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Bunding

Contour Bunding

Contour Bunding are carried out in many parts of India- notably in Maharashtra, Gujarat, Tamilnadu, Karnataka and Andhra Pradesh.

Definition:

It consists of building earthen embankments across the slope of the land, following the contour as closely as possible. A series of such bunds divide the area into strips and act as barriers to t5he flow of water, thus reducing the amount and velocity of the runoff.

Peripheral bunds:

Bunds area also constructed along field boundaries without reference to contour. These bunds are called peripheral bunds. They serve as fences, and give protection from water and wind erosion in low rainfall areas. They are not suitable in heavy rainfall areas.

No cultivation is allowed on the earthen embankments of contour bunds. Therefore under contour bunds an area of about 5 percent is lost under the bunds and is not available for cultivation.

Contour bunds can save soils from erosion to the extent of 25 to 162 tones/ hectare annually. It maintains soil fertility and increases water infiltration into the soil considerably,

Contour bunds in deep black soils have been a failure because of the nature of soil, which cracks during hot weather and cakes during the monsoon. So they are not stable in black soils. Further the poor drainage properties of deep black soils gives raise to long stagnation of water against contour bunds and make it unstable. Contour bunds are also not successful in very shallow soils having a depth loss than 7.5cm.

General principles & design of contour bund:

1. Spacing of Contour bund:

Bund spacing is expressed as the vertical or the horizontal distance between corresponding points on two adjacent bunds. Although the horizontal spacing is useful in determining the row arrangement. Vertical distance is commonly known as the vertical interval or V. I.

Bund spacing should not be so wide as to cause excessive soil erosion between adjacent bunds. Spacing may be increased or decreased 10 to 20% to suit local conditions.

Table-1 Spacing of Contour bunds: Recommended by Gadkary

Slope of land [ c ] vertical interval [ m ] Approx horizontal distance [ m ]
0 to 1 1.05 105
1 to 1/2 1.2 98
1/2 to 2 1.35 75
2 to 3 1.5 60
3 to 4 1.65 52

2. Bund Grade:

Since the contour bunds are laid along the contours, they are level bunds.

3. Bund length:

In general, 400 to 500m is the maximum length of bund. The bund retains the runoff and carries it over the distance equal to bund length in one direction. The length of bund should be such that the velocity of water flowing between bunds should be non- erosive.

4. Bund cross section:

The height of bund should provide sufficient storage above the bund to handle the expected runoff. In normal practice sufficient practice is provided to take care of runoff from rains expected in 10 year recurrence interval. The cross section area of of the storage space required can be calculated by the following formula

Cross section area of storage space = [Runoff, cm] X [Bund horizontal interval in m] / 100

The height of bund should permit frees board of about 20% as design depth [after allowing settlement of the ridge.] Specific at bund cross section are given in table

Design Criteria for Bunds:

The following factors are to be considered while developing design criteria for contour bunds.

1. Allowable submergence of land:

The amount of land submerged due to pending and duration of pending will affect crops. Therefore the level of waste weir and the amount of land to be submerged should be decided by the cropping practice to be followed and the infiltration rate for the soil.

2. Moisture Conservation:

For paddy lands it is desirable to store all the rain water for the use of the plants. Therefore the bunds should be of such dimensions as to permit no runoff. For other crops, the capacity of the bund should be decided by the average consumptive use of the crop proposed and the maximum length of dry period in growing season. The heights of waste weirs should be such that the bunds store just sufficient water to meet requirement of crop.

3. Economy in Construction:

The cost of Bunding includes two main atoms which vary according to the spacing of the bunds.
i. Expenses of the earthwork
ii. Value of land lost permanently due to construction of bunds.
The sum of these two should be minimum

4. Critical Length:

Another approach in fixing the spacing of bunds by determining the critical length of land between adjacent bunds. Increase in drainage area increases both velocity and amount of runoff gathering in narred channel. But the critical length approach, the attempt is to space bunds in such a way that the velocity remains within non-erosive limit.

5. Seepage consideration:

While designing the bund cross- section, the seepage through bunds due to accumulation of water behind it should be taken into account. The seepage rate is affected mainly by the head of water impounded, the side slopes of the bound and the permeability of the soil.

Location of contour bund:

Moderate slopes (510%) with light or medium soil texture and less than 700 mm of rain per year.

Purpose of contour bund:

- To reduce soil erosion.
- To increase the amount of water the soil can hold.

Advantages of contour bund:

- Simple to build.
- Bunds conserve topsoil and improve productivity.
- They keep water in the soil, allowing chemical fertilizers to be used effectively.
- They can be used both on cultivated and uncultivated land.
- Farmers can build contour bunds themselves without outside help.

Limitations of contour bund:

- The bunds take some land away from cultivation (though some types of crops can be grown on the bunds to stabilize them). - May create temporary waterlogging problems in heavy soil.
- May interfere with the farm operations if the bunds are too close together.
- A lot of labour is needed to maintain and repair the bunds.

Source :Dr. H. L. Shirsath
Assistant Professor of Agronomy
College of Agriculture
Dr. Vithalrao Vikhe Patil Foundation, Ahmednagar

Bunding

Bunding is one of the mechanical measures of erosion control, requiring engineering techniques and structures. They reduce run off and impound water for longer time to help infiltration into the soil. Their construction and design will depend upon rainfall, soil type, slope, and such other factors. These measures are costly but if properly maintained will improve the land over a long period of time.

There are two types of bunding.

Block bunding: - here construction of big bunds across large blocks of sloping lands. These bunds are constructed of earth or stone or both, at a great cost, to impound water and arrest soil washed from the fields lying above. They are high and broad enough to withstand the force of water from the catchment. Water is let out at the end of the monsoon and land, which has received fertile silt, is sown with crops. At present such type of big block bunds are not constructed.

Contour-bunding: - This practice consists in making a comparatively narrow-based embankment at intervals across the slope of the land on a level that is along the contour. It is an important measure that conserves soil and water in arid and semi-arid areas with high infiltration and permeability, and is commonly adopted on agricultural land up to a slope of about 6%.

It may be noted here that as no specifications for bunding in deep black soil are available so far, no large-scale bunding on them can be recommended. As regards spacing between bunds. It should not exceed 150cm vertical drop or 67.5cm horizontal spacing, whichever is less. The following schedule, which is used in Maharashtra, can be used as a fair guide.

Percentage slopeVertical drop
0 to 11.05m
1 to 11.20m
1 to 21.35m
2 to 31.50m

The size, cross-section and interbund spacing depend upon the nature of rainfall, soil and slope of the area.

Where the rainfall is more than 62.5mm and moisture conservation is important, bunds may be constructed at shorter intervals. In the alluvial soils of Gujarat, a vertical interval of 1.83m and a cross-section of 1.3sq m were found to be suitable for land with the slopes ranging from 6-12% and for slopes less than 6% contour bunds with cross-sections of 0.9 to 1.3 sq m spaced at 0.9 to 1.2m vertical interval were found to be effective. The low rates of permeability and infiltration in these soils cause a prolonged impounding of water on the upstream side, and the crops are consequently damaged.

Graded bunding or channel terraces: - Graded bunding has been used in areas receiving rainfall of more then 80cm per year, irrespective of the soil texture. In clay soils, graded bunding has to be used even for areas having less than 80cm of annual rainfall. Graded bunds may be terrace consists of a wide-low embankment constructed on the lower edge of the channel from which the soil is excavated. The channel is excavated. The channel is excavated at suitable intervals on a falling contour with a suitable longitudinal grade. Broad-based terracing is recommended where farming is practiced with tractors.

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