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.

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.

Ag.
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Engineering)