These are discontinuous types of narrow bench terraces built across the hillslopes. Their spacing for cultivation purposes is determined by the slope of the land, climate and soil conditions.
Although, in some countries, trenches are used instead of narrow benches, the benches are preferable because they act both as an access road and as an interception drain; they are easier to maintain; and part of the bench can be cultivated if necessary.
Criteria for selection
- In humid tropical regions: Use reverse sloped narrow benches (or ditches)
- In semi-arid or arid regions: use outward sloped or levelled narrow benches (or ditches).
- On shallow soils: Use 1.5 m wide benches (flat strips)
- On deep soils: Use 2 m wide benches.
- To break up long slopes into several shorter slopes, thereby reducing erosion.
- To intercept excess run-off from the land above and drain it safely.
- On slopes of up to 25 degrees (47%).
- In order to grow semi-permanent or perennial crops on the land between two ditches.
- For small farms using manual cultivation methods.
- As a minimum land treatment measure to be supplemented with auxiliary and agronomic conservation measures for safe cultivation.
Length: Same as for bench terraces, i.e. 100 m for humid regions and 200 to 300 m for arid regions.
Width: 2 or 1.5 m platform plus cut part and filled part (riser).
Gradients: for horizontal gradients, 0.5 to 1% for humid regions and 0 to 0.5% for arid
or semi-arid regions as explained in the previous section on bench terraces. For reverse
grades, 10% (humid regions). For outward grades, 3% (semi-arid regions).
Risers: riser slope is 0.75:1 for hand-made and 1:1 for machine-made. Riser height is only
one-half of the bench terraces because they are the discontinuous type. Riser width is
riser height multiplied by 0.75 or 1.
Vertical intervals and spacings: The following formulas can be used for calculating VI between two ditches:
USA
VI (feet) = aS + b
S : slope of the land, in %
a : varies from 0.4 in the humid South to 0.8 in the North
b : 1 to 4 according to soil and cover.
Jamaica, for instance, uses 0.3 for "a" and 2 for "b" A spacing list is produced and incorporated in the specification tables.
Taiwan (Republic of China)
El Salvador and Thailand
Israel
In humid tropical countries where the condition is critical, the El Salvador formula should be used. However, in cases where normal conditions prevail, the Taiwan (Republic of China) formula is recommended.
After calculating the VI, the horizontal distance (HI) and inclined distance (D) can be obtained as follows:
D = VI / Sin 0
0 : slope angle in degrees
The calculated distance can be adjusted within a range of 10 to 20% to fit cropping or local needs.
Cross-section and volume: For cross-section, the same formula should be used as for the bench terrace assuming the height of riser (Hr) is full and not divided in half. The volume to be cut and filled is calculated by multiplying linear length by the cross-section.
Ditch area and linear length: The ditch area in a hectare can be calculated by multiplying the linear length of the ditches by their assumed width (either 2 m or 1.5 m). The linear length in a hectare can be obtained in two ways, depending on actual needs (see Surveying, below):
L : linear length
HI : horizontal distance between two ditches
Wt : the total width of a ditch
Fig. 5 shows the cross-section and the formulas for calculations.
Table 2 provides a set of specification tables calculated according to the following site conditions:
Reverse-sloped, 2 m wide ditches
Different sets of tables need to be worked out for other site conditions.
Layout
The general layout is similar to that for bench terraces. Hillside ditches require a simple road to connect them and a natural drainage area or a waterway system to safely drain the run-off. Consideration should be given to the sites of farm building and/or windbreaks. Waterways can be sited in the middle or at the end of the field, depending on the terrain and actual needs. Fig. 6 shows a hypothetical layout plan.
Surveying
The survey preparation is as same as for bench terraces. A dumpy level should be used for long ditches (100 m) and a hand or an A-level for short ditches (less than 30 m). Calculate the representative slope and establish an up-and-down base line as indicated in the surveying section for the bench terraces. Use inclined distance (D) calculated previously, or taken from the specification tables to make markings along the base line.
SYMBOLS AND COMPUTATIONS
In heavy rainfall regions and on erosive sites, use the exact values of the inclined
distance (D). In light rainfall regions, on clay soils or for crops with good cover, use
the values of the HI plus the width of the ditch (Wt). The distance can be reduced or
extended within a range of 10 to 20% to fit local crop needs.
Graded contours should be run from each staked point along the base-line in order to drain run-off. These are the centre lines of the ditches and should be marked with a colour (e.g. red). The top bottom stakes of the ditches should be set up using the widths indicated in the specification tables. For example, on a 20% slope with a 2 m ditch, the width (Wt) for the reverse-sloped type is 2.52 m. Separate colour markings should also be made on the top and bottom stakes, on the end of the contour line close to a waterway and on the contour near a road.
By manual labour
Hillside ditches are ususally built with manual labour in the developing countries. Start cutting ditches from the upslope. A 10% reverse slope or 3% outward slope should be observed. Allowances must be made for settling. The riser should be built layer by layer and requires firm compacting. After the initial rough cutting, the gradients should be carefully checked. Any part of the ditch going across depression areas must be made particularly strong. A team of 3 persons is recommended for this work. For more details, refer to the construction section for handmade bench terraces.
Mechanized construction
Only small machines fitted with a 2 to 2.5 m wide angle blade are suitable for this task. The construction principles are similar to those for bench terracing, except that there is less room to manoeuvre the machine. The risers should be shaped by hand.
A man can cut and fill 3.5 to 4 cubic m a day under average conditions. Expressed another way, it means a 17 to 19 m length of 2 m wide hillside ditches on a 25% (14 degree) slope can be completed in a man-day. on slopes of 30%, it will take about 50-60 man-days to construct a hectare of hillside ditches under normal conditions. A small machine (70 to 80 H.P.) can complete 20 cubic m per hour.
The cost of hillside ditching is equal to 1/5 or less of the cost of bench terracing in a unit area. Nonetheless, hillside ditches can reduce erosion by 80% if they are supplemented by auxiliary conservation measures.
Ditches
Cultivation on hillside ditches is not usually recommended as they are part of a drainage or road system. If small farmers do need to cultivate them, every alternative ditch (terrace) should be left uncultivated so that access can still be maintained, and at least 30 cm should be left unplanted to act as a toe drain.
Spacing
Auxiliary and/or agronomic conservation measures should be applied to the spacings between two ditches in order to achieve effective erosion control. These measures include the following: contour cultivation, contour mounds and contour furrows; staggered and close planting; mulching, including the use of residues, trashes and grass. grass barriers and stone barriers.
Protection
A line of thick, tall grass should be planted and maintained above the cut banks in order to restrain soils from upslope. The protection of the risers and outlets is the same as that for bench terraces explained earlier.
Maintenance
Remove any deposit in the ditches and ensure that water flow in the toe drains is kept free from blockages. maintain a proper crosssection and grades. Keep cattle away from the ditches and carry out frequent inspections during the first two rainy seasons after establishment.
Hillside Ditch (1)
(Hand Made)
Width of platform : 2m
Riser slope : 0.75 :1
Reverse slope:0.10
VI : (S+4)/10
|
Slope |
S P E C I F I C A T I O N |
|||||||||||
|
% |
Grade |
TVI m | Hr m | Wr m |
Wt m |
VI m |
L m |
A m² |
Pb % |
C m² |
V m³
|
D |
|
1 |
0.6 |
0.02 | 0.11 | 0.08 |
2.16 |
0.5 |
200 |
400 |
4 |
0.06 | 12 |
48 |
|
2 |
1.1 |
0.04 | 0.12 | 0,09 |
2.18 |
0.6 |
333 |
666 |
7 |
0.06 | 20 |
31 |
|
3 |
1.7 |
0.06 | 0.13 | 0.10 |
2.20 |
0.7 |
429 |
858 |
9 |
0.07 | 30 |
24 |
|
4 |
2.3 |
0.08 | 0.14 | 0.11 |
2,22 |
0.8 |
500 |
1000 |
10 |
0.07 | 35 |
20 |
|
5 |
2.9 |
0.10 | 0.15 | 0.11 |
2.22 |
0.9 |
556 |
1112 |
11 |
0.08 | 45 |
18 |
|
6 |
3.4 |
0.13 | 0.17 | 0.13 |
2.26 |
1.0 |
600 |
1200 |
12 |
0.08 | 48 |
17 |
|
7 |
4.0 |
0.15 | 0.18 | 0.14 |
2.28 |
1.1 |
636 |
1272 |
13 |
0.09 | 57 |
16 |
|
8 |
4.6 |
0.17 | 0.19 | 0.14 |
2.28 |
1.2 |
667 |
1334 |
13 |
0.09 | 60 |
15 |
|
9 |
5.1 |
0.19 | 0.20 | 0.15 |
2.30 |
1.3 |
692 |
1384 |
14 |
0.10 | 69 |
15 |
|
10 |
5.7 |
0.22 | 0.21 | 0.16 |
2.32 |
1.4 |
714 |
1428 |
14 |
0.11 | 79 |
14 |
|
10 |
6.3 |
0.24 | 0.22 | 0.17 |
2.34 |
1.5 |
733 |
1466 |
15 |
0.11 | 81 |
14 |
|
12 |
6.8 |
0.26 | 0.23 | 0.17 |
2.34 |
1 6 |
759 |
1500 |
15 |
0.12 | 90 |
14 |
Hillside Ditch (2)
(Hand Made)
|
Slope |
S P E C I F I C A T I O N |
|||||||||||
|
% |
Grade |
TVI m |
Hr m |
Wr m |
Wt m |
VI m |
L m |
A m² |
Pb % |
C m² |
V m³ |
D |
| 13 | 7.4 | 0.29 | 0.25 | 0.19 | 2.38 | 1.7 | 764 | 1528 | 15 | 0.12 | 92 | 13.0 |
| 14 | 8.0 | 0.31 | 0.26 | 0.20 | 2.40 | 1.8 | 781 | 1562 | 16 | 0.13 | 102 | 13.0 |
| 15 | 8.5 | 0.34 | 0.27 | 0.20 | 2.40 | 1.9 | 787 | 1574 | 16 | 0.14 | 110 | 13.0 |
| 16 | 9.1 | 0.36 | 0.28 | 0.21 | 2.42 | 2.0 | 801 | 1602 | 16 | 0.14 | 112 | 12.5 |
| 17 | 9.6 | 0.39 | 0.30 | 0.23 | 2.46 | 2.1 | 805 | 1610 | 16 | 0.15 | 121 | 12.5 |
| 18 | 10.2 | 0.42 | 0.31 | 0.23 | 2.46 | 2.2 | 818 | 1636 | 16 | 0.16 | 131 | 12.5 |
| 19 | 10.7 | 0.44 | 0.32 | 0.24 | 2.48 | 2.3 | 822 | 1644 | 16 | 0.16 | 132 | 12.5 |
| 20 | 11.3 | 0.47 | 0.34 | 0.26 | 2.52 | 2.4 | 833 | 1666 | 17 | 0.17 | 142 | 12.0 |
| 21 | 11.9 | 0.50 | 0.35 | 0.26 | 2.52 | 2.5 | 843 | 1686 | 17 | 0.18 | 152 | 12.0 |
| 22 | 12.4 | 0.53 | 0.37 | 0.28 | 2.56 | 2.6 | 845 | 1690 | 17 | 0.18 | 152 | 12.0 |
| 23 | 12.9 | 0.56 | 0.38 | 0.29 | 2.58 | 2.7 | 848 | 1696 | 17 | 0.19 | 161 | 12.0 |
| 24 | 13.5 | 0.59 | 0.40 | 0.30 | 2.60 | 2.8 | 858 | 1716 | 17 | 0.20 | 172 | 12.0 |
| 25 | 14.0 | 0.62 | 0.41 | 0.31 | 2.62 | 2.9 | 860 | 1720 | 17 | 0.21 | 181 | 12.0 |
| 26 | 14.6 | 0.65 | 0.43 | 0.32 | 2.64 | 3.0 | 868 | 1736 | 17 | 0.21 | 182 | 12.0 |
| 27 | 15.1 | 0.68 | 0.44 | 0.33 | 2.66 | 3.1 | 870 | 1740 | 17 | 0.22 | 191 | 12.0 |
| 28 | 15.6 | 0.71 | 0.46 | 0.35 | 2.70 | 3.2 | 873 | 1746 | 18 | 0.23 | 201 | 12.0 |
| 29 | 16.2 | 0.74 | 0.47 | 0.35 | 2.70 | 3.3 | 880 | 1760 | 18 | 0.24 | 211 | 12.0 |
| 30 | 16.7 | 0.77 | 0.49 | 0.37 | 2.74 | 3.4 | 883 | 1766 | 18 | 0.24 | 212 | 12.0 |
| 31 | 17.2 | 0.81 | 0.51 | 0.38 | 2.76 | 3.5 | 884 | 1768 | 18 | 0.25 | 221 | 12.0 |
Hillside Ditch (3)
(Hand Made)
|
Slope |
S P E C I F I C A T I O N |
|||||||||||
|
% |
Grade |
TVI m |
Hr m |
Wr m |
Wt m |
VI m |
L m |
A m² |
Pb % |
C m² |
V m³ |
D |
| 32 | 17.7 | 0.84 | 0.52 | 0.39 | 2.78 | 3.6 | 887 | 1774 | 18 | 0.26 | 231 | 12.0 |
| 33 | 18.3 | 0.88 | 0.54 | 0.41 | 2.82 | 3.7 | 894 | 1788 | 18 | 0.27 | 241 | 12.0 |
| 34 | 18.8 | 0.91 | 0.56 | 0.42 | 2.84 | 3.8 | 896 | 1792 | 18 | 0.28 | 251 | 12.0 |
| 35 | 19.3 | 0.95 | 0.58 | 0.44 | 2.88 | 3.9 | 898 | 1796 | 18 | 0.29 | 260 | 12.0 |
| 36 | 19.8 | 0.99 | 0.60 | 0.45 | 2.90 | 4.0 | 900 | 1800 | 18 | 0.30 | 270 | 12.0 |
| 37 | 20.3 | 1.02 | 0.61 | 0.46 | 2.92 | 4.1 | 903 | 1806 | 18 | 0.31 | 280 | 12.0 |
| 38 | 20.8 | 1.06 | 0.63 | 0.47 | 2.94 | 4.2 | 905 | 1810 | 18 | 0.32 | 290 | 12.0 |
| 39 | 21.3 | 1.10 | 0.65 | 0.49 | 2.98 | 4.3 | 907 | 1814 | 18 | 0.33 | 209 | 12.0 |
| 40 | 21.8 | 1.14 | 0.67 | 0.50 | 3.00 | 4.4 | 909 | 1818 | 18 | 0.34 | 309 | 12.0 |
| 41 | 22.3 | 1.18 | 0.69 | 0.52 | 3.04 | 4.5 | 912 | 1824 | 18 | 0.35 | 319 | 12.0 |
| 42 | 22.8 | 1.23 | 0.72 | 0.54 | 3.08 | 4.6 | 914 | 1828 | 18 | 0.36 | 329 | 12.0 |
| 43 | 23.3 | 1.27 | 0.74 | 0.56 | 3.12 | 4.7 | 917 | 1834 | 18 | 0.37 | 339 | 12.0 |
| 44 | 23.7 | 1.31 | 0.76 | 0.57 | 3.14 | 4.8 | 917 | 1834 | 18 | 0.38 | 349 | 12.0 |
| 45 | 24.2 | 1.36 | 0.78 | 0.59 | 3.18 | 4.9 | 918 | 1836 | 18 | 0.39 | 358 | 12.0 |
| 46 | 24.7 | 1.40 | 0.80 | 0.60 | 3.20 | 5.0 | 920 | 1840 | 18 | 0.40 | 368 | 12.0 |
| 47 | 25.2 | 1.45 | 0.83 | 0.62 | 3.24 | 5.1 | 923 | 1846 | 19 | 0.41 | 378 | 12.0 |
| 48 | 25.6 | 1.50 | 0.85 | 0.64 | 3.28 | 5.2 | 923 | 1846 | 19 | 0.43 | 378 | 12.0 |
| 49 | 26.1 | 1.55 | 0.88 | 0.66 | 3.32 | 5.3 | 924 | 1848 | 19 | 0.44 | 407 | 12.0 |
| 50 | 26.6 | 1.60 | 0.90 | 0.68 | 3.36 | 5.4 | 928 | 1858 | 19 | 0.45 | 418 | 12.0 |
TVI = Theoretical Vertical Interval
Hr = Height of Riser
Wr = Width of Riser
Wt = Width of terrace
VI = Vertical Interval
L = Length of terrace per ha
A = Area of platform per ha
Pb = Percentage of platform
C = Cross Section of the ditch
V = Volume of cut per ha
D = Incline Distance
These are primarily narrow bench terraces built across slopes where fruit trees or food trees are planted.
Their spacings differ from the hillside ditches and are determined by the distance between each planted tree.
Orchard terraces are normally used on steep slopes and for this reason, permanent vegetative cover such as grass should be maintained on the spaces between each terrace.
Criteria for selection
- On steep slopes (25 to 30 degrees), use 1.75 m wide benches (flat strips).
- For orchards on moderate slopes where mechanization is used, use 3.5 m wide benches.
The selection of the terraces, whether reverse or outward-sloped, depends on climate zones as explained earlier in the section on hillside ditches.
- To provide erosion control practices for orchards situated on sloping land.
- To facilitate the management of orchards on steep slopes.
- To serve both as access roads and as drains.
- on steep slopes, 25 to 30 degrees.
- on gentle slopes where cultivation of orchards is planned.
Length: 100 m for one direction - in humid regions.
Width: 1.75 or 3.5 m, as explained earlier.
Gradient: Same as for hillside ditches except for 3.5 m wide benches; the reverse slope is
5%.
Risers: Riser slope is 0.75:1 for hand-made terraces and 1:1 for machine made terraces. Riser width is riser height multiplied by 0.75 or 1.
Spacing: The planting distance of the particular fruit or food trees is us to determine the spacing of the ditches. Normally it is 10.5 m to 14 m for two-row trees.
Terrace area and linear length: The linear length per ha is obtained by the following formula:
L = 10 000 / HI
L : Linear length, in m
HI : Horizontal distance between two terraces
Terrace area is linear length multiplied by the width of the benches (1.75 m or 3.5 m).
Volume and cross-section
Same calculations as for hillside ditches (Fig. 5).
Fig. 7 shows the cross-section and plan views of two-row orchard terraces. Note that one terrace serves two rows of trees.
Fig. 8 shows all the formulas for calculating 1.75 m wide reverse-sloped orchard terraces.
Table 3 provides a set of specification tables for steep slopes and for hand-made 1.75 m wide reverse-sloped terraces.
The planting distances of trees are 6, 7 and 8 m. For orchards on gentle slopes the distance should be 3.5 m; where mechanized methods are used, specification tables should be recalculated.
Layout
General layout considerations are similar to those for bench terraces and hillside ditches. Drainage can be situated in the middle of the terrace so that the grass strips can be used as protection in order to reduce costs. Tree spacing must be taken into consideration in order to determine whether there is enough space for two rows or only for one. For crops such as coffee, multiple rows are required.
Surveying
Surveying procedures are similar to those for hillside ditches. Individual tree sites should be marked with stakes. The trees should not be situated too close to the terraces. If the slope becomes steeper and the terraces are therefore spaced closer together, only one row of trees should be staked out.
By manual labour
The method is the same as that used for constructing hillside ditches.
1. Width of Platform Wb : 1.75 m
2. Theoretical Vertical Interval
TVI = S * Wb/100 - S x U
(S: Slope in %
U: 0 -75)
3. Reverse Height : RH = Wb x 0.10
4. Height of Riser : Hr = (TVI + RH)/2
5. Width of Riser : Wr = Hr x U
6. Width of Terrace : Wt = Wb + 2 Wr
7. Horizontal. Interval : HI = 10.5 m, 12 m or 14 m
8. Linear Length : L = 10000/HI (per ha)
9. Area of Platform : A= L x Wb
10. Percent of Platform : Pb = (A/10000) x 100
11. Cross Section of Terraces : C = Wb x 2 Hr/8
12. Volume of Cut and Fill : V = L x C (per ha)
Mechanized construction
For 3.5 m wide benches on moderate slopes, a medium-size machine can be employed. The
procedures are similar to those used for constructing bench terraces and hillside ditches.
Output is similar to hillside ditches when manual labour is used (an average of 3.5 cubic m per man-day). on gentle slopes, a medium-size machine (100-150 H.P.) can complete 40-50 cubic m per hour.
On the same range of slopes, the cost of orchard terracing is fairly similar to hillside ditching. However, on gentle slopes, orchard terraces are more expensive than hillside ditches because the spacings between two terraces are closer and the width is greater (3.5 m compared to 2 m).
Benches
The same as for hillside ditches.
Access roads
For hand cultivation, a simple road is required. For orchards on gentle slopes where mechanized methods are to be used, a hexagon-shaped road network that surrounds the orchard terrace area is needed to enable the machines to manoeuvre freely.
Protection
Same as for hillside ditches plus the following auxiliary measures:
-Individual basins for fruit trees.
-Shade-tolerant grass or cover crops should be grown between the basins.
Maintenance
Grass or vegetative cover should be kept low in the spaces between terraces. Refer to maintenance section of hillside ditches for details of other measures.
Orchard Terraces (1)
(Hand made, for 6 m x 6 m tree spacings)
Width of Platform: 1.75
Horizontal Interval: 10.5m
|
Slope |
Specification |
||||||||||||
| % |
Grade |
U |
TVI (m) |
RH ( m) |
Hr(m) |
Wr(m) |
Wt(m) |
HI (m) | L(m) | A (m²) | Pb (%) | C(m²) | V (m³ ) |
| 47 |
25.2 |
0.75 |
1.27 |
0.18 |
0.73 |
0.55 |
2.85 |
10.5 | 952 | 1667 | 16.67 | 0.32 | 305 |
| 48 |
25.6 |
0.75 |
1.31 |
0.18 |
0.75 |
0.56 |
2.87 |
10.5 | 952 | 1667 | 16.67 | 0.33 | 314 |
| 49 |
26.1 |
0.75 |
1.36 |
0.18 |
0.77 |
0.58 |
2.91 |
10.5 | 952 | 1667 | 16.67 | 0.34 | 324 |
| 50 |
26.6 |
0.75 |
1.40 |
0.18 |
0.79 |
0.59 |
2.93 |
10.5 | 952 | 1667 | 16.67 | 0.35 | 333 |
| 51 |
27.0 |
0.75 |
1.45 |
0.18 |
0.82 |
0.62 |
2.99 |
10.5 | 952 | 1667 | 16.67 | 0.36 | 343 |
| 52 |
27.5 |
0.75 |
1.49 |
0.18 |
0.84 |
0.63 |
3.01 |
10.5 | 952 | 1667 | 16.67 | 0.37 | 352 |
| 53 |
27.9 |
0.75 |
1.54 |
0.18 |
0.86 |
0.65 |
3.05 |
10.5 | 952 | 1667 | 16.67 | 0.38 | 362 |
| 54 |
28.4 |
0.75 |
1.59 |
0.18 |
0.89 |
0.67 |
3.09 |
10.5 | 952 | 1667 | 16.67 | 0.39 | 371 |
| 55 |
28.8 |
0.75 |
1.64 |
0.18 |
0.91 |
0.68 |
3.11 |
10.5 | 952 | 1667 | 16.67 | 0.40 | 381 |
| 56 |
29.2 |
0.75 |
1.69 |
0.18 |
0.94 |
0.71 |
3.17 |
10.5 | 952 | 1667 | 16.67 | 0.41 | 390 |
| 57 |
29.7 |
0.75 |
1.74 |
0.18 |
0.96 |
0.72 |
3.19 |
10.5 | 952 | 1667 | 16.67 | 0.42 | 400 |
| 58 |
30.1 |
0.75 |
1.80 |
0.18 |
0.99 |
0.74 |
3.23 |
10.5 | 952 | 1667 | 16.67 | 0.43 | 499 |
U = Riser Slope
TVI = Theoretical Vertical Interval
RH = Reverse Height
Hr = Height of Riser
Wr = Width of Riser
Wt = Width of Terrace
HI = Horizontal Interval
L = Linear Length per ha
A = Area of Platform
Pb = Percent of Platform
C = Cross Section of terrace
V = Volume of Out and Fill per ha
Orchard Terraces (2)
(Hand made, for 7 m x 7 m tree spacings)
Width of Platform: 1.75m
Horizontal Interval: 12m
|
Slope |
Specification |
||||||||||||
| % |
Grade |
U |
TVI(m) |
RH(m) | Hr(m) |
Wr(m) |
Wt(m) |
HI(m) | L(m) | A(m²) | Pb(%) | C(m²) | V(m³) |
| 47 |
25.2 |
0.75 |
1.27 |
0.18 | 0.73 |
0.55 |
2.85 |
12 | 833 | 1458 | 1458 | 0.32 | 267 |
| 48 |
25.6 |
0.75 |
1.31 |
0.18 | 0.75 |
0.56 |
2.87 |
12 | 833 | 1458 | 1458 | 0.33 | 275 |
| 49 |
26.1 |
0.75 |
1.36 |
0.18 | 0.77 |
0.58 |
2.91 |
12 | 833 | 1458 | 1458 | 0.34 | 283 |
| 50 |
26.6 |
0.75 |
1.40 |
0.18 | 0.79 |
0.59 |
2.93 |
12 | 833 | 1458 | 1458 | 0.35 | 292 |
| 51 |
27.0 |
0.75 |
1.45 |
0.18 | 0.82 |
0.62 |
2.99 |
12 | 833 | 1458 | 1458 | 0.36 | 300 |
| 52 |
27.5 |
0.75 |
1.49 |
0.18 | 0.84 |
0.63 |
3.01 |
12 | 833 | 1458 | 1458 | 0.37 | 308 |
| 53 |
27.9 |
0.75 |
1.54 |
0.18 | 0.86 |
0.65 |
3.05 |
12 | 833 | 1458 | 1458 | 0.38 | 317 |
| 54 |
28.4 |
0.75 |
1.59 |
0.18 | 0.89 |
0.67 |
3.09 |
12 | 833 | 1458 | 1458 | 0.39 | 325 |
| 55 |
28.8 |
0.75 |
1,64 |
0.18 | 0.91 |
0.68 |
3.11 |
12 | 833 | 1458 | 1458 | 0.40 | 333 |
| 56 |
29.2 |
0.75 |
1.69 |
0.18 | 0.94 |
0.71 |
3.17 |
12 | 833 | 1458 | 1458 | 0.41 | 342 |
| 57 |
29.7 |
0.75 |
1.74 |
0.18 | 0.96 |
0.72 |
3.19 |
12 | 833 | 1458 | 1458 | 0.42 | 350 |
| 58 |
30.1 |
0.75 |
1.80 |
0.18 | 0.99 |
0.74 |
3.23 |
12 | 833 | 1458 | 1458 | 0.43 | 358 |
U = Riser Slope
TVI = Theoretical Vertical Interval
RH = Reverse Height
Hr = Height of Riser
Wr = Width of Riser
Wt = Width of Terrace
HI = Horizontal Interval
L = Linear Length per ha
A = Area of Platform
Pb = Percent of Platform
C = Cross Section of Terrace
V = Volume of jut and Fill per ha
Orchard Terraces (3)
(Hand made, for 8 m x 8 m tree spacings)
Width of Platform: 1.75m,
Horizontal Interval: 14m
|
Slope |
Specification |
||||||||||||
| % | Grade | U |
TVI(m) |
RH(m) | Hr(m) | Wr(m) | Wt(m) | HI(m) | L(m) | A(m²) | Pb(%) | C(m²) | Vm³ |
| 47 | 25.2 | 0.75 |
1.27 |
0.18 | 0.73 | 0.55 | 2.85 | 14 | 714 | 1250 | 12.50 | 0.32 | 228 |
| 48 | 25.6 | 0.75 |
1.31 |
0.18 | 0.75 | 0.56 | 2.87 | 14 | 714 | 1250 | 12.50 | 0.33 | 236 |
| 49 | 26.1 | 0.75 |
1.36 |
0.18 | 0.77 | 0.58 | 2.91 | 14 | 714 | 1250 | 12.50 | 0.34 | 243 |
| 50 | 26.6 | 0.75 |
1.40 |
0.18 | 0.79 | 0.59 | 2.93 | 14 | 714 | 1250 | 12.50 | 0.35 | 250 |
| 51 | 27.0 | 0.75 |
1.45 |
0.18 | 0.82 | 0.62 | 2.99 | 14 | 714 | 1250 | 12.50 | 0.36 | 257 |
| 52 | 27.5 | 0.75 |
1.49 |
0.18 | 0.84 | 0.63 | 3.01 | 14 | 714 | 1250 | 12.50 | 0.37 | 264 |
| 53 | 27.9 | 0.75 |
1.54 |
0.18 | 0.86 | 0.65 | 3.05 | 14 | 714 | 1250 | 12.50 | 0.38 | 271 |
| 54 | 28.4 | 0.75 |
1.59 |
0.18 | 0.89 | 0.67 | 3.09 | 14 | 714 | 1250 | 12.50 | 0.39 | 278 |
| 55 | 28.8 | 0.75 |
1.64 |
0.18 | 0.91 | 0.68 | 3.11 | 14 | 714 | 1250 | 12.50 | 0.40 | 286 |
| 56 | 29.2 | 0.75 |
1.69 |
0.18 | 0.94 | 0.71 | 3.17 | 14 | 714 | 1250 | 12.50 | 0.41 | 293 |
| 57 | 29.7 | 0.75 |
1.74 |
0.18 | 0.96 | 0.72 | 3.19 | 14 | 714 | 1250 | 12.50 | 0.42 | 300 |
| 58 | 30.1 | 0.75 |
1.80 |
0.18 | 0.99 | 0.74 | 3.23 | 14 | 714 | 1250 | 12.50 | 0.43 | 307 |
U = Riser Slope
TVI =Theoretical Vertical Interval
RH = Reverse Height
Hr = Height of Riser
Wr = Width of Riser
Wt = Width of Terrace
HI = Horizontal Interval
L = Linear Length per ha
A = Area of Platform
Pb = Percent of Platform
C = Cross Section of Terrace
V = Volume of Cut and Fill per ha
These are small round benches for individual plants. This treatment alone is usually not sufficient for minimizing erosion and should be supplemented by the use of hillside ditches, orchard terraces and other measures.
Criteria for selection
- Small trees or plants on steep slopes: use 1.5 m diameter basins.
- Big trees or plants on gentle slopes: use 2.1 m diameter basins.
- Direct seeding of forest trees: use 0.9 m diameter basins.
- To minimize soil erosion on slopes when used in conjunction with other treatments.
- To provide a flat platform for moisture conservation, better weed control and fertility management of tree crops.
- On slopes up to 30 degrees.
- On dissected lands with variable soil depth and the occasional presence of stones.
- For the initial period of establishing tree crops and semi-permanent plants.
- For direct seeding of forest trees.
Riser slopes: 0.75:1.
Reverse slope: 10 %
Spacing: According to the required tree spacings.
Volume: The volume of a basin can be estimated as a semi-wedge of a truncated cone.
Outlet: Around the upper part of the basin, a 15 cm wide and 10 cm deepoutlet is needed to intercept and drain the run-off from the slope above in humid regions.
Fig. 9 shows the cross-sectional view of an individual basin.
Table 4 shows the specifications of the basins and their respective volumes.
Rows of individual basins should be laid out along contours. Each basin should be marked and staggered according to the required crop spaces. They should not be placed too close to the risers or cut-banks of hillside ditches or orchard terraces.
Basins should be built reverse-sloped. Risers must be compacted firmly and if available, stones should be used to protect them. If an outlet is needed, it should be made slightly outward-sloped (3%) for drainage purposes. If basins are to be built around existing plants, filling instead of digging is necessary to avoid disturbing the root systems.
Table 4 provides output figures detailing the number of basins of varying diameters which can be built per man-day on different slopes. Field experiments showed similar results as those indicated in the table.
The cost of basins on steep slopes may be two or three times the cost of those situated on gentle slopes. If the slope gradient is the same, the cost per ha increases according to the number of basins and their diameter.
Basins
Mulch should be applied (grass, leaves, crop residues or gravel) in order to conserve moisture.
Spacing
In humid climates, individual basins should be supplemented by hillside ditches or orchard terraces for the safe draining of excess run-off. Cover crops, especially perennial, non-aggressive legumes, are recommended for growing on the spacings.
Protection
The same as for other kinds of terraces. No grazing should be allowed.
Maintenance
The basins must be shaped back after tillage or the application of fertilizer. Basin outlets should be kept open and grass kept out of the basin platform. Young shoots such as bananas should be kept in the middle of the basins.
| Degree of slope (°) | 0.9 m (3') Day |
1.5 m (5') Day |
2.1 m (7') Day |
|||
|
|
Vol. | No/Day | Vol. | No/day | Vol. | No/day |
| m³ | m³ |
m³ |
||||
|
10 |
0.024 |
112 |
0.112 |
24 |
0.307 |
9 |
|
12 |
0.028 |
96 |
0.132 |
20 |
0.561 |
7 |
|
14 |
0.033 |
82 |
0.146 |
18 |
0.420 |
6 |
|
16 |
0.038 |
71 |
0.177 |
15 |
0.484 |
6 |
|
18 |
0.044 |
61 |
0.202 |
13 |
0.554 |
5 |
|
20 |
0.050 |
54 |
0.230 |
12 |
0.630 |
4 |
|
22 |
0.056 |
48 |
0.261 |
10 |
0.715 |
4 |
|
24 |
0.064 |
42 |
0.295 |
9 |
0.810 |
3 |
|
26 |
0.072 |
38 |
0.334 |
8 |
0.917 |
3 |
|
28 |
0.082 |
33 |
0.379 |
7 |
1.041 |
3 |
|
30 |
0.093 |
29 |
0.431 |
6 |
1.184 |
2 |
Remarks:
1. Riser slope: 0.75:1
2. No/day: No. of basins per manday
3. Average output: using 2.7 m³ per manday
