What are the steps involved in the design of the contour bunds? How these parameters can be estimated?by Madan Mohan
Bunding or construction of small embankment is carried out to reduce the length of slope, to reduce the velocity of runoff water and to hold the water in the catchment for a longer period. Thus more water infiltrates into the ground and less run-off and soil erosion take place. Different types of bunds are used for erosion control and moisture conservation.
When the bunds are constructed along the contours with some minor deviation to adapt to practical situations, they are known as contour bunds. If the bunds are constructed with some slope, they are known as graded bunds, Side bunds are constructed along the slope at the two sides of the contour bund. Lateral bunds are constructed along the slope in between two side bunds to reduce the length of the contour bund. This reduces the concentration of runoff water along one side. Supplemental bunds are constructed between two contour bunds to limit the horizontal spacing of the contour bunds. Peripheral bunds are constructed along the field boundaries and may not conform to the contours.
In India, contour bunding or simply bunding has been practiced for a long time and the Indian farmers have very good knowledge about it. First thing that is done to control a rill or a gully is to do bunding. The former state of Bombay did notable works in contour bunding and in other states like Andhra Pradesh, Tamil Nadu and Karnataka vast areas were put under contour bunds. From the experience gained through these constructions, it was known that bunds could stand well only in shallow, medium and medium deep soils. Deep black soils show cracks in dry conditions and the bunds fail. Through these cracks water continues to flow and big breaches are caused. This results in severe damage to the fields. Although various erosion problems exist in black cotton soils, contour bunding cannot be taken up in such soils successfully.
Design of Contour Bunds
The design of contour bund includes determination of spacing, both horizontal and vertical and bund cross-section. The bund cross-section includes base width, side slope and bund height. The bund height should be sufficient to store the expected runoff from a rainfall of 10 years recurrence interval. Over this depth, extra depth should be provided for the design depth of water over the weir and the free board. The base width, side slope and top width are decided by the nature of soil.
Spacing of Contour Bunds
As the water flows through a sloping land, it attains erosive velocity. The bund should be spaced in such a way so as to intercept the erosive velocity. Again, the spacing should not be too close to interfere with the farming operations. Different relationships have been developed for the spacing of bunds.
Ramser conducted experiments in sub-humid areas with good infiltration rates and developed the following relationship for vertical, interval of contour bunds.
The above formula does not take into account soil and rainfall characteristics and its applicability cannot be genera1ized. When the above formula is used for soils with high infiltration rate and good conservation practices such as contour farming, growing of cover crops etc., then 25% extra spacing can be used. On the other hand, in soils of low infiltration capacity arid unfavoub1e conservation measures, the spacing should be reduced by 15%. Similar adjustments are required for rainfall variation. For high rainfall areas, the interval should be reduced and vice-versa. In fact, a general relationship of the following form may be used and the constants should be evaluated for the specific site.
M.P. Cox, a water management specialist of United States Agency for International Development (USAID) gave the following formula for spacing of contour bunds.
y = infiltration and crop cover factor.
The values of x and y are given in Tables 15.1 and 15.2 respectively.
Table 15.1.Values of the Rainfall Factor (x)(Source: Mal, 1995)
Value of x
Annual Rainfall, cm
Table 15.2. Values of the Infiltration and Crop Cover Factor (Source: Mal, 1995)
|Intake rate||Crop cover during critical period||Value of y|
|Below average||Low average||1.0|
|average or above||Good average||2.0|
|One of the above two factors is|
|favourable and the other is unfavourable||1.5|
For the purpose of moisture conservation mainly, the spacing of the bunds can be selected as given in Table 15.3. The recommendation is based on the works carried out by Gadkary in the former Bombay State.
Table 15.3. Spacing of Contour Bunds,(Source: Mal, 1995)
|land slope, %||Vertical interval, m||Approx. horizontal distance, m|
|0 to 1||1.05||105|
|1 to 1.5||1.20||97|
|1.5 to 2||1.35||76|
|2 to 3||1.50||61|
|3 to 4||1.65||52|
|4 to 5||1.80||39|
|5 to 6||1.95||36|
Alignment and Construction of Contour Bunds
For the purpose of alignment and construction of contour bunds, a map showing the plan of the area is necessary. The map should either be available or prepared using any standard method like plane table survey. All natural features like streams, gullies, field boundaries etc., should be shown in the map. The area is then divided into blocks of suitable size (say 50 ha) in which soil conservation programmes can be taken up at a time. Detailed map of each block with a larger scale should be prepared. On these maps, the locations of the contour bunds are marked.
For drawing the position of the contour bonds on the map, at first lever survey is conducted by taking a suitable grid distance so that contours can be drawn on the map at an interval of 30 cm. From the contour map, average slope of the land is calculated. Knowing the slope, infiltration and rainfall characteristics, the vertical and horizontal intervals are calculated for the bunds. Planning is started from the top of the watershed. Contour bunds are located on the map as per the calculated horizontal interval. The positions are then transferred to the field. Slight modification is made to eliminate the sharp curvatures and changing of existing field boundaries. Another method in which the contours are directly located in the field can be used for alignment of contour bunds. This is known as direct contour method.
While using direct contour method, the slope of the land is determined approximately by taking flying levels. The vertical and horizontal intervals are calculated as describe earlier. Locating of the bunds is started from the top of the watershed. The dumpy level is set up at a convenient position. Leveling staff is held at one corner of the proposed location of the first bund and the reading is taken. The staff is shifted to different positions from where the same level readings are obtained. These positions are marked with pegs. Thus the position of the first contour bund is obtained. This procedure is repeated to obtain the positions of all the remaining contour bunds.Contour Bunding
Counter Bunding are carried out in many parts in India- notably in Maharashtra, Gujarat, Tamilnadu, Karnataka and Andhra Pradesh.
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 the flow of water, thus reducing the amount and velocity of the runoff.
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
- 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|
- Bund Grade:
Since the contour bunds are laid along the contours, they are level bunds.
- 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.
- 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
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]
Design Criteria for Bunds:
The following factors are to be considered while developing design criteria for contour bunds.
- 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.
- 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.
- 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.
- 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.
- 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 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”.
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.
- 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.
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