TERRACING.docx

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TERRACING
Introduction
 Terraces are ancient erosion control practice that facilitates cultivation on steep
slopes.
Eg:-Production of grains such as Rice and Wheat on steep slopes
 A Terrace system consists of multiple continuous lines of embankments constructed
across a field slope to check erosion under the most intense cropping system
planned for the field.
 Runoff water intercepted by the embankments is conducted to a stable outlet
through non erosive stable channels or stored and released through soil infiltration.
 It reduces the length of hill side slope, thereby reducing sheet and rill erosion and
prevents formation of gullies.
Objectives of Terracing
 Land use
To facilitate farming on steep slopes
 Soil conservation
To check soil erosion
 Water Conservation
Reduce runoff
Increase infiltration rate
Functions of Terracing
 to reduce erosion by
o shortening length of slope
o conducting runoff water on a non erosive grade to a stable
slope
 to conserve moisture by
o increasing infiltration opportunity time
 facilitate cultivation and tillage operation on sloping land
Limitations
 can be used in all soils except
too storny, sandy or shallow to permit practical and economical
constructions and maintenance-due to earth moving limitations and
the detrimental effect of the loss of farmability after disturbance of
these soils.
 Not advisable
o Slope of the land is either too slight/too excessive

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o Topography is extremely irregular
o Soil depth is not sufficient-earth moving during construction
may place subsoil on top of the fertile top soil and affect
productivity.
Classification of Terraces
In India ,the Terraces are classified into two major types:
 Broad base Terraces-adopted where the main purpose is either to remove /to retain
water on slopping lands that are suitable for cultivation.
 Bench Terraces-are mainly adopted to reduce the land slope.
і)Broad-base Terraces
 Terraces for which all constructed and excavated slopes are flatter than 5:1 and the
entire terrace is cropped.
 Earthfill for the Terrace ridge is normally obtained from excavation of the Terrace
channel.
 Adopted for land slope<8%
 Preferred due to ease of construction with any earthmoving equipment simply by
moving the soil downhill to form a channel/ridge.
 Flat slope allow farm equipment to operate on the constructed ridge slope,
therefore allowing cropping of the entire field.

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A.Graded/Channel type
 Constructed by cutting a shallow channel on the uphill side and using only these soil
to build the embankment.
 Primarily removes excess water and minimises erosion by
1. Reducing slope length
2. Conducting the intercepted runoff to a safe outlet as a non erosive
velocity.
 Side slope of both channel and ridge kept as possible to facilitate farming operations.
B.Level/Ridge type
 Primarily used for moisture conservation.
 In low to moderate rainfall regions they trap and hold rainfall for infiltration into soil
profile.
 Embankment is constructed of soil from both sides of the ridge.

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іі)Bench Terrace
 Bench Terrace consist of series of platforms along contours cut into hill slope in a
step like formation.
 Platforms are separated at a regular intervals by vertical drops/by steep sides and
protected by vegetation and sometimes by stone retaining.
 It convert a steep slope to 20-30% to a series of level/nearly level benches
 Costly to construct.
A.Absorption/Level/Table top Terraces
 Constructed exactly on the contours and provided with sufficient freeboard so that
they can trap all the rain that falls.
 Suitable for areas having evenly distributed medium rainfall and having deep and
highly permeable soils.
B.Outward sloping Terraces
 Bench Terraces sloping outward are effective only in low rainfall areas with a
permeable soil of medium depth.
 Shoulder bund is essential.
 Useful for slopes upto 15%.
 Suitable for Orchards.

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C.Inward sloping Terraces
 Bench terraces of sloping inward type are more effective in heavy rainfall areas.
 Suitable drain at the inward end of each of these terraces is to be provided to drain
runoff.
 Prevent ponding of water and useful for crops that are susceptible to water logging.
D.Puerto type Terraces
 Levelling is done gradually by cultivation.
 Bunds are constructed on the place where the terrace wall would have been
positioned.
 Instead the land is ploughed ,harrowed and cultivated from the lower side of each
plot turning the sliced soil each plough line towards the lower side. Gradually the
plot becomes levelled automatically.
Design of Bench Terraces

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 The design of bench terraces consists of determining the type of the bench
terrace, terrace spacing or the depth of the cut, terrace width, and terrace
cross section.
 Terrace spacing is generally expressed as the vertical interval between two terraces.
The vertical interval (D) is dependent upon 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 (W) should be as per the requirement (purpose) 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 using the following formulae
Case 1: When the terrace cuts are vertical
Case 2: When the batter slope is 1:1
Case 3: When the batter slope is ½: 1
(S is the land slope in percent; D/2 is the depth of cut and W is the width of terrace)
Submitted by,
Farzana Haris

TERRACING.docx

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