Last reviewed 1 Oct 2026 · 6 min read
Why sub-surface drainage?
When the water table rises into the root zone, surface drains are not enough, because they do not remove the water held in the soil pores. Sub-surface drainage removes this excess gravity water below the surface, lowers the water table, and keeps the root zone aerated. It is also necessary to control salinity, because it carries away leached salts.
The water table is the upper surface of the saturated zone. Sub-surface drainage aims to hold it at a safe depth below the ground — typically about 1.0–1.5 m for most field crops (less for shallow-rooted crops and more where salinity is a risk), and deeper in arid, saline regions where capillary rise must be controlled.
Classification
| Group | System | Principle |
|---|---|---|
| Horizontal | Open ditches (deep drains) | Deep open drains cut below the water table; simple but waste land and interfere with farming |
| Tile / pipe drains | Buried pipes (clay tile, concrete, corrugated plastic) receive water through their joints or perforations | |
| Mole drains | Unlined channels formed in the subsoil by a bullet-shaped plough — cheap, temporary (a few years), in stable clay | |
| Vertical | Tube-well (vertical) drainage | Pumping groundwater from tube-wells to lower the water table and use or dispose of the water |
| Relief wells | Release artesian pressure | |
| Bio-drainage | Planting trees such as eucalyptus to take up water by transpiration |
Horizontal sub-surface drainage
Components
- Laterals (field drains) — the closely spaced pipes that collect water from the soil.
- Collectors (sub-mains) — receive water from laterals.
- Main drain — carries the water to the outlet.
- Outlet — a gravity outlet to an open drain, or a sump and pump where gravity outfall is not possible.
- Envelope (filter) — gravel, sand or synthetic material around the pipe to prevent soil entering and to improve inflow (see the materials note).
- Manholes / inspection chambers, silt traps, breather pipes — for access, cleaning and air.
Layout patterns
| Pattern | Description | Use |
|---|---|---|
| Random | Drains laid only in the wet spots, connected to a collector | Scattered depressions, irregular topography |
| Parallel (gridiron) | Laterals parallel at a uniform spacing, discharging into a collector at right angles | Flat, uniform land; the commonest |
| Herringbone | Laterals join the sub-main at an angle (about 45°) on both sides, forming a "fishbone" | Narrow depressions or valleys |
| Double main | Two parallel mains with a strip of land between them | Where the main drain divides the field (a wide depression) |
| Interceptor (cut-off) | Drain laid across the slope to intercept seepage | At the foot of a canal or hill |
Design parameters
- Depth of laterals — usually 1.0–2.0 m (deeper laterals can be placed at a wider spacing, but cost more); in saline areas often 1.5–2.0 m.
- Spacing — determined by the soil's hydraulic conductivity, the depth of the drain, the depth to the impermeable layer, and the drainage coefficient (see the drain-spacing note). Spacing may range from about 10–20 m in heavy soils to more than 100 m in sandy soils.
- Pipe diameter — from discharge and slope: laterals typically 80–100 mm (corrugated PVC), collectors 150–300 mm and mains larger. Use Manning's formula for pipe flow.
- Slope — about 0.1–0.3 % (minimum 0.05 %); the velocity should be at least about 0.3–0.5 m/s to avoid silting.
- Drainage coefficient — the rate at which water is removed to maintain the target water table; in humid regions about 7–15 mm/day and in irrigated arid regions about 1–3 mm/day (indicative; use the project design value).
- Pipe length — laterals up to 100–250 m; avoid very long laterals because of the head loss.
Discharge of a lateral
where = drainage coefficient (m/day), = length of the lateral (m) and = spacing (m); in m³/day.
Laterals of length 200 m are spaced 40 m apart, and the drainage coefficient is 6 mm/day.
(0.56 L/s). A 100 mm corrugated pipe at 0.2 % slope carries far more than this, so the diameter is governed by minimum practical size and sediment management, not by capacity.
Installation
- Survey and layout; check the outlet level and the invert levels.
- Trenching to the design grade (by trencher, chain digger or laser-controlled trenchless machine).
- Laying the pipe with an envelope to the proper grade; test the grade with a laser.
- Backfilling with selected soil, compaction near the pipe; avoid damage.
- Outlet protection (rodent screen, riprap, flap gate).