Transcript TERRACING - agronomy society of pakistan

TERRACES
Haris Khurshid
2008-ag-1775
What is Terrace. . ?
“A terrace is an embankment or
ridge of earth constructed across a
slope to control runoff and
minimize soil erosion”
TERRACING
“It is the practice of making such
structures like embankments along with
flattened portions of land on a slope to
utilize it precisely, keeping in mind the
protection and improvement of soil”
Or
“Terracing is the mechanical practice
used on slopping agricultural land to
reduce the length and /or the degree of
slope”
TERRACE FARMING
“Cultivation of crops on terraces
keeping suitability of a particular
crop and good crop husbandry
practices in mind to utilize that
land maximum to get wholesome
returns”
Objectives of Terracing
 Land use

Soil conservation
 To check soil erosion
 To improve soil structure
 To maintain fertility status

Water conservation
Increase infiltration rate
Decrease runoff
Storage of water by water
harvesting occasionally
Types of Terraces

Ridge Type Terrace:
Which remove or retain water on sloping land

Level terrace or contour Bunding

Bench Terrace:
Which reduce the land slope
Ridge type Terraces
Depending on the width of the base,
ridge type terrace may be classified
as:
Narrow Based Terraces
 Medium based Terraces
 Broad Based Terraces


Narrow-base terraces;
Where soil movement is limited to about
three meters

Medium-base terraces;
Where soil movement is three to six meters

Wide or broad-based terraces;
Where soil is moved more than six meters,
but normally less than 12 meters
Broad based Terraces
A broad base terrace has a ridge 25 to
50cm high with gently slopping sides and
a dish stopped channel along the upper
side constructed to control erosion by
diverting runoff at a non –erosive velocity.
based on greed; it is divided or classified
as:


Level Terrace
Graded Terrace
Level terrace:
A level terrace follows the contour line, in
control
to
a
graded
terrace
and
recommended in areas having permeable
soil.
Graded terrace:
A grads terrace has a constant or variable
grade along its lengthened used to convey
excess runoff at safe velocity into a vegetated
waterway or channel.
Bench Terracing:
A bench terrace is shelf like
embankment of earth with a level
or nearly level top and a step or
vertical downhill face constructed
along the contour of sloping land.
In very stable soils, the walls may be
held by vegetation, and in others
some structural walls are necessary,
as brick, stone or timber.
Cross section showing benched effect
.A
sketch of a typical bench terrace
Classification of bench terraces
based on slope
1)
Absorption or Level Terraces

These are constructed exactly on the contour
and provided with sufficient freeboard so that
they can trap all the rain that falls.
They are principally used in dry areas where
rainfall is low and moisture for crop
production is, therefore, at a premium.

Absorption or Level Terraces
2)
Outward-sloping terraces

Bench terraces sloping outwards are effective only
in low rainfall areas with a permeable soil of medium
depth at lower ends graded channels are provided
for safe disposal of runoff.

Suitable for orchards

Better on steep slopes

Used to mild slope

Slope up to 15%
Some terraces have a grassed waterway outlet. . .
While other terraces channel runoff water to a
storage section where it is released slowly through a
surface inlet and underground pipe or tile. . .
3) Reverse/Inward sloping Terraces
In heavy rainfall areas, bench terraces of sloping inwards
type are more effective. It prevents in-ponding of water
and useful for crops susceptible to water logging.

No super-saturation of
profile

Back slope stabilization

Runoff towards hillside
4) Conservation bench terraces
o
Modified form of level terrace
o
In limiting soil moisture
Use moisture of
Uncropped area by runoff
o
Conservation bench terracing
Pictorial comparison of slope based bench terraces
Classification of bench terraces
based on use
Depending upon the purpose for which they
are used, bench terraces are classified as
follows:

Hill type Bench Terraces

Irrigated Bench Terraces

Orchard Bench Terraces

Hill type Bench Terraces:
It is used for hilly areas whit a reverse
grad towards the hill.

Irrigated Bench Terraces:
Level benches are adopted under
irrigated conditions. The level table
top terraces are referred to as
irrigated bench Terries.

Orchard Bench Terraces:
Narrow width terraces for individual
trees are prepared in this type. These
are also referred as intermittent
terraces and step terraces. The
conversion of land into bench
terraces over a period of time is
referred as gradual bench terracing.
Designing & construction of
Bench terraces
Factors of Site suitability:
Soil depth
• Land slope
• Implement widths
• Back slope stabilization
• Runoff disposal
•
Steps in design of bench Terraces
Step I-Type of the bench terrace:
Selection of the type of bench terrace depends upon
the rainfall and soil, condition.
Step II -Terrace Spacing:
It is normally expressed in terms of the vertical interval
between two terraces. It depends upon soil, slope,
surface condition, grade and agriculture use.
Considerations for spacing:




Find Out the maximum depth of productive soil,
maximum admissible cutting depth
Given land slope
From this, The width of the terraces can be computed for
a given slope
Step III: Terrace cross section:
Design of terrace cross consists of deciding
1] The slope
2] Dimensions of shoulder bund
3] Inward slope in case of terraces
sloping inward, and
4] Outward slope in case of terraces
sloping outward.
Step IV:
The cross- section of shoulder bund
along outer edge of terrace should
also be designed suitably to make the
bund stable against slipping.
TERRACING COSTS
The costs that will be incurred in
constructing terraces is largely a function of:
 Width
of terrace,
 Slope of land,
 Type of protection for the back slope
 construction machinery.
Construction using a disc plough
C LINE=CUT LINE
F LINE=FILL LINE
BENEFITS OF TERRACING
 Avoid soil erosion
Up to 25% reduction in soil loss
Terraces in China (photo by UN - World Food Programme)
Benefits . . .





Reduction in Runoff(up to 30-35%)
Soil structure integrity
Proper land use
Feasibility of crop production in hilly areas
Increase in soil moisture level
Benefits . . .

Maintenance of soil fertility

Water conservation(up to 45-55% saving)

Enhanced infiltration rate

Improved water holding capacity
TERRACE FARMING WORLDWIDE
Terraces in northern Luzon Philippines
2000-B.C
Level terracing in JAPAN
Tea on bench terraces, BHUTAN
KOREA
TERRACES ON ERODED SOILS OF ISRAEL
TERRACED FIELDS IN SA PA, VETNAAM
RICE TERRACE PENORAMA IN BALI
CHINA
FLAT TERRACES IN HUNZA VALLEY, GILGIT
SWAT VALLEY, PAKISTAN
GRASSY TERRACES IN CHITRAL, PAKISTAN
KASHMIR, PAKISTAN
FUTURE OF TERRACE FARMING
THANKS
Questions ???