MECHANICAL MEASURES

MECHANICAL MEASURES The mechanical or engi­neering methods most widely used include:
(a) excavation of different types of ditches and con­struction of terraces for the removal of excess of water from the fields.
(b) construction of dams for checking the erosive velocities of the water.

The main objectives are to increase the time of con­centration by intercepting the runoff and thereby providing an oppottunity for the infiltration of water; and to divide a long slope into several short ones so as to reduce the velocity of the runoff and thus prevent erosion.

Basin Listing Small interrupted basins are made along the contour with a special implement called a basin lister. It helps to retain rain water as it falls and is specially effective on retentive soils having mild slopes.

Subsoiling It involves in breaking with a subsoiler the hard and impermeable subsoil to conserve more rain water by improving "the physical conditions of a soil. This opera­tion does not involve soil inversion but promotes greater moisture penetration into the soil and reduces both runoff and soil erosion.

Contour Bunding It involves making a comparatively narrow-based embankment at intervals across the slope of the land on a level that is along the contour. It conserves soil and water in arid and semi-arid areas.

Graded Bunding or Channel Terraces This method is used in areas receiving rainfall of more than 80 cm per year irrespective of soil texture. It may be narrow or broad based. Generally in India the terraces are broad-based, that is, wide and low embankments constructed on the lower edge of the channel from which the soil is excavated.

Bench Terracing It consists of a series of platforms having suitable vertical drops along contours or on suitably graded lines across the general slope of the land. The vertical drop may vary from 60 to 180 cm, depending upon the slope and soil conditions.