These are narrow trenches built along contours to collect run-off and conserve moisture for trees or tree crops. The trenches can be built as a continuous line across an entire slope, over shorter distances, or for individual plants.
Criteria for selection
- On gentle slopes (up to 30%), use outward-sloped or level trenches.
- On steep slopes (30 to 70%), use reverse-sloped trenches.
- For dissected slopes, use shorter or individual gradoni.
- For uniform slopes, use long gradoni.
- To promote absorption and storage of water in the soil for tree crops or plant use.
- To prevent flash floods or dry mantle floods.
- For erosion control on slopes where plant cover has deteriorated and revegetation is required.
- In semi-arid or arid regions where rainfall intensities are not too high.
- For soils with relatively high permeability (Test soil infiltration rates and compare with the records of an automatic rain gauge, using 10 year return period and 0.5 to 1 hr duration).
For denuded watershed slopes where revegetation is planned.
For development of orchards on slope lands.
Length: Long gradoni - according to the terrain but not over 350 m; shorter gradoni - 3 to 6 m long; individual gradoni - 1 to 2 m long and crescent-shaped.
Width: Gradoni built by hand - 1 to 1.5 m; gradoni built by machine 2 to 2.5 m or more.
Gradient: Horizontal - virtually level or with no gradient; reverse or outward grades - 2:1 to 2.5:1 (Fig. 17).
VI and spacing: This depends on the slopes. For long gradoni, the VI formula for hillside ditches used in semi-arid or arid regions can be applied. Shorter or individual gradoni can be added between two long gradoni if necessary. The most important consideration is the capacity to hold rainwater from storms the size of 10-year return period. In the western USA, for instance, the gradoni are designed to hold 50% of a 50 mm (2") storm of 1 hour's duration.
Cross sections and volume. The cross-sections can be either of the triangle (V-shaped) or trapezoid type, both of which have been discussed earlier. For the triangle type, simple trigonometry functions should be used for determining the cross-section and volume. The calculations for broad-based terraces and hillside ditches, explained earlier, can also be used. It is important to remember that the cut and fill should be equal so that when the volume is calculated, only the cut part is accounted for. For total volume calculations, the barriers and dykes must be added. For the trapezoid type, refer to the section on diversion ditches. For the crescent type, the approximate volume can be obtained by using the corresponding diameters of individual basins (Table 4). A factor of 75% should be used since the volume is less.
Shape: Longer gradoni - straight trenches, either V-shaped, or trapezoid shaped; individual gradoni - crescent-shaped.
Barriers or dykes: For long gradoni, low barriers or small dykes for holding r should be built every 3 to 4 m.
Fig. 17 shows the three main types of cross-sections i.e. triangle (Vshaped), trapezoid and crescent ones.
Fig. 18 shows a hypothetical field arrangement.
For longer gradoni, the same principles and layout procedures for hillside ditches can be applied. For individual gradoni, the distance should be staked out by the required intervals of trees or orchard trees (often staggered).
By manual labour
For shorter or individual gradoni, the construction techniques for Vshaped and trapezoid cross-sections are similar to those for hillside ditches and diversion trenches respectively. The construction procedure for crescent-shaped gradoni is similar to that for individual basins explained earlier except that the crest for the former is built higher like a dyke in order to retain run-off.
Mechanized construction
For long and V-shaped trenches: on gentle slopes (up to 30%), a crawler tractor, fitted with an angled blade tilted away from the slope of the land should be used to cut outward-sloped or level trenches. Two mediumsized machines (100-150 h.p) working in unison are recommended for this operation. On steep slopes (30 to 70%), the machine should make a level base first and then make a reverse-sloped trench on the inside of the road by tilting the angle blade on the opposite side to the slope.
The physical output for constructing V-shaped trenches should be similar to that for bench terraces and hillside ditches, i.e. 3 to 4 cubic m per man-day, or, when using tractors, 40 to 50 cubic m per hour depending on site conditions and soils (gradoni are cut with a single operation; the time spent on smoothing the bench terraces in order to obtain precise horizontal grades can be saved by placing barriers in the gradoni). The deep trapezoid trench made by hand requires extra time for digging. The output may vary from 2.5 to 3 cubic m per man-day. The output for crescent shaped gradoni should be equal or similar to the output for individual basins (Table 4).
Long gradoni are generally more cost-effective when constructed by machine in most advanced developing countries. Short or crescent-shaped gradoni on the other hand, are more economical when built by hand. When constructing gradoni by hand, the V-shaped type is cheaper than the trapezoid type while the outward V-shaped one is normally the cheapest of all. In common with other land treatments, gradoni are more expensive to construct on steep slopes than on gentle ones.
When the spacing between two gradoni is widened because of slope changes, the cross-section of the gradoni should be built proportionately bigger to accommodate additional run-off. When rehabilitating watershed slopes, the spaces between the two gradoni should be ploughed and seeded immediately after construction.
The risers of the gradoni can be either seeded, or temporarily covered with bushwood and dead leaves before natural vegetation starts to grow. It is necessary to maintain the proper height of the gradoni's edge in order to avoid overflowing. Any settling of the edge should be repaired or strengthened. The barriers or dykes that separate the trenches should be kept lower than the edge of the gradoni to allow overflowing to the adjacent sections during heavy rains (Fig. 17).
These are short ditches or pits, dug for the purpose of temporarily storing run-off water and gradually infiltrating it into the soil. The ditches can be either in the form of trenches or simple pits and may be built at varying intervals depending on site conditions.
Criteria for selection
- For rehabilitating rangeland, use small pits dug by a machine such as a disk-type pitter.
- In cultivated fields, use deep trenches which are normally dug by manual labour.
- To intercept and temporarily store run-off.
- To improve soil moisture conditions and stimulate plant growth.
- To reduce the amount of flood water, thereby minimizing erosion.
- In semi-arid regions where rainfall it not less than 400 mm but where the intensity is low.
- On sites where the soil has moderate or relatively low ''permeabilities".
- For the rehabilitation of rangeland.
Trench-type ditch
Dimensions: These depend on rainfall, soil characteristics, vegetation and land use. The capacity of the ditch should be sufficiently large to hold water from a major storm. In some flat areas, for example, 2.5 m wide, 1.5 m deep and 40 m long ditches have been used. The dimensions can be much smaller than this on steep slopes and when the ditches are to be built at regular intervals. The calculations have been explained in previous sections.
Horizontal grade: No grade or virtually level.
Spacings: Regular or irregular depending on site needs. For extensive farming
on moderate slopes, the distance can be similar to
that for hillside ditches, i.e. 15 to 20 m
Pits
Dimensions: These depend on the mechanized equipment used for construction. If a disc-type pitter is used, for instance, shallow, discontinuous pits measuring approximately 20 by 50 cm should be scooped out. The distance between each pit should be roughly 40 cm. No calculation of the cross-section or volume is needed.
Depth: This depends on the moisture content of the soil and weight of the machine.
See the diagrams for trapezoid gradoni (Fig. 17) or diversion ditches (Fig. 15). The pits are small holes or openings.
The same principles used for planning and staking out hillside ditches or gradoni can be applied when the ditches are to be laid out at regular intervals. If an irregular layout is planned, the ditch sites should be determined by the terrain and the farmer's needs.
By manual labour
Similar to diversion ditches. When constructing deep trenches, the dug out soil should be placed in the immediate downslope area and must be well compacted.
Mechanized construction
A disc-type pitter can be used for herbaceous and shrub cover on slopes up to about 20% which are free of extensive gullies, and on sites where the disk can penetrate freely. A custom-made lister-type pitter with a heavy ripper tooth is recommended for sites with heavy root systems and harder soils.
By manual labour
The deep digging makes output similar to diversion ditches.
Mechanized construction
The pitting work depends on the intervals between each pit, plant roots, terrain, soils and the skills of the machine operator.
By manual labour
The deeper the trench, the more costly the construction will be.
Mechanised construction
The cost will be much higher on steep slopes, uneven topography, or on sites with dense
roots, hard soils and rocks.
Trench-type: The bank built by the fill must be well compacted. Wines of grasses should be planted above the cut to keep the ditch sediment free.
Pits: When planning rangeland rehabilitation, seed should be sown immediately while the soil surface is still loose.
For the trench-type, the most important maintenance work is to clear sediment deposits in order to maintain the ditch's capacity at a constant level. Grasses should be planted to protect the fill banks from eroding. Any settling of the bank must be repaired immediately. The bank's top should be kept level and at a sufficient height.
