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TERRACING - Coggle Diagram
TERRACING
Introduction
One of the oldest means of efficiently utilising soil and water also conservation practice throughout the world for agriculture & slope protection method
Terraces serve the functions of stopping downslope soil and water movement and also give the advantage of providing a flat surface for the planting of crops, thereby further reducing the possibility of erosion.
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properly maintained, if not they can provoke land degradation
Terracing has to be combined with additional soil conservation practices, of which the most important one is the maintenance of a permanent soil cover
There are several disadvantages to terracing, therefore hedgerows and vegetation ridges could be good alternatives for terraces
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Refers to building a mechanical structure of a channel and a bank or a single terrace wall, such as an earthen ridge or a stone wall
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Created to intercept surface runoff, encourage it to infiltrate, evaporate or be diverted towards a predetermined and protected safe outlet at a controlled velocity to avoid soil erosion
Before doing the terracing, you need to know about
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Slope and gradient, exp: 1:500, 30% etc.
Topographical landscape i.e. River, valley etc.
Type and soil profile i.e. Laterite, sandy or clayey
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Drainage, Water Flow And Water Table
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Types Of Terraces
Their Main Function
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also called absorption or level terraces, designed to accumulate and retain runoff in the terrace channel so that it will eventually infiltrate and the sediment accumulates
recommended for low rainfall areas, permeable soils, and for land of less than 8% slope,
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these are sloping terraces, designed to intercept or divert runoff into protected waterways.
recommended for high rainfall regions, for slightly or moderately permeable soils, and for slopes of between 8 and 20%
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The Construction Process
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constructed by excavating soil from the upper side only to form a channel, and depositing it downhill to form a bank
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used in high rainfall regions, and in soils of low or medium permeability
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a long, low ridge of earth with gently sloping sides and a shallow channel along the upper side
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recommended for slopes <20%, for low rainfall regions, and for permeable soils
control erosion by diverting surface runoff across the slope instead of permitting it to flow uninterrupted down the slope.
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The Shape
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consists of a ridge or bank and a channel, which may be constructed on a gradient or level.
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these terraces form a series of level or nearly level strips of earth and a steep or vertical downhill face, constructed on or nearly on the contour
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constructed by cutting and filling, and are used in land with slopes > 20%.
one of the oldest forms of terraces, other types of terraces have been derived from this terrace type
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Anti Erosion Maintenance
Silt Pits
In more nearly flat areas, adequate soil conservation may be achieved by constructing silt pits and bunds
Silt pits - consist of the digging of trenches some 2.5m long, 60cm wide and 60cm deep along the marked contour, with a gap of 60cm to 90cm between trenches
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It has to be regularly maintained to remain effective by clearance of the silt pits, placing the trapped silt in debris back over the land and effecting repairs as necessary to the bunds
Contour Ditches
Individual terraces are constructed to provide a level platform for the growth of an individual tree. They are always used in combination with another type of conservation structure, such as contour ditches, since the small platforms by themselves do not appreciably control surface water movement.
Terraces Maintenance
Maintenance Of Terraces
If terraces are necessary to protect an area, a study should be carried out on the nature of the soils
record the location of natural drainage lines, low- lying areas, and sites suitable for constructing runoff and storm water diversion drains
identify where runoff occurs in neighbouring areas, whether along tracks or in gullies
take notes on the slope gradients, slope lengths, the presence of rill erosion or gullies, and the location of roads and tracks
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Optimal distance between terraces on slopes with different gradient is not really known, for example, although 50% slope is usually considered a threshold for making terraces, in Nepal terraces are made on slopes exceeding this limit and cultivation is practiced even on 100% slope by means of contour terracing
A practical guide for installing terraces that deals with issues such as slope steepness and terrace size is provided by FAO (2000).
An alternative for constructing terraces, which requires a lot of work, time and material, is the use of permanent vegetation barriers, whether or not combined with small contour ridges made by stone or earth.
These are strips of vegetation planted along the contour at certain intervals within the main crop, and consist of perennial species that develop a dense cover capable of reducing the velocity of runoff
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An example of permanent vegetation barriers are contour hedgerows of nitrogen-fixing trees or shrubs
Terraces degrade naturally by erosion and sediment, and can be damaged by machinery, animals, and settling
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The best time to check is after rains, when erosion, sedimentation, and unevenness in elevation are easiest to see
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removing earth from borrow pits dug at the back of the terraces, in between the planted crop, and using this earth to make the repair