Water Erosion Control Measures for Hilly Areas, Terracing, Bench terrace, Contour bunds, graded bunds, contour trench etc.

1. WATER EROSION CONTROL
MEASURES FOR HILLY AREAS
Lecture-8
Dr. Ajay Singh Lodhi
Assistant Professor
College of Agriculture, Balaghat (M.P.)
Jawahar Lal Krishi Vishwa Vidyalaya, Jabalpur (M.P.)

2. TERRACING
 It it an engineering soil conservation practice, used to control the soil erosion in
highly sloped areas.
 Terracing involves the construction of embankment or ridge and steps like
structure, across the land slope to check the flow of surface runoff and to reduce
the soil loss.
 In this system, the effective length of land slope is reduced to a large extent.
 From experimental evidence it has been found that the soil loss is directly
proportional to the slope length of power 0.5. According to this statement, if
the length of slope is increased as twice the soil erosion increases in proportion
of 1.4 times.
 Terracing practice is not feasible in flat and cropped land.

3. The terracing involves following main features:
 These are constructed across the slope to intercept the surface runoff and
convey it to a suitable outlet, at non erosive velocity.
 They reduce the length of slope by splitting the slope length in different parts.
 The terracing practice is adopted for soil and water conservation in that area,
where land slope is greater than 6%, soil is more erodible and prevails high
rainfall intensity.
 Terraces, not only control the soil loss caused by sheet flow, but also play an
important role in trapping the splashed soil particles.
 This practice is not possible, particularly on those hill sloped areas, where soil
depth is not sufficient. Since it involves greater earthwork, therefore it proves a
costly work.

4.  In India the terraces are classified in two major types, given as under:
 Bench terraces and
 Broad base terraces.
 BENCH TERRACES
 Bench terraces are the platform like construction which are constructed along
the contour of the sloping land.
 This type of terraces are generally constructed on the land of 6 to 33% slope.
 Bench terraces play a significant role to make the hilly land suitable for
cultivation.
 In this system the hilly land is modified in the form of several steps, which
intercept the flowing water through the soil surface.
 These steps are also used for cultivation purposes. Generally tea, coffee,
sugarcane etc. are successfully grown on these terraces, depending upon the
climatic factors such as rainfall etc.

5. Components of bench Terrace

6. Bench terraces are again classified into following three types (Bali, 1978)
based on the purpose of their use.
 Hill type bench terraces
 Irrigated type bench terraces and
 Orchard type bench terraces.
 Hill type bench terraces are generally used in those hilly areas which have
reverse land slope towards the hill.
 Irrigated type bench terraces are also known as level bench terrace and are
generally adopted in irrigated conditions.
 Orchard type bench terraces are constructed in the form of narrow strips and are
widely used for orchard purposes.

7. Rama Rao (1974) has also classified the bench terraces on the basis of the slope of
bench, given as under :
 Level bench terrace
 Bench terrace sloping outward, and
 Bench terrace sloping inward.
LEVEL BENCH TERRACES
 This type of bench terraces consist of level top surface.
 Level bench terraces are generally used in the areas which receive medium rainfall and
have highly permeable soils. Since the soils are highly permeable, therefore it is
expected that most of flowing surface runoff passing through these terraces are
absorbed by the soil and remaining portion is drained into the drain.
 The level bench terraces are also known as irrigated bench terraces provided that they
must be under irrigation.
 Sometimes level bench terraces are also called as table top or paddy terraces, because
such terraces have level top surface that can be easily impounded with water and
plantation of paddy crop can be performed. The level bench terraces used for paddy
cultivation, the bench slope is kept as mild as 1%, so that uniform water impounding
over them can be easily made.

8. Leveled bench terrace. (Source: Das, Ghanshyam., 2002)

9. BENCH TERRACES SLOPING OUTWARD
 Such bench terraces are adopted in low rainfall areas with permeable soil.
 Outward sloping bench terraces are laid out in areas where the depth of soil is
not sufficient for leveling work. At deeper cuts, the infertile subsoil is likely to be
exposed to the surface.
 Bench terraces sloping outward are also known as orchard type bench terraces.
 For these terraces a shoulder bund is essential even though this bund is also
provided in other two types. The main function of shoulder bund is to provide the
stability to the outer edge of the terrace. In addition, this bund also helps in
retaining the surface runoff on the benches that is either absorbed by the soil or
drained.
 For outwardly sloping bench terraces constructed on soils having poor
permeability, the provision of graded channel at lower end is most essential for
disposing surplus surface water to the grassed water way. Whereas, in very less
permeable soil case, a strong bund along with spillway arrangement should be
essentially equipped, for making the terrace safe against heavy storm and
allowing the water very safely, downward to the next terrace.

10. Slopping outward bench terrace (Source: Das, Ghanshyam., 2002)

11. BENCH TERRACES SLOPPING INWARD
 Bench terraces slopping inward are preferred to construct in the areas of heavy
rainfall and deep permeable soils, from where large portion of rain water is
drained as surface runoff.
 Such type of bench terraces have a provision to drain the runoff from their inner
side by constructing a drainage channel. The drain ultimately leads to a suitable
outlet (grassed water ways).
 This type of bench terrace is also known as hill type bench terrace.
 The inwardly sloping bench terraces are usually preferred for growing those
crops, which are extremely susceptible to water-logging, such as potato.
 These terraces are widely used in Nilgiri hills of Tamil Nadu, steep Himalayan
slopes of Himachal Pradesh and North-Eastern hill regions.

12. Slopping inward bench terrace (Source: Das, Ghanshyam., 2002)

13. Limitations of Bench Terraces
The following limitations are observed in case of bench terraces, used for erosion
control:
 Although, it is more or less a permanent improvement of land, but requires heavy
expenditure of money. Therefore, it is usually not recommended until the area is
either under irrigation or under heavy rainfall to facilitate the cultivation of
agricultural crops.
 For the land slope ranges upto 15%, this practice is recorded as suitable. In
special conditions it can also be extended for the land slope upto 33%.
 This measure is possible at the sites, where soil depth is sufficient to permit
levelling operation. Normally it is recommended that, minimum of 15 cm greater
soil depth than the half of VI (vertical interval), should be existing there.

14. BROAD BASE TERRACE
 A broad base terrace is defined as a surface channel or embankment type
construction, which is formed across the land slope.
 Broad base terraces may be classified in two types, based on their primary
functions i.e. to retain or remove the runoff water. These two types are as:
 Graded terrace and
 Level terrace.
 Graded Terrace: This terrace is also known as channel-type terrace.
 To drain excess runoff water from the land, is the primary objective of this
terrace.
 It is constructed by digging a shallow channel on the uphill side and putting the
excavated soil to build the embankment, beside the channel. Generally, the side
slope of channel as well as embankment is kept as flat as possible to facilitate
cultivation activities.
 Graded terrace also reduces the length of slope and thereby the soil erosion as
well as soil loss both are reduced. Similarly, it also removes the surface runoff
with a non-erosive velocity and drains into the outlet.

15. Cross sectional view of a broad-base terrace. (Source:
Das, 2002)
Triangular design
Trapezoidal design

16. Sometimes, graded terrace is also used as drainage channel and referred as
drainage terrace. The drainage terraces are suitable for the areas, where,
 Land slope varies from 3 to 10%.
 Permeability of soil is low.
 Fields are distracted by gullies or depressions.
 Contour cultivation is insufficient to control the surface runoff and soil erosion.

17. Level Terrace
 Level terraces are constructed for conserving the moisture and controlling the
soil erosion.
 These terraces are most suitable in low to moderate rainfall regions because
they trap and hold the rain water on their top, which gets infiltrated into the lower
soil profile.
 Such terraces may also be used on permeable soils for the same purpose.
 Embankment of this terrace is constructed with the help of excavated soil from
the earthwork of channel by keeping them both sides of the channel. The height
of embankment should be such that the runoff water cannot overtop it.
 This type of terrace is also known as conservation terrace. The conservation
terraces are normally found most suitable, where:
 Rainfall is low.
 Rainfall intensities are not excessive.
 Land slope is less than 6%.
 Infiltration as well as soil permeability are enough to absorb the water.

18. BUNDING
 Bunds are simply embankment like structures, constructed across the land slope, used for
retaining the water, creating obstruction and thus to control erosion.
 When they are constructed on the contour of the area called as contour bund and when a
grade is provided to them, then they are known as graded bund.
 By bunding practice, the entire area is divided into several small parts, thereby the effective
slope length of the area is reduced. The reduction of slope length causes not only to reduce
the soil erosion but also retain the runoff water in the surrounded area of the bund.
 Contour bunds are generally used in relatively low rainfall areas (<600mm per year) for the
purpose to control the soil erosion and to store the rain water, while graded bunds are
constructed in relatively medium to high rainfall areas (>700mm year) for the same purpose
as the contour bunds.
 Generally, bunding practice is found most suitable for the land having the slope, ranges from
2 to 8 percent. However, it can also be used beyond 8 percent land slope but there must be
required to provide a close spacing of bund, which results high cost of construction.
 The bunding is reasonably successful in the shallow and medium- deep soils. Black soils are
not suitable for bund construction.

19. CLASSIFICATION OF BUNDING SYSTEM
The bunding systems are divided according to their function, which they perform. It is
given as under:
 Contour Bunding
 Narrow based
 Broad based
 Graded Bunding
 Narrow based
 Broad based
 Contour Bunding
 The formation of bunds passing through the points of equal elevation (i.e. on contour) of
the land, is defined as contour bunding. The contour bunds and level terraces are the
synonymous term. Therefore, sometimes contour bunding is also denoted as level
terrace.

20.  The contour bunding can be adopted on all types of permeable soils such as alluvial,
red, laterite brown soil, shallow and medium black soil but clayey or deep black cotton
soils are strictly avoided for this work, as these soils have the problem of crack
development, which causes the bund failure against its function. In addition, clayey soils
also born the problem of water-logging condition near the bund section, which makes the
bund construction unfeasible.
The main functions of contour bunds are given as under:
 It reduces the length of slope, which in turn to reduce the soil erosion.
 Impound the water at u/s portion and permits more water to recharge into the soils that is
utilized for crop cultivation.
The various limitations of this practice are as follows:
 This practice is suitable for those areas, which receive the annual rainfall upto 600 mm
and existing soil involves greater permeability.
 It is not used in clayey soils.
 Contour bunding is not technically feasible on the land slopes, greater than 6%.

21.  The contour bunding is further sub-divided into following two types.
 Narrow based contour bunding: In this bunding system, an obstruction is created for
crossing of farm implements; natural vegetations cover the sides and more height is
attained for same cross-section. It has limitations that, there is considerable area is lost
in constructing the bund; bund section is also liable to get affected by erosion due to rain
drop impact, hence required a sincere maintenance.
 Broad based contour bunding: The broad based contour bunding is concerned, it
does not create hinderance in farming operations; entire area can be kept under
cultivation. But it has some limitations too, such as disturbance of bund's section due to
crossing of farm equipments as a result there is required an attentive care and
maintenance.
 Apart from above, the soil of bund is also loosened during movement of farm
machineries, causing reduction in the size of bund in a very short period, unless there
some proper maintenance is adopted.

22. Limitations:
 It requires establishment of grassed waterway as an outlet for safe disposal of surplus water,
accumulated over bunded area.
 Graded contour bunding is not recommended on the land slopes less than 2% or on the
slopes greater than 8%.
 The grassed waterway needs an extra care about control of grazing of grasses, grown in the
section of waterway.

23. Graded Bunding
 The graded bund is also referred as channel terrace, used in the areas having rainfall
greater than 700 mm per year.
 However, it can also be used in less rainfall areas, provided that the soil of area should
be in heavy texture. In such areas what happens when rainfall takes place a large
portion of water is ponded over the surface; to remove this water some grade is provided
to the channel.
 The grade, should lie within non-erosive limit. Normally minimum as 0.1 to maximum
0.5% is allowed.
 It should also be noticed that, the grade of channel must be less than the actual land
slope. In this condition, the water flows at slow speed to that of flowing directly down the
slope, causing not taking place of soil erosion from there.
The main functions of graded bund are given as follows:
 It reduces the length of slope as a result the soil erosion gets reduced.
 It also disposes the excess water very safely to a suitable point.
 Contour bunding is followed with the construction of bunds to remove excess water
which is not required by the crop; through the graded bunds.

24. Contour Trenching
 The contour trenching is also one of the practice, used as a soil conservation measure to
intercept the runoff.
 In this system, several trenches are excavated along the contour in a uniform level
across the land slope on the top portion of the catchment. The excavated earth materials
are placed at down-stream side of the trench, to make bund along that.
 The function of contour trench is to break the runoff velocity and intercept the runoff. The
rain water collected into these trenches is slowly and slowly percolated into lower soil
strata and travels horizontally or laterally, as a result the lands of middle and lower
sections of the catchment, are enriched by moisture content.

25. Thank You