In this article we will discuss about:- 1. Types of Bund 2. Classification of Bunding System 3. Design Criteria 4. Planning 5. Construction.
The different types of bund used in practice are given as under:
1. Contour Bund – The bunds constructed exactly on contour or with permissible deviation from the contour, are called contour bunds.
2. Side Bunds – These bunds are formed at the extreme ends of the contour bund, running along the land slope.
3. Lateral Bunds – Lateral bunds are constructed between two side bunds along the slope for preventing the concentration of water at one side, and also to break the length of contour bund into convenient bits.
4. Supplemental Bunds – The bunds constructed between two contour bunds to limit the horizontal spacing by its maximum extent, are nomenclatured as supplemental bunds.
5. Marginal Bund – These bunds are formed at the margin of the field, road, river etc., to demarcate their boundary.
6. Shoulder Bund – Such type of bunds are formed at the outer edge of the bench terraces, to hold the runoff over the top of the terrace, and also to provide stability to the terrace.
The bunding systems are grouped according to their function, which they perform.
The classification of bunding system is given as under:
(A) Contour Bunding:
(i) Narrow base; and
(ii) Broad base
(B) Graded Bunding:
(i) Narrow base; and
(ii) Broad base
All these are described as under:
The bunds passing through the points of equal elevation (i.e. on contour) of the land, are known as contour bunding. The contour bunds and level terraces are about to synomynous. Because of this reason, sometimes contour bund is also denoted as level terrace. The practice of contour bunding was firstly-introduced in Maharashtra (India) as famine relief work. For sloping cultivated fields, it is a good practice for controlling the runoff and soil erosion.
The contour bunding can be done on all types of permeable soils such as alluvial, red/laterite brown soil, shallow and medium black soil, but clay or deep black cotton soils are strictly avoided for this work, as these soils have the problem of crack development, which causes the bund failure. In addition, the clay soils also create the problem of water-logging condition near the bund section, which makes the bund construction non-feasible.
The main functions of contour bunds are given as under:
1. It reduces the length of slope, which in turn to reduce the soil erosion.
2. Impounds the water at u/s portion, and permits more water to get recharge into the soils that is utilized for crop cultivation.
Various limitations of this practice are as follows:
1. This practice is suitable for those areas, which receive the annual rainfall upto 600mm and soil involves greater permeability.
2. It is not used in clay soils.
3. Contour bunding is not technically feasible on the land slopes, greater than 6%.
Contour bunding is an important measure to conserve the soil and water in arid and semi-arid areas with high infiltration rate and permeability. It is commonly used on agricultural land up to 6% slope.
The dimension of bund (top width, bottom width, height, side slope and cross sectional area) adopted in the dry areas of Maharashtra (India) is shown in Table 12.3. The dimension of bund mentioned in Table 12.3 can be used as guidelines for similar areas. The vertical spacing of contour bund should not exceed 150 cm, and 67.5 m horizontal spacing.
The vertical spacing (drop) used in Maharashtra is given in Table 12.4, can be used as a guide for other similar areas. In the areas where rainfall is more than 62.5mm and moisture conservation is essential, then bunds may be constructed at shorter intervals.
In alluvial soils of Gujarat with slope ranging from 6 to 12%, the vertical interval 1.83 m and cross-sectional area 1.3sqm has been found suitable; and for the land with slopes less than 6%, the contour bunds with cross-sectional area of 0.9 to 1.3sqm and spacing 0.9 to 1.2 m have been found very effective.
Types of Contour Bunding:
The contour bunding is sub-divided into following two types:
(i) Narrow base contour bunding; and
(ii) Broad base contour bunding
In narrow base contour bunding system, there is an obstruction for crossing of farm implements; natural vegetations cover the sides and more height is allowed for same cross-section. It has limitations that, there is considerable area is lost in constructing the bund; the bund section is liable to get affected by erosion due to rain drop impact, hence requires a sincere maintenance. The narrow base contour bund also causes obstruction in farming operations.
The broad base contour bunding is concerned, it does not create hindrance in farming operations; the entire area can be kept under cultivation. It has some limitations, such as disturbance of bund’s section due to crossing of farm equipments, as result there is required an attentive care and maintenance. Apart from above, the soil of bund is also loosened during movement of farm machineries, causing reduction in the size of bund in a very short period, unless some proper maintenance is adopted.
The graded bund is also known as channel terrace, used in the areas having annual rainfall greater than 700 mm. However, it can also be used in less rainfall areas, provided that the soil of area is in heavy texture. In such areas when rainfall takes place a large portion of water is ponded over the surface. To remove this water at fast rate some grade is provided to the channel.
The grade should be within non-erosive limit. Normally, minimum 0.1 to maximum 0.5% is allowed. It should also be taken care that the grade of channel must be less than the actual land slope. In this condition, the water flows at slow speed to that which is flowing directly down the slope, causing not taking place of soil erosion from there.
The main functions of graded bunds are given as follows:
i. It reduces the length of slope, as result the soil erosion gets reduce.
ii. It also disposes the surplus water very safely to a suitable point.
iii. Contour bunding is followed by the construction of graded bunds to remove excess water which is not required by the crop.
Apart from above functions, the graded bund has some limitations also, which are cited as under:
i. It requires establishment of grassed waterway as an outlet for safe disposal of surplus water, accumulated in the bunded area.
ii. Graded contour bunding is not suitable on the land slopes less than 2% or on the slopes greater than 8%.
iii. The grassed waterway needs an extra care about control of grazing of grasses, grown- in the section of waterway.
In India, the graded bunds have been constructed in the areas receiving the annual rainfall more than 80cm, irrespective of soil types. However, in clay soils the graded bunds have been constructed even in the areas having the annual rainfall less than 80 cm.
In a study at Dehradun (India) on channel terraces in alluvial soils with 3 to 4% slope for two different Vis, i.e. 0.3 (S + 2) and 0.3 (S + 3) it was found that there is no much difference in runoff; however the soil loss per mm of runoff was found more from the vertical interval of 0.3 (S + 3), but was within permissible limit. The study indicates that the V.I. of 0.3 (S + 3) can be adopted safely, there. The findings are presented in Table 12.5.
These bunds are constructed on the eroded or abandoned lands. For better performance, they are also constructed at the contour of the area. The contour lines are marked on the ground with the help of hoe.
The contour interval is kept 15 to 30 m. After deciding the position of contour lines, a shallow foundation trench is dug. The foundation depth is kept 5cm and width from 30 to 40cm. For construction of bund, initially large size stones are packed in the foundation.
And smaller stones are placed to build the rest of the bund height. The stones are packed carefully, especially at the bottom. After packing the stone pieces, the earth materials obtained from the foundation trench are piled in front of the bund. If the land is abandoned plot, then stone bunds are left for a year to catch the eroded soil particles. In stone bunded area, the cultivation should be started from the second season.
Types of Graded Bunding:
It is divided in following two types:
i. Narrow base graded bunding; and
ii. Broad Base Graded Bunding
The narrow base graded bunding makes obstruction in crossing of farm implements; the natural vegetation covers the sides; more height is attained for the same cross-section; though additional height may not be required. The main limitation of this practice is to get lost a large cultivable area.
The broad based grade bunding is concerned; it creates no hindrance in movement of farm machineries and entire area is available for cultivation purposes.
This system also involves some limitations, which are as follows:
1. Due to crossing of farm implements the soil of bund’s section gets loosened and lost.
2. The original cross-section of bund gets disturbed in very short period of time due to running of farm machineries. To maintain the bund section in proper form a proper maintenance is required.
Design Criteria of Bund Construction:
The design criteria of bund construction is based on following points:
1. Land Submergence:
Submergence of land for longer duration affects greatly to the crop yield. Therefore, the design of outlet or surplus arrangement for the bunds should be done on the basis of allowable submergence of land, cropping practice to be adopted and infiltration rate of the soil.
2. Water Conservation:
The water conservation for growing some crops like paddy which requires enough water for its growth, the dimension of bunds should be fixed in such a range that the total surface runoff passing through the area must be harvested by the bund.
For other crops such as wheat, maize etc., the storage capacity of bund should be decided on the basis of their average consumptive use and maximum length of dry period of the area. The height of waste weir should be fixed based on the crop water requirement.
3. Economy of Construction:
The economy of bund construction mainly depends on the following two points:
(i) Total earthwork involved in construction; and
(ii) Average land lost under bund construction.
As for as possible the sum of above two items cost should be minimum for any bunding project.
4. Critical Length:
It is that spacing of bund, beyond which erosion gets start, depending on the slope and soil condition. Theoretically, the spacing of the bund should be equal to the critical length to control the erosion. Use of closer spacing increases the construction cost. In addition to this, the field also becomes very narrow that hinders the agricultural operations.
5. Seepage Consideration:
Seepage through bund is mainly affected by the depth of water collected towards upstream face of the bund, side slope and permeability of the construction material. The design of bund cross-section should strictly be followed by considering the seepage rate through the bund due to accumulation of surface runoff behind it.
The planning of graded bunding is carried out, using same guide lines that are used in contour bunding system. It is performed either by making the plan on contour map of the area and then implementing the same into the field, or directly into the field. However, the direct alignment requires a lot of field experience.
The first step of planning of graded bunding is to decide the appropriate location of outlets to be used for draining the surplus water from the bunded area. The outlet may be the grassed waterway. It should be kept in view that, if bunding is not done at the top of catchment, then this portion of land should be utilized for constructing the diversion channel.
The elevation of outlet is fixed on the basis of vertical interval (VI) and highest elevation of the field; for example – if VI of first bund is taken as 0.8 m and highest elevation is 531.6 m, then the elevation of outlet will be 531.6 – 0.80 = 530.8 m for first bund.
For deciding the grade of the bund, 20 to 25 m distance is measured along the contour by steps and elevation of the alignment is raised in same proportion. Let, if it is desired to provide the grade 0.4% in 25 m length and elevation of outlet at upper end is 530.8, then elevation of outlet at 25 m distance would be 530.9 m [i.e. (0.4/100) x 25 + 530.8 = 530.9].
Likewise, in next 25 m successive distance, the outlet elevation will be 531.0, 531.1, 531.2 and so on. For providing variable grade to the bund the same procedure is adopted, except the rise in level is kept in proportionate to the grade. In the same way the elevation of outlet of second bund is also determined by deducting the VI from the elevation of outlet obtained for first bund. In present case the elevation of outlet for IInd bund will be 530.8 – 0.8 = 530.0, and for next bund it will be 530.0 – 0.8 = 529.2, 528.4 and so on.
The alignment of second bund is done, using same technique as described in case of first bund. The same technique is also extended for other graded bunds. In case, if land feature is irregular and exact alignment is not possible, then minor adjustments can also be made to bring the bunds alignment in reasonable limit; and accordingly the grade of channel is also modified.
The plan, prepared so, is transferred in the field, using the following steps:
1. Set the levelling instrument at higher elevation than the highest elevation of the graded bund to be marked.
2. Find out the height of instrument (HI) by taking the back sight of the bench mark.
3. Determine the staff reading, by deducting the desired elevation from the HI.
4. Instruct the staff to move ahead towards the outlet till the desired staff reading for first outlet of graded bund is obtained.
5. Also, instruct the staff man to move across the slope up to a distance of about 25 m and to do the staff up and down the slope, keeping approximately the previous marked station as centre and 25 m as radius. The staff reading is reduced in the proportion of rise in the elevation. Suppose, if rod reading at previous station was 4.80, and if 0.4% grade is desired to provide, then the rod reading at 25 m intervals from the outlet, should be 4.70, 4.60 and so on. Similarly, if 0.2% grade is to provide, then at each 25 m distance, the staff reading would be 4.75, 4.70, 4.65 and so on.
6. Make adjustment, if exact alignment is not possible due to topographical irregularities. Normally, 15 cm up or down followed by cutting or filling, to avoid the accumulation of water, is permitted.
7. The rest outlets and graded bunds are also positioned in the same way.
8. Lastly, by making earth work the bunds are constructed.
The construction of bund should be started from the highest point and proceeded down to the valley part of the area, which provides an assurance to protect the bund against occurrence of heavy rains during construction period. The base width of bund is marked on the down-stream side and field is cleaned from existing vegetations. During construction the soil of the area is thoroughly mixed for getting good bonding effect when bund is formed over it. The burrow pits should be located on the upstream face of the bund with a uniform depth of about 30 cm.
The width of burrow pit may vary as per requirement, but it must be in continuous form. The burrow pits should be avoided from the gully or depression parts of the field. At the time of digging the soil, if there is found some clods, they should be broken first and then allowed for bund construction. The clods more than 3 cm in size and stones should be removed from the soil.
For constructing the bund, the soil should be kept in layer wise not more than 15 cm thickness and compacting them by tramping. The templates of specified dimensions should be used for checking the bund’s cross section. At last, the bund’s section is finally shaped, trimmed and slightly rammed form its top and side slopes. After the construction work of bund is finished, the field should be ploughed for filling the burrow pits, made during bund construction.