Last Updated on August 4, 2018 by Naeem Javid Muhammad Hassani
TECHNIQUES FOR SOIL CONSERVATION:
Contour bunds are small walls constructed along the contour, depending upon the availability of material. Generally, two types of bunds are constructed.
STONE CONTOUR BUNDS:-
If stones are available in the catchments area stone contour bunds are the best choice. Stones are collected & bunds are constructed along the contour. The stone contour bunds are preferred on steep slopes. The first step in bund construction is the alignment of contour lines with the help of A. frame. After the alignment, a foundation of 0.5 ft deep & 2 ft wide is excavated. Then soil so excavated is kept on the uphill side at a distance to be used later on if needed.
The larger stone is placed on downhill side & comparatively smaller stones on the uphill side in the first layer of stones. Well-graded stones should be used for constructing contour bunds to decrease the porosity of the bunds. The use of only large stones causes large spaces for the easy flow of water through the bunds & the purpose of contour bund is not served/achieved well. The bunds constructed only with small stones are not stable. It is important to use a mixture of large & small stones. The normal height of contour bund is 2 ft. A slope of 1:4 should be given to the wall on both sides; so that at the top the width should be left only one foot. Well-protected spillway or outlets are provided at both ends near the drainage lines (streams) for the safe disposal of extra runoff water. The outlets are protected with stone pitching. The soil excavated from the foundation is spread behind the wall structure & seed of local grasses is sown. The contour bunds trap the runoff water & sediment yield from the slopes in between the consecutive contour bunds. The velocity of water is reduced, so the kinetic energy of water is reduced resulting in the mitigation of surface erosion. The sediment trapped behind the bund proves to be ideal soil for plant growth & from here the process of succession also starts naturally.
EARTHEN CONTOUR BUNDS:
On gentle slopes where stones are not available, Earthen dams of generally 2/ height are constructed with the front blade of a tractor or manually. The soil is taken from the uphill side & piled on contour lines. At both ends, spillways pitched with stone are left. The structure of bund is planted with grass tufts for its stability. The alignment along the contour is much important otherwise chances of failure will be more. In the storage area behind the bund structure, the grasses, shrubs & plants of indigenous nature are planted. The choice of species depends upon the environmental conditions & the objective of the management. The function of earthen dames is the same as that of stone contour bunds discussed above in detail.
The earthen ditches of specific dimensions for the purpose of exerting some control on runoff & erosion along with the improvement of soil moisture with its subsequent benefit to vegetative cover.
The main purpose of contour trenches is to aid the recovery of vegetative cover to the extent that vegetation will eventually exert the maximum possible control on surface runoff & erosion. Trenches are especially valuable in situations where surface flow & erosion are preventing revegetation of the area.
DESIGN & CONSTRUCTION:-
The design & construction of contour trenches should be based on the specific objectives for the treatment of the area. The physical characteristics of the site, types of equipment & know how available; following are the general guidelines for design.
- Rainfall intensity record information of at least 25years from local resources.
- The duration of storm & estimated runoff should be based on soil, vegetation & topographic factors of the area.
- Generally smaller contour trenches at more frequent intervals result in better vegetative recovery, fewer hazards & reduced costs as compared to larger contour trenches.
- Check dikes or dividers should be placed in the trenches at 50 ft –100 ft intervals.
- All disturbed area should be seeded & reseeds in the event the first seeding fails.
- The treated area should be visited continually; the broken, rodent damaged & other vulnerable trenches must be repaired timely.
Normally Acacia modesta in the scrub zone & local vegetation of the concerned zone is preferred for planting in the trenches. The design of contour trenches i.e. length, width & depth varies according to different climate & site conditions & the tree species for which these are used. e-g in scrub zone trenches of (6 ft x 1 ft x 1 ft) are used for planting & sowing Acacia modesta. The earth excavated from trenches is piled on the downhill side & extended on both sides. Sowing is done on three rows. The first row at the bottom, second at the sloping surface & the third near the top of the berm. This is done to make sure the germination of at least one row of seeds according to the rainfall received and moisture availability at different levels of the trench.
CONTINUOUS CONTOUR TRENCHES:
A continuous trench along the contour having 0.4 m width & 0.4m depth is excavated. The length varies according to the direction of slope & terrain configuration. At the 4m interval, a plant pit of 0.5 m diameter & 0.5m depth is excavated. The center point of trench slopes towards both the pits for the trapping of runoff water to feed the pits. The center of the trench is kept shallow (0.2m) deep; outlets are provided near drainage line on both ends of the trench for the safe disposal of overflow in case of runoff volume is more than the storage/retaining capacity of the pits & trench. If these trenches are not along the contour; the erosion process may be further accelerated.
MODIFIED CONTOUR TRENCHES:-
4 m long, 0.3 m wide & 0.4 m deep trenches are excavated at the 4m interval along the contour. The center of the trench is kept only 0.2m deep sloping towards both ends. At the ends pits of 0.5m depth and 0.5m m diameter are excavated for planting. The excavated soil is piled on the downhill side & extended upward & outward in a crescent shape for trapping the runoff water. The rows are made in the staggering arrangement. In this way, the erosive power of runoff water is reduced by reducing its volume, velocity & kinetic energy.
This method of modified trenches is very useful for the establishment of plantations in arid & semi-arid conditions; because with a few mm rainfalls, the pits are filled with water. If more rainfall is received the whole trench is filled and as the water is reduced with the passage of time, water goes on concentrating in the plant pits at both ends and moisture remains available for a longer period. This technique helps in the establishment of tree species under rainfed conditions where survival is impossible otherwise.
Small catchments (V-shaped) are very useful & effective for soil & water conservation in arid &semi-arid areas where the main objective is Afforestation. The plant pit having a 0.8m diameter at the top, 0.5m at the bottom and 0.5 m depth are excavated. Two very narrow & shallow trenches of (0.1 x 0.1m) are excavated on both sides the plant pit extending outward and upward up to a distance 4m & making an angle of 900 with each other. The excavated soil is piled downhill side of the plant pit extending up to 1m along the trench on both sides. The level of the berm is kept highest on the lower side of the plant pit & gradually decreased towards both the ends of the berm. The staggered Microcatchment field looks like a checker board. If the runoff is more, the excess is drained from sides of the berm.
SIMPLE PIT PLANTING:
The pits of 2.5 ft diameter and 1.5 ft depth are dug. The surface fertile soil has put aside and is used for refilling after planting the plant. Rest of the dugout soil is placed against the direction of flow of water. This method is generally used in comparatively plain areas having hard and compact soil. The pits should be in staggered arrangement.
EYEBROW PLANT PITS:-
Simple pits the soil of which is deposited downhill side of the pits. So making a shape of eyebrow hence called eyebrow pits. The length of the pit is 1m & the width at middle point is 0.5 m while depth is 0.4m. The spacing of plant pits in the rows is kept according to the requirement of the species planted. The pits are staggered in the adjacent rows for the collection of water & sediment. This moisture is retained & used by the plants for a longer period of time in between two successive rainfalls.
For reducing the flow velocity of channel water, slope length and slope gradient are modified by putting check dams. The check dams change the gully gradient due to deposition of sediment behind the check dam. Secondly, the check dams break the velocity of water at each check dam & velocity comes to zero. The velocity accelerates afresh up to the next check dam. By decreasing the velocity silt and debris are deposited in the gully instead of additional material being eroded away. When the catch basin is filled, a relatively level surface or delta is formed over which water flows with a non-eroding gradient. The additional water flows over the dam through a spillway into a defended area or apron. By constructing a series of such check dams along the gully, a stream channel of comparatively steep slope or gradient is replaced by a “stair-stepped” channel consisting of a succession of gentle slopes with “cushioned” cascades in between.
Porous check dams of indigenous material rocks and posts can be used instead of nonporous check dams of steel & concrete. These porous dams release part of the flow through the structure, & thereby decrease the head of flow over the spillway and the dynamic and hydrostatic forces against the dam porous check dams are simpler & more economical in construction.
CONSTRUCTION OF CHECK DAMS:-
The quality, shape, size, and gradation of rock used in the construction of check dams substantially affect the success and life of the structure. Easily disintegrating, Flat & round (river cobbles) should be avoided. Broken & quarried rock having well anchor capability should be used. If only small rocks are used in a dam, they may be moved by the impact of the first large water flow. Similarly, large boulders leave large voids in the structure and allow destructive piping through these voids. These voids allow the sediment to pass through; the deposition of which is desirable because it increases the stability of the structure reduces effective gradient permits the establishment of vegetation & enhances stabilization of the gully.
Required size & gradation of rock for check dams depends upon the size & type of dam and magnitude of expected flows. The majority of the rock should be large enough to resist expected flow with a blend of smaller stones to fill the voids.
Check dams should have a spillway and apron. The spillway is simply a provision to pass flood water safely over the dam. It requires a low center or notch which draws the overflow towards the middle of the channel. An apron is a protected area below the downstream face of the dam which is essential to prevent falling water from undercutting the base of the dam. Sometimes wire mashing of the structure is done for strength & long life of the structure. The minimum width at the top should be at least 0.4m. Generally, the width at the top should be half of the effective height of the check dam. The upstream slope should be vertical while the downstream slope should have a gradient of 1: 10.
If the wooden material is used for check dam construction, any good fence post material is satisfactory. Use of live willow is recommended where possible because it will root & grow. Deadwood should be treated if used.
The hillside ditch is a discontinuous kind of narrow, reverse-sloped terrace built across the land in order to break long slopes into a number of short slopes so that the runoff will be safely intercepted and drained before causing erosion. The cross-section of this kind of narrow bench is more convenient for maintenance than the conventional type of ditch. They can also be used simultaneously as roads. The distance between two ditches is determined by the degree or percent of the corresponding slope.
They are narrow trenches built along contour lines with a V-shaped or trapezoidal cross-section. They are practiced to intercept and collect the excessive runoff water for increasing infiltration and conservation of moisture for growing trees on these trenches. Gradoni is usually built across the entire slope. Small dikes and barriers are built at certain intervals for conservation purposes. In addition to moisture conservation, they prevent flash floods and torrents. The Gradoni are suitable for soils with relatively high infiltration capacity and on slopes of 40-50% if the subsoil is non-erodable and somewhat permeable.
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NJMH is working as Deputy Conservator of Forests in Balochistan Forest & Wildlife Department (BFWD). He is the CEO of Tech Urdu (techurdu.net) Forestrypedia (forestrypedia.com), Majestic Pakistan (majesticpakistan.pk), All Pak Notifications (allpaknotifications.com), Essayspedia, etc & their YouTube Channels). He is an Environmentalist, Blogger, YouTuber, Developer & Vlogger.