Monday, January 7, 2013

SEVAI Watershed Contour Bunding in Dasilnaikanoor enables Moisture Conservation


SEVAI Watershed contour bunding

Counter Bunding was carried out in Dasilnaickanoor watershed project in Tamilnadu as implemented by SEVAI. Contour Bunding in SEVAI watershed project 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 the flow of water, thus reducing the amount and velocity of the runoff. Contour bunds save soils from erosion. It maintains soil fertility and increases water infiltration into the soil considerably in this SEVAI watershed project. Bund spacing is expressed as the vertical or the horizontal distance between corresponding points on two adjacent bunds. Bund spacing is not so wide as to cause excessive soil erosion between adjacent bunds .Since the contour bunds are laid along the contours, they are level bunds. 300 to 400m 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 is designed well such that the velocity of water flowing between bunds should be non- erosive. The height of bund provides 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 height of bund permits frees board of about 20% as design depth [after allowing settlement of the ridge. The amount of land submerged due to pending and duration of pending affects crops. Therefore the level of waste weir and the amount of land is submerged has been decided by the cropping practice to be followed and the infiltration rate for the soil. Moisture Conservation is an important role of the contour bunds, for paddy lands it is designed to store all the rain water for the use of the plants. Therefore the bunds are of such dimensions as to permit no runoff. For other crops, the capacity of the bund is 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 are put up such that the bunds store just sufficient water to meet requirement of crop. The cost of Bunding includes two main atoms which vary according to the spacing of the bunds, Expenses of the earthwork, Value of land lost permanently due to construction of bunds; the sum of these two is minimum. While designing the bund cross- section, the seepage through bunds due to accumulation of water behind it is 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.-Govin

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