains away by gravity
Permanent Wilting Point (PWP): minimum soil moisture at which plants can no longer extract water
Available Water Capacity (AWC) = FC − PWP

Depth of water required: d = (FC − θ_actual) × γ_b / γ_w × depth of root zone
γ_b = bulk density of soil; γ_w = density of water
📝 GATE Tip: Definitions of field capacity, PWP, and available moisture are directly asked. Canal irrigation accounts for the largest irrigated area in India.
2Duty, Delta & Base Period►

2.1 Definitions

TermDefinitionUnits
Duty (D)Area of land that can be irrigated by unit discharge (1 cumec) flowing continuously for the base periodhectares/cumec
Delta (Δ)Total depth of water required by a crop for its full growth (entire base period)metres
Base period (B)Total time from first to last watering of a cropdays

2.2 Relationship Between D, Δ, and B

Δ = 8.64 × B / D
or equivalently: D = 8.64 × B / Δ

Derivation: 1 cumec for B days delivers 1 × 86400 × B m³ = 86400B m³
This covers D hectares = D × 10⁴ m² to depth Δ m
So: 86400B = D × 10⁴ × Δ → Δ = 86400B / (D × 10⁴) = 8.64B/D

Crop deltas (approximate for India):
Rice (Kharif): Δ ≈ 1.0–1.2 m; base period ≈ 120–150 days
Wheat (Rabi): Δ ≈ 0.35–0.40 m; base period ≈ 120–130 days
Sugarcane: Δ ≈ 0.9–1.2 m; base period ≈ 360 days

2.3 Gross Command Area (GCA) and Net Command Area (NCA)

GCA = total area within the irrigation boundary
CCA (Culturable Command Area) = area fit for cultivation within GCA
NCA = actual area irrigated in any crop season

Intensity of irrigation = NCA / CCA (expressed as %)
Typically 40–60% for Kharif; 30–40% for Rabi

Canal discharge Q = A_ir × Δ / (B × 86400) where A_ir = area to be irrigated (m²)
📝 GATE Tip: The formula Δ = 8.64B/D is the single most tested in Irrigation Engineering. Always check units — D in ha/cumec, B in days, Δ in metres. GCA vs CCA vs NCA distinction is asked in ESE.
3Canal Design — Kennedy's Theory►

3.1 Kennedy's Silt Theory (1895)

R.G. Kennedy studied canals in Punjab and concluded that a canal is in regime (no silting or scouring) when the critical velocity equals the mean velocity of flow. Critical velocity is the velocity that keeps silt in suspension without silting or scouring.

3.2 Kennedy's Formula

V_0 = C × m × y^0.64
V_0 = critical velocity (m/s)
y = depth of flow (m)
C = 0.55 for channels in India (original value)
m = critical velocity ratio (CVR) — depends on silt grade
m = 1.0 for standard silt; m < 1 for fine silt; m > 1 for coarse silt

Design procedure:
1. Assume depth y; calculate V_0 = 0.55 × m × y^0.64
2. Compute area: A = Q / V_0
3. For trapezoidal section with side slope 1H:2V (standard): solve for B and y
4. Check with Manning's equation: V = (1/n) × R^(2/3) × S^(1/2)
5. Adjust until V = V_0 and Manning's V are consistent

3.3 Limitations of Kennedy's Theory

Does not account for bed load transport; does not define a unique section (infinite combinations of B/y satisfy the formula); does not consider bed slope directly; gives only velocity criterion, not shape of channel.

📝 GATE Tip: Kennedy's formula V_0 = 0.55 × m × y^0.64 and the CVR concept are directly asked. If m = 1 (standard silt) and y is given, V_0 is straightforward to calculate.
4Canal Design — Lacey's Theory►

4.1 Lacey's Regime Theory (1930)

Gerald Lacey recognised that a channel is in true regime when it is carrying a particular discharge with a particular silt load — no silting or scouring. He introduced the concept of silt factor f related to grain size.

4.2 Lacey's Regime Equations

Silt factor: f = 1.76 × √(m_r) [m_r = mean particle diameter in mm]
f = 1.0 for standard Kennedy silt (d = 0.323 mm)

Regime velocity: V = 0.6385 × (f × Q)^(1/6) or V = 10.8 × R^(2/3) × f^(1/3) / 16

Flow area: A = Q / V

Hydraulic radius: R = 0.4715 × (Q / f²)^(1/3)
or: V² = f × R / 2.5 → R = 2.5V²/f

Wetted perimeter: P = 4.75 × √Q

Bed slope: S = f^(5/3) / (3340 × Q^(1/6))

Channel shape: semicircular in theory; practical trapezoidal with side slope 1H:2V
Width-to-depth ratio: B/y ≈ 3.8 to 4.5 for typical channels

4.3 Comparison: Kennedy vs Lacey

FeatureKennedy's TheoryLacey's Theory
Silt parameterCVR (m) — qualitativeSilt factor f — quantified from particle size
SlopeExternal input (design assumed)Determined from regime equations
Section shapeNot uniquely definedSemi-circular (regime); unique section
Wetted perimeterNot addressedP = 4.75√Q
AccuracyLess accurateMore accurate; widely used in India
📝 GATE Tip: Lacey's P = 4.75√Q and bed slope formula are frequently tested as numericals. f = 1.76√(d_mm) is the silt factor formula — memorise it. For given Q and f, R can be found from V²=fR/2.5.
5Canal Distribution Systems►

5.1 Canal Network Hierarchy

Canal LevelTakes off fromFunction
Main canalRiver headworks / reservoirCarries bulk discharge; no direct irrigation
Branch canalMain canalFeeds distributaries; may have direct irrigation
DistributaryBranch canalCarries water to watercourses; Q typically 0.03–0.3 m³/s
MinorDistributarySmaller channel for a group of villages
WatercourseMinor / distributaryLast channel before farm; Q < 0.03 m³/s
Field channelWatercourseOn-farm; directly irrigates plots

5.2 Canal Lining

Benefits of lining: reduced seepage (saves 20–40% water), higher velocity, reduced maintenance, weed control

Types: (1) Concrete lining — most durable; (2) Boulder/stone masonry; (3) Brick lining; (4) Shotcrete; (5) Plastic film (polythene)

Seepage loss from unlined canal: 1–3 m³/s per million m² of wetted perimeter (typical India)
Seepage from lined canal: 0.1–0.3 m³/s per million m² (10× reduction)

5.3 Canal Losses

Transit losses = Seepage + Evaporation
Evaporation loss ≈ 0.25–1.0% of flow (small compared to seepage)
Seepage loss: 15–25% of water at head; 50–60% in old unlined systems

Irrigation efficiency:
Water application efficiency: η_a = water stored in root zone / water delivered at farm
Water conveyance efficiency: η_c = water delivered at farm / water released at headworks
Overall project efficiency: η = η_a × η_c (typically 35–50%)
📝 GATE Tip: Canal hierarchy (main → branch → distributary → minor → watercourse) and irrigation efficiency definitions are directly asked. Seepage is the dominant canal loss — evaporation is minor.
6Weirs & Barrages►

6.1 Definitions

A weir is a low-head dam built across a river to raise the water level for diversion, with no gates (or fixed crest). A barrage has adjustable gates spanning the full width — allows discharge control at all river stages. Barrages are preferred for modern irrigation headworks.

6.2 Bligh's Creep Theory

Bligh (1910): seepage follows the base profile of the weir.
Creep length L_c = length of seepage path along the contact between structure and foundation
Head loss: h per unit creep length = H / L_c
Safe hydraulic gradient: i_safe = 1 / C_B (C_B = Bligh's creep coefficient)
Required creep length: L_c = C_B × H

Bligh's C_B values (approximate):
Fine micaceous sand: C_B = 15; Fine sand: 12; Coarse sand: 9; Boulders/gravel: 4–6

Exit gradient is NOT addressed — Bligh assumes uniform loss.

6.3 Lane's Weighted Creep Theory

Assigns different weights to horizontal and vertical creep:
Weighted creep: L_w = (1/3) × L_H + L_V
L_H = total horizontal creep length; L_V = total vertical creep length
Safe weighted creep coefficient C_L: similar to Bligh's but uses L_w
Required: L_w / H ≥ C_L

6.4 Khosla's Theory (Seepage — Exact Solution)

Khosla (1936): uses theory of finite seepage under structures (conformal transformation)

Exit gradient: G_E = H / (d × λ) [for sheet pile at downstream end]
H = total head; d = depth of downstream pile; λ = function of α = b/d
λ = (1 + √(1 + α²)) / 2 where α = b/d (b = base width of structure)

Safe exit gradient values: Fine sand: 1/6; Medium sand: 1/5; Coarse sand: 1/4; Gravel: 1/3

Pressure at key points (Φ values) obtained from Khosla's curves or formulae for:
(a) Sheet pile at either end of impervious floor
(b) Intermediate pile (correction for mutual interference applied)
📝 GATE Tip: Bligh's creep theory and Khosla's exit gradient formula are very frequently asked. Exit gradient G_E = H/(d·λ) — must know λ formula. Khosla superseded Bligh; Bligh underestimates exit gradient.
7Gravity Dams►

7.1 Forces Acting on a Gravity Dam

ForceNatureActs at
Water pressure (FWS)Horizontal triangular; stabilising normal pressureH/3 from base
Self-weight (W)Vertical; stabilisingCG of dam cross-section
Uplift pressure (U)Vertical upward; destabilisingVaries — full at heel, tail water at toe (or zero if no drainage)
Silt pressure (F_s)Horizontal; destabilisingH_s/3 from base (H_s = silt height)
Earthquake forceHorizontal + vertical; dynamicCG of dam
Wave pressureHorizontal; from reservoir wavesNear FRL
Ice pressureHorizontal; in cold climatesNear FRL

7.2 Elementary Profile

Elementary (theoretical) profile: right-angled triangle with vertical upstream face and base width B
For no tension at any point and no overturning:
B = H × √(S_c) / (S_c − 1) approximately B ≈ H / √(S_c) for no flotation
More precisely: B ≥ H / [(S_c − 1) × √(no-tension criterion)]

For limiting stress (no tension at heel):
B = H / √(G − 1) (G = specific gravity of dam material; typically 2.4–2.5 for concrete)

7.3 Stability Criteria

1. No overturning: FOS_overturning = ΣM_R / ΣM_O ≥ 1.5
ΣM_R = sum of restoring (stabilising) moments about toe
ΣM_O = sum of overturning moments about toe

2. No sliding: FOS_sliding = μ × ΣV / ΣH ≥ 1.0 (or 1.5 with cohesion)
μ = coefficient of friction (typically 0.7–0.75 for concrete on rock)

3. No overstress (crushing):
Normal stress at base: p = ΣV/B × (1 ± 6e/B)
e = eccentricity = B/2 − x̄ (x̄ = position of resultant from toe)
For no tension: e ≤ B/6 (resultant within middle third)

4. No piping / seepage failure: exit gradient controlled by drainage gallery
📝 GATE Tip: FOS against overturning and sliding, middle-third rule (e ≤ B/6), uplift pressure distribution, and Bligh/Lane creep for foundation are the core gravity dam topics. Base stress formula p = ΣV/B × (1 ± 6e/B) is frequently asked.
8Waterlogging & Drainage►

8.1 Waterlogging

Waterlogging occurs when the water table rises to within the root zone of crops, reducing soil aeration and damaging agricultural productivity. In India, over 8.4 million ha are affected.

8.2 Causes of Waterlogging

CauseDetails
Over-irrigationMore water applied than crop need; excess percolates to water table
Canal seepageUnlined canals lose 15–30% water; raises surrounding water table
Poor natural drainageFlat topography; impermeable layers preventing downward movement
Intensive croppingYear-round irrigation keeps water table high

8.3 Measures to Prevent Waterlogging

Preventive measures:
1. Lining of canals and distributaries
2. Adoption of correct duty and delta values (efficient water use)
3. Growing crops with high consumptive use in waterlogged areas
4. Conjunctive use of surface and groundwater

Curative measures:
1. Open surface drainage — drains collect excess water
2. Sub-surface tile/pipe drainage — buried perforated pipes
3. Pumping from tube wells to lower water table

8.4 Salt Balance and Leaching Requirement

Leaching requirement (LR) = fraction of irrigation water that must percolate to control salinity
LR = ECw / (5 × ECe − ECw) [ECw = EC of irrigation water; ECe = EC of saturated soil extract at threshold]
Net irrigation requirement (NIR) = ET − effective rainfall
Gross irrigation requirement (GIR) = NIR / (1 − LR) / η_a
📝 GATE Tip: Causes of waterlogging, remedial measures, and distinction between surface and sub-surface drainage are directly asked. Canal lining is the most effective preventive measure.
9Command Area Development►

9.1 Command Area Development Programme (CADP)

The CADP (India) aims to bridge the gap between irrigation potential created and utilised. Key components: field channel construction, land levelling and shaping, on-farm development, and conjunctive use of surface and groundwater.

9.2 Warabandi System

Warabandi is a rotational water distribution system used in north-west India (Punjab, Haryana). Each landowner gets water for a fixed duration (in proportion to land area) once a week on a scheduled roster. Reduces disputes and ensures equitable distribution.

Duration of turn (hours/week) = 168 × A_i / A_total
A_i = land holding of farmer i; A_total = total area served by the outlet

9.3 Irrigation Project Classification

CategoryCulturable Command Area (CCA)
Major irrigation projectCCA > 10,000 ha
Medium irrigation project2,000 ha ≤ CCA ≤ 10,000 ha
Minor irrigation projectCCA < 2,000 ha
📝 GATE Tip: Major/medium/minor project classification by CCA is frequently asked. Warabandi formula for turn duration is occasionally tested in ESE.
★Quick Revision — Key Formulae►
TopicKey Formula / Value
Duty–Delta–Base periodΔ = 8.64B/D (Δ in m, B in days, D in ha/cumec)
Kennedy's critical velocityV_0 = 0.55 × m × y^0.64
Lacey's silt factorf = 1.76√(d_mm)
Lacey's regime velocityV² = fR/2.5
Lacey's wetted perimeterP = 4.75√Q
Lacey's bed slopeS = f^(5/3) / (3340Q^(1/6))
Bligh's creepL_c = C_B × H
Khosla exit gradientG_E = H/(d·λ); λ = (1+√(1+α²))/2
Dam — no tensione ≤ B/6 (middle-third rule)
Dam — FOS overturningΣM_R / ΣM_O ≥ 1.5
Dam — FOS slidingμΣV/ΣH ≥ 1.0
Project classificationMajor: CCA > 10,000 ha; Minor: CCA < 2,000 ha