Last reviewed 16 Sept 2026 · 8 min read
Cross-drainage works
A cross-drainage work is a structure at the crossing of a canal and a natural drain (stream, nala, river), allowing both to flow without interfering with each other. They are costly, so canal alignment tries to minimise their number (ridge alignment avoids them).
Types
Canal passes over the drain:
- Aqueduct — the drain's HFL is below the canal trough bed; the drain flows freely under the canal (open-channel flow), the canal is carried in a trough on piers.
- Syphon aqueduct — the drain's HFL is above the canal bed; the drain flows under pressure through barrels beneath the canal (the drain floor is usually depressed).
Drain passes over the canal:
- Super passage — the canal's FSL is below the underside of the drain trough; the canal flows freely below (open flow).
- Canal syphon (syphon) — the canal's FSL is above the underside of the drain trough; the canal flows under pressure through barrels below the drain.
Drain and canal at the same level:
- Level crossing — both waters intermix at the crossing; regulators on the drain and canal control flow (used when a large drain meets a canal at nearly the same level).
- Inlets and outlets — for small drains: the drain water is admitted into the canal through an inlet and, if necessary, released downstream through an outlet.
| Structure | Flow in drain | Flow in canal |
|---|---|---|
| Aqueduct | Free (open) | Free (in trough) |
| Syphon aqueduct | Under pressure (barrels) | Free (in trough) |
| Super passage | Free (in trough) | Free (open) |
| Canal syphon | Free (in trough) | Under pressure (barrels) |
Selection of type
Relative bed levels and water levels of the canal and drain; discharges; availability of suitable foundations; need to avoid heavy earthwork; importance of navigation or road crossing; comparative cost. An aqueduct or syphon aqueduct is generally preferred to a super passage or canal syphon, because the canal (the more important and permanent channel) remains open for inspection and the drain's floods do not endanger the canal flow.
Design considerations
- Drainage waterway — high flood discharge of the drain estimated hydrologically; the required waterway is taken from Lacey's wetted perimeter: (for wide streams width).
- Canal waterway and fluming — the canal is contracted (flumed) at the crossing to reduce the length and cost of the trough; fluming is limited so that velocity stays below about 3 m/s and the flow remains subcritical. Transitions (contraction and expansion) keep losses low — e.g. Mitra's hyperbolic transition and Chaturvedi's semi-cubical parabolic transition; typical splay 2 : 1 in contraction and 3 : 1 in expansion.
- Head loss through syphon barrels = entry loss + friction loss + exit loss (the velocity head lost at the outlet), which fixes the afflux in the drain.
- Uplift on the barrel floor — due to the water table below the floor (static uplift) and seepage from the canal (seepage uplift, by Bligh's or Khosla's theory); the floor is made thick enough or reinforced to resist the worst condition (e.g. drain empty, canal full).
- Uplift on the roof of barrels — when the drain flows under pressure; the roof is designed for this upward pressure against its weight.
- Scour protection — cut-off walls and pitching; foundation depth below the maximum scour depth.
- Bank connections — wing walls and returns to guide flows and retain earth.
Canal regulation works
Regulation works control discharge, water level and velocity in canals.
Canal falls (drops)
A fall is provided where the natural ground slope is steeper than the designed canal bed slope; the excess level is dropped at a fall so the canal does not run in high embankment.
Location is decided by balancing earthwork (cut and fill); falls may be combined with bridges, cross regulators and outlets. A fall may also generate small hydropower.
| Type of fall | Features |
|---|---|
| Ogee fall | Convex and concave curves (ogee) — gradual drop; early type, energy not properly dissipated, downstream erosion |
| Rapids | Long sloping glacis (roughly 1 in 10 to 1 in 20) with pitching; expensive, needs stone |
| Stepped fall | Series of vertical drops in steps; for large drops (older works) |
| Trapezoidal notch fall | Trapezoidal notches in a breast wall across the canal; maintains upstream depth for all discharges |
| Vertical drop fall (Sarda type) | Raised crest with vertical downstream face, water falls into a cistern that dissipates energy; developed on the Sarda canal (UP) where the upper soil was sandy with a clay/loam base; commonly a rectangular crest for smaller discharges and trapezoidal for larger |
| Glacis fall (straight glacis) | Sloping glacis on which a hydraulic jump forms; efficient energy dissipation; can be flumed (meter fall) |
| Montague type fall | Parabolic glacis profile |
| Inglis fall | Straight glacis with a baffle wall downstream to help jump formation |
- Meter falls (flumed, with a known crest) are used also to measure discharge; non-meter falls only drop the water.
- A fall should not cause silting upstream or scour downstream; crest level is set to maintain normal depth upstream at full supply.
Cross regulator
A gated structure across the parent canal, just downstream of an offtaking distributary:
- Heads up the water in the parent canal during low supply so the distributary can draw its share.
- Helps close supply to the downstream reach for repairs.
- Absorbs fluctuations in the canal system; combined with a road bridge and often with a fall.
Cross regulators are provided at intervals (and at major offtakes) along main and branch canals.
Distributary (head) regulator
A structure at the head of an offtaking channel (distributary or branch):
- Regulates and meters supply into the distributary.
- Controls silt entry (crest raised above the parent canal bed, with silt control devices).
- Shuts off supply when not needed.
Canal escape
A structure to dispose of surplus water from a canal into a natural drain — to protect the canal from breaches when excess water enters (e.g. rain runoff, sudden closure of outlets), and to empty the canal for repairs.
- Surplus (weir-type) escape — crest at FSL; excess spills automatically.
- Regulator (sluice) type escape — gated, crest at canal bed level; can empty the canal and flush silt (scouring escape).
Other works
- Bed bars — masonry walls across the canal bed at bed level, to monitor bed changes (silting/scouring).
- Metering flumes — standing-wave flumes (critical-depth flumes) to measure discharge.
- Canal outlets — see Canal Irrigation & Design of Alluvial Canals.
Silt control at offtakes
The offtaking channel should draw surface water (less silt) from the parent canal:
- Skimming platform — a raised platform at the head of the offtake so that only top water enters.
- King's vanes (curved bed vanes) — deflect bottom silt-laden water away from the offtake.
- Gibb's groyne wall — a curved wall in the parent channel guiding top water into the offtake.
- Offtake alignment — a small angle (about 30° or less) to the parent canal and location on the outer side of a bend are favourable.