Types of Bench Terraces | Erosion Control | Soil Management

The following are the common types of transitional terraces used for soil conservation: 1. Convertible Terraces 2. Intermittent Terraces.

1. Convertible Terraces:

These are the bench terraces, which are alternated with the original land slope. Slope limit varies from 12 to 36% or 7 to 20 degrees. These can be used for mixed cultivation comprising annual crops and tree crops.

Suitability of this type of terraces is mainly for following conditions:

i. Humid tropical regions.

ii. Semi-arid regions – the out- ward-sloped benches are being good.

In humid tropical regions the reverse-sloped benches are prefer­red, while in semi-arid regions the outward-sloped benches are found suitable.

The location and conditions for their use are mentioned as under:

i. On the slopes less than 36% (20 degrees).

ii. For mixed farming systems.

iii. For the farmers who have not finalized their long-term plan for using the land.

Objectives:

Various objectives of this type of terraces are outlined as under:

i. To make compatibility of hilly topography with the mixed farming systems.

ii. To create scope for future land development, and flexibility in future land use.

iii. To protect sloping lands from soil erosion.

Design Specifications:

Its various design specifications are described as below:

i. Length and Width:

The length and width of convertible terrace can be up to 100 m and 3.5 m, respectively.

ii. Gradients:

The requisite gradients are as below:

a. Horizontal grade 0.5 to 1% in humid regions, and level to 0.5% in arid and semi-arid regions.

b. Reverse grade 5%.

c. Outward grade 3%.

iii. Riser Slope:

In case of manually constructed terraces the riser slope may be 0.75:1; and 1:1 in machine constructed terraces.

iv. Vertical Interval:

It is normally same to the hillside ditches.

Layout:

It is done under following steps:

i. Use the same spacing and principle for staking as in case of hillside ditches.

ii. Set a base line; and start the staking by considering the space required for the fruit/food trees. On gentle slope, the inclined distance or horizontal distance (centre-to-centre) should be taken into consideration for staking.

iii. Determine the width of benches from the specification table of bench terraces. Normally, 3.5 m wide benches on different slopes are used.

iv. If layout is started from the centre stakes of the terraces, then add top and bottom stakes, which is the terrace width obtained from the specification table.

v. The trees, either planted in one or two rows, should not create the shade over the terraces, where food crops are grown.

vi. If trees are of tall variety or the site is in narrow form then only one row of trees should be staked. And where wider spacing and small trees are there, then two or more rows can be staked.

Construction:

Its construction procedure is the same to the bench terraces. Since, the width of terraces is 3.5m, there­fore, they can be constructed either by hand or by a medium-size machine. For draining the excess rainwater, an individual basin should also be provided between plant spaces.

2. Intermittent Terraces:

Intermittent terraces are also the bench terrace. In a cluster total three terraces are constructed, in which only one terrace out of three is constructed at the initial (1st year); and the other two terraces are constructed over the next two years or even over a longer period of lime.

The criterion for their construction are different for following regions:

i. Humid tropical regions.

ii. Semi-arid regions.

In humid tropical regions the reverse slope benches are recommen­ded for construction, while in semi-arid regions the level bench terraces with dyke for runoff storage towards u/s land are found suitable for cons­truction.

Objectives:

The construction objectives of intermittent type terraces are outlined as below:

i. To protect the cultivated slopes from erosion by terracing 30% of the total area, initially.

ii. To spread the total expenditure under terrace construction over a year time period; and thus to reduce the sudden involvement of cost and labour under terrace operation.

iii. To provide a quick rehabilitation of the area.

Design:

Its design is performed based on the following considerations:

i. For runoff collection; and

ii. For general use.

If terrace is to construct for runoff collection then its design should be done by adopting the same principle as for the design of irrigation or level bench terraces. And for general use, the design should be done similar to the reverse-slope bench terraces.

Layout:

The layout is prepared on the same principle as the bench terraces.

However, few important points are outlined as under:

i. After setting the base line, the terrace width should be determined. However, it can also be taken from the specification table.

ii. Staking should be started from the top of the slope to downwards.

iii. The graded contours for the top and bottom lines of the first terrace should be on upward slope.

iv. For future terracing, a space equal to the double of the terrace width should be left.

v. The 4th terrace should be staked in the same way as the first terrace, but leaving the space afterwards for the next two terraces.

vi. Stake other terraces in the same way till to complete the entire area.

Construction:

The construction of intermittent type terraces is done by the same procedure as followed in reverse slope bench terraces. However, if they have been planned for runoff collection then a 15 to 20 cm dyke must be provided to the level benches.

Maintenance:

The protection measures are the same to the bench and convertible terraces. Especially, in humid regions to restrain the soils from down slope, a line of tall thick grass should be planted above the cut bank of the terrace on cultivated slope. The maintenance of reverse slope and runoff collection type terraces is done in the same way as the bench terraces. However, in runoff collection type terraces, it is also very important to maintain the dyke, properly.

Discontinuous Type Terraces:

These terraces are basically the hillside ditches, orchard terraces: or individual basins.

These are described as under:

Hillside Ditches:

The hillside ditches are the discontinuous type narrow bench terraces. These are constructed across the hill slope. Spacing between two ditches is fixed on the basis of cultivation purposes which depends on the land slope, climate and soil conditions.

Sometimes, these ditches are also used as the bench terrace for the purpose of access road and as interception drain. Maintenance of this structure is very easy; and also a part of it can be used for cultivation, if required.

Selection Criteria:

The followings are the important criterion, used for constructing the hillside ditches:

i. For humid tropical regions the reverse slope narrow benches or ditches are suitable.

ii. In semi-arid or arid regions the outward slope or leveled narrow benches or ditches should be used.

iii. On shallow soils relatively wide benches are preferred. The width of bench should be about 1.5 m.

iv. In deep soils relatively wide benches should also be used, but the width of bench should be about 2 m.

Objectives:

The objectives of construction of this structure are outlined as under:

i. To break the longer slopes into several shorter slopes, and reducing the soil erosion.

ii. To intercept the excess runoff from u/s area; and draining the same to a safe point.

Locations and Conditions for Use:

The hillside ditches are found suitable for following locations:

i. On the slopes up to 25 degrees or 47%.

ii. For taking the crops (semi-permanent or perennial crops) between two ditches.

iii. In small size fields under manual cultivation methods.

iv. Where use of a minimum land treatment measure in the form of agronomic conservation measures.

Design Specifications:

View of hillside ditch with different notations is presented in Fig. 13.12. Various specifications of this structure are mentioned below; and also presented in Table 13.8.

1. Length:

Its length is generally kept equal to the length of bench terraces, which is 100 m for humid regions and 200 to 300 m for arid regions.

2. Width:

Its width varies up to 2 m; or 1.5 m plus cut and filled parts (riser).

3. Gradient:

Its gradient is taken as per Table 13.8.

4. Riser:

The specifications of riser in hillside ditches are given as under:

i. Slope – 0.75:1 for hand-made, and 1:1 for machine-made ditches.

ii. Height – It is one-half of the bench terrace.

iii. Width – It is equal to the riser height multiplied by 0.75 or 1.

5. Vertical Interval:

The vertical spacing between two hillside ditches can be computed by using the following formulae:

i. In USA the following formula is used for the purpose –

VI (feet) = aS + b … (13.59)

in which, S is the land slope (%); a is the constant, its value varies from 0.4 for humid South to 0.8 for North; and b is the constant, its value varies from 1 to 4 depending on the soil type and cover conditions or type. In Jamaica the value of a is taken as 0.3, and 2 for b.

ii. In Taiwan (Republic of China) the following formula is used for computing the vertical intervals –

VI (m) = (S + 6)/10 … (13.60)

This formula can also be used for determining the VI, where normal condition prevails.

iii. In El Salvador and Thailand the VI is computed by using the following formula –

VI(m) = (S + 4)/10 … (13.61)

In humid tropical regions, where condition is critical, this formula can also be suitably used.

iv. In Israel, the vertical interval is calculated by the formula, given as under –

VI (m) = (S + 2)/10 … (13.62)

6. Horizontal Interval:

The HI depends on the VI and percent land slope. The following formula can be used for HI –

7. Inclined Distance (Fig 13.12):

It is determined based on the vertical interval and slope angle of land surface. The following formula can be used for its computation –

D = VI/sin θ … (13.64)

In which, Q is the slope angle in degree. In heavy rainfall regions and on erosive sites, the exact value of inclined distance (D) is used for construction. However, for adjusting the cropping pattern and local needs, the computed value of D can be adjusted within 10 to 20% range.

8. Cross-Section and Volume:

The cross sectional area is computed by using the same formula as for the bench terrace, assuming the total height of riser (Hr). The amount of earthwork (cut and fill) is calculated by multiplying the total ditch length and its cross-sectional area.

9. Ditch Area and Total length:

The ditch area per hectare is calculated by multiplying the total length of ditch per hectare and their width. The width of ditch may be taken as 2 or 1.5 m. The total length of ditch per hectare can be computed by using the following formula –

L = 10000/HI … (13.65)

In which, L is the total length of ditch per hectare, and HI is the horizontal distance between two ditches. If total width of ditch is taken as the sum of HI and Wt, then total length of ditch per hectare is given by the following formula –

L = 10000/ (HI + Wt) … (13.66)

In which, Wt is the total width of ditch. In light rainfall regions in clay soils or the crops with good cover, the HI plus the width of ditch (Wt) provides better value of L.

Layout:

Layout of hill side ditches is similar to the bench terraces. These ditches require road connection and a natural drainage or waterway system to safely drain the runoff from the area. Also, if there is farm building and/or windbreaks, then they must be considered in preparation of layout. For proper drainage, the waterway should either be in the middle or at the end of field, depending on the terrain of the area.

Construction Procedure:

1. Manual Method:

The hillside ditches are normally constructed, manually, where labours are easily available at less cost.

Construction is carried out as per below:

i. Start cutting of ditch from the upslope with 10% reverse slope (in reverse sloping ditch) or 3% outward slope (in outward sloping ditch). Allowances should also be given for settling.

ii. Construct the riser, layer by layer, along with thorough compaction.

iii. Check the gradients of ditch as per recommended specification.

iv. If the ditch is to cross any depressed area, then it must be constructed strong.

2. Mechanical Method:

For construction of hillside ditch system using machines, the small earth cutting machines fitted with 2 to 2.5 m wide-angle blade are suitable. The construction method is similar to the bench terraces. Since, the risers cannot be shaped by the machine; therefore it should be done by hand.

3. Operation and Management:

This system mainly consists of ditch and in-between space as the compartment.

The operation and maintenance of these two components is done separately, described as under:

Ditch:

In hillside ditch system the function of ditches is to drain the runoff from the area. Ditches should not be used for cultivation. However, if the farmers need to cultivate them, then every alternative ditch (terrace) should be kept undisturbed so that their access can be maintained. Also, at least 30 cm width should be left uncultivated to act as toe drain.

Inter Space:

The spaces between ditches are either in open condition or under cultivation. These portions are affected by the soil erosion problem. To check the soil erosion from these land segments the erosion control measures are essential. Generally, the agronomical measures are used for this purpose. The agrono­mical measures could be the contour cultivation, contour furrows, staggered and close planting; mulching with residues, trashes and grasses, grass barriers and stone barriers.

Protection:

The protection of ditch system can be done as under:

i. Above the cut bank a line of thick and tall grass should be planted to restrain the soils of upslope.

ii. The protection of risers and outlets should be done in the same way as the bench terraces.

Maintenance:

The maintenance of ditch system should be accomplished under following points:

i. If there is any deposit in the ditches, then that should be removed, to ensure proper water flow through them.

ii. The maintenance should also be done to ensure proper cross section and grade of the ditches.

iii. The cattle should not be allowed for grazing.

iv. A frequent inspection should be made during the first two rainy seasons after construction.

Orchard Terraces:

The orchard terraces are mainly the narrow bench terraces. These are constructed across the land slope, especially where trees can be planted. The trees may be of the fruit or food trees.

Normally, the steep slopes are preferred for construction of orchard terraces. Since these terraces are constructed at steep slopes, so for their protection the grasses of suitable species are also planted between terrace spaces.

The criterion for its selection are mentioned as under:

i. Steep slopes (25 to 30 degrees). At such slopes the width of bench should be about 1.75 m.

ii. Moderate slopes, especially recommended where machineries are used. At such slopes, the width of orchard bench is kept about 3.5 m.

iii. Climate also decides whether the slope of bench should be reverse or outward-sloped.  

Objectives:

Various objectives of construction of orchard type of terraces are outlined as under:

i. To control soil erosion by planting trees across the steep slopes.

ii. To manage the orchards on steep slopes.

iii. Also, to serve as access road and also as drain for water disposal.

Design Specifications:

Its various design specifications are given below:

i. Spacing:

It differs from the hillside ditches. It is determined based on the distance between each planted row of trees. Normally, the spacing varies from 10.5 to 14m for 2-row trees plantation.

ii. Length:

In one direction its length can be up to 100 m in humid regions. However, the length of terrace per ha can also be determined by using the following formula –

L = 10000/HI … (13.68)

Where,

L = linear length of terrace (m)

HI = horizontal distance between two terraces

iii. Terrace Area:

It can be determined by multiplying the linear length of terrace with its width.

iv. Width:

Width of orchard terraces is either 1.75 m or 3.5 m.

v. Gradient:

Its gradient is same to the hillside ditches, except for 3.5 m wide benches. The reverse slope of terrace may be 5%.

vi. Risers:

The provision of risers in such type of terrace is essential, because these terraces are constructed at steep slope. In manually constructed terraces the slope of riser is 0.75:1, while in machine constructed terraces it is 1:1. The width of riser can be determined by multiplying its height to 0.75 or 1.

Table 13.9 enlists various formulae used for computing design specifications of orchard type terraces. The values obtained based on Table 13.9 are valid for orchard terraces (manually constructed) for 6 m x 6 m tree spacing; width of bench as 1.75 m and horizontal interval 10.5 m.

Layout:

Layout of this type of terrace is prepared based on the same considerations as followed in bench terraces and hillside ditches.

However, few important considerations are pointed as under:

i. Drainage system should be located in the centre of the terrace field so that the grass strips can be easily planted to protect the terrace.

ii. The tree spacing should be in such a way that there is available sufficient space for 2-rows or for only 1-row plantation. However, in case of coffee plantation multi-rows are required.

Construction Procedure:

The construction of orchard type terraces is carried out, manually or by machines. In manual method, the procedure is the same as followed for constructing hillside ditches. Mechanical method is used for constructing 3.5 m wide benches on moderate slopes, for which a medium-size machine is used. Construction procedure is same to the bench terraces and hillside ditches.

Maintenance:

Maintenance of orchard type terrace system is normally the same to the hillside ditches important amongst them are outlined as under:

i. Maintenance should be done at individual basin level.

ii. The shade-tolerant grass or cover crops should be grown between the basins.

iii. The height of grass/vegetative cover should be kept small.