The following points highlight the top mechanical measures adopted to control erosion of soil caused by water. The mechanical measures are: 1. Contour Bunding 2. Graded Bunding 3. Bench Terracing 4. Conservation Ditching 5. Contour Trenching 6. Vegetative Barriers.
Mechanical Measure # 1. Contour Bunding:
It is most popular in the country. Contour bunding consists of narrow based trapezoidal bunds on contour to impound runoff water behind them such that it can gradually infiltrate into the soil for crop use. It is, generally, recommended for areas receiving less than 600 mm rainfall and for permeable soils upto slopes of about 6 per cent in agricultural lands.
Important principles to be kept in view while deciding the spacing between bunds are:
1. Seepage zone below the upper bund should meet the saturation zone of the lower bund.
2. Bunds should check the water at a point when the water attain erosive velocity.
3. Bund should not be major obstruction for agricultural operations.
The following formula is used for determining spacing of bunds:
VI = S/a+ b
where, VI = vertical interval (m) between consecutive bunds
S = land slope (%), and
a and b = constants depending on soil and rainfall characteristics.
Low rainfall areas: VI = 30S/3+ 60
Medium and high rainfall areas: VI = 30S/2+ 60
Recommended sections of contour bunds are given in Fig. 4.3. Height of the bund depends on slope of the land, spacing of bunds and maximum rainfall intensity expected. Once the height is determined, base width, top width and side slopes depend on the nature of soil.
Base width of bund depends on the hydraulic gradient of water in the soil. A general value of 1: 4 hydraulic gradients is usually assumed. Side slopes depend on the angle of response of the soil.
The cross sections of bunds adopted in the dry areas of Maharashtra are given in Table 4.9. These cross sections of the bunds can be used as a guide in similar situations.
As regards spacing between bunds, it should not exceed 150 cm vertical drop or 67.5 cm horizontal spacing, whichever is less.
The following schedule, which is used in Maharashtra, can be used as a fair guide:
Where the rainfall is more than 62.5 mm and moisture conservation is important, bunds may be constructed at shorter intervals.
Situations for various bunding options are given below:
In the alluvial soils of Gujarat, a vertical interval of 1.83 m and a cross section of 1.3 m2 were found to be suitable for land with slopes ranging from 6-12 per cent and for slopes less than 6 per cent, contour bunds with cross sections of 0.9 to 1.3 m2 spaced at 0.9 to 1.2 m vertical interval were found to be effective.
Observations and experiments at Sholapur, Bellary and Kota have shown that even in semiarid climate, contour bunding in deep black soils, with montmorillonitic type of clay is not suitable. The low rates of permeability and infiltration in these soils cause a prolonged impounding of water on the upstream side and the crops are consequently damaged.
Black soils develop cracks when the soil is dry and these cracks extend into the bunds. These cracks serve as channels for easy passage of water leading to breaching of bunds due to high intensity rains. As such bunding is not successful for soil conservation in heavy black cotton soils.
Mechanical Measure # 2. Graded Bunding:
Graded bunds or channel terraces are constructed in high rainfall areas of more than 600 mm, where excess rain water has to be removed safely out of the field to avoid water stagnation.
Water flows in graded channels constructed on upstream side of bunds at non-erosive velocities and is led to safe outlets or grassed waterways. Channel portion of the graded bunds is put under cultivation and the graded waterways are permanently kept under grass. Sections of graded bunds are given in Fig. 4.4.
Even in scarce rainfall areas, high rainfall intensities are common during months of peak rainfall. High runoff invariably cause over saturation and damage on upstream of bunds and to some extent on downstream side in medium to deep soils. Such damage may be insignificant in shallow soils or sandy loam soils with high rates of infiltration.
Keeping these problems in view, it is good practice to provide surplus arrangements in all the cases except in areas with less than 375 mm annual rainfall. Clear over-fall stone weir, channel weir, cut outlet, pipe outlet and grass outlet are some of the surplus arrangements for surplus runoff disposal for contour bunding scheme.
Mechanical Measure # 3. Bench Terracing:
A terrace is a combination of ridge and channel built across the slope on a controlled grade. Excess water is led at non-erosive velocity into grassed waterways. Bench terracing is practiced in steep hill slopes, where mere reduction of slope length is not adequate to reduce the intensity of scouring action of runoff flowing down. In addition to slope length reduction, degree of slope is also modified.
Bench terracing consists of transforming relatively steep land into a series of level strips or plant forms across the slope to reduce slope length and consequently erosion. The field is made into a series of benches by excavating soil from upper part and filling in the lower part of terrace. It is normally practiced in 16-33 per cent slope range (Fig. 4.5).
Depending on soil, climate and crop requirement, bench terraces may be table top (level), sloping outwards or sloping inwards. Table top or level terraces are suitable for medium rainfall (750 mm) areas with even distribution and permeable soils. These are ideal where irrigation facilities are available. Bench terraces sloping outwards are constructed in low rainfall (less than 750 mm) areas with permeable soils.
Terraces with slopes inward are adopted in heavy rainfall (more than 750 mm) areas where a major portion of rainfall is to be drained as surface runoff. Suitable drain at inward end of each terrace is provided to drain the runoff through suitable outlet.
Vertical interval depends on land slope, soil type, surface conditions and soil depth.
The formulae evolved for Nilgiris are:
When the batter slope is 1: 1
VI = WS/ 100 -S
When the batter slope is 1.5: 1.0
VI = WS/200 -S
where, VI = vertical interval (m)
W = width of terrace (m)
S = land slope (%)
The minimum width of terrace recommended for Nilgiri is 4.5 m for slope between 15 and 25 per cent and 3.0 m between 25 and 33 per cent. Considering the depth of cutting (d), width of terrace (W) may be computed by the formula
W = 200d/S
where, W and d are in m and S is in per cent.
a. Puertorican Bench Terrace:
Puertorican or natural type bench terrace comprises laying an earthen bund (30-45 cm height) or a vegetative barrier of 1.0 m width along the contour at 1.0 m vertical interval. The space between barriers is cultivated leading to formation of terraces through induced deposition of soil along the barriers in about 4-5 years.
This is a fairly less expensive practice than bench terracing on slopes up to 12 per cent. Vegetative barriers should be established by staggered planting of 2-3 lines of grasses across the slope at appropriate vertical intervals. Recommended grass barriers are guinea grass, napier, bhabar, vetiver and guatemala. This practice has given excellent results on hilly tracts of TN for increasing potato yields.
b. Conservation Bench Terrace (CBT):
This system has been successfully applied to mildly sloping lands in arid, semiarid and sub-humid regions for erosion control, water conservation and improving crop productivity. The CBT system consists of a terrace ridge to impound runoff on a level bench (recipient area) and a donor watershed, which is left on its natural slope and produces runoff that spreads on the level bench.
Crops such as maize, sorghum, pearl millet etc. which require drainage are cultivated on sloping area while water intensive crops like rice is grown on the level bench. When the slopping area is kept under grass, horticulture or bushy plantation, this practice is known as Zingg terracing. This CBT system harvests runoff from upper area for the benefit of crops grown in the lower leveled portion of the field.
Thus, even in low rainfall conditions, assured crop can be taken on conserved moisture. The ratio of donor to recipient area varies from 1:1 to 3:1. At the end of receiving area, a shoulder bund is provided to impound runoff up to 20 cm depth. The CBT system has been successfully demonstrated in semiarid region at Bellary (Karnataka) and Kota (Rajasthan) and sub-humid region at Dehra Dun (Uttarakhand).
Mechanical Measure # 4. Conservation Ditching:
It is also known as inverted contour bonding has been evolved by Bellary center;, of the CSWCRTI for black soils in which contour bunds fail due to cracking of deep black soils. It consists of trapezoidal ditches laid out on contours at the same interval as contour or graded bunds to serve as a terrace as well as storage structure in low rainfall deep black soils.
Stored runoff water in the ditches (200-300 m ha-1 of inter terrace land) can be utilised for crop production during dry periods.
An agave filter can be established on upstream side of ditch to trap fine silt and prevent rilling. Conservation ditches have been found effective for reducing runoff and soil erosion in vertisols and increasing rabi sorghum yield due to stored soil moisture.
Mechanical Measure # 5. Contour Trenching:
It consists of excavating trenches along the contour. They are practiced on hill slopes as well as on degraded lands for soil and water conservation. These trenches break the slope length, reduce the velocity of runoff leading to retarded scouring and carrying capacity. Water retained in trenches aids in conserving moisture for sowing and planting. Trenching can be practiced on slopes not exceeding 20 per cent.
Trenches are usually 60 x 45 cm with a spacing of 10-30 m between trenches. The trenches are half refilled diagonally with excavated material and the remaining half of the soil form spoil bank on which planting is done. Trenches are not continuous, but broken at 60 m interval. Rain water held in trenches for some time facilitate its conservation in subsoil for plant use.
Mechanical Measure # 6. Vegetative Barriers:
Although bunding is an effective method of soil and water conservation, it is cost intensive and requires technical skill and maintenance. As an alternative to bunding, biological control of erosion by means of vegetative barriers is advocated.
A vegetative barrier is a permanent strip of closely spaced grass grown on contours across the slope (Fig. 4.6).
Usually, the grass strip is 1.0 m wide with 4 to 8 lines of grass clumps so as to provide a dense and sturdy barrier against runoff. Importance of these vegetative barriers in soil conservation is reattributed to their rapid growth, ease of establishment, capacity to survive under harsh environment, extensive root system and soil binding capacity, improved water infiltration and soil aggregate stability.
The barriers are relatively cheaper and farmer friendly to provide direct benefits like fodder and thatching material.
Suitable grasses for different regions are:
1. Semiarid regions of Central India—khus (Vetiveria zizamioides) and munj (Saccharum spontaneum).
2. Subhumid lower Himalayas—guinea, napier and munj.
3. Shiwaliks—khus, bhabar (Eulaliopsis binata) and munj.
4. Black soil regions—khus, guinea and maroel.
5. Arid regions—dhaman, anjan and sewan.
6. Southern hill regions—napier and guatemala.