How to Design a Bund? | Soil Management

In this article we will discuss about how to design a bund to control soil erosion.

A bund consists of following parameters for determining under design:

1. Choice of Bund:

The choice of bund (i.e. contour or graded bund) to be constructed depends on the rainfall, soil condition and types of the outlets used. In low rainfall areas with the annual rainfall less than 500 mm, and where soil moisture is a limiting factor for crop production, the contour bunds are preferred for construction. While in heavy and medium rainfall areas, the graded bunds are recommended for construction. The grade to be provided to the bund, should be from 0.2 to 0.3%.

2. Spacing of the Bund:

The basic criterion, used to decide the bund’s spacing, are given as under:

i. The phreatic zone of the upper bund should coincide the saturation zone of the immediate lower bund.

ii. The bund should be able to check the surface runoff at the point where flow attains an erosive velocity; and

iii. The bund should satisfy all the requirements of agricultural operations.

Following formula can be used for determining the spacing of bunds:

V.I. = (S/a) + b … (12.1)

Where,

V.I. = vertical interval between two successive bunds

S = percent land slope

a, b = constants, depend on the soil and rainfall characteristics of the area.

The above equation can be modified for the specified areas having different rainfall amounts.

The modified forms of above equation may be as under:

For the areas of heavy rainfall –

V.I. = (30 S/3) + 60 … (12.2)

For the areas of low rainfall –

V.I. = (30 S/2) + 60 … (12.3)

In which, VI is in cm and S in percent.

C.E. Ramser also proposed a general relationship for computing the spacing of contour bund for sub- humid areas of United States. The formula was developed on the basis of field observations and experimental data. The equation is given by –

Where,

V.I. = vertical interval (m)

S = land slope (degree)

The above equation was derived by considering only land slope; not by taking the other factors such as soil infiltration, permeability and rainfall into consideration. In order to account all these factors the above equation is required to adjust.

In this respect, the possible adjustments are described as under:

(a) Adjustment for Infiltration and Permeability:

The soil infiltration and permeability affect the spacing of bund, significantly. Higher value of infiltration and permeability increase the extent of vertical interval. Normally, 25% extra spacing is given to the mean value of VI for the soils having high infiltration rate and permeability; and also for the soils, engauged under agronomical practices, like contour farming, frequent use of legume cover crops etc. On the contrast, in the soils which have very low rate of infiltration and permeability or when soil is engauged under unfavorable cropping condition, the vertical interval is reduced by 15% than the mean VI.

(b) Adjustment for Rainfall:

The rainfall plays an important role to decide the limit of vertical interval of contour bund. In lesser rainfall condition comparatively greater spacing is used and vice-versa. The reason may be justified as – a lesser rainfall is capable to produce less volume of surface runoff; virtually the soil loss is also reduced accordingly. In this situation, the bund constructed at farther interval can be sufficient to handle the small runoff and to control the soil erosion.

Incorporating the effect of infiltration rate and rainfall amount on VI of bund, Cox developed following equation –

VI = 0.3 (XS + Y) … (12.5)

In which, X is the rainfall factor; Y is the infiltration rate and crop cover factor and S is the percent land slope. By above equation the value of VI is obtained in the unit of ‘m’. The X and Y values are cited in Table 12.6 and 12.7, respectively.

Deviation of VI:

The ‘vertical interval’ calculated by any above equation, is theoretical; it may be and may not be exact for different locations. If it is exact for a given location, then no problem, otherwise the calculated value of VI is required to change by some extent.

Normally, the vertical spacings are increased by 10% or maximum 15 cm. Apart from above, if a small strip of land is left between the last bund and boundary line of the field, then this space may be distributed to all the bunds by increasing the VI, rather only in last bund.

It can be explained more clear by the example as: if contour bunds are formed at 80 cm VI, and a space of 40 cm is left between field boundary and last bund, then rather constructing the last bund at 120 cm fall, it would be better to construct all the bund at some greater distance than the 80 cm, say at 85 cm or so, depending on the area and number of bunds to be constructed. However, it should always be kept in view that 40 cm surplus space should be equally distributed amongst all the bunds.

The ‘horizontal spacing’ of the bund is decided by the land slope. For this purpose the following formula can be used –

In which, H.D. indicates the horizontal distance between the bund and V.I. is the vertical interval.

3. Size of the Bund:

The size of bund includes its height, top width, side slopes and bottom width. For deciding bund size, the height of it, is fixed, first. Once the height of the bund is fixed, the other dimensions such as top width and base width can be easily obtained. The height of bund mainly depends on the land slope, spacing of the bund and expected maximum rainfall intensity of the area. The side slopes of the bund depend on the nature of the soil (i.e. internal friction angle) used for construction.

Height:

It is determined on the basis of amount of water to be intercepted by the bund.

The determining procedure is shown by the following derivation:

Let, h is the height of the bund; x is the horizontal distance between two successive bunds; W is the width of water spread behind the bund and v is the vertical interval between two bunds.

This is the desired expression for calculating the height of bund, in which x is in meter and S in meter per 100 m. The above equation yields the theoretical value of bund’s height. For getting the practical value of bund’s height, there should be added some additional height to the theoretical one, called free board.

Apart from above derivation of bund’s height, the following two more criterion are also adopted for determining the height of contour bund.

They are:

1. Height of bund with 30-cm depth of water impounding; and

2. Height of bund to store 24-h rainfall of a particular frequency.

1. Bund’s Height with 30-cm Impounding Depth:

It is divided into two parts:

(a) For Narrow Base Contour Bund.

The narrow base contour bund with 30-cm depth of water impounding is practiced in many parts of the India. In this case, the total bund’s height.is determined by adding 30 cm as the depth of water impounding; 30 cm as depth of flow over the outlet and additional 20 cm as free board, which born 80 cm as the total height of bund. The top and bottom widths are fixed as 50 cm and 200 cm, respectively. For this specification the side slope of bund is obtained approximately 1:1. And total cross-sectional area is furnished with 1. sq m, i.e.

The bunds with cross-sectional area 1 sqm are found suitable for low rainfall regions with permeable soils. In few states, the farmers maintain the bund’s cross-sectional area less than 1 sq. m, because of the reason that the depth of water impounding or depth of flow over the outlet is kept constant as 30 cm.

The bunds with less than 1 sqm cross-sectional area involve several demerits; the important ones are as follows:

i. The bunds constructed with cross-sectional area less than 1 sqm do not last for longer duration.

ii. By tillage operations and rainfall occurrence, the bund’s section is always liable to get reduced; and if proper maintenance at proper time is not being done, then bund may be distracted very soon from the field.

Considering above points, it is therefore recommended that the cross-sectional area of narrow base contour bund should not be less than 1 sqm.

(b) For Broad Base Contour Bund:

In case of broad base contour bunds the size of bund is kept larger than the narrow base bund. The fields laid with broad base bunds can be operated by using tractors like wide machines for farming works. For determining bund size, the settled height of bund is counted as greater than 30 cm (most preferably up to 50 cm), which is measured from the ground surface.

The side slopes are used as 5:1. However, on steep land slopes where bunds are constructed at closer intervals, the side slopes can be reduced to 4:1. The cross-sectional area of broad base contour bunds with their heights and side slopes are cited in Table 12.8. It is desirable to have a minimum cross-sectional area 1 sqm, either by increasing the height or side slope of the bund.

2. Bund’s Height to Store 24-h Rainfall of a given Frequency:

The height of contour bund to store 24-hour rainfall of a particular frequency, is described under following derivation. Referring the Fig. 12.3.

Let,

VI = vertical interval between consecutive bunds (m)

HI = horizontal interval (m)

R_e = 24-h rainfall excess (cm)

W.S = water spreading length behind the bund (m)

h = depth of water impounding near the bund (m)

Assuming the storage area formed behind the bund is in triangle shape; hence storage capacity of bund is thus given as –

After having the depth of water ponding (h), the practical bund’s height (h_n) can be obtained as –

h_n = Depth of impounding (h) + Depth of flow over outlet + Free board as 25% of h.

The depth of flow over the weir (i.e. outlet) is taken as 30 cm. Once the height of contour bund has been determined, then considering 50 cm as top width and 1:1 as side slope, the cross-sectional area of contour bund can be obtained.

Other Details of Contour Bunding:

1. Length of Contour Bund:

It is determined based on the horizontal interval of the bund. The length of bund per hectare area is given as –

2. Earth Work:

The earthwork of bunding system includes the sum of earthworks under main contour bunds, side bunds and lateral bunds formed in the field. The earthwork of any type of bund is obtained by multiplying the cross-sectional area to its total length.

The total earthwork can be expressed by the following equation –

E_t = E_m + E_s + E_l … (12.19)

Where,

E_t = total earthwork

E_m = earthwork of main bunds

E_s = earthwork of side bunds

E_l = earthwork of lateral bunds

For calculation point of view, the E_s + E_l are taken as 30% earthwork of the main contour bund. Thus,

In above calculation the value of E_s + E_l is taken as 30% earth work of main contour bund (E_m) by assuming that, the length of side bunds and lateral bunds to be as 30% of the length of main bund and their cross-sectional area is also being equal to the main bund.

4. Side Slope of the Bund:

The side slopes of the bund are dependent on the internal frictional angle of the fill material. Recommended values of side slopes for different types of soil are listed in Table 12.9. Generally, in large scale of bunding projects, it is a difficult task to fix the dimension of the individual bund. In this circumstances a suitable size of bund is selected on the basis of soil characteristics and rainfall intensity, and is used throughout the field.

Grade:

The contour bunds are constructed along the contour of the land. For this type of bund the grade is kept as zero. Particularly, when field is irregular where exact alignment of bund on contour is not possible, then some deviation is required in construction. Normally, the limit of deviation is prescribed as 10 cm at higher elevation sides and 20 cm for crossing the depressions. The deviation used so, causes uneven impoun­ding of water, by virtue of which the bund involves lesser storage capacity, as a result there is increase of pro­bability of bund breaching due to unexpected intense rainfall.

Therefore for avoiding such type of happening, it is always desirable to remove all irregularities and depressions from the field prior to make alignment of contour bunds. As the contour bunding is recommended for semi-arid areas mainly where intensive farming is not feasible without irrigation, the levelling of land by removing local depressions and ridges is not possible due to cost factor.

In the circumstance, when there is no other alternative than to allow some deviations for getting better alignment, then deviations are permitted only for crossing the narrow ridges and depressions, not for larger depressions or wider ridges, which are the part of the land already taken into consideration.

The grade is provided to the graded bunds. In this case the grade to be given in graded bunds should be sufficient to cause good drainage, and also to develop adequate flow without causing erosion from there. A lesser grade causes the problem of water-logging which in turn not to permit the farming operations at suitable time. On the contrast, a higher grade is responsible to cause erosion problem from the channel section. Therefore, the limit of grade should neither be less nor more, but within an optimum range.

In usual way, the grade ranging from 0.2 to 0.4 percent is provided to the bund, depending on the soil types. For graded bunding system, someone prefer to provide variable grade rather uniform grade throughout the length. Bunds having variable grades throughout their length, are called variable graded bunds.

The range of variable grade varies from soil to soil. In permeable soils, it may vary from 0% at upper end of field to 0.4% at the outlet end, whereas in impermeable soils, it may be 0.2% at upper end of field and 0.4% at the outlet. For last 20 or 25 m length of bund, the grade is doubled to the bund for eliminating the restrictions caused by outlet and also allowing some extra depth to the graded bund.

After deciding the range of grade to be given the bund, the next is to decide that at what interval the grade should be change. It is customarily changed at the interval of 100 to 150 m. In this way, in permeable soil if a bund which has total 400 m length, then as per custom 0% grade should be given for the first 100 m length; 0.1% for second 100 m; 0.2% for third 100 m and 0.3% for the last 100 m length of the bund.

One point should also be kept in mind that, if maintenance of grassed outlets and bunds is not anticipated, then greater length of bund is not appropriate; it is advisable to use shorter length and more number of outlets, rather longer length and few outlets. The experience also revealed that the length of bund may vary from 100 to 200 m as-minimum to maximum 400 m, depending on the soil types and topographical features of the area.

Choice on Uniform and Variable Grade:

The variable and uniform grades are the two types of grade, provided to the bunding or terracing systems. Now question arises which type of grade should be provided.

The choice making on uniform grade or variable grade for providing to the bunding system, depends on the following points:

1. Alignment:

A variable grade provides better alignment, because it increases the channel capacity as approaches towards outlets. The creation of such features in channels is very essential, because as the channel approaches towards down-stream side, the net volume of runoff flow gets increase. The increase in flow rate at farther distance can be easily accommodated by increasing the grade which is only possible by providing the variable grade to the bund.

2. Cross-Sectional Area and Length of Bund:

When cross-sectional area of bund is fixed as 1 sqm and length is kept in a reasonable range, then uniform grade is better to provide sufficient capacity (i.e. use of uniform grade is suitable) to the channel. Apart from making the choice on types of grade to be given based on bund’s alignment, the length of bund should also be considered as a factor for deciding the same. As a thumb rule, for shorter length, uniform grade and for longer length of bund a variable grade is recommended to the bunding system.

Cross-Section of Graded Bund.

An ideal cross-section of graded bund should satisfy the following features:

1. Should have sufficient carrying capacity.

2. The side slopes should be sufficiently flat to permit the farming operations without having the problem of undue breakage of bund’s section.

The channel capacity of graded bund depends on the cross-sectional area and flow velocity. The cross-sectional area is determined based on the peak runoff rate generated from the contributing area. The field experience shows that, the cross-sectional area should not be less than 1 sqm.

The depth of settled channel terrace (i.e. graded bund) is taken as minimum 45 cm, measured from bottom of the channel to the top of the ridge. Many individuals prefer to construct the graded bund, to conduct the flow only from the dug portion and the bund height to serve as ‘free board’.

The flow velocity in the channel is determined by calculating the hydraulic radius of the channel, appropriate roughness coefficient (usually 0.04) and grade of the channel. The velocity should be within non-erosive limit. The velocity greater than 0.50 m/s in sandy soil and 0.65 m/s in erosion resisting soils are not threshold for safe disposal of runoff.

Some typical dimensions of bunds to fix the cross-sectional area, are given in Table 12.10. As precaution, initially the bunds should be designed for a greater size on the basis of hydraulic considerations so that they can accommodate the requirement of settlement allowance and poor maintenance of the bund.

5.  Alignment of Bund:

The alignment of bund should be done by considering the following points:

i. Bunds should be constructed in straight line, and if possible the sharp curves should be avoided.

ii. If there occur gullies or surface depressions, then bund should be constructed at right angles to them. However, a deviation up to 15 cm in crossing the ridges and up to 30 cm in case of gully is permissible. Also, when it is necessary to construct the bund by crossing the depression, then there should be made a provision for straightening the bund section.

iii. Longer length of graded bund should be avoided. The length of bund should not exceed 350 m in one direction of surface flow.

iv. Odd bits at the land surface should not be left in the surroundings of the individual holdings on down-stream side of the bund.

v. As far as possible, there should be provided a passage for movement of carts and catties.

Generally, the alignment of bund is carried out with the preparation of plan of the area purposed for construction of bunds, first. If the plan of the area is not prepared, then it should be carried out either by chain survey or plane table survey.

The irregularities like surface depressions, natural drains such as water courses, gullies and field embankments, recognized cart tracts etc., existing in the field should be fully indicated in the plan. Similarly, if there is provision to use the grassed waterways as an outlet for the bunding system, then their position should also be fully decided and indicated on the plan.

To locate the position of bunds in the field, basically two approaches are frequently used. In which, first is the preparation of contour map of the area, considering 30 m as bund interval and 0.3 m as contour interval. The contour plan of the area helps in finding the average field slope and required vertical and horizontal intervals of the bund. With the help of these informations, the correct position of the bund is marked on the plan.

After that the position of the bund is checked in the field. If it requires some modification, then should also be incorporated, accordingly. The second approach involves, the direct marking of bunds position in the field, using levelling instruments, but it requires sufficient experience. Alignment of the graded bund is generally done by this method.

The alignment of bund should be started from the ridge line of the area. For alignment, the first bund is located directly in the field by providing a suitable vertical and horizontal interval. After that, the field is smoothened to avoid undesirable surface irregularities. The position of second bund is now marked as per plan. Similarly, the subsequent bunds are also marked and proceeding towards the valley parts of the area. This work is continued until the alignment of entire bund gets finished.