Contour Bunding
Counter Bunding are carried out in many parts in India- notably in Maharashtra, Gujarat, Tamilnadu, Karnataka and Andhra Pradesh.
Contour Bunding:
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 of design
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
[Runoff, cm] X [Bund horizontal interval in m]
Cross section area of storage space = ——————————————
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:–
Table: specification for bund cross-sections
Depth of soil | Base width ‘m’ | Top width ‘m’ | Height ‘m’ | Side slope |
1. Shallow soils [7.5 to 22.55cm] | 2.67 | 0.38 | 0.75 | 1 1/2 : 1 |
2. Medium soils [ 22.5 to 45cm ] | 3.12 | 0.6 | 0.85 | 1 1/2 :1 |
3. medium deep soil [ 45 to 90 cm] | 4.25 | 0.6 | 0.9 | 2:01 |
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.
Graded Bunding:
Definition:
“Graded bunds or graded terraces or channel terraces are the bunds or terraces laid along a pre-determined longitudinal grade very near the contour but not exactly along contour”.
Suitability:
The graded bunds, commonly used in India are comparable to the narrow base terraces. They are used for the safe, disposal of excess runoff high rainfall areas and rigious where the [Clay] soil is relatively impervious. Farming operations are not done on bunds or bund channels.
Function:
These terraces act primarily as drainage channel to regulate and conduct runoff at non erosive velocity.
To make the runoff water to trickle rather than to rush out.
