How to Control Soil Erosion at Hill Slopes? | Conservations | Soil Science

The upper reaches of a watershed usually consist of hilly areas. These areas have an undulating topography and are foci for soil erosion. In areas with steep slopes and subjected to soil erosion vegetation do not get established.

Due to lack of vegetative cover soil erosion is accelerated transporting large quantities of silt into the streams below. The uncontrolled runoff from the sloping areas causes extensive damage to the adjoining agricultural lands.

When hill slopes are put under cultivation, erosion becomes more severe due to the soil working required for agriculture. Shifting cultivation is an example for the same. Shifting cultivation as practiced in several hilly regions is a system of farming land under which forests are cut and burnt to raise crops.

After a few years of cropping the area is abandoned. The practice of shifting cultivation removes the natural protective cover of vegetation resulting in accelerated soil erosion. The problems of shifting cultivation and management need to be looked from the socio-economic aspects of the people in hilly areas. In some locations, landslides on the hill slopes could be a problem.

It is necessary to treat the steep hill slopes under soil erosion with appropriate soil conservation measures so that these are reclothed with vegetation and the surface runoff from these areas is controlled. Contour trenching and afforestation, bench terracing and stone terracing are measures suitable for such areas.

Controlling Soil Erosion in Hill Slopes

Contour Trenching:

Contour trenching implies excavating trenches along the contour or along a uniform-level. Contour trenches are used both on hill slopes as well as on degraded and bare waste lands for soil and moisture conservation and afforestation purposes.

These trenches break the slope lengths, reduce the velocity of surface runoff and consequently retard its scouring action and carrying capacity. The water retained in the trenches help in conserving the moisture and provides advantageous sites for sowing and planting.

Even though contour trenches have been used on all slopes, trenching on slopes exceeding 20% is not advisable either technically or economically. Contour trenches have been profitably used in both high rainfall and low rainfall conditions, varying soil types and depths, for soil and water conservation and afforestation.

Specification of Trenches:

Contour trenches may be continuous or interrupted. The interrupted trenches may be constructed in series or in a staggered manner. Continuous trenches are essentially used for moisture conservation in low rainfall areas and require careful layout. Intermittent trenches are adopted in high rainfall areas.

The size of the trenches depends upon the soil depth available at the site. Normally sizes between 1000 sq. cm to 2500 sq. cm in cross-section are adopted. The dimensions of the trench may be 30 cm base with 30 cm top width and square in cross-section.

The trench may also be trapezoidal in cross- section with side slopes of 1 : 1. Knowing the amount of rainfall to be retained in a unit area, it is possible to calculate the size and the number of trenches required.

As far as the length of the trenches are concerned, shorter lengths viz. 3, 5 or 7 m lengths are generally adopted for convenience of layout and construction. However, if careful layout is done longer lengths of trenches (upto 300 m) can be adopted. In areas for pasture development such long trenches are used.

Alignment and Construction:

The alignment of the trenches (i.e., marking the position of trenches on the ground) should start from the ridge. After fixing the necessary horizontal interval, contour lines can be laid out with the help of a levelling instrument. A simple device known as the levelling frame or A-frame level can also be used for this purpose.

This device consists of a triangular wooden frame with a base of 3 to 4 m and of a convenient height. A cross bar is fixed half way between the base and the tip of the triangle. In this cross bar, an ordinary spirit level is counter sunk as shown in the figure.

To use this A-frame, a peg is driven at the starting point, one end of this frame is placed against this peg and the other end is moved up and down on the ground, till the bubble of the spirit level comes to the centre.

A second peg is driven at the forward toe of the frame and the frame is moved through 180° so that the leading toe becomes the hind toe. The forward toe is moved again until the bubble is again at the centre.

By repeating this process, a line along the contour is laid passing through the first, second, third etc. pegs. After the contour line is stalked the trenches may be marked with the help of a measuring tape.

Similarly the alignment of the subsequent contours may be done, fixing the requisite horizontal interval between the contours and the position of the trenches marked on them.

Construction of Trenches:

Construction of trenches should be started from the ridge and progressively extended towards the valley. In no case the trenches in the downstream areas be constructed unless the areas above are covered with trenches.

While excavating the trench the top soil should be kept towards the trench as it can be used for refilling if necessary or otherwise it can be put on that portion of the spoil-bank where planting is to be done. Boulders and gravel from excavation should be stalked on the lower side of the trench to serve as a toe of the spoil-bank.

Trenching should preferably be carried out during winter and spring so that the site is ready for sowing and planting during the first monsoon season. Protection of the trenched area (from animal and human interference) till it is fully covered by vegetation is essential to achieve the desired results.

Bench Terracing:

Bench terracing consists of construction of series of platforms along contours cut into hill slope in a step like formation. These platforms are separated at regular intervals by vertical drops or by steep sides and protected by vegetation and sometimes packed by stone retaining walls. In fact, bench terraces convert the long uninterrupted slope into several small strips and make protected platform available for farming.

In several hilly areas bench terraces have been used for the purpose of converting hill slopes to suit agriculture. In some areas where the climatic conditions favour the growing of certain cash crops like potato, coffee etc., the hill slopes are to be bench terraced before the area cart be put for cultivation of these crops. Bench terraces have also been adopted for converting sloping lands to irrigated fields or for orchard plantations.

Design of Bench Terraces:

For the design of the bench terraces for a particular tract, the average rainfall, the soil type, soil depth and the average slope of the area should be known. In addition the purpose for which the terraces are to be constructed should also be known.

The design of bench terraces consist in determining the-

(1) Type of the bench terrace,

(2) Terrace spacing or the depth of the cut,

(3) Terrace width, and

(4) Terrace cross-section.

Selection of the type of the bench terrace among the three types depends upon the rainfall and soil conditions.

Terrace spacing is generally expressed as the vertical interval between two terraces. The vertical interval is dependent upon the depth of the cut and since the cut and fill are to be balanced it is equal to double the depth of the cut.

The factors that limit the depth of cut are the soil depth in the area and the slope. The depth of cut should not be too high as to expose the bed rock which makes the bench terraces unsuitable for cultivation. In higher slopes greater depth of cuts result in greater heights of embankments which may become unstable.

The width of the bench terraces should be as per the requirement for which the terraces are to be put after construction. Once the width of the terrace is decided, the depth of cut required can be calculated as per the following formulae (Fig 21.8).

After deciding the width required the depth of cut can be calculated from one of the above formulae. An adjustment has to be made between the width and the depth depending upon the land slope and soil conditions.

The design of the terrace cross-section consists in deciding-

(1) The batter slope,

(2) Dimensions of the shoulder bund,

(3) Inward slope of the terrace and the dimensions of the drainage channel in case of terraces sloping inward, and

(4) Outward slope in case of terraces sloping outward.

The batter slope is mainly for the stability of the fill or the embankment.

The flatter the batter slope, the larger the area lost due to bench terracing. Vertical cuts are to be used in very stable soils and when the depth of the cut is small (up to 1 m). Batter slopes of 1/2: 1 can be used in loose and unstable soils.

The size of the shoulder bunds in case of terraces sloping inward be nominal. In case of terraces with flat top and sloping outwards larger sections of shoulder bunds are required as water stands against these bunds.

The bund cross-section depends upon the terrace width and the soil conditions. The inward slope of the terrace may be between 1 in 50 (2 per cent) to 1 in 10 (10 per cent) depending upon the soil conditions. In case of these terraces a drainage channel is to be provided at the inner edge of the terrace to dispose the runoff.

Alignment of Bench Terraces:

Alignment of the bench terraces should start from the ridge and progress towards the valley. The average land slope of the area to be terraced should be determined by taking levels and then the specifications of the terrace worked out.

Contour lines may be marked with the help of the A-frame or a levelling instrument. Taking a contour line as the centre line, the terrace width may be marked on the ground. The alignment may now be examined with all the considerations to the local conditions that exist at the site like depressions, sharp turns, field boundaries etc. and suitable adjustments may be made wherever necessary.

Constructions of the bench terraces may be started from the highest terrace and proceed downwards. By this method, the top soil and the subsoil get mixed up and the top soil may not be available for the terrace surface. In cases where the subsoil condition is not good, it is necessary to keep the top soil apart and again spread it on the terrace.

This can be accomplished by starting the construction of the terraces from the lower most one. After the construction of the first terrace, the top soil from the second terrace may be spread on the first terrace and the process continued for subsequent terraces.

In bench terraced areas, suitable outlets should be provided to dispose the runoff safely. In most of the cases one of the sides of the hill slope where vegetation is well established can be used as the outlet. Where such outlets are not available or feasible, waterways are to be formed to dispose the runoff.

Stone Terracing:

Stone terracing, also known as stone wall terraces are small embankments constructed with stones across the hill slopes (Fig. 21.12). These can be adopted on any slope where stones are available in plenty at the spot. By intercepting the surface runoff, these stone terraces help in retarding the soil loss and conserving soil and moisture.

At the same time, the formation of the stone terraces help in removing the stones that lie scattered on the field and otherwise hinder agricultural operations like ploughing, interculture etc.

Specifications:

The spacing adopted for stone terraces need not be rigid and suitable spacing can be adopted depending upon local conditions. Spacing from 10 m to 30 m can be adopted depending upon the slope. Stone terraces should be as far as possible straight.

Local depression should be crossed straight in the alignment and whenever required a neat curve may be adopted. Deviation from the contour is allowed provided that the bunds are strengthened at depressions by additional stone work to bring the height of the bund at the depressions to the same level as the rest of it. The last stone terrace should coincide with the field boundary.

For the construction of the terraces, a shallow trench has to be dug and the stones collected and packed directly into the foundation and in the super structure to form the terrace. The stones are now arranged to form the shape of the bund. The stones should be properly interlocked.

The soil excavated to form the foundation for the terrace is used for forming a small bund on the upstream side of the terrace. In order to stabilize this bund and the terrace suitable vegetation may be planted on this bund.