Irrigation and Drainage

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This section covers crop and irrigation water requirements, command area concepts, duty and delta, irrigation efficiencies, effective rainfall, the soil-moisture-irrigation relationship, and design discharge for canals.

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Crop & Irrigation Water Requirements Crop water requirement (CWR) is the total water needed by a crop over its growth period, dominated by evapotranspiration, plus any special needs (e.g. land preparation/puddling for paddy).

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Irrigation water requirement (IWR) is the water that must be supplied by irrigation, obtained by subtracting effective rainfall and any contribution from stored soil moisture/groundwater from the crop water requirement, and then accounting for application/conveyance losses.

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Water availability for irrigation depends on the source's discharge and its seasonal variation, which must be compared against the crop calendar's water demand to assess adequacy.

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Command Area & Irrigation Intensity Term Meaning Gross command area (GCA) The total area that can physically be irrigated by a canal system, including cultivable and uncultivable land Culturable command area (CCA) The portion of the GCA that is culturable (fit for cultivation), excluding unculturable land (e.g. ponds, forests, built-up areas) Irrigation intensity The ratio of the area actually irrigated in a given season to the culturable command area, expressed as a percentage; can exceed 100% where multiple cropping (more than one crop per year on the same land) occurs Duty, Delta & Their Relationship Duty (D) is the area of land (in hectares) that can be irrigated by continuous flow of one cumec of water throughout the crop's base period, reflecting the efficiency of water use (a higher duty means less water is needed per unit area).

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Delta (Δ) is the total depth of water (in metres) required by a crop over its entire base period, obtained by summing all individual irrigation depths applied during that period.

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Base period (B) is the time (in days) from the first watering at sowing to the last watering before harvest.

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Relationship: Δ = 8.64B/D (Δ in metres, B in days, D in hectares/cumec) — a higher duty corresponds to a smaller delta for a given base period, and vice versa.

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Losses, Efficiencies, Effective Rainfall & Soil Moisture Water losses occur during conveyance (seepage and evaporation in canals/channels) and during field application (deep percolation beyond the root zone, surface runoff from the field).

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Irrigation efficiencies: conveyance efficiency (water delivered to the field / water released at the source), application efficiency (water stored in the root zone / water delivered to the field), and overall (project) efficiency (the product of the individual efficiencies along the path from source to crop root zone).

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Effective rainfall is the portion of total rainfall that is actually available to and used by the crop (excluding the portion lost to surface runoff and deep percolation beyond the root zone).

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The soil-moisture-irrigation relationship involves field capacity (maximum moisture soil can hold against gravity after free drainage), permanent wilting point (moisture level below which plants can no longer extract water and wilt permanently), and available moisture (the range between field capacity and wilting point usable by the crop, with irrigation typically applied once a “readily available” sub-portion of this range is depleted).

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Depth and frequency of irrigation are interrelated: a deeper irrigation depth allows a longer interval (lower frequency) between waterings (suited to deep-rooted crops/soils with high water-holding capacity), while a shallow depth requires more frequent irrigation (suited to shallow-rooted crops or sandy soils with lower water-holding capacity).

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Design discharge for canals is determined from the peak crop water demand across the command area (considering the cropping pattern and its most water-demanding period), adjusted for conveyance losses, so the canal can reliably deliver the required discharge to the full command area at peak demand.

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This section covers canal types, network and alignment principles, the tractive force approach to canal design, alluvial canal design via Kennedy's and Lacey's theories, and lined canals.

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Canal Types, Network & Alignment Canals are classified by function (irrigation, power, navigation canals), by alignment (contour/ridge canal — aligned along a ridge/watershed line so it can command land on both sides by gravity without crossing drainage; side-slope canal — aligned along the general slope of the land, roughly parallel to the natural drainage, commanding land on one side only and often needing cross-drainage works), and by discharge/hierarchy (main canal, branch canal, distributary, minor, watercourse, in decreasing order of discharge).

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Alignment along a ridge/watershed line is generally preferred where feasible since it avoids or minimizes the number of cross-drainage structures (which are costly), and maximizes the area that can be commanded by gravity flow.

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Tractive Force Approach The tractive force (tractive stress) approach designs a stable, non-erodible canal section by analysing the distribution of shear stress (tractive force per unit area) exerted by flowing water on the bed and sides of the channel, ensuring this shear stress nowhere exceeds the critical tractive stress the channel boundary material can resist without eroding — a more fundamentally hydraulics-based method than the empirical regime theories.

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Alluvial Canals — Kennedy's & Lacey's Theories Theory Key Concepts Kennedy's theory Based on the concept of “critical velocity” — the velocity at which the canal neither silts (deposits sediment) nor scours (erodes its bed); silt is kept in suspension by vertical eddies generated by friction along the bed.

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Vo = 0.55 m y0.64 (y = depth of flow in metres, m = critical velocity ratio, a factor depending on silt grade).

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Does not directly address the canal's side slopes/width, and requires a separate assumption for the section's bed width-to-depth ratio Lacey's theory The “regime theory”: assumes a channel in true regime has reached equilibrium in bed slope, wetted perimeter, and velocity for a given discharge and silt grade, with both the bed and sides in a state of dynamic balance (not just the bed, as in Kennedy's theory).

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P (m) = 4.75√Q (Q in cumecs).

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Uses a silt factor (f) reflecting the average particle size of the bed/boundary material Lined Canals A lined canal has an impermeable lining (commonly concrete, masonry, or a geomembrane) along its bed and sides, substantially reducing seepage losses compared to an unlined (earthen) canal.

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Because the lining resists erosion, a lined canal can be designed for a higher permissible velocity than an unlined alluvial canal, allowing a smaller cross-section for a given discharge; lining involves a higher initial construction cost but generally lower long-term water losses and maintenance (e.g. reduced weed growth, reduced risk of breaches).

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This section covers the components of a diversion headwork, seepage theories (Bligh's, Lane's, Khosla's), silt control structures, and the design of weirs/barrages and energy dissipators.

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Components of a Diversion Headwork A diversion headwork typically comprises: the weir/barrage (raises the river's water level to divert flow into the canal), undersluices (maintain a deep clear-water channel near the canal head regulator and help flush sediment/pass low flows), a divide wall (separates the undersluice bay from the main weir bay, ensuring smooth flow conditions near the undersluices), a fish ladder (allows fish migration past the barrier), the canal head regulator (controls discharge admitted into the canal and prevents excessive silt/coarse sediment entry), silt excluder/ejector arrangements, and river training works (e.g. guide banks) to guide flow properly toward the structure.

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Seepage Theories Theory Key Idea Bligh's creep theory Assumes seepage water “creeps” along the contact profile of the impervious floor and any sheet piles, losing head uniformly per unit length of this creep path, regardless of whether the path is horizontal or vertical; safety against piping requires the total creep length to be adequate relative to the applied head (via an empirical creep coefficient).

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A simple but conservative/approximate method that does not distinguish the differing effectiveness of horizontal vs. vertical creep Lane's weighted creep theory An improvement on Bligh's theory: assumes vertical (or steeply inclined) creep is more effective at dissipating head than horizontal creep, so vertical creep length is given a higher weight (roughly three times) than horizontal creep length when computing the total weighted creep length against the applied head Khosla's theory The most rigorous of the three: applies potential flow theory (using the method of independent elementary profiles, combined via superposition/correction) to determine the actual uplift pressure distribution beneath the impervious floor and the exit gradient at the downstream end of the floor; safety requires the exit gradient to remain below a safe/critical value for the founding soil, and the floor thickness at each point is designed to resist the computed uplift pressure there Silt Control Structures A silt excluder is located in the river bed upstream of the canal head regulator; it diverts the silt-laden bottom layers of river flow back into the river (away from the canal off-take) before they can enter the canal, so it works on excluding silt before entry.

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A silt ejector is located a short distance downstream of the head regulator, within the canal itself; it extracts the silt-laden bottom layer of flow that has already entered the canal and returns it to the river via a separate escape channel, so it works on removing silt after entry.

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Settling basins are enlarged sections of channel where flow velocity is deliberately reduced so suspended silt settles out; the deposited silt is then periodically flushed out.

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Design of Weir/Barrage & Energy Dissipators The crest level of a weir/barrage is set to provide the required pond level (upstream water level) for diverting the design discharge into the canal, considering permissible afflux (rise in upstream water level caused by the structure); the waterway (length) is commonly determined using Lacey's regime perimeter relationship; the thickness of the impervious floor at each section is designed to resist the uplift pressure computed via Khosla's theory, with an adequate margin of safety provided by the floor's self-weight.

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Energy dissipators (commonly a hydraulic-jump stilling basin) are provided downstream of a weir/barrage/regulator to safely dissipate the excess kinetic energy of the high-velocity, supercritical flow leaving the structure before it re-enters the natural channel, typically using a depressed cistern, friction/chute blocks, and an end sill, with the basin's design based on the Froude number of the incoming flow.

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This section covers river stages and the need for river training, the design of guide bunds, launching aprons, levees and spurs, and the principles of watershed management.

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River Stages & Need for River Training Rivers can be broadly characterized by their planform/behaviour as straight, meandering (developing sinuous bends that migrate over time), or braided (dividing into multiple interlacing channels separated by sand/gravel bars, typical of rivers carrying heavy sediment loads with erodible banks and variable discharge).

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River training is needed to control a river's course and behaviour near structures (bridges, barrages, intakes), to confine flow within a defined waterway, to protect banks and adjoining land/structures from erosion and outflanking, and to maintain a stable, adequately aligned channel approach to hydraulic structures.

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Guide Bunds & Launching Aprons A guide bund (guide bank) is an earthen embankment (typically with stone pitching for protection) constructed to guide and confine flood flow towards the designed waterway of a bridge/barrage, preventing the river from outflanking the structure; guide bunds are commonly built in pairs (one on each bank), often bell-mouthed/curved in plan at the upstream end.

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A launching apron is a flexible apron of loose stone (or similar material) laid on the riverbed at the toe of a guide bund/spur; as the riverbed scours during high flows, the apron progressively “launches” (tumbles down and self-adjusts) to form a protective, stable slope (commonly around 2 horizontal :

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1 vertical) that continues to protect the toe of the embankment against undermining as scour progresses.

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Levees & Spurs Structure Function Levees Embankments constructed roughly parallel to a river's banks to confine flood flow within a defined channel width, protecting adjoining low-lying land from inundation during high flows Spurs (groynes) Structures projecting from the riverbank into the flow, used to deflect flow away from the bank, induce silt deposition near the bank, and protect the bank from erosion; classified by orientation/function as attracting, repelling, or deflecting spurs, depending on whether they draw flow toward, push flow away from, or simply redirect flow near a bank Watershed Management Watershed management takes an integrated, catchment-scale approach to controlling runoff, erosion, and sediment production at their source (rather than only treating symptoms downstream at the river/structure), using measures such as soil conservation practices, afforestation, contour bunding, and check dams; effective watershed management reduces sediment loads and peak flows reaching the river, complementing direct river training works.

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This section covers the functions and design of canal regulators and escapes, pipe outlets, vertical drops, and the different types of cross-drainage structures.

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Regulators & Escapes A head regulator is located at the off-take point of a canal/distributary from its parent channel, controlling the discharge admitted into the off-taking channel and helping to exclude excessive silt/coarse sediment.

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A cross regulator is built across the parent canal (typically just downstream of an off-take), maintaining an adequate upstream water level so the off-taking channel(s) can draw their required discharge, especially important when the parent canal's discharge is reduced downstream of the off-take.

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Both regulator types are hydraulically designed in a manner similar to a small weir/barrage: crest level (to pass the required discharge at the design water level), waterway/length (adequate for the design discharge), and impervious floor thickness (to resist uplift pressure, via seepage theory such as Khosla's), all sized for the regulator's specific discharge and head conditions.

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An escape is a structure that allows surplus or excess water in a canal (e.g. from unanticipated rainfall inflow, operational adjustments, or emergency conditions) to be safely diverted out of the canal into a natural drain or river, protecting the canal banks from overtopping/breaching.

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Pipe Outlets & Vertical Drops A pipe outlet delivers water from a distributary/minor canal into a watercourse via a pipe passing through the parent channel's bank; it may be free (unsubmerged) — discharging freely with the outlet unaffected by the downstream watercourse's water level — or submerged — where the discharge is influenced by the downstream water level in the watercourse, requiring the outlet's design/rating to account for this backwater effect.

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A vertical drop structure is provided where a canal must drop to a lower bed level over a short distance (e.g. due to a sudden change in ground slope); its design is similar in principle to a small weir, with crest level, length, and impervious floor thickness determined analogously, and typically includes a stilling arrangement (cistern) downstream to dissipate the energy of the falling water.

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Cross-Drainage Structures Structure Relative Levels / Arrangement Aqueduct The canal passes over the natural drain via a trough/flume; the canal bed is above the drain's high flood level (HFL), so the drain flows freely (open channel) beneath the canal trough Syphon aqueduct The canal again passes over the drain via a trough, but the canal bed is below the drain's HFL, so the drain is made to flow under pressure (as a syphon/pressure conduit) beneath the canal trough during high drain flows Super passage The natural drain passes over the canal via a trough; the drain's bed is above the canal's full supply level (FSL), so the canal flows freely (open channel) beneath the drain trough Canal syphon (syphon super passage) The drain passes over the canal via a trough (as in a super passage), but the canal's FSL is high enough that the canal is made to flow under pressure (as a syphon) beneath the drain trough Level crossing Used only for very small discharges, where the canal and drain are allowed to intermingle/cross at essentially the same level, with regulating structures provided on both the canal and the drain to manage flows appropriately

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This section covers the causes, effects, and preventive measures for water logging, and the design principles of surface and sub-surface drainage systems.

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Causes & Effects of Water Logging Causes: over-irrigation (applying more water than the crop/soil can use), seepage losses from unlined canals and water bodies, impeded natural drainage (e.g. due to flat topography, obstructions, or inadequate drainage infrastructure), a naturally high water table, poor land grading, and excessive rainfall/flooding, particularly on low-permeability soils that drain slowly.

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Effects: reduced crop yield due to poor root-zone aeration (roots suffocate in saturated soil), progressive soil salinity/alkalinity buildup (as capillary rise brings dissolved salts to the surface where evaporation concentrates them), damage to infrastructure (foundations, roads) from prolonged saturation, and increased mosquito breeding/associated public health hazards from stagnant water.

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Preventive Measures Adopting efficient irrigation practices (avoiding over-irrigation, lining canals to reduce seepage), providing adequate drainage infrastructure (both surface and sub-surface, as needed) to remove excess water, practising controlled/scheduled irrigation matched to actual crop water needs (rather than fixed excessive quantities), improving land grading to promote proper surface runoff, and, where necessary, groundwater pumping to actively lower an elevated water table.

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Surface & Sub-Surface Drainage Design Surface drainage systems consist of a network of open drains/ditches following the natural land slope, designed to collect and remove excess surface runoff and ponded water before it can infiltrate excessively or cause prolonged waterlogging; sizing is based on an appropriate design storm and a drainage coefficient (the depth of water to be removed within a specified time).

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Sub-surface drainage systems consist of buried perforated pipe drains (or gravel-filled trenches) installed at an appropriate depth and spacing below the ground surface, intercepting and removing excess water from within/below the crop root zone to lower the water table to a safe depth; drain spacing is commonly determined using drainage equations (e.g.

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Hooghoudt's equation), which account for the soil's hydraulic conductivity, the depth to any impermeable barrier layer, and the design drainage rate (the rate at which the water table must be lowered/maintained).