Contour Bunding and Graded Bunding

Bunding is an engineering measure of soil conservation that involves placing an impediment in the course of surface runoff to slow down the flow of water. It helps to reduce soil erosion by retaining the runoff water in the watershed. Bunds are simple embankment-like constructions that run the length of a land slope. In watersheds, many forms of bunds are used to reduce erosion and conserve moisture. Contour bunds are defined as bunds that are built along the contours with small deviations to respond to practical situations. Graded bunds are those that have a slight incline to them.

By holding runoff water in the watershed, it helps to decrease soil erosion. Bunds are straightforward embankment-like structures that run the length of a land slope. Bunds are used in watersheds to minimise erosion and store moisture in a variety of ways. Bunds built along contours with modest deviations to respond to practical requirements are known as contour bunds. Bunds with a small inclination are known as graded bunds.

Design Specification of Bunds

The following factors should be addressed when designing a bund:

1. Bund Type: The type of bund to be built is determined by the rainfall and soil conditions. In locations where yearly rainfall is less than 600 mm and soil moisture is a limiting factor for crop productivity, contour bunds are preferred. In locations with heavy or moderate rainfall, graded bunds are advised.
2. Bund Spacing: The following basic principles should be followed when determining bund spacing: (1) the seepage zone below the upper bund should meet the saturation zone of the lower bund; (2) the bunds should check the water at a point where the water attains erosive velocity; and (3) the bund should not cause agricultural operations any inconvenience.
3. Bund Size: The height, top width, side slopes, and bottom width all contribute to the bund’s size. Bund height is primarily determined by the slope of the land, bund spacing, and the maximum amount of rainfall forecast in the area. Following the height of the bund, the remaining parameters of the bund, such as the base width, top width, and side slopes, are calculated using the information on the soil’s type.

Uses

1. Many portions of India, including Maharashtra, Gujarat, Tamil Nadu, Karnataka, and Andhra Pradesh, practice contour bundling.
2. It doesn’t work with black soil since it shrinks in dry conditions, resulting in fissures.
3. Contour bunding, among other techniques, has proven to be a successful strategy for watershed management.
4. It not only aids in the avoidance of soil erosion but also the creation of water pools for groundwater rejuvenation.
5. During forest plantation on slopes, the same procedure is utilised.
6. Near the roots of trees, crescent-shaped depressions are formed to aid in the sinking of water in the roots.
7. Castor oil crops can be put on bunds to provide extra income while also holding the bunds in place.

Difference between Contour bunding and differential Bunding

Contour Bunding

Contour bunding refers to land bunds that travel through points of equal elevation (i.e. on contour). The level terraces and contour bunds are about to become synonymous. As a result, the contour bund is sometimes referred to as a level terrace. The practice of contour bunding was first implemented as a famine relief measure in Maharashtra, India. It is an effective strategy for preventing runoff and soil erosion in sloping cultivated areas.

Contour bunding can be done on a variety of permeable soils, including alluvial, red/laterite brown soil, shallow and medium black soil, and clay or deep black cotton soils, but these soils have a problem with crack growth, which leads to bund collapse.

The contour bundling is classified into two categories:

1. Bundling with a narrow base contour;
2. Bundling with a broad base contour.

Natural vegetation covers the sides of the narrow base contour bunding system, preventing farm implements from crossing; additional height is allowed for the same cross-section. It has limitations in that a significant amount of space is wasted in the construction of the bund; the bund portion is prone to erosion owing to raindrop impact, necessitating careful maintenance. Farming operations are further hampered by the shallow base contour bund.

When it comes to the broad base contour bunding, it does not obstruct agricultural operations; the entire area can be kept under cultivation. It has some limitations, such as the disruption of the bund’s section caused by farm equipment crossing it, necessitating careful attention and maintenance. Apart from the aforementioned, the soil of the bund is loosened with the operation of farm machinery, resulting in a rapid loss in the size of the bund unless regular maintenance is implemented.

Graded Bunding

The graded bund, also known as a channel terrace, is utilised in locations when annual rainfall exceeds 700 mm. It can, however, be employed in locations with less rainfall as long as the soil is of a heavy texture. When it rains in these locations, a considerable amount of water accumulates on the surface.

The grade must stay within the non-erosive range. Normally, a range of 0.1 to 0.5 percent is allowed. It’s also important to remember that the channel’s grade must be lower than the real land slope. The water flows slowly to the water going directly down the hill, preventing soil erosion from occurring there.

The following are the principal roles of graded bunds:

1. It shortens the length of the slope, which lowers soil erosion.
2.  It also safely disposes of any excess water to a suitable location.
3. After contour bunding, graded bunds are built to eliminate any extra water that the crop does not require.

Conclusion

Contour and graded bunding have been performed in India for a long time, and Indian farmers are well-versed in the technique. Bunds have been shown to work successfully in shallow, medium, and medium-deep soils based on experience. Bunds fail in deep black soil because of cracks in dry conditions. Water continues to pour through these fissures, resulting in large breaches. The fields are severely damaged as a result of this. Even though black cotton soils have a variety of erosion issues, contour bunding cannot be properly implemented in such soils.