Bench Terraces: Types and Maintenance | Hill Slopes | Soil Science

In this article we will discuss about:- 1. Types of Bench Terraces 2. Maintenance of Bench Terraces 3. Bench Terraces for Rice Cultivation.

Types of Bench Terraces:

Depending upon the purpose for which they are used bench terraces are also classified as follows:

1. Hill-type bench terraces – used for hilly areas with a reverse grade towards the hill.

2. Irrigated bench terraces – level benches adopted tinder irrigated conditions.

3. Orchard bench terraces – narrow width terraces (about 1 m) for individual trees. These are also referred to as intermittent terraces and step terraces.

The conversion of land into bench terraces over a period of time is referred to as gradual bench terracing.

Bench terraces are classified depending upon the slope of benches.

The different types are –

(i) Bench terraces sloping outward;

(ii) Bench terraces sloping inward, and

(iii) Bench terraces with level top.

Bench terraces with slopes inside are to be adopted in heavy rainfall areas where a major portion of the rainfall is to be drained as surface runoff. In the case of these terraces, a suitable drain at the inward end of each of these terraces is to be provided to drain the runoff. These drains ultimately lead to suitable outlet. These are also known as hill-type bench terraces.

Bench terraces with level tops are suitable for areas of medium rainfall, evenly distributed and having deep and highly permeable soils. Due to the fact that no slope is given to the benches it is expected that most of the rainfall coming over the area is to be absorbed by the soil and very little water is to go as surface drainage. These types of terraces are also used where irrigation facilities are available and referred to as irrigated bench terraces.

Bench terraces sloping outward are to be used in low rainfall areas with permeable soils. For bench terraces sloping outward a shoulder bund even though such a bund is provided in the other two types also for giving stability to the edge of the terrace.

In these terraces, the rainfall coming over the area is to be conserved by retaining the shoulder bund and the rainfall thus conserved will have more time for soaking into the soil. Bench terraces with narrow width (about 1 m) are sometimes constructed for orchards and are known as orchard bench terraces. These terraces are referred to as step terraces when a series of step like formations are made.

Maintenance of Bench Terraces:

Proper maintenance of the bench terraces is important. The shoulder bund should be planted with permanent vegetation and ploughing of the toe of bund should be avoided. The batter slope of the terraces should be stabilized and protected by establishing deep rooted and soil binding and spreading type of grasses.

Bench Terraces for Rice Cultivation:

When agricultural lands are to be converted for rice cultivation, the fields should be fairly flat and bounded by bunds to retain water. The subsoil should not be exposed to such an extent that may make the fields unproductive for some years.

The bench terraces constructed for rice cultivation will be much wider than the ones constructed on hill slopes. Converting the land to bench terraces involves construction of bunds and levelling the area in between the bunds. Generally about 60 cm high bunds with a top width by 30 cm and side slopes of 1 : 1 at the time of construction are satisfactory.

The horizontal spacing of the bunds should be selected such that the spacing of the bunds is not too close and at the same time the total quantity of earthwork for construction of the bunds and filling the depressions is a minimum. A method for calculating the width of the terraces to obtain minimum earthwork was given by Subha Raju (1969) and is outlined here.

Consider a field surface of length ‘L’ along the slope and a unit width across the slope. In Fig. 21.9, XY represents the sloping surface of length ‘L’ which has to be shaped to horizontal benches.

In Fig. 21.10 portion MPQR represents the earth used for construction of the bund.

From the above it is seen that earth from one third of the width between bunds will go to bund construction, and earth cut in the middle one third of the width will go to fill the depression in the bottom one third.

It should be noted that in above calculations, the base width of the bund is neglected. The earth work per hectare can be calculated as follow –

For planning of the terraces before construction, a topographic plan of the area is prepared showing the contours at 50 cm interval. The location of the irrigation and drainage channels is marked. The direction of flow of irrigation water is marked in each field.

A key bund is marked on the field, with a slope of 0.2 to 0.3 per cent along the direction of the flow of irrigation water. After the location of the key bund, other bunds are marked parallel to the key bund, at a horizontal distance determined by Eq. 21.12. A typical layout is shown in Fig. 21.11.