Soil And Water Conservation Methods – Mechanical Practices
The above measures control erosion by good management practices. Bunding, terracing, gully or nala control, and construction of tanks and bandharas are mechanical measures 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 slope and such other factors. These measures are costly but if properly maintamed will improve the land over a long period of time.
i) Block bunding: Bunding for control of soil erosion and conservation of surface run off was known to farmers for centuries. It was not uncommon to find Tals i.e., 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 catchments. Water is let out at the end of the monsoon and land which has received fertile silt is sown with crops. Such type of big bloods bunds are not constructed now as contour bunding has been taken up on catchments basis.
ii) Contour bunding: It consists of construction of a series of earthen bunds of suitable sizes along contours at a lateral distance of every 60 mm or a fall of 1 to 1.5 m. The shope of land is thus broken into smaller and more level compartments which hold soil as well as rain water. It has been estimated that about 75 million hectares of land i.e. about one fourth of the common land surfaces suffering from soil erosion. In Maharashtra State, the problem is more acute and it is estimated that out of 186 lakh hectares about 144 lakh hectares require bunding. The planning Commission has, therefore laid great stress on contour bunding programme, because bunding alone has been found to increase crop yield by 20 to 30 p.c.
The size, cross- section and interbund spacing depend upon the nature of rainfall, soil and slope of the area. In order to improve the technique of bunding, studies have been carried out in respect of spacing of bunds, shrinkage of bund sections and hydraulic gradients and kind and location of outlets etc. in different soil and rainfall conditions of Maharashtra State. On the basis of such studies it has been observed that the spacing between bunds should not be allowed to exceed 1.5m. Vertical drop or 67.5 m. lateral spacing. The following table21 is a fair guide.
iii) Graded bunding: In high rainfall areas, while conservation of soil is important, drainage of surplus water has to be attended to, for avoiding waterlogged condition of soil. The bunds are therefore, slightly graded longitudinally about 7.5 cm. per running 33 m. to prevent safe disposal of water into the nala. The cross sections into for safe removal of excess run off water it is essential to provide suitable waste water or outlet structures at proper places so that no damage is done to bunds in case heavy precipitation is received on any single day. Normally stone outlets are provided low rainfalls are as. Channel weirsor pipe outlets may also be provided. Grass outlets have been found to be effective and cheaper in heavy soil. The crest wall should be 30 cm. above the contour level and its length should be so designed as to discharge the surplus water from the maximum intensity of rainfall with are asonable period. 1,250 mm. Terrace bunds consist of comparatively narrow embankments constructed at intervals across the shope and the vertical spacing between bunds may vary from 1to 2 m., depending upon the slope, type of soil, rainfall etc. Bench terracing is done when gradient is stceper than 10 p.c. as in hilly ranges of Himalayas, Sahyadry etc. and consists of a series of step like platforms along contours. These terraces are like table tops sloping outwards and are provided with stone wateweirs to drain away surplus water. Angular and big boulders should be used for terrace outlets because round and small boulders will slip and get dislodged under the gushing water.
Gully or nala control: Gully or nala control is very essential to prevent its extension and further destruction of cultivated lands and grasslands. The sloping sides are planted with grass and trees. Suitable temporary and permanent structures such as check dams, overflow dams, drop structures are also provided. Small gullies can be stabilized by converting them into paddy fields. So far 17,034 nalas have been controlled and the target for sixth plan (1980 – 85) period is 2005.
Control of stream and river banks: Vulnerable sharp bends nalas by the sides of the rods and river bends near village sites cause considerable damage to property. These should be protected by providing spurs, jetties, rivets and retaining walls. Adjoining areas should be stabilized under permanent vegetation. Spurs are constructed at an angle to reduce the velocity of water and there by enabling the flood water to flow away but deposit coarse sand which will cause obstruction to successive water currents from cutting into the bank and thus straightening their course.