In this article we will discuss about:- 1. Classification of Bench Terraces 2. Objectives of Bench Terraces 3. Locations and Conditions for Use 4. Area Lost 5. Maintenance 6. Limitations.
- Classification of Bench Terraces
- Objectives of Bench Terraces
- Location and Conditions for Use of Bench Terraces
- Area Lost in Bench Terracing
- Maintenance of Bench Terrace
- Limitations of Bench Terraces
1. Classification of Bench Terraces:
Bench terraces are the platform like construction, which are constructed along the contour of the sloping land. This type of terraces are generally constructed on the land slope ranging from 6 to 33%. Bench terraces play a significant role to make the hilly land suitable for cultivation.
In this terrace system the hilly land is modified in the form of several steps, which intercept the flowing water on the soil surface. These steps are also used for cultivation purposes. Generally, tea, coffee, sugarcane etc., are successfully grown on these terraces, depending on the rainfall.
Bench terraces are again classified into following three types based on the purpose of their construction:
1. Hill type bench terraces
2. Irrigated type bench terraces; and
3. Orchard type bench terraces.
Hill type bench terraces are generally constructed in those hilly areas which have reverse land slope towards the hill. Irrigated type bench terraces are also known as level bench terrace, and are generally constructed in irrigated conditions. Orchard type bench terraces are constructed in the form of narrow strips, are widely used for orchard plantation purposes.
Rama Rao (1974) has also classified the bench terraces on the basis of the slope of bench, given as under:
1. Level Bench Terraces:
This type of bench terraces consist of level top surface. Level bench terraces are generally used in those areas, which receive medium rainfall and have highly permeable soils.
Since, the soils are highly permeable, therefore, it is expected that most of the flowing surface runoff passing through these terraces are absorbed by the soil, and remaining portion is drained through a suitable drain. These terraces are also known as irrigated bench terrace provided that they must be under irrigation.
Sometimes, level bench terraces are also called table top or paddy terraces, because such terraces have level top surface that can be easily impounded with water and plantation of paddy crop can be performed. The level bench terraces for paddy cultivation have mild slope top, i.e. 1 %, so that a uniform water impounding over them can be easily made.
2. Bench Terraces Sloping Outward:
These bench terraces are used in low rainfall areas with permeable soil. In these terraces a shoulder bund is essential. The main function of shoulder bund is to provide stability to the outer edge of the terrace. In addition, shoulder bund also helps in retaining the surface runoff on the benches that is either absorbed by the soil or drained. Bench terraces sloping outward are also known as orchard type bench terrace.
For such type of bench terraces constructed on the soils having poor permeability, the provision of graded channel at lower end is most essential for disposing surplus surface water to the grassed waterway (outlet). On the other hand, in very less permeable soil, a strong bund along with spillway arrangement is essentially provided for making the terrace safe against heavy rainfall and allowing the water very safely, downward to the next terrace.
3. Bench Terraces Sloping Inward:
The bench terraces sloping inward are preferred to construct in the areas of heavy rainfall and less permeable soils, from where large portion of rain water is drained as surface runoff. Such type of bench terraces have the provision to drain the runoff from their inner side by constructing a drainage channel, as shown in Fig. 13.2. The drain has connection to a suitable outlet (grassed water ways).
This type of bench terrace is also known as hill type bench terrace. The inwardly sloping bench terraces are preferred for growing those crops, which are extremely susceptible to water-logging. These terraces are very common in Nilgiri hills of Tamil Nadu, steep Himalayan slopes of Himachal Pradesh and North-Eastern hill regions.
4. California Type Terrace:
This type of bench terrace is also known as Puertorican type bench terrace. These are constructed by excavating the, soil in little amount at the time of each ploughing, and simultaneously making bench by pushing the soil down-hill against vegetative or mechanical barriers laid on the contour. These barriers should be placed across the slope at suitable intervals. Construction of these terrace is very slow; it may be throughout the year.
2. Objectives of Bench Terraces:
Various objectives of bench terracing are given as below:
i. To reduce the quantum of overland flow/sheet flow or runoff, and their velocity.
ii. To minimize the soil erosion.
iii. To conserve soil moisture.
iv. To conserve soil fertility, and to facilitate farming operations such as ploughing, irrigation etc., and transportation on sloping land.
v. To promote intensive land use, permanent agriculture and check shifting cultivation on steep lands.
3. Location and Conditions for Use of Bench Terraces:
The bench terraces are particularly suitable under following conditions:
i. Areas with severe erosion hazards.
ii. Areas with small-holdings and dense population.
iii. Areas with food shortage or high unemployment.
iv. Areas, where crops require flood irrigation.
v. In the areas, where relatively deep soil is there.
vi. On the slopes not exceeding 25 degrees.
vii. The sites, not dissected by the gullies, and not too stony.
Bench terracing involves the loss of cultivable area. The additional area engaged (Lost area) under bench terrace is only due to batter slope. It can be calculated by using the following relationships.
The newly constructed terraces are required to protect their risers and outlets, mainly, especially during first two years of construction, because of un-stabilized filled soils. If measures are not taken about their protection then there is possibility of getting damage; and even their destruction, too. Normally, the protection and maintenance of newly constructed bench terraces are performed in following aspects.
i. Riser Protection:
After constructing the bench terraces, their protection is very essential, otherwise they get destroy. Especially, the risers require immediate protection. The risers should be shaped to a proper shape; and planted with grass, immediately after construction. Normally, for this purpose the sod forming or rhizome-type grasses are being better than the tall or bunch-type vegetations.
However, preference should always be given to the local species. The rhizome-type grass is being very good for protecting the risers. However, if stones are readily available in nearby area, then they can also be used for protecting and supporting the risers. A recent method, called hydro seeding can also be used for protecting the bench terrace risers.
ii. Outlet Protection:
Outlet is an important component of terraced land. It receives the excess rainwater of the terraced area, and drains to a suitable point. The outlet may be the grassed waterway or any other suitable points where excess rainwater could be safely stored. The outlet’s grade is normally kept more, as result the runoff flows with high velocity. At high flow velocity there is possibility of soil scouring, which may damage the outlet.
To avoid this happening the outlet section should also be planted with suitable grasses. The stones can also be used for this purpose. The stones are packed in the outlet section, which dissipate the KE of flowing water, as result the erosive power of water flow gets reduce; and the outlet remains free from erosion.
After construction of the bench terrace, the next step goes to its maintenance. Normally, the following parts of bench terrace are required very careful maintenance.
Their maintenance is as below:
1. Shoulder bund should be planted with permanent grasses.
2. Toe part of the bund should be avoided from ploughing operation.
3. Batter slope should be protected by establishing deep rooted grasses.
The bench terraces require regular care and maintenance. On bench terrace maintenance it is said that, if a small break is neglected, then there can be large-scale damage. Very careful maintenance of bench terrace is required during first 2 to 3 years period, after construction.
The maintenance of bench terrace as whole of its section, is done as below:
Maintenance of Benches:
It is done in following respects:
i. The toe drains should be kept open and properly graded.
ii. Water should not be allowed to accumulate at any part of terrace.
iii. All the runoff should be allowed to get drain into a protected waterway.
iv. The obstructions such as continuous mounds etc., should be removed at regular time intervals, so that the runoff could pass to the toe drain, easily.
v. The grasses and weeds should be removed from the bench sections.
vi. A proper gradient should also be maintained; and reshaped after harvesting of crops.
vii. The tillage operations should be performed with care, so that the toe drains could not get damage.
The following points are taken into consideration regarding maintenance of risers of bench terraces:
i. The planted grasses on riser should be well in growing condition.
ii. The weeds and vines, which threats the survival of grasses, should be removed from there.
iii. The grasses should not be allowed to grow, very much
iv. Any small break or fall in the riser must be repaired, immediately.
v. The cattle should not be allowed to graze the grasses or trample the risers.
vi. The runoff should not be allowed to flow over the risers, in inwardly sloping terraces.
The maintenance of outlet is performed under following points:
i. The outlets should be checked whether they are adequately protected or not.
ii. Make sure that the water should not get flow around the outlet; it should always flow from the outlet, only.
iii. If there is any break, that must be repaired, immediately.
In order to keep the productivity of bench soil in proper limit, the following measures can be used:
i. Improve the bench soil by deep ploughing, ripping or sub-soiling.
ii. Use green manuring, and add compost or sludge to improve the soil fertility.
iii. Adopt proper crop rotation practices to improve soil productivity.
iv. Use the fertilizers in proper dose as per soil condition.
6. Limitations of Bench Terraces:
The following limitations are observed in case of bench terraces, used for erosion/soil loss control:
1. Although, the construction of bench terraces is for permanent improvement of land, but they require heavy expenditure of money. Therefore, it is usually not recommended until the area is either under irrigation or heavy rainfall to facilitate the cultivation of agricultural crops.
2. For the land slope ranging up to 15%, this practice is recorded as suitable. In special conditions it can also be allowed for the land slope up to 33%.
3. This measure is possible at the sites, where soil depth is sufficient to permit levelling operation. Normally, it is recommended that after construction a minimum of 15 cm greater soil depth than the half of VI, should be existing there.