AEG-101-Class-13 – Bench Terracing

Introductory Soil and Water Conservation Engineering

II Semester 3^rd Feb to 30^th June 2020, 2019-20

Teacher Information

Professor	Email	Phone
Dr. K. C. Shashidhar	shashidhar.kumbar@gmail.com	9448103268

Class-13 – Reference Material

Bench Terracing

Bench terracing is one of the most popular mechanical soil conservation practices adopted by farmers of India and other countries for ages. On sloping and undulating lands, intensive farming can be only adopted with bench terracing. It consists of construction of step-like fields along contours by half cutting and half filling. Original slope is converted into level fields and thus all hazards of erosion are eliminated. All the manure and fertilizers applied are retained in the field. In slopping irrigated land, bench terracing helps in proper water management.

TYPE OF BENCH TERRACES AND THEIR ADAPTABILITY

The bench terraces are of four types 

 

Level Bench Terrace

Paddy fields require uniform impounding of water. Level bench terraces are used for the same. Contrary to usual concept that bench terraces are to be adapted on slopes steeper than 6 to 7%, level bench terraces are required in paddy growing areas on slope as mild as 1%, to facilitate uniform impounding. Sometimes this type of terraces and referred as table top, or paddy terraces, conveying the sense that such bench is as level as top of the table.

Inwardly Sloping Bench Terraces

Crops like potato are extremely susceptible to water logging. In that case the benches are made with inward slope to drain off excess water as quickly as possible. These are especially suited for steep slopes, it is essential to keep the excess runoff towards hill (original ground) rather than on fill slopes. These inwardly sloping bench terraces have a drain on inner side, which has grade along its length to convey the excess water to one side, from where it is disposed-off by well stabilized vegetated waterway. These are widely used in Nilgiri hills of Tamil Nadu state as well as on steep Himalayan slope in Himachal Pradesh and North –Eastern hill regions.

Outwardly Slopping Bench Terraces

Farmers many a times carry out the levelling process in phases, doing part of the job every year. As such outwardly sloping bench is usually a step towards construction of level or inwardly sloping bench terraces. In places of low rainfall or Shallow soils, the outwardly sloping bench terraces are used to reduce the existing steep slope to mild slope (say from 8% to 4%). In this type of terraces constructed on soils not having good permeability, provision of graded channel at lower end has to be kept, to safely dispose of surplus water to some water way. In very permeable soils a strong bund with spillway arrangement may take care for most of the rainfall events, while during heavy rainfall storm, the excess water may flow from one terrace to another. Attempt is usually made to some waterway at an earliest possible spot.

Puertorican or California Type of Terraces

In case of Puertorican type of terraces, the soil is excavated little by little during every ploughing and gradually developing benches by pushing the soil downhill against a vegetative or mechanical barrier laid along contour. The terrace is developed gradually over years, by natural levelling. It is necessary that mechanical or vegetative barrier across the land at suitable interval has to be established.

Strip Terrace

This type of terraces is constructed in districts of Kangra and Kulu for growing fruit trees on contour, Widths of strip terraces are much smaller than those adopted for growing paddy.

DESIGN OF BENCH TERRACES

Condition of soil depth, slope, rainfall, farming practices, etc., have all a direct bearing on terrace design and therefore, very careful consideration should be given to all these factors.

The design of bench terraces consists of the following:

• Terrace spacing
• Terrace grade length; and
• Terrace cross-section

Terrace spacing

It is normally expressed in terms of the vertical interval at which the terraces are constructed. It depends upon factos like slope, soil and surface condition, grade and agricultural use. Therefore, no hard and fast rule can be prescribed in this regard for all conditions. In bench terracing, the vertical interval affects the depth of cutting and the height of shoulder bund and thereby the total height of vertical drops.

The important factors limiting the spacing in this case are therefore, the soil and slope. At the same time, the width of terraces should be such as to enable convenient and economic agricultural operation. Determination of vertical interval, therefore, depends upon the practical experience of the area and good judgement with due consideration of prevalent local practices.

Many workers estimate vertical interval by adopting the formula, as used for contour or graded bunding viz., VI=0.3((S/2) + 2) where, S=percentage of slope, VI=vertical interval(m). In bunding, only length of slope is reduced, while the degree of slope remains unchanged. On the contrary, in bench terracing, the degree and length of slope are both changed. As such, the above-mentioned formula should not be used.

Step 1: Find out the maximum depth of productive soil range(D). This is a very important factor, as the lesser the cutting made, the greater will be the depth of productive soil available for cultivation.

Step 2: Having the above consideration in view, find out the maximum admissible cutting (d), for the desired land slope (S) and crops to be grown. This cutting should, at the same time, enable construction of terraces with convenient widths.

Step 3: Having fixed depth of cutting, the width of terrace (W), can be computed for a given slope(S) by the formula W= (200d / s) , where, W and d are in meters and S in per cent.

Step 4: For a batter of 1:1 riser, the vertical interval (VI) can be computed by the formula  VI = WS/(100-s) and for a batter of 0.5:1 for risers VI = (2WS)/(200-S)

For a given slope greater the VI, greater would be the width and for a given VI, higher the slope, smaller the width of terrace.

Terrace gradient

In high rainfall areas, for the quick disposal of the excess water, a suitable gradient has to be provided for newly laid out terraces.

Step 1: To estimate the peak rate of runoff (cumecs) from the bench terraces, rational formula

Q=CIA / 360 is used.

Step 2: The area drained, A(ha) can be calculated by the formula

A= LXW / 10000,

where, L = length of terrace(m); and W = average width of terrace (m)

Step 3: Permissible velocity of flow (V) for different soils

Step 4: Compute approximate area of cross-section of the channel using the formula Q=A,V, where V is the permissible velocity as obtained under step 3 and Q = peak rate of runoff.

Step 5: Compute hydraulic radius (R) for the cross-section obtained as in Step 4.

Step 6: Compute the grade of the terrace/channel using the Manning’s Formula, V = (1/n)R^2/3 S^1/3

where, V = permissible velocity (Step 3); R = hydraulic radius (m) (Step 5); n = Coefficient of rugosity or roughness coefficient

Step 7: The grade of the terrace/channel obtained under step 6, may be rounded off for convenience of layout.

Step 8: For the rounding of grade (S), recalculate the velocity of flow for the section under consideration (if needed cross-section is to be adjusted) and verify whether the obtained velocity is less than the velocity as assumed under Step 3.

Terrace cross-section

The construction of bench terrace is such that the earthwork excavated from the upper half is deposited over the lower slope. As such, the deposited portion forms an embankment, and therefore, care should be taken to secure tis well on the slope, by providing suitable key trenches and clearing the surface of all vegetation. Precaution should also be taken to provide suitable batter in cutting and embankment, so that the embankment may be well-seated on this slope. The height of the embankment should be increased sufficiently to provide for shrinkage of soils, so that the ultimate slope after consolidation, confirms to the specification.

The cross-section of the shoulder bunds along outer edge of terrace should also be suitably designed to be stable against slipping and over-topping.

Alignment of terraces

While aligning bench terraces on slopes, the tillage convenience and field boundaries are of primary consideration. The alignment should be so made that the minimum convenient width of terrace is always available for cultivation. So also, proper adjustment and necessary deviation can be made near the field boundary, in order to avoid inconvenient width of terrace strips at the field boundary.

All sharp and convenient curves should be conveniently eased out, deviating if necessary for the contour. Acute bends and straight junction should also be avoided as far as possible


Terrace execution

The contour lines were actually marked on the site by the method of direct contouring and with due reference to these lines, the alignment of terraces as per plan was staked out. The alignment was carefully examined with all consideration to the local conditions that existed at the site, like depressions, sharp turns, fields, boundaries and the like and were then finalized making deviation wherever necessary.

When the alignment has been finalized, the seat of embankment was prepared, ploughed and cleared of all vegetation and roots; and wherever necessary, key trenches were excavated. The excavation was then commenced approximately at the middle upwards and excavated earth was gradually pushed towards the lower slope until the desired level was obtained. The required gradient was then marked with dumpy level and the final scraping and levelling were attended to.

In case tractor bulldozer is used, the dozer blade may be tilted to provide grade inwards and worked along a smooth curve, pushing the soil excavated from the upper half towards the lower one. After the rough levelling was over, the end of terraces was leveled with manual labor. Proper gradients were marked with a dumpy level and the final scraping with the bulldozer was attended to carefully.

The sectioning of shoulder bunds to the required size and trimming the side slopes to proper batter had to be done with manual labor. Grass may be planted on batter for protection.

Usually the construction is taken up from top to bottom. But in case it is desired to keep the top soil on top, then leveling is needed to be done from the bottom. After leveling of first terrace, the top soil of higher adjoining aligned bench terrace is pushed on to the constructed terrace.

Then the leveling of the terrace from where top soil is borrowed is done, followed by pushing of top soil from land above to the lower terrace. Thus, all terraces will have top soil of adjoining above terrace and only last top terrace will be without top soil. But this involves additional earthwork. For example, if 25 cm thick soil is scraped off, the additional earthwork per ha will be 10000 X(1/4) = 2500 m³. This may be a must, in some shallow soils having unfertile sub-soil, while in other case, it may be chapter to do post reclamation works to build up the fertility of bench terrace constructed area.

Computation of width of terrace and earthwork for bench terracing

The width (w) of terrace can be computed for a given depth of soil (d) as follows:

(V1/2) / ( V1/2 + W/2 ) = d/(W/2) ,  V1 / (V1+W) = (2d)/W   ,  (2d)/W = S/100  , W = (200d) / S

or

Thus

The earthwork per ha is computed by using the formula

E (Earthwork per ha) = (100/8) W S where, W = width of terrace (m); and  S = land slope (%)

Area available for cultivation = 100(100 – ns) where, n = batter slope (%); and s = land slope (%)

Area lost due to benching = 100 n.s.

Percentage of area lost in

benching = n.s.

Batter area to be sodded   = 100 √ (1 + n²)

No. of terrace outlets/ha  = L/(2K) where, L = total length of terrace/ha; and K = critical length of terrace.

Cost of bench terracing

The actual cost of bench terracing depends upon conditions of soil and sub-soil; land surface including vegetation cover on it, undulation, depression, mounds, etc., slope of the area; specification of bench terraces and deviation in its alignment and terrace outlets.

The principal item that affects cost of bench terracing/ha is the total quality of earthwork involved and rate and the manner as to how it is executed (manual labor of bulldozer, etc.).

Example 14: On a 20% hill slope, it is proposed to construct bench terrace. If the interval is 2 m, calculate (i) length per hectare, (ii) earthwork, and (iii) area lost for vertical cut. The cut should be equal to fill, when the terrace cuts are vertical.

Solution- 

W = 100 D / S = (100 X 2) / 20 = 10m= 10,000 / 10 = 1000m 

Length per hectare

Since it is vertical cut, no area is lost for cultivation, except the area for shoulder bund.

when the batter slope 1:1 using  W = D(100-S)/s = 2(100-20) / 20 = 8 m 

Length per hectare  = 10,000 / (8 + 1 + 1) = 1,000 m

Earthwork per hectare = (1/2) X 4 X 1 X 1000 = 2000 m³

Example 15: In an area of one ha lying in a hilly region, bench terracing has been proposed to bring the land under cultivation. The land has a general slope of 30%. The average depth of soil as observed by an auger is about 0.9 m. The crops to be growth after benching require at least a soil depth of 0.25 m. Risers to be laid on 1:1 gradient and will be planted with local varieties of grasses. The critical length of terrace is approximately equal to 100 m. Work out (a) cost of bench terracing only at Rs. 2 per m3 (b) compute the number of outlets and the cost of earthwork excavation for the vertical drains; (c) cost of grass plantation at Rs. 4 per 100 m on the risers and (d) total cost of bench terracing outlets. Mode of execution is mostly manual labor.

Solution-

The depth of cut  d = 0.7 x 0.9 = 0.63 m

Still the depth of soil available = 0.27 m which is higher than 0.25 m as indicated. Bench widt (W) to be adopted

w = 200 d / s = (200 X 0.63) / 30 = 4.2m

VI = (w x s) / (100 - s) = (4.2 x 30) / (100 -30) = 1.8 M

HI = W + VI = 4.2 + 1.8 = 6.0 m

Earthwork involved/ha  = (100/8) w  s = (100/8) x 4.2 x 30 = 1575 m³


The critical length of terrace (k)=100m

L= Length of terrace/ha = 10,000 / Hi = 10,000/ 6 = 1666m

Number of outlets (assuming surplus water drained from both sides)  = L/2K = 1666/(2 x 100) =~ 8 (aprox)

Where N= number of outlets/ha; and Hi= horizontal interval

I=8 x 6= 48m

The cross section of disposal drains:

Bottom width = 0.3m

Top width = 1.05 m

Height = 0.38m

Area of cross section of disposal = ((0.3 + 1.05) / 2) x 0.38 = 0.256m² drain

The earth work involved in the vertical disposal drains

Cost of earth work at Rs. 2 = 48 x 0.256 = 12.3m³ = 12.3 x 2 = ₹ 24.60 or say ₹ 25.00

Cost of sodding = Rs. 4per 100m length

Total cost of = (4 / 100) x (10000 / HI)  sodding/ha  = (4 / 100) x (10000 / 6.0) = 400/6 = ₹ 66.66/ha or ₹ 67.00

Total cost of bench terracing outlet including sodding = (1) +(2) +(3) = 3150 + 25 + 67 = ₹ 3242 or ₹ 3240 per/ha

Bench terraces with stone walls

The construction of bench terraces with stone walls is justified where stone can be found in adequate quantities close to the site, and potential productivity of the land justifies the expense. However, where there are many surface stones, cultivation may be restricted.

Bench terraces withe stone walls can be used for annual crops and perennial tree plantations. Tha latter are likely to produce the highest return on the investment, except in cases of special high value annual crops. Furthermore, tree crops require less cultivation.

All remarks regarding the use of natural features in designing terraces already dealt, apply to terraces with stone walls.

Determining the interval between bench terraces of level cross-section

The horizontal distance is a function of:


-the height of the retaining wall; and
-The slope of the land.

This can be expressed as

Hl = (100 x H) / S


where, HI = horizontal distance (m); H = height of retaining wall (m); and S = Slope, (m/100m).

The formula applies only to those bench terraces constructed by building a retaining wall and filling it with soil brought in from outside. This method, however, is expensive and is only used when there is insufficient soil depth on site to be reclaimed. Usually, bench terraces of level cross-section are constructed by moving soil from the upper part down to the lower part.

This can be expressed as  HI = ((H1 + H2)/s) x 100

where, HI = horizontal distance (m); H1 = depth of excavation in upper part of terrace (m); H2 = height of fill in lower part of terrace (m); and S = slope, (m/100 m).

This method, though cheaper than the previous one, is still fairly expensive. Level bench terraces have many advantages, but suffer from the following drawbacks:

They are narrow and implements cannot be easily used on them;

Many high retaining walls are needed, thus steeply raising the cost of reclamation on the one hand, and on the other hand, making access to the benches more difficult; and

Where the soil is initially shallow, the terrace, after its construction, has relatively deep soil on its lower part, while on the upper part, the soil is often too shallow for the plant’s requirements.

Determining the interval according to the presumed stable slope

The disadvantages mentioned above may be largely overcome by designing bench terraces on the principle of the stable slope. This design is especially useful in the case of rotation crops, where an artificial stable slope of 3.8% can be maintained by using a reversible plough or disk, as already strongly recommended.

The stable slope changes for different soils. Local observations and experience should be noted and applied. It should be possible to reach fairly exact conclusions from reviewing the condition of a given cultivated soil and comparing its stability against erosion seen on different slopes. The terrace interval can be calculated as follows: 

H1 = 2 ( D1 - D2 ) / ( S1 - S2 )

where, HI = horizontal distance (m); D1 = present depth of soiul on the area (m); D2 = minimum soil depth required by the crops (m)*; S1 = present slope (m/m); and S2 = presumed stable slope (natural or artificial) (m/m),

The stable slope may be achieved in two ways:

by transporting the soil from the upper part of the terrace to its lower part with earth moving equipment; and

By leaving it to be done by cultivation and nature, over a period of years.

When the area is being reclaimed for orchards, it may sometimes be worthwhile to sub-bench the individual rows before planting.

With rotation crops, it is advisable to let nature and cultivation do the job of benching. The cost is thereby reduced and it is easier to build up and maintain soil fertility.

The retaining walls

The wall is constructed (Fig. 3.23) from stones collected on the spot. As a rule, close and low retaining walls are preferable to high walls set far apart.

The height of a stone retaining wall is given by:

H = (H1 + X1) + (H2 + X2)

where, H = height of wall; H1 = depth of soil, presumed to be lost by creep, at the foot of the wall;

* with rotation crops this depth is that of the upper part of the terrace, at the foot of the retaining wall. In the case of orchards, the depth is that of the root zone of the trees in the uppermost row of the terrace.

H2 = depth of soil presumed to be deposited near the wall; X1= Safety factor (usually 30 cm); and X2 = safety factor (usually 10 cm).

The formula is readily understood when it is considered that the wall on its lower side must be sunk into the ground by an amount equal to the depth of soil, which is expected to be removed by creep (plus the 20 cm safety factor), and on its upper side must rise above the ground surface by an amount equal to the depth of deposition expected (plus the 10 cm safety factor).

Drainage of gravitational water

It may be frequently observed, following rainfall, how water percolating through the retaining wall wets the terrace below it. In order to drain this water, furrows are opened at the foot of every retaining wall. These furrows need not have a large capacity, since they only conduct small quantities of water, but they need a continuous longitudinal grade. For this reason, it is important to grade the strip along which this furrow is to run.

Disposal of runoff water

In many countries a common practice is to divert the runoff from the cultivation furrows and retaining walls into deep, narrow, ditches made of stone, concrete or earth, which run straight down the slope or diagonally down it. Since such ditches cannot be crossed by machinery, they cannot be used in areas where cultivation is mechanized. On the other hand, wide, shallow, vegetated waterways, crossable by machinery, which are effective on moderate slopes, are not a practical proposition on slopes exceeding 15%, since the vegetation will not supply the necessary protection. There are two possible solutions:

Combining vegetated waterways with drop structures of stone of other material

The drop structures in the waterway are an extension of the retaining walls. Their height is lower than that of the wall and is determined by the amount of flow in the waterway. Where the retaining walls are set far apart, it may be necessary to construct additional drops in the waterway itself is sown or planted with perennial grasses for complete protection.

Paving the waterways with stone

Paving the water with stone is rather costly, but the method has the particular advantage that the waterway can then be used as a field track. The provision of tacks on steep areas is a substantial fraction of the total cost of reclamation, so that combining the waterway with the field track is a promising solution.

Cost of Construction

The cost of Construction stone walls is high. It is possible to give standard costs per ha, because in each location and for each crop different requirements prevail. However, a few indications can be given for estimating the cost.

Stone walls should have a width of about, 0.3m on the top and not less than 0.8m at the bottom, if they are 1 m high. At least, 0.3m should be buried in the ground as a safe foundation. This depth of foundation has to be calculated after the removal of soil for the fill in the lower terrace. A length of 1 m of terrace will require 0.55m3 of stones.

Building of stone terrace is a specialized job required experience. One skilled laborer should be able to use 1 m3 of stone (including excavation of the foundation) per day.

Classroom Video

 

---