Last reviewed 1 Oct 2026 · 7 min read
Why terraces and bunds?
Terraces and bunds shorten the slope length and reduce runoff velocity, intercept runoff and either hold it (to soak in) or lead it safely to an outlet, so that erosion falls and soil moisture rises. They are the main mechanical measures under USLE's support-practice factor .
Choice by site conditions
| Situation | Suitable measure |
|---|---|
| Gentle slope (up to about 6 %) in low-rainfall (below about 600 mm), permeable soils | Contour bunds (retain water) |
| Gentle slope in high-rainfall areas or impermeable (black, heavy) soils | Graded bunds / graded terraces (drain safely) |
| Moderate slope (up to about 8–10 %) in cultivated fields | Broad-base terraces (level or graded) |
| Slopes about 10–25 %, farmed with small machines | Narrow-base terraces (rare for farming), bench terraces at steeper slopes |
| Steep slopes (about 16–33 %), hills, rice cultivation | Bench terraces |
| Orchards on slopes | Orchard (individual) terraces, half-moon terraces, contour trenches |
| Very steep or non-arable land | Afforestation, grass, contour trenches |
(The slope ranges are guides; local state guidelines differ.)
Contour bunds
An earthen embankment built along the contour to hold runoff and sediment and increase infiltration; used in semi-arid areas on lands of up to about 6 % slope.
- Spacing by the vertical interval (VI); the bund is level (so water spreads), with cross-bunds (spurs) to stop water moving sideways; surplus weirs at lower ends of bunds prevent over-topping.
- Typical section: height about 0.5–0.6 m, top width about 0.3–0.45 m, side slopes 1:1 to 1.5:1, depending on soil.
Graded (channel) bunds and terraces
- Built on a slight grade (about 0.2–0.4 %, often less than 0.5 %) along the contour so that runoff drains slowly to a grassed waterway or a safe outlet without ponding; suited to high-rainfall or slow-permeability soils.
- The channel above the ridge carries the flow; the ridge supplies the earth.
Vertical and horizontal intervals
A widely used form:
where = land slope (%), = vertical interval, = horizontal interval, and , are constants depending on the region, soil and rainfall (taken from the state or national guidelines; for example, and are used in some tables).
For a 6 % slope using the constants , (assumed for illustration):
. .
Bunds are therefore laid out every 20 m along the slope, each on the contour.
Channel capacity (graded terrace)
The channel is designed to carry the peak runoff from the area between terraces by the rational formula:
( in m³/s, in mm/h, in hectares, the runoff coefficient). Velocity from Manning's equation should stay below the non-erosive limit (about 0.6–0.9 m/s in a cultivated channel).
A terrace channel drains 2 ha with and a design intensity mm/h:
.