Terraces or trenches? A shallow-soil orchard on an 8% slope
A farmer in the Vindhyan foothills has 1.6 ha on a uniform 8% slope, 200 m long along the slope and 80 m across, with 30–35 cm of gravelly sandy loam over weathered sandstone; the auger refuses at 40 cm in about a third of the field. Rainfall is 900 mm, 85% from June to September. He grows sorghum and black gram sown up and down the slope and wants to convert to a mango orchard with a black gram intercrop, keeping his two shallow dug wells that yield only in the monsoon. Two gullies, 1.2 m and 0.8 m deep, have cut in from the lower boundary and are advancing. A contractor has quoted for two alternatives: full bench terracing with a tractor-drawn terracer, or contour bunds at a 2.7 m vertical interval. The farmer also asks whether he should simply level the field with a laser leveller and grow paddy instead. His budget covers about 250 man-days and 400 m3 of masonry or gabion work.
1. Check the contractor's two options numerically - compute the vertical and horizontal interval, the number of bunds, and the depth of cut implied by bench terracing - and state which option is physically feasible.
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Contour bunds:
- Vertical interval = 0.3 × 8 + 0.3 = 2.7 m (matching the contractor's figure).
- Horizontal interval = 2.7/0.08 = 33.75 m.
- Number of intervals along 200 m = 200/33.75 = 5.9, so five to six bunds, total length about 5 × 80 = 400 m. This is feasible: a 0.5 m high, 0.45 m top-width bund can be built from a 0.3 m trench, keeping the section within the 30–35 cm of soil.
Bench terracing: for a level bench whose width equals the horizontal interval (33.75 m), balanced cut-and-fill puts the cut at the back edge at roughly half the vertical interval, about 1.35 m. That is nearly four times the 35 cm soil depth, so the terracer would cut into weathered sandstone over most of the bench and leave a rock-floored, infertile strip.
Even a narrow 6 m bench, which needs only about 6 × 0.08/2 = 0.24 m of cut, consumes roughly 70% of the soil depth, and the auger already refuses at 40 cm over a third of the field - in those places the cut reaches refusal. Level benches also concentrate runoff at the riser ends, which is exactly where the two gullies are already advancing.
Conclusion: reject full bench terracing. Use contour trench-cum-bund or staggered contour trenches with individual tree basins, which need no cut below about 20 cm and harvest moisture in situ.
2. Given 1.6 ha with mango at 6 m × 6 m, lay out the orchard and give numbers: tree basins, trench dimensions and number, and gully treatment, all within 250 man-days and 400 m3 of masonry/gabion.
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Tree layout: 16,000 m2 / 36 m2 = 444 positions; deduct boundary rows, leaving about 420 trees, planted staggered along the contour at 6 m × 6 m.
Basins: each tree in a ring basin 1 m diameter × 0.2 m deep with a 15 cm earth rim on the downhill side: 0.157 m3 × 420 ≈ 66 m3 of earthwork. Break any compact layer under each basin before planting and mix 10–15 kg FYM per basin.
Contour trenches: staggered trenches 4 m long, 0.45 m × 0.45 m section, in alternate inter-row spaces at about 6 m horizontal spacing; over 80 m across the slope that is about 13 lines, and 80/7 ≈ 11 trenches per line, so about 140 trenches. Volume = 0.81 m3 × 140 ≈ 113 m3. Keep trenches at least 3 m away from gully banks.
Vegetative barriers: khus or vetiver slips on the contour between trench lines, 0.3 m apart, roughly 250 slips per 80 m line - about 3,000 slips.
Gullies (1.2 m and 0.8 m): shape both headwalls to a 1:1 batter and protect each with a gabion or masonry drop of about 6–8 m3; with a 0.5 m effective check height and 3% bed slope, spacing is about 17 m, so three checks in the longer gully and two in the shorter, stabilised with khus and fenced from grazing. A stone-pitched drop where the waterway leaves the field completes the outlet.
Budget check: earthwork ≈ 180 m3 at about 1.5–2 m3 per man-day ≈ 100–120 man-days; basin work and planting ≈ 60 man-days; masonry/gabion ≈ 30–40 m3, well inside 400 m3. The remaining labour goes to fencing and slip planting.
3. Justify rejecting laser levelling plus paddy, and explain how your layout changes the slope-length effect on erosion.
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Why levelling plus paddy fails here:
- Levelling a 200 m long field at 8% means a total elevation difference of 200 × 0.08 = 16 m. Even split into four basins of 50 m, each requires about 2 m of cut - six times the 35 cm of soil - so the leveller would cut through the soil into sandstone and bury the remainder under sterile fill.
- Paddy needs 15–20 cm of ponded water on a level basin. On a shallow, gravelly, permeable profile over sandstone, seepage losses would be severe and the dug wells, which only yield in the monsoon, cannot support ponding.
- Puddling would further degrade the already thin A horizon, and the farmer would lose the option of the orchard, which is more suitable to a sloping, shallow, well-drained soil.
Effect on slope length: the soil-loss LS term rises roughly with the square root of slope length, and steeply with slope angle. Contour trenches and vegetative barriers at about 6 m horizontal spacing cut the effective slope length from 200 m to roughly 6 m - a factor of about (200/6)0.5 ≈ 5.8 reduction in the length term - while cultivation across the slope and the barriers also flatten the effective gradient. That is why a dense, cheap grid of trenches and grass strips outperforms a single large earthwork on this land.
Water disposal: the layout must still deliver surplus runoff safely. Take the trench overflows and the inter-row flow into a 4–6 m wide grassed waterway with a gradient under 5%, pitched where it drops, and discharge into the natural drainage line only after the two gully heads are stabilised.