In this article we will discuss about:- 1. Size of the Bunds 2. Alignment of Bunds 3. Construction of Bunds 4. Surplussing Arrangements 5. Types of Surplus Weirs.
Size of the Bunds:
The height of the bund depends upon the slope of the land, spacing of the bunds and the maximum intensity of rainfall expected in the area. Once the height is determined, other dimensions of the bund viz., base width, top width and side slopes depend upon the natural of the soil.
Depending on the amount of water to be intercepted, the height of the bund can be calculated as shown below –
L is in metres and S metres per 100 metres and x will be given in metres. This is the theoretical height and suitable free board is added to arrive at the practical height.
Base width of the bund is dependent upon the hydraulic gradient of water in the soil. Side slopes are dependent upon the angle of repose of the soil. A general value of the hydraulic gradient assumed is 1 : 4. Side slopes of the bunds recommended for different soils are given in Table 22.2.
In large scale bunding projects, it will be difficult to calculate the dimensions of the bund in each individual case. Therefore, a particular size of bund is chosen and adopted in all fields.
Table 22.3 shows some of the typical cross-sections of bunds adopted. Usually, a higher size of the bunds than required by the hydraulic considerations is adopted to allow for settlement and the poor maintenance by the cultivators.
Alignment of Bunds:
A plan of the area proposed to be bunded is necessary. In case the plan is not available, it has to be prepared either by chain survey or plane table survey. All field subdivisions, natural features such as existing water courses, gullies, field embankments, recognized cart tracks etc., may be marked in the plan. If grassed waterways are to be used as outlets, the position of the waterways may be decided by reconnaissance survey and marked on the plan.
For locating the position of bunds in the field two approaches are adopted. In the first one, a contour plan of the area is prepared. For this purpose levels are taken at 30 m interval and contours are drawn at 0.3 m interval.
From the contour plan, the average slopes of the fields are calculated and the required vertical and horizontal spacings are decided. Taking these into consideration the position of the bunds is marked on the plan. The position of the bunds is now checked under fields conditions and suitable modifications are made.
In the other approach the position of the bunds is directly marked on the field using the levelling instrument. Alignment of the graded bunds is efficiently done by this method. The slope of the area is fixed by taking fly levels. The first bund is marked on the field directly by giving the necessary vertical and horizontal spacing.
Marking of the bunds is started from the outlet point and the levels taken at every 30 m or closer. After making the bund position, it is smoothened to avoid undesirable curves and cuttings of field boundaries. The position of the bund is now marked in the plan for record. The subsequent bunds towards the valley are aligned in a similar manner.
The following points may be observed for the alignment of bunds:
1. As far as possible, the bunds should be straight and sharp curves should be avoided.
2. Bunds should cross a gully or depression at right angles to the gully or depression. Deviation upto 15 cm in crossing a ridge and upto 30 cm in crossing a gully is allowed. Where the bund crosses a depression provision should be made to strengthen the bund section.
3. In case of graded bunds long length of bunds should be avoided. The length of bund in one direction of flow of water not exceeds 350 m.
4. Odd bits of land should not be left on the down streamside of the bunds at the periphery of individual holdings.
5. Ramps (for passage of carts and cattle) may be provided wherever necessary.
Construction of Bunds:
Construction of bunds should start with the bund nearest the ridge and continued down the valley. This will ensure protection of the bunds if rains occur during construction. The bund position shown in the plan indicates the upstream toe of the bund.
The full base width is marked on the downstream side and the area is cleared of vegetation. The soil in this area is also slightly disturbed so that good binding can be effected when the bund is formed over it.
The burrow pits for the soil are generally located on the upstream side of the bund. It should have a uniform depth of 30 cm and width varied as per necessity. The burrow pit is to be continuous and no breaks need be left.
The burrow pits should not be located in a gully or depression. When the soil is dug, all the clods should be broken in the burrow pit itself before putting in the bund. Clods more than 3 cm or stones should not be put in the bund. The earth should be put in layers of 15 cm and consolidated by trampling.
Templates of the specified dimensions be used for checking the bund section. The bund section should be finally shaped, trimmed and slightly rammed on the top and the side surfaces. After the bund formation it is desirable to plough the field and the burrow pit.
Surplussing Arrangements:
Both in case of contour bunds and graded bunds, arrangements for safely draining the run-off are necessary. Even though the contour bunds are designed to withhold all the rainfall, surplussing arrangements are necessary as the high rainfall intensities occurring sometimes produce high rates of runoff which the bunds may not be able to withstand. At times it is also not desirable to keep the water standing in the field for a long time. Two types of surplussing arrangements are used in bunding.
These are –
(1) Surplus weirs (also known as waste weirs), and
(2) Grassed waterways.
The surplus weirs do not require any land to be taken away from cultivation and they can be located at the desired location. Because of these reasons, the surplus weirs are used with contour bunds. The main problem observed in case of the surplus weirs is the tendency to develop flow channels as water flows from one field to another through these weirs.
Grassed waterways have been conveniently used for water disposal in case of graded bunds as well as the channel type of broad bed terraces. However, they occupy some land area.
Types of Surplus Weirs:
The various types of surplus weirs are –
(1) Clear overfall weirs,
(2) Pipe outlets,
(3) Grass outlet or ramp-cum-waste weirs,
(4) Cut outlets, and
(5) Prefabricated structures.
The clear overfall weir is a permanent masonry structure. Several local variations of the structure are made at different locations.
The locally available construction materials are used for structure. The height of the crest wall controls the depth of the water ponded upstream of the weir. A 30 cm height above the contour is generally kept.
The length of the weir is calculated using the weir formula –
Where, Q is the peak rate of runoff, and C is a coefficient.
L is the length of weir required and H is the head of flow. The runoff is estimated from the catchment area of the weir. As the cumulative catchment area increases progressively towards the valley, the length of the weir also correspondingly increases.
The head of flow over the weirs is generally limited to 30 cm. It is not desirable to provide waste weirs for catchment areas beyond 40 hectares as in such a case the water flowing from field to field will be very considerable. In such cases suitable diversion of the water should be planned.
The pipe outlet consists of an upstream and downstream stilling wells and a conduit of concrete pipe. The stilling wells are constructed with brick or stone masonry or with cast concrete pipes.
Grassed Weir:
This type of weir consists of a simple opening in the bund protected with grass. Stone pitching is done if stones are available at the site. This weir is simple in construction but cannot handle large volumes of water. This type of weir is also used as a ramp for the passage of cattle and for taking agricultural implements from one plot to another.
Cut Outlets:
As their name indicates, the cut outlets are simple openings left out at the end of bunds to allow the water to escape. Protection from erosion is achieved either by stone pitching or by planting grasses or both.
Pre-Fabricated Surplus Weir:
A pre-fabricated surplus weir, similar to the shape of a drop structure used in gully control can conveniently be used in bunding and land levelling projects. The advantages are that it can be fabricated at a .convenient location and transported in parts, assembled and installed at the desired locations.
Thus considerable saving in construction costs are effected compared to the construction of these structures in-situ. Later if desired or necessary these structures can be removed and reinstalled at any other location without any damage to the components of the structure.
Fabrication of the Structure:
For the purpose of fabrication and manufacture, the structure is made into four components.
These are –
(1) Head wall,
(2) Apron,
(3) Right side wing wall and
(4) Left side wing wall.
Each of the component can be cast separately. Mild steel flats are used as forms to cast the components. In order to get different sizes of the components the same forms can be suitably modified. The wing walls through similar in shape have to be cast separately as one wing wall is to be used on the right side and the other on the left side.
All the components are made with reinforced cement concrete. Reinforcements of 3 to 5 mm dia. bars can be used. Fine gravel has to be used and 1:2:4 ratio is recommended for the concrete. After the casting of the concrete it has to be cured properly as per the standard practice.
Dimensions of the Structure:
The dimensions of the structure can be decided to suit the particular site conditions. The length of the weir is calculated using the weir formula for a trapezoidal weir having 1 : 4 side slope – 
To get different drops, the position of the apron in the head wall has to be altered suitably. In any land levelling or a field bunding project, the possible installation sites can be identified and dimensions of the weir needed for each installation can be worked out.
A range of the weir dimensions for the particular project can be decided. Depending upon the number of ranges decided the head wall and the apron can be cast to suit these ranges while the wing walls can be the same for different dimensions of the head wall and apron.