How to Design and Construct Bench Terrace? | Soil Management

In this article we will discuss about how to design and construct bench terrace.

Design of Bench Terrace:

The components of bench terrace are determined based on the average annual rainfall, soil types, soil depth and average slope of the land. In addition, the purpose of bench terrace is also considered for the design.

The design of bench terrace system involves the following parameters for determinating:

1. Type of bench terrace

2. Spacing of bench terrace

3. Width of bench terrace; and

4. Terrace cross-section.

The type of bench terrace to be used among different types, is based on the rainfall and soil conditions of the area. For example – if an area receives medium average rainfall, and soil is highly permeable, then bench terrace with level top is recommended for construction.

Terrace Spacing:

Terrace spacing is the vertical distance between two successive bench terraces. It is equal to the twice of depth of cut. It depends on the soil depth and land slope. The depth of cut should not be so deep as to expose the bed rock, because rocky layer makes the bench terraces unsuitable for cultivation.

Furthermore, a greater depth of cut makes wider spacing of bench terrace, which may be unstable, especially at higher slopes. However, there is no hard and fast rule that can be used for determining the terrace spacing for all the conditions.

Usually, on the basis of practical experiences and good judgment with due consideration of prevalent local practices, the spacing should be fixed. The important factors deciding the spacing of terraces are the soil and land slope. At the same time, the terrace width should also be such that as to enable convenient and economic agricultural operations. However, the terrace interval (i.e. spacing) can be computed, using the formula of contour or graded bunds. The formula is given as under –

In which, S is the percent land slope and VI is the vertical interval of bund. Theoretically, use of this formula for computing the terrace interval is not justified, because by bunding practice only length of slope is reduced, while the degree of slope steepness remains un-change; but by bench terracing the degree and length of slope, both are required to change.

The computation of terrace interval can be accomplished under following steps:

1. Determine maximum depth of productive soil. It is one of the most important factors, as it makes the benches suitable for cultivation.

2. Find out the maximum admissible depth of cut for the existing land slope and the crops to be grown. Normally, the depth of cut should be such that as not to expose the unfertile soil layer. One point is also kept in mind that, the cutting depth should enable the construction of terraces with convenient width.

3. After determining the depth of cut, find out the width of terrace by using the following equation:

W = 200 D/S … (13.2)

Where,

W = width of terrace (m)

D = depth of cut (m)

S = slope (%)

4. Determine the VI of bench terrace, using the following formula –

(i) For batter slope 1:1 –

VI = W.S/100 – S … (13.3)

(ii) For batter slope 1/2:1 –

VI = 2 W.S/200 – S … (13 4)

The above equations revealed that, for a given land slope a greater VI makes wider width of terrace; and for a given VI value if slope is more than smaller will be the terrace width.

Morgan (1986) has also reported the following formulae for determining the spacing of terraces.

Morgan’s Formula:

Assuming steady-state conditions, if

Q_w = runoff

L = length of slope on hill side

R = rainfall intensity

i = infiltration capacity of the soil

θ = slope angle

In which, r is the mean hydraulic radius and n is the Manning roughness coefficient. The r can be expressed by the following equation –

Using this equation the value of L can be computed. The R and i are used for 1-hour rainfall of 10-years return period; V is the maximum permissible flow velocity for the given soil, and the value of n is taken as 0.20 for bare soil.

If value of L has been determined, then VI of terrace can be computed by using the equation, given as –

VI = L sin θ … (13.10)

Empirical Formulae:

For computation of bench terrace interval, there have also been developed various empirical formulae, few of them are given as under:

i. USSCS Formula:

VI = a.S + b … (13.11)

In which, VI is expressed in feet; a & b are the constant. The value of a varies from 0.3 for South to 0.6 for North U.S.A regions, and b is 1 or 2, depending on the limit of soil erodibility.

ii. Zimbabwe Formula:

VI = S.f/2 … (13.12)

in which, VI and S are the same as mentioned in previous formula and f is the constant varies from 3 to 6, depending on the soil erodibility value.

iii. South African Formula:

VI = (S/a) + b … (13.13)

In this equation VI and S are the same as in equation 13.12; the value of ‘a’ varies from 1.5 for low to 4.0 for high rainfall areas and b is from 1 to 3 depending on the soil erodibility.

iv. Algerian Formula:

VI (m) = (S/10) + 2 … (13.14)

v. Israelian Formula:

VI (m) = X.S + Y … (13.15)

Where,

X and Y are the constants, in which X varies from 0.25 to 0.3 depending on the rainfall and Y ranges from 1.5 to 2.0 depending on the soil erodibility and S is the land slope (%).

vi. Kenyian Formula:

VI (m) = [0.3 (S + 2)]/4 … (13.16)

vii. New South Wales Formula:

HI (m) = K.S^-0.5 … (13.17)

in which, HI is the terrace spacing (m) and K is the constant varies from 1.0 to 1.4 depending on soil erodibility.

Other Formulae:

There are several others formulae for terrace spacing, developed in different countries.

They are as:

i. Algeria/Morocco:

(i) For the slope ranging from 10 to 25% –

VI = (260 S)^–0.3 … (13.18)

(ii) For the slopes more than 25% –

VI = (64.S)^–0.5 … (13.19)

(VI is expressed in ‘m’)

ii. Taiwan/Jamaica:

VI (m) = S.W_b/[100 – (S.U)] … (13.20)

Where,

W_b = width of shelf (m)

U = slope of riser, expressed as the ratio of horizontal to vertical distance, taken as 1.00 or 0.75.

iii. Taiwan:

This equation is valid for inwardly sloping bench terraces.

Bangar et.al (1983) reported following values of horizontal and vertical intervals of bench terraces, for hilly tracts of Maharashtra state. They also mentioned that, while constructing the bench terraces, about 50% of total width of terrace should be cut and deposited on the rest 50% area. 0.5% back slope should also be given to the terraces to avoid suppression in the shoulder bund to be constructed at the outer edge of the terrace.

Dutta (1986) has also reported the VI for level terraces on various slopes with different widths, given in Table 13.2.

Terrace Gradient:

The provision of terrace gradient, especially in high rainfall areas is very essential to dispose the excess water very quickly. The gradient should be in such a range, so that the water can be moved off without causing erosion problem. It can be determined on the basis of peak runoff rate produced from the upstream bench terraces.

The computation of terrace gradient can be accomplished under following steps:

1. Compute the peak runoff rate produced from the upstream bench terraces. For this purpose the rational formula can be used, which is given as under –

Q_peak = CIA/360 … (13.22)

In which, Q_peak is the peak runoff rate (m³/s); C is the runoff coefficient (dimension less); I is the rainfall intensity for the duration equal to time of concentration and A is the watershed area, i.e. the area under u/s terraces.

2. Calculate the watershed area of terrace (A) by the following equation –

A = L.W/10,000 ha … (13.23)

In which, L is the length of terrace (m) and W is the average width of terrace (m).

3. Select permissible flow velocity (V) for the given soil of the area using the Table 15.3.

4. Find out the approximate value of cross-sectional area of the channel, using the following formula –

Q = A’.V … (13.24)

in which, Q is the peak runoff rate, computed in step (1); A’ is the cross-sectional area of the channel which is required to compute and V is the permissible flow velocity determined in step (3).

5. Calculate the mean hydraulic radius (R) of the obtained cross-sectional area of channel. It is the ratio of cross-sectional area to the wetted perimeter of the channel (A/p).

6. Compute the value of terrace grade, using the Manning’s formula, given as under –

V = 1/n. R^2/3. S^1/2 … (13.25)

In this equation the value of R is known from step (5); V from step (4) and n is taken from the table 15.5 and S has to determine, which is carried out by substituting the above values in equation 13.25 and solving that.

7. The grade of terrace obtained so. should be round off for convenience of layout of terrace system, which is performed by recalculating the flow velocity for the channel cross-section under consideration and verifying the same whether the calculated flow velocity is less than the value selected in step (3) or not.

Width of Bench Terrace:

The width of bench terraces varies with their need for which they are to be used after cons­truction.

Once the width of bench terrace is decided, the depth of cut or spacing of bench terrace can be easily calculated using the following formulae for different cases, given as:

Case (1) – When terrace cut is vertical, then from the Fig. 13.3 the depth of cut may be given by –

D = W.S/100 …(13.26)

In which, W is the width of bench terrace; D/2 is the depth of cut and S is the percent land slope.

Case (2) – When terrace cut has 1:1 slope, then depth of cut may be given as –

D = W.S/(100 – S) …(13.27)

Case (3) – When terrace cut has 1/2:1 slope, then depth of cut is given by –

D = 2 W.S/(200 + S) …(13.28)

However, depending on the land slope and soil conditions, few adjustments are also made between the depth of cut and width of the bench terrace.

Terrace Cross-Section:

The cross section of bench terrace includes following parameters for determining:

1. The batter slope

2. Dimension of shoulder bund

3. Inward and outward terrace slopes; and

4. Size of drainage channel.

1. Batter Slope:

The batter slope is given for providing stability to the filled material or the embankment. The range of batter slope is dependent mainly on the soil material to be used for construction. If batter slope is kept flat then larger area is lost in terracing.

Vertical slopes are generally used in very tough and stable soil conditions, and also when depth of cut is limited to a very small depth say up to 1m. Similarly, the batter slope 1/2:1 is given in loose and unstable soils, depending on the internal friction angle of the soil.

2. Size of Shoulder Bund:

The size of shoulder bund depends on the type of bench terrace. In case of terraces sloping inward the size of shoulder bund is kept nominal, while in the terraces sloping outward and level top, the shoulder bund comprises a larger cross-section for holding the rain water on terrace top. The terrace width and soil condition also affect the size of shoulder bund.

The dimension of shoulder bund used in bench terraces sloping outward is given as under:

Top width = 30 cm

Bottom width = 120 cm

Height = 45 cm

Bangar et.al. (1983) have reported the dimensions of shoulder bunds for different soil depths for Maharashtra state, cited in following table –

3. Inward and Outward Slopes:

The inward and outward slopes of bench terrace depend on the soil type and average rainfall of the area. The inward slope varies from 2 to 10%, and outward slope is from 2 to 8½%.

4. The size of drainage channel is determined on the basis of runoff rate to be disposed from the terrace area. The channel’s grade is provided in the range of 0.5 to 1.0%. Specifications of cross-sections of drainage channels as per size of catchment are given in Table 13.3.

Planning of Bench Terrace System:

Principle:

The planning of bench terrace system is done with the view to keep the earthwork minimum; to avoid longer movement of earth material, and to keep the terrace boundary parallel to the contour in level bench terrace, while in inwardly sloping bench terrace the boundary to be on grade.

For alignment of bench terrace the following points are taken into consideration:

1. The field boundaries and tillage convenience should always be counted for alignment.

2. Alignment should be such that, at least minimum width of terrace is available there for performing agricultural operations.

3. The adjustment or deviation if required, that should be done near to the field boundary.

4. All sharp or inconvenient curves should be conveniently eased out; and if some deviation is necessary then it should be compensated from the contour line.

Normally, the alignment of bench terraces is done for following two conditions:

1. When boundary of bench terrace does not follow the contour then the earth work will be more, and to get proper construction of terrace, the land levelling is required across the slope and in longitudinal directions, both.

2. When boundary of bench terrace is at the contour then construction of bench terrace involves minimum earthwork, and levelling is done only in cross-wise direction.

The boundary of level bench terrace is aligned along the contour line, but in case of inwardly sloping bench terraces having same bench boundary, they are executed on the longitudinal grade, facing towards outlet.

Procedure:

It is performed under following steps:

Step (1) – Determine the percent land slope and depth of productive soil.

Step (2) – Fix the depth of cut suitable to cultivation. It should be half of the vertical interval. In addition, after construction the depth of soil must be at least 15 cm deeper than VI to ensure sufficient soil depth on the cut side after levelling work.

In case, if soil is very deep then vertical interval (VI) should be fixed in such a way that the cost of earth work for levelling work is not very high.

Step (3) – Calculate vertical interval, based on the percent land slope, width of terrace and depth of soil. Usually, it is kept greater than 2 (depth of soil – 0.45).

The width of bench terrace decides the horizontal interval at a particular slope.

It is fixed as per following conditions:

(a) When a land which has steep slope, and on which all agricultural operations have to accomplish manually; and vegetable crops are desired to cultivate on them, then a narrow bench terrace of 1.5 to 3.0 m width is recommended for construction.

(b) When land slope is gentle, then width should not exceed 30m, but not be less than 10m for the condition when all cultivation operations are desired to perform by using bullock power.

Construction of Bench Terrace:

Construction of bench terrace is generally started from the highest point of the area, and proceeded towards lowest point. In other words, it is started from the highest point and moving downward. In this method the top soils and sub-soils are thoroughly mixed, especially when top soil is not available in enough quantity to make the terrace.

In contrast, when sub-soil is not good, then it is necessary to keep them apart and should be spread on the terrace after construction. The construction of terraces can also be started from the lower point and proceeded upper side of the area. In this case, after completion of first terrace the top soil of second terrace is spread on the top of first terrace. This process is continued for subsequent terraces, till completion of entire terraces in the field.

The bench terraced areas should have linkage with suitable outlet for safe disposal of surface runoff from the terraces. Generally, the land depression where vegetation has been developed in good form, can be used as outlet. But, where such types of outlets are not available or not feasible, then grassed waterways can be constructed.

Construction Procedure:

The construction of bench terraces is performed cither manually or by using machines.

These two methods are described as under:

1. Construction by Manual Labour:

The construction of terraces using manual labours is preferred, especially when soil is neither too dry nor loo wet. The construction is stared from the top of hill and proceed down slope. However, if there have been used conservation measures in the field then it should be started from the bottom of hill; and proceeded upward. But in this case, a temporary protection measures is essential to use. Before starting construction, the positions of terraces should be staked in the field.

The initial cut should be done below the top stakes; and filling should be done at the bottom stakes to ensure a correct grade without over cutting. If the rocks or soil clods are available at the site then they should be placed along the bottom line of stakes in the form of foundation, before filling.

Also, during filling, the soil should be compacted using beater for every 15cm filled layer. If the filled soil layer is thick, then compaction is difficult. As precautionary measure, if terrace is to construct across a depression then it should be constructed strong. For this particular case, the edge of terrace should be kept little higher than the planned height as settlement allowance. In terrace system the settlement allowance is given about 10% of the depth of soil fill.

During construction the reverse and horizontal grades should be checked using level; and if there is any correction, that must be done, immediately. The riser slope should be given to a proper range, which is normally 0.75:1. Also, the terraced area should be connected with the waterway to drain the excess rainwater from there. The elevation of terrace outlet should be higher than the bottom of waterway to provide a gradient for flow of water from terraced area to the waterway.

2. Construction by Machines:

The bulldozers are used for constructing the terrace. By this machine the soil cutting is done parallel to and about 50cm apart from the top line of stakes. The cut soil is dumped just above the bottom line. The angled blade should be used for cutting the soil to the depth of 40 to 50cm. The filled soil should be compacted by the bulldozer. The compaction should be done by passing the bulldozer from one end of the terrace to the other end.

The passing of bulldozer should always be done along the edge of bench for compacting purposes. The cutting and filling of soil is continued until the terrace has been constructed for the required length in the area, with a proper vertical and horizontal intervals and grades. During construction the grades of terrace should also be checked using dumpy level. And the riser slope should be shaped by hand to 1:1.

Topsoil Preservation:

In bench terrace construction the soil gets exposed to the infertile sub-soil layer. Which can result reduced crop production, unless few improvement measures are undertaken. In this condition, the exposed soil of terraced land should be treated for making improvement in fertility status of the soil.

This work can be carried out by adopting following methods:

i. The terraces should be constructed from the bottom of slope; and mowed upward. The cut soil from bottom terrace should be kept somewhere, and should be spread on the top of bench terrace, after construction. Same procedure is followed for constructing the next terrace, up the slope; and is continued till completion of entire terrace construction in the field.

ii. By pushing the topsoil horizontally to the next section before cutting the terrace, is another way to maintain the top soil of bench terrace, fertile. In case of manual construction, the topsoil can be piled along the centre line, provided that the width of bench is sufficient.

Earthwork Per Hectare:

The earthwork involved in construction of different types of bench terraces, can be computed using the following formulae:

(a) For level bench terrace –

E_w = 1250 VI

= 1250 x (W.S/100) = 12.50 W.S … (13.29)

(b) For inwardly sloping bench terrace –

E_w = 1250 x VI = 1250 x W (S + s)/100

= 12.50 W (S + s) … (13.30)

(c) For outwardly sloping bench terrace –

E_w = 1250. VI = 1250 x W (S – s)/100

= 12.50 … (13.31)

Where,

E_w = earth work per hectare (m³)

W = width of terrace (m)

S = land slope (%)

s = inward or outward slope of the bench (%).

The cost of earth is calculated by multiplying the rate of earth work (i.e. Rs/m³) to the total volume of earthwork made in terrace construction, i.e.

Cost/ha = Earth work x Cost

Cost of Bench Terracing:

The cost of bench terracing is governed by the volume of earth work, its rate and method employed for earth work (i.e. manual or machines). The total volume of earth work made in the construction depends on various factors, such as condition of the surface soil and sub-soil, vegetations existing on the land surface, surface irregularities (i.e. undulations, depressions, mounds etc.), land slope, terrace specification, deviations made for better alignment, terrace outlet and maintenance by grass planting.

The cost of bench terrace construction is determined on the basis of volume of terrace per unit area, which can be obtained from the Table 13.3. The terrace volume is the total volume of earth work made under cut and fill works. The following formula can be used for computing the cost of terrace construction, based on the volume of soil cut and fill, output per man-day or machine and unit wages of man and machine used under construction work-

C = (V/T) R … (13.32)

in which, C is the cost of construction of bench terrace based on the volume of cut and fill; T is the output per man-day or per machine hour and R is the wages per man-day or per hour. If there has been taken measure for topsoil preservation, then its cost should also be taken into consideration at the rate of 15 to 20 hours per hectare in case of medium size machine; and 40 man-days per hectare in case of manual construction.

Other Computations Related to Bench Terrace System:

Bench Terrace Supported by Stone Wall:

It is a special type of bench terrace, which is constructed by filling the soil behind the stone wall constructed at the outer edge of the terrace width, as shown in Fig. 13.4. The function of stone wall is to create a space towards upstream face for filling the soil over natural ground slope, and also provide stability to the outer edge of the filled materials.

The construction of this type of bench terrace is technically feasible for those areas, where stones are available in large quantities close to the construction site, and where depth of soil is very poor but for human lives the cultivation of crops is very essential. However, the lands accompanied by stone boulders or stony surfaces are avoided for this practice. Such type of terraces are generally constructed in Telangana and Coastal areas of Andhra Pradesh.

The construction details are described as under:

Horizontal Interval:

It is determined on the basis of following parameters:

(i) Retaining wall’s height (i.e. stone wall); and

(ii) Land slope

By knowing above two parameters the following formula can be used for computing HI –

HI = 100 H/S … (13.39)

In which, HI is the horizontal interval (m); H is the height of retaining wall (m) and S is the land slope (m/100 m). This formula is applicable when bench terraces are constructed by making the stone retaining wall over the slope and up-stream area of the wall is filled by the soil brought from some other places.

This particular method is used in that condition when slope does not contain sufficient soil depth. However, when there is sufficient soil depth, then this type of terraces can also be formed by cutting and moving the soil from upper part of slope area to the lower part for construction. For this specific case the horizontal interval of bench terrace supported by stone wall can be computed by using the following equation –

Where,

H₁ = depth of excavation at upper part of land slope (m)

H₂ = height of soil filled in lower part.

This method involves less construction cost as compared to the previous method, because transportation of soil from other places to the construction site is not there, but soil is excavated and filled simultaneously on the site, itself.

The above two methods have some draw backs, by virtue of which they require some modifications in construction procedure.

The important drawbacks are mentioned as follows:

(a) These terraces are in narrow shape, as result the use of agricultural implements on them becomes very difficult.

(b) The construction cost gets increase with increase in the height of stone wall.

(c) This type of terraces are constructed at the site where soil depth is not sufficient, and are formed either by filling the slope by bringing the soil from outside or by cutting upper part and filling at lower part. In both the conditions, the soil depth is not being uniform throughout terraced area; at lower part the soil depth is more as compared to the upper part, which causes effect on crop yield.

(d) The soils filled at lower part (i.e. close to the stone wall) are likely to get removed during rainfall through the holes formed in the wall. The removed soils are deposited towards next down-stream terraces, which cause the top of terrace, uneven.

(e) It is generally observed that, during rainfall the water gets move through the wall and wets the terrace below. In this condition, the wet areas are required to drain; for which additional drainage arrangement is required.

Modifications in Terrace Construction:

Modification in terrace construction includes:

(a) Construction of stone wall by making its half height into the soil and other half above the soil. By doing so, the lower half height of wall acts as foundation which provides structural stabi­lity to the terrace and wall, both. The top of bench surface should also be modified into presumed stable slope.

For this condition the horizontal interval can be computed by using following equation –

Where,

D₁ = present soil depth (m)

D₂ = minimum soil depth required for crop cultivation (m)

S₁ = present land slope (m/100 m)

S₂ = presumed stable slope (m/100 m)

(b) For eliminating the problem of soil wetting at d/s of the wall, the furrows (drainage channel) are opened at the foot of stone wall. The capacity of furrow is kept nominal, because a little volume of water is needed to drain. More importantly the furrows should be provided by continuous longitudinal slope, so that the water entering the furrow can be drained, immediately. To make the water drainage more effective, it is also very important to grade the strip, along which the furrows run.

Height of Stone Wall:

Referring Fig. 13.5 the height of stone wall is the sum of following components:

1. Depth of soil presumed to be lost by the surface flow from the foot of the stone wall (m), let it is H₁.

2. Depth of soil presumed to be deposited near the wall (m) as H₂.

3. Safety factor towards toe of the wall (X₁).

4. Safety factor at top of the wall (X₂).

It is thus, given as –

H = (H₁ + X₁) + (H₂ + X₂) … (12.41)

The values of X₁ and X₂ are normally taken as 30 and 10 cm, respectively.

Disposal of Runoff:

The runoff disposal from terraced land can be performed by several means such as by constructing deep and narrow ditches using stones, concrete or simply by earth materials. These ditches are constructed in straight line down the slope or diagonally down.

The use of such ditches is not find suitable, where cultivation is done by using machines such as tractor etc., because these ditches cause difficulty in crossing machineries. However, the drainage work can also be performed by making grassed waterway of wide and shallow cross-section, able to get easily crossed by the farm machineries.

However, it has limitation that, the construction of grassed ways is not practically possible on the slopes greater than 15%, as on higher slope the vegetations are not able to provide necessary protection. In Retaining wall Retaining wall such situation, the use of grassed waterway is advised to accomplish along with drop structures or paving the same using stones.

The runoff disposal using combination of drop structure and grassed water way is shown in Fig. 13.6. The drop structure is constructed by making extension of the retaining wall, which height is kept lower than the wall. Normally, the height of drop structure is fixed as per amount of flow passing through the waterway.

One point is also kept in view that, when retaining walls are constructed at farther intervals then additional drops should be essentially constructed in the water way to dissipate the erosive effect of flowing water. Grass plantation in water way is also recommended for complete protection.

The stone paving is also done for safe disposal of water through the waterway. Although, the stone paving involves heavy cost, but it also makes the waterway to use as field track, in addition to safe disposal of water. The stone paving is normally recommended, when stones are readily available in nearby areas.

Construction Cost:

It includes following cost items for estimation of construction cost:

(i) Cost of stone.

(ii) Cost of labour charge for making wall.

(iii) Cost of earth work.

The cost of stone is determined on the basis of quantity of stones required to construct the wall of given specification. Normally, the following specification is used in practice for constructing the stone wall of 1.0 m height.

(i) Bottom width = not less than 0.8 m

(ii) Top width = about 0.3 m

(iii) Depth of foundation = 0.3 m

Usually, one meter terrace length involves about 0.55 m³ of stones for constructing the wall using above dimensions. The cost of labour charge is concerned, one skilled labour can dispose 1 m³ of stone including excavation of the foundation, per day. The cost of earth work is determined on the basis of volume of earth work made under cutting and filling of the soil and transportation charges of soil, when it is brought from some other places.